Integrated Materials and Construction Practices for Concrete Pavement.pdf

Technical Report Documentation Page

1. Report No. 2. Government Accession No. 3. Recipient’s Catalog No.

FHWA HIF - 07 - 004 4. Title and Subtitle 5. Report Date

[December 2006] October 20076. Performing Organization Code

Integrated Materials and Construction Practices for Concrete Pavement: A State-of-the-Practice Manual

7. Author(s) Peter C. Taylor, Steven H. Kosmatka, Gerald F. Voigt, et al. 8. Performing Organization Report No.

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) National Concrete Pavement Technology Center/ Center for Transportation Research and Education Iowa State University 2711 South Loop Drive, Suite 4700 Ames, IA 50010-8664 515-294-8103 (voice)

11. Contract or Grant No. ISU/FHWA Cooperative Agreement No. DTFH 61-01-X-00042 Project No. 10

12. Sponsoring Organization Name and Address 13. Type of Report and Period Covered Manual 14. Sponsoring Agency Code

Federal Highway Administration Office of Pavement Technology 400 7th Street S.W. Washington, D.C. 20590 15. Supplementary Notes FHWA Cooperative Agreement Technical Representative: Gina Ahlstrom 16. Abstract At the heart of all concrete pavement projects is the concrete itself. This manual is intended as both a training tool and a reference to help concrete paving engineers, quality control personnel, specifiers, contractors, suppliers, technicians, and tradespeople bridge the gap between recent research and practice regarding optimizing the performance of concrete for pavements. Specifically, it will help readers do the following: Understand concrete pavement construction as a complex, integrated system involving several discrete practices that

interrelate and affect one another in various ways. Understand and implement technologies, tests, and best practices to identify materials, concrete properties, and

construction practices that are known to optimize concrete performance. Recognize factors that lead to premature distress in concrete, and learn how to avoid or reduce those factors. Quickly access how-to and troubleshooting information.

17. Key Words 18. Distribution Statement Concrete pavement, concrete materials, construction, hydration, materials incompatibilities, QA/QC, optimizing pavement performance

No restrictions. This document is available to the public through the Center for Transportation Research and Education, Iowa State University (address above).

19. Security Classification (of this report)

20. Security Classification (of this page)

21. No. of Pages 22. Price

Unclassified. Unclassified. 350 N/A

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

Integrated Materials and Construction Practices for Concrete Pavement: A State-of-the-Practice Manual

FHWA Publication No. HIF - 07 - 004

[December 2006]October 2007

Co-Principal Investigators Mr.DaleHarrington,P.E.(IA),Snyder&Associates Mr.JimGrove,P.E.(IA),NationalConcretePavementTechnologyCenter,IowaStateUniversity

Technical Editor Dr.PeterC.Taylor,P.E.(IL),CTLGroup

Associate Technical Editors Mr.StevenH.Kosmatka,PortlandCementAssociation Mr.GeraldF.Voigt,P.E.(IL),AmericanConcretePavementAssociation

Managing Editor and Co-Principal Investigator Ms.MarciaBrink,CenterforTransportationResearchandEducation,IowaStateUniversity

Authors ChaptersThefollowingcontentexpertscontributedsignificantlytooneormorechapters:

Dr.MichaelE.Ayers,AmericanConcretePavementAssociation 2,8,9,10 Dr.AllenDavis(deceased),CTLGroup 10 Mr.GaryJ.Fick,TrinityConstructionManagementServices,Inc. 8,9,10 Mr.JohnGajda,P.E.(multiplestates),CTLGroup 5 Mr.JimGrove,IowaStateUniversity 4,9 Mr.DaleHarrington,P.E.(IA),Snyder&Associates 4,5 Ms.BeatrixKerkhoff,PortlandCementAssociation 3,5,6 Mr.StevenH.Kosmatka,PortlandCementAssociation 3,5,6 Dr.H.CelikOzyildirim,P.E.(VA),VirginiaTransportationResearchCouncil 3,5,9 Mr.JamesM.Shilstone,Sr.,P.E.(TX),TheShilstoneCompanies,Inc. 3,6 Mr.KurtSmith,P.E.(IL),AppliedPavementTechnology,Inc. 2,3 Mr.ScottM.Tarr,P.E.(multiplestates),CTLGroup 8,10 Dr.PaulD.Tennis,Consultant 3,5,6 Dr.PeterC.Taylor,P.E.(IL),CTLGroup All Dr.ThomasJ.VanDam,P.E.(IL,MI),MichiganTechUniversity 9 Mr.GeraldF.Voigt,P.E.(IL),AmericanConcretePavementAssociation 1,8,10 Mr.SteveWaalkes,AmericanConcretePavementAssociation 7

Graphic Designer Ms.AlisonWeidemann,CenterforTransportationResearchandEducation,IowaStateUniversity

Illustrator Ms.JaneSterenberg,UnlimitedDesigns

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ThismaterialisbasedonworksupportedbytheFederalHigh-wayAdministrationundergrantno.DTFH61-01-X-0042.

Thispublicationisintendedsolelyforusebyprofessionalpersonnelwhoarecompetenttoevaluatethesignificanceandlimitationsoftheinformationprovidedhereinandwhowillaccepttotalresponsibilityfortheapplicationofthisinformation.Anyopinions,findings,andconclusionsorrecommendationsexpressedinthismaterialdonotnecessarilyreflecttheviewsoftheFederalHighwayAdministrationorIowaStateUniversity.

CenterforTransportationResearchandEducation,IowaStateUniversityResearchPark,2711SouthLoopDrive,Suite4700,Ames,IA50010-8664

PrintedintheUnitedStatesofAmerica,[December2006]October2007secondprintingwithnominalcorrections1514131211100908072345

CenterforTransportationResearchandEducationIntegratedMaterialsandConstructionPracticesforConcretePavement:AState-of-the-PracticeManualSelectedbibliography:pages293–299Includesabbreviatedsubjectindex

LibraryofCongressCatalogCardNumber:2006936814ISBN:978-0-9652310-9-1

TheFederalHighwayAdministration,IowaStateUniversity,principalinvestigators,authors,editors,designers,andillustratorsmakenorepresentationsorwarranties,expressedorimplied,astotheaccuracyofanyinformationhereinanddisclaimliabilityforanyinaccuracies.

TheUnitedStatesGovernmentdoesnotendorseproductsormanufacturers.Tradeormanufacturers’namesappearhereinsolelybecausetheyareconsideredessentialtotheobjectiveofthisdocument.

vii

Acknowledgments

Theprojectteamisgratefultomanyprofessionalsandorganizationsinthenationalconcretepavementcommu-nitywhobelievedinthevalueofthismanualandwhosecontributionsarereflectedherein.Firstandforemost,weareindebtedtotheFederalHighwayAdministration(FHWA)foritsgeneroussupportandsponsorship.Inparticular,GinaAhlstrom,JonMullarky,andSuneelVanikar,OfficeofPavementTechnology,andRickMeininger,OfficeofInfrastructureR&D,gavethisprojectsignificantattentionandenergyduringthecourseofthemanual’sdevelopmentandpublication.

Thecontentofthismanualreflectstheprofessionalexpertiseof15technicalauthors(seetheinsidetitlepage).Collectively,theyrepresentthestateofthescienceandartofconcretepavementdesign,materials,andconstruc-tionintheUnitedStates.Theyalsorepresentthewidevarietyofdiscreteprocessesandvariablesthatmustbeintegratedinordertooptimizetheperformanceofconcreteinpavements.Theauthorshelpedtheprojectteamassemblearesourcethathelpsbridgethegapbetweenrecentresearchandcommonpractice.Thankyou,all.

Wealsogratefullyacknowledgethecontributionsoftheproject’stechnicaladvisorycommittee.Themembersofthiscommitteewereresponsibleforthetechnicaldirectionofthemanual.Theyhelpeddeveloptheoriginalout-lineandcriticallyreviewedseveralheftydrafts.Theirfeedbackandsuggestionswereinvaluable.Thecommitteememberswerethefollowing:

Mr.FaourH.Alfaour,P.E.(OH),OhioDOTMr.AhmadArdani,ColoradoDOT(retired)Dr.Jamshid(Jim)Armaghani,P.E.(FL),Florida

ConcreteandProductsAssociation,Inc.Mr.C.MichaelByers,AmericanConcrete

PavementAssociation(INchapter)Mr.EdDenton,DentonEnterprisesMr.FredFaridazar,FHWAMr.BenjaminJ.Franklin,HeadwatersResourcesMr.MaxG.Grogg,P.E.(VA),FHWA–IowaDivisionMr.ToddHanson,IowaDOTMr.KeithHerbold,MidwestResourceCenter,FHWADr.KennethC.Hover,P.E.(NY),CornellUniversityMr.CraigHughes,CedarValleyCorporationMr.AllenJohnson,W.R.GraceCo.Mr.KevinJones,IowaDOTMr.KeithKeeran,OhioDOTMr.KevinKline,GOMACOCorporationMs.SandraQ.Larson,P.E.(IA),IowaDOT

Ms.FrancesA.McNeal-Page,MaterialsConsultant

Mr.DaveMeggers,P.E.(KS,NE),KansasDOTMr.RichardC.Meininger,P.E.(MD,AZ),FHWAMr.JonI.Mullarky,P.E.(MD),GlobalConsult-

ing,Inc.Mr.KimA.Murphy,KossConstructionCo.Mr.SteveOtto,HolcimDr.AntonK.Schindler,AuburnUniversityMr.GordonL.Smith,P.E.(IA,NE),Iowa

ConcretePavingAssociation.Mr.DaveSuchorski,P.E.(KS,WI),AshGrove

CementCo.Mr.ShannonSweitzer,P.E.(NC),NorthCarolina

DOTMr.JimmieL.Thompson,AshGroveCement

CompanyDr.ThomasJ.VanDam,P.E.(IL,MI),Michigan

TechnologicalUniversityMr.JohnB.Wojakowski,P.E.(KS),Hycrete,Inc.

Finally,butcertainlynotleast,wewanttoacknowledgetheimportantroleprovidedbytheMidwestConcreteConsortiumandthe16-StatepooledfundstudyatIowaStateUniversity,“MaterialsandConstructionOptimiza-tionforPreventionofPrematurePavementDistressinPortlandCementConcretePavements”(TPF-5[066]).Theconsortium’sagency,industry,andacademicpartners—specifically,theirleadershipinstartingthepooledfundstudy—weretheimpetusbehindthepreparationofthismanual.Theysawacriticalneedwithintheconcretepavementcommunityandmadethecommitmenttofillit.

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Table of ContentsAcknowledgments ............ vii

List of Figures .................. xvii

List of Tables .....................xxi

Chapter 1

Introduction ....................... 1

Purpose of This Manual ...............................................................2

Today’s Construction Environment .............................................2

Principles of Concrete Pavement

as an Integrated System ..............................................................2

Optimizing Concrete for Pavements ..........................................3Variables That Affect Concrete Performance..................3Mix Properties That Correspond to Concrete Performance ...................................................3

Organization of This Manual .......................................................4Critical Details: Chapters 3, 4, 5, 6, 9 ..................................4Quick References: Chapters 4 and 10 ...............................5

Chapter 2

Basics of Concrete Pavement Design ............... 7

Integrated Pavement Design ......................................................8Key Points ..............................................................................8Mechanistic-Empirical Design Guide ................................9Basic Concrete Pavement Types .......................................9

JPCP ................................................................................9JRCP ..............................................................................10CRCP .............................................................................10

Design Considerations: What Do We Want? .........................11Key Points ............................................................... 11Pavement Performance .....................................................11

Structural Performance .............................................12Functional Performance ............................................13

Service Life ..........................................................................15Concrete Properties ...........................................................16

Concrete Strength ......................................................16Elastic Modulus ...........................................................17Drying Shrinkage and Thermal Expansion/Contraction...............................................17Durability ......................................................................18

Design Considerations: What Site Factors Do

We Have to Accommodate? .....................................................19Key Points ............................................................... 19

Support .................................................................................19Environmental Factors .......................................................19Traffic Considerations ........................................................19

Design Procedures: Getting What We Want,

Given Site Factors .......................................................................20Key Points ...........................................................................20Mechanistic-Empirical Design Procedure .....................20Constructability Issues ......................................................21

Mix Design ...................................................................22Pavement Design ........................................................22Construction ................................................................22Curing and Opening to Traffic ...................................22Maintenance ...............................................................22

Concrete Overlays ......................................................................23Key Points ...........................................................................23Two Families of Concrete Overlays .................................23

Concrete Overlays on Concrete Pavements ..........23Concrete Overlays on Asphalt Pavements .............24Concrete Overlays on Composite Pavements........25

Concrete Overlay Design Considerations .......................25

References ...................................................................................26

Chapter 3

Fundamentals of Materials Used for Concrete Pavements ........................ 27

Cementitious Materials ..............................................................28Key Points ............................................................................28Hydraulic Cement ...............................................................29

Portland Cement (ASTM C 150 / AASHTO M 85) ..................................29Blended Cements (ASTM C 595 / AASHTO M 240) ................................30Performance Specification for Hydraulic Cements (ASTM C 1157) ..........................30Selecting and Specifying Hydraulic Cements........30

Supplementary Cementitious Materials .........................31Types of Supplementary Cementitious Materials..............................................32Effects of Supplementary Cementitious Materials in Concrete ................................................36

Aggregates...................................................................................39Key Points ...........................................................................39Aggregate Types .................................................................39

Carbonate Rock ...........................................................40Granite .........................................................................41Gravel and Sand ..........................................................42

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Manufactured Aggregate .........................................42Recycled Aggregates.................................................43

Physical Properties of Aggregates ..................................44Aggregate Gradation..................................................44Aggregate Particle Shape ........................................46Aggregate Surface Texture .......................................47Aggregate Absorption................................................47Aggregate Coefficient of Thermal Expansion .......47Aggregate Durability ..................................................47

Water ............................................................................................52Key Points ...........................................................................52Sources of Water in the Mixture ......................................52Water Quality .......................................................................52Recycled Water...................................................................53

Chemical Admixtures .................................................................55Key Points ...........................................................................55Air-Entraining Admixtures .................................................56

Function of Air Entrainment in Concrete.................56Air-Entraining Agent Mechanisms ..........................57Side Effects of Air-Entraining Admixtures ..............57

Water Reducers ..................................................................58Function of Water Reducers .....................................58Water-Reducing Agent Mechanisms ......................58Side Effects of Water Reducers ...............................59

Set-Modifying Admixtures ................................................59Primary Function of Set-Modifying Admixtures ........................................59Set-Modifying Agent Mechanisms ..........................59Side Effects of Set-Modifying Admixtures .............59

Other Admixtures ................................................................60

Dowel Bars, Tiebars, and Reinforcement ...............................61Key Points ...........................................................................61Dowel Bars (Smooth Bars) ................................................61Tiebars (Deformed Bars) ...................................................61Reinforcement .....................................................................62Materials for Dowel Bars, Tiebars, and Reinforcement .............................................................62

Steel ..............................................................................62Epoxy-Coated Bars .....................................................63Stainless Steel Bars ...................................................63Stainless-Clad Bars ....................................................63Fiber-Reinforced Polymer .........................................63Synthetic Fibers ..........................................................63Other Materials ..........................................................63

Curing Compounds......................................................................64Key Points ...........................................................................64Types of Curing Compounds .............................................64

References ...................................................................................65

Chapter 4

Transformation of Concrete from Plastic to Solid ............................. 69

Stages of Hydration: Overview .................................................70Mixing ...................................................................................71Dormancy .............................................................................71Hardening .............................................................................72Cooling ..................................................................................72Densification ........................................................................73Summary ..............................................................................73

Stages of Hydration: Details......................................................75Primary Compounds in Unhydrated Cement ..................75Primary Hydration Products ..............................................75Basics ...................................................................................76Effects of Supplementary Cementitious Materials (SCMs) ................................................................78Effects of Chemical Admixtures .......................................79Implications of Cement Hydration for Construction Practices ................................................80Incompatibilities: Early Stiffening/Retardation ..............81Implications of Cement Hydration for Cracking .............82Implications of Cement Hydration for the Air-Void System ......................................................83

Hydration Compounds ................................................................84Key Points ............................................................................84Portland Cement Compounds ...........................................84

Portland Cement Clinker ...........................................84Gypsum .........................................................................85

Portland Cement Hydration .......................................................88Key Points ............................................................................88Aluminate and Sulfate Reactions .....................................88Silicate (Alite and Belite) Reactions ................................89

Alite Reactions ............................................................90Belite Reactions ..........................................................91

Primary Products of Hydration .........................................91Factors Affecting Hydration Rates ...................................91Pores .....................................................................................92Concrete Strength Gain, Tensile Stress, and the Sawing Window ...................................................93Hydration and Concrete Permeability .............................93

Reactions of Supplementary

Cementitious Materials ..............................................................94Key Points ............................................................................94Hydraulic SCMs ..................................................................94Pozzolanic SCMs ................................................................94

Table of Contents, continued

xi

Potential Materials Incompatibilities ......................................97Key Points ............................................................................97Stiffening and Setting .........................................................98

Sulfate-Related Setting and Stiffening ...................98Other Stiffening and Setting Incompatibilities .........................................................98

Air-Void System Incompatibilities ..................................100Testing for and Prevention of Incompatibilities ................................................................100Potential Solutions to Incompatibilities ........................102

References .................................................................................103

Chapter 5

Critical Properties of Concrete ...................... 105

Uniformity of Mixture................................................................106Key Points ..........................................................................106Simple Definition ...............................................................106Significance .......................................................................106Factors Affecting Uniformity ...........................................106

Raw Materials ..........................................................106Batching Operations ...............................................106Mixing Operations ...................................................106

Uniformity Testing .............................................................106

Workability .................................................................................108Key Points ............................................................. 108Simple Definition ...............................................................108Significance of Workability .............................................108Factors Affecting Workability .........................................108

Water Content ..........................................................109Aggregates ...............................................................109Entrained Air .............................................................109Time and Temperature ............................................109Cement .......................................................................109Supplementary Cementitious Materials ..............109Admixtures .................................................................109

Workability Testing ...........................................................109Slump Test ..................................................................110Compacting Factor Test ...........................................110Vebe Test ....................................................................110Penetration Tests ......................................................110

Segregation of Concrete Materials .......................................111Key Points ............................................................. 111Simple Definition ...............................................................111Significance of Segregation ...........................................111Factors Affecting Segregation .......................................111Segregation Testing..........................................................111

Bleeding......................................................................................112Key Points ..........................................................................112Simple Definition ...............................................................112Significance of Bleeding .................................................112Factors Affecting Bleeding .............................................113

Water Content and Water-Cementitious Materials (W/CM) Ratio ..........................................113Cement .......................................................................113Supplementary Cementitious Materials ..............113Aggregate ..................................................................113Chemical Admixtures ...............................................113

Testing for Assessing Bleeding ......................................113

Setting .........................................................................................114Key Points ..........................................................................114Simple Definition ...............................................................114Significance of Setting .....................................................114Factors Affecting Setting .................................................114

Temperature/Weather .............................................114Water-Cementitious Materials (W/CM) Ratio .....114Cement Content ........................................................114Cement Type .............................................................114Chemical Admixtures ..............................................114Timing of Addition of Admixtures ...........................114Mixing .........................................................................114Supplementary Cementitious Materials ..............115

Testing for Setting Time ...................................................115

Strength and Strength Gain ....................................................116Key Points ..........................................................................116Simple Definition ...............................................................116Significance of Strength and Strength Gain ................116Factors Affecting Strength and Strength Gain ............116

Water-Cementitious Materials (W/CM) Ratio .....117Degree of Hydration ................................................117Cement .......................................................................118Supplementary Cementitious Materials (SCMs) 118Air Content ................................................................118Aggregate Bond .......................................................118Handling and Placing ..............................................118

High Early-Strength Concrete .........................................118Strength Testing: Mix Design Stage ..............................119

Mix Design Testing: Flexural Strength ..................119Mix Design Testing: Compressive Strength .........119Mix Design Testing: Splitting Tensile Strength Tests ...........................................................120

Strength Testing: Field Tests ...........................................120Maturity Testing ...............................................................121

Modulus of Elasticity and Poisson’s Ratio ............................123Key Points ..........................................................................123Simple Definition ...............................................................123


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Significance .......................................................................123Factors Affecting Elasticity .............................................123Testing for Modulus of Elasticity and Poisson’s Ratio ..................................................................124

Shrinkage ...................................................................................125Key Points ..........................................................................125Simple Definitions of Shrinkage ....................................125Significance .......................................................................125Factors Affecting Shrinkage ...........................................125Testing for Shrinkage ......................................................126

Temperature Effects ................................................................127Key Points ..........................................................................127Simple Definition ...............................................................127Significance of Thermal Properties ...............................127

Effects on Hydration .................................................127Effects on Cracking ..................................................127

Factors Affecting Thermal Properties ...........................128Heat of Hydration of Cementitious Materials ......128Initial Temperature ..................................................128Environmental Factors ............................................129Thermal Expansion/Contraction .............................129

Testing for Thermal Properties .......................................129

Permeability ...............................................................................131Key Points ..........................................................................131Simple Definition ...............................................................131Significance .......................................................................131Factors Affecting Permeability .......................................131Testing.................................................................................131

Frost Resistance........................................................................132Key Points ..........................................................................132Simple Definition ...............................................................132Significance .......................................................................132

Freezing and Thawing Damage ..............................132Salt Scaling ................................................................133D-Cracking .................................................................133Popouts .......................................................................133

Factors Affecting Frost Resistance ...............................133Air-Void System .........................................................133Strength ......................................................................135Aggregates ................................................................135Cementitious Materials............................................135Finishing .....................................................................135Curing ..........................................................................135

Testing.................................................................................135D-Cracking .................................................................135Air-Void System .........................................................136Rapid Freezing and Thawing...................................137Salt Scaling ................................................................137

Sulfate Resistance ....................................................................139Key Points ..........................................................................139Simple Definition ...............................................................139Significance .......................................................................139Factors Affecting Sulfate Attack ....................................139Testing.................................................................................140

Alkali-Silica Reaction ...............................................................141Key Points ..........................................................................141Simple Definition ...............................................................141Significance .......................................................................141Factors Affecting Alkali-Silica Reaction .......................142Testing.................................................................................142

Aggregate Testing ....................................................142Mitigation Measures Testing ..................................143Concrete Testing .......................................................143

Abrasion Resistance ................................................................146Key Points ..........................................................................146Simple Definition ...............................................................146Significance .......................................................................146Factors Affecting Abrasion Resistance ........................146

Aggregate Type ........................................................146Compressive Strength..............................................146Surface Finishing ......................................................146Curing Methods ........................................................146

Testing.................................................................................146

Early-Age Cracking ...................................................................148Key Points ..........................................................................148Simple Definition ...............................................................148Significance .......................................................................148Factors Affecting Early-Age Cracking ...........................148

Volume Change and Restraint ...............................149Curling and Warping: A Variation of Volume Change .....................................................150Strength and Stiffness .............................................151Lack of Support .........................................................151Early Loading .............................................................151

Controlling Early-Age Cracks ..........................................152Controlling Early-Age Cracks with Joints .............152Controlling Early-Age Cracks with Curing ............154

Preventing Early-Age Cracks ..........................................154Using Joints to Prevent Uncontrolled Early-Age Cracks .....................................................154Selecting Materials to Prevent Early-Age Cracks .....................................................154Preventing Early-Age Cracks with Moisture Control ......................................................155Preventing Early-Age Cracks with Temperature Control ...............................................155


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Preventing Early-Age Cracks with Uniform Support ...............................................155

Testing for Cracking Risk .................................................156Summary of Preventable Early-Age Cracks: ................158

Plastic Shrinkage Cracks ........................................158 Map Cracking (Crazing) ..........................................160Random Transverse Cracks ....................................161Random Transverse Cracks continued .................162Random Longitudinal Cracks ..................................163Corner Breaks ...........................................................164

References .................................................................................165

Chapter 6

Development of Concrete Mixtures........................... 171

Sequence of Development ......................................................172Key Points ..........................................................................172Pre-Construction ...............................................................172

Bidding ........................................................................172Laboratory Mixes ......................................................173Anticipating Responses to Field Conditions .........173

Field Trials ..........................................................................173

Aggregate Grading Optimization ............................................176Key Points ..........................................................................176Coarseness Factor Chart ................................................176Coarseness Factor Zones ...............................................1760.45 Power Chart ..............................................................177Percent Aggregate Retained Chart ..............................178

Calculating Mixture Proportions ............................................179Key Points ..........................................................................179The Absolute Volume Method ........................................179

Adjusting Properties .................................................................185Key Points ..........................................................................185Workability .........................................................................185Stiffening and Setting .......................................................186Bleeding .............................................................................186Air-Void System .................................................................187Density (Unit Weight) .......................................................187Strength ..............................................................................187Volume Stability .................................................................188Permeability and Frost Resistance ................................188Abrasion Resistance ........................................................188Sulfate Resistance ............................................................189Alkali-Silica Reaction .......................................................189

References .................................................................................190

Chapter 7

Preparation for Concrete Placement ........................ 191

Subgrades ..................................................................................192Key Points ..........................................................................192Uniform Support ................................................................192

Expansive Soils .........................................................193Frost Action ................................................................193Pumping ......................................................................194Grading and Compaction .........................................194Pre-Grading ...............................................................194Moisture-Density Control ........................................194

Improving Soil Characteristics .......................................194Trimming .............................................................................195Proof-Rolling ......................................................................195

Bases ..........................................................................................196Key Points ..........................................................................196Types of Bases ..................................................................196

Unstabilized (Granular) Bases ................................196Stabilized (Treated) Bases ......................................196

Grading Control .................................................................197Compaction Requirements ..............................................197Materials ............................................................................197Construction ......................................................................197Drainage in the Base Layer .............................................198Trackline .............................................................................198Trimming to Grade.............................................................199

Preparation for Overlays..........................................................200Key Points ..........................................................................200Preparing for Bonded Concrete Overlays ....................200

Bonded Concrete Overlays on Concrete Pavements ................................................200Bonded Concrete Overlays on Asphalt Pavements ...................................................201

Preparing for Unbonded Concrete Overlays ................201Unbonded Concrete Overlays on Concrete Pavements ................................................201Unbonded Concrete Overlays on Asphalt Pavements ...................................................201

References .................................................................................202

Chapter 8

Construction ....................203

Field Verification .......................................................................204


xiv

Key Points ..........................................................................204

Concrete Production ................................................................205Key Points ..........................................................................205Setting Up the Plant ..........................................................205Handling Materials ...........................................................206Stockpile Management ....................................................206Batching .............................................................................207Mixing Concrete ................................................................209Delivering Concrete ..........................................................209Field Adjustments .............................................................210

Ambient Temperatures ............................................210Materials Variability .................................................211Material Supply Changes ........................................211

Paving .........................................................................................212Key Points ..........................................................................212Placement (Fixed Form) ...................................................213Consolidation (Fixed Form) ..............................................214Placement (Slipform) ........................................................214Consolidation (Slipform) ..................................................215String Lines ........................................................................215Alternatives to String Lines .............................................216Edge Slump ........................................................................217

Dowel Bars and Tiebars...........................................................218Key Points ..........................................................................218Pre-Placed Bars ................................................................218Inserted Bars .....................................................................219

Finishing......................................................................................220Key Points ..........................................................................220

Texturing and Smoothness ......................................................221Key Points ..........................................................................221Texturing Options ..............................................................221

Drag Textures ............................................................222Longitudinal Tining ....................................................222Transverse Tining ......................................................222Diamond Grinding .....................................................222Innovative Techniques .............................................222

Texture Variability .............................................................222Pavement Smoothness ....................................................222

Curing ..........................................................................................224Key Points ..........................................................................224Curing Compound .............................................................224Other Curing Methods ......................................................225

Weather Considerations ..........................................................226Key Points ..........................................................................226Hot-Weather Concreting .................................................226Cold-Weather Placement ................................................228Protection From Rain ........................................................229

Crack Prediction with HIPERPAV ...........................................231Key Points ..........................................................................231

Joint Sawing ..............................................................................233Key Points ..........................................................................233Saw Timing .........................................................................233Mixture Effects ..................................................................235

Joint Sealing ..............................................................................237Key Points ..........................................................................237

References .................................................................................239

Chapter 9

Quality and Testing ..........241

Quality Assurance/Quality Control Records .........................242Key Points ..........................................................................242Quality Assurance ............................................................242Quality Control ...................................................................242Quality of Testing...............................................................243Record Keeping .................................................................244

Batching .....................................................................................246Key Points ..........................................................................246Aggregate Moisture .........................................................246Water-Cementitious Materials Ratio ............................246Batching Tolerances ........................................................246

Construction Monitoring ..........................................................247Key Points ..........................................................................247Temperature.......................................................................247Air Content .........................................................................248Vibration Monitoring ........................................................248Dowel Bar Tolerances ......................................................248

Alignment ...................................................................249Embedment Length ...................................................249Dowel Depth ..............................................................249

Test Methods .............................................................................250Key Points ..........................................................................250Differential Scanning Calorimetry (DSC) ......................251Blaine Fineness .................................................................252Combined Grading ............................................................253Penetration Resistance (False Set) ...............................255Cementitious Materials Temperature Profile (“Coffee Cup Test”) ..............................................256Water-Cementitious Materials Ratio (Microwave) .....257Unit Weight ........................................................................258Heat Signature (Adiabatic Calorimetry Test) ...............259Concrete Temperature, Subgrade Temperature, and Project Environmental Conditions ..........................260Concrete Maturity .............................................................261


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Flexural Strength and Compressive Strength (Seven Day) .......................................................263Air-Void Analyzer (AVA) ...................................................265Air Content (Plastic Concrete, Pressure Method) .......266Air Content (Hardened Concrete) ..................................267Chloride Ion Penetration ..................................................268Coefficient of Thermal Expansion ..................................269

References .................................................................................270

Chapter 10

Troubleshooting and Prevention .......................273

Key to Troubleshooting Tables ................................................274

Table 10-1. Problems Observed Before the

Concrete Has Set ......................................................................275Mixture and Placement Issues .......................................275

Slump is Out of Specification..................................275Loss of Workability/Slump Loss/ Early Stiffening ..........................................................275Mixture is Sticky .......................................................276Mixture Segregates ..................................................276Excessive Fresh Concrete Temperature ...............276Air Content is Too Low or Too High ........................277Variable Air Content/Spacing Factor .....................277Mix Sets Early ............................................................278Delayed Set ................................................................278Supplier Breakdown, Demand Change, Raw Material Changes ............................................279

Edge and Surface Issues .................................................279Fiber Balls Appear in Mixture .................................279Concrete Surface Does Not Close Behind Paving Machine ..........................................280Concrete Tears Through Paving Machine ............280Paving Leaves Vibrator Trails .................................280Slab Edge Slump .......................................................281Honeycombed Slab Surface or Edges ..................281Plastic Shrinkage Cracks ........................................281

Table 10-2. Problems Observed in the

First Days After Placing ...........................................................282Strength ..............................................................................282

Strength Gain is Slow...............................................282Strength is Too Low ..................................................282

Cracking .............................................................................283Early-Age Cracking ..................................................283

Joint Issues ........................................................................285Raveling Along Joints ..............................................285

Spalling Along Joints ...............................................285Dowels Are Out of Alignment .................................285

Table 10-3. Preventing Problems That Are

Observed at Some Time After Construction .........................286Edge and Surface Issues .................................................286

Clay Balls Appear at Pavement Surface...............286Popouts .......................................................................286Scaled Surface ..........................................................286Dusting Along Surface .............................................286Concrete Blisters ......................................................287Surface Bumps and Rough Riding Pavement ...................................................................287Surface is Marred or Mortar is Worn Away ...........................................................................288

Cracking .............................................................................289Cracking ....................................................................289

Table 10-4. Assessing the Extent of

Damage in Hardened Concrete ..............................................290

References .................................................................................291

Bibliography ....................293

Glossary ........................... 301

(based on ACPA definitions)

Abbreviated Index .........323


xvii

Chapter 11-1. Concrete pavement construction is an

integrated system, with the concrete material at the center. ................................................3

Chapter 22-1. Pavement features (ACPA) ........................................8

2-2. Concrete pavement types (ACPA) ............................92-3. Pavement condition as a function of

time or traffic (ACPA) ................................................112-4. Corner cracking in jointed concrete

pavement (ACPA) ......................................................122-5. Transverse crack that represents

structural distress (ACPA) .......................................122-6. Longitudinal crack representing

structural distress (ACPA) .......................................132-7. Shattered slab representing

structural distress (ACPA) .......................................132-8. Faulting in jointed concrete pavement

(ACPA) .........................................................................152-9. Comparison of compressive and

flexural strength correlations, based on equations 2.1 and 2.2 ..........................................17

2-10. Examples of bonded overlays ..................................242-11. Examples of unbonded overlays ..............................24

Chapter 33-1. Concrete is basically a mixture of

cement, water/air, and aggregates (percentages are by volume). (PCA 2000) .............27

3-2. Scanning electron micrograph of fly ash particles. (PCA) ..................................................33

3-3. Typical size distributions of cementitious materials (CTLGroup) ...............................................33

3-4. Scanning electron micrograph of GGBF slag particles. (PCA) .........................................................34

3-5. Family of carbonate minerals showing rock and mineral names (CTLGroup) .....................42

3-6. Aggregates produced by crushing operation (top) have a rougher surface texture and are angular compared to round river gravel (bottom). (PCA) .........................43

3-7. The level of liquid in the cylinders, representing voids, is constant for equal absolute volumes of aggregates of uniform (but different) sizes. When different sizes are combined, however, the void content decreases. (PCA) ........................45

3-8. An example of a smooth grading curve that would be preferred (red lines define

List of Figuresan acceptable envelope) and an actual

aggregate system that was not optimum (blue) (CTLGroup) ......................................................46

3-9. Moisture conditions of aggregates (PCA) ............47

3-10. An aggregate particle that has cracked due to alkali-silica reaction (PCA) .........................48

3-11. D-cracking (Grove, CP Tech Center) .....................49

3-12. A popout at the surface of concrete (PCA) ..........50

3-13. Effect of compressive strength and aggregate type on the abrasion resistance of concrete (ASTM C 1138-97) ............50

3-14. Recycled water and reclaimed aggregate at a ready-mixed concrete plant (PCA) .................54

3-15. Entrained air bubbles in concrete (PCA) ..............56

3-16. Spacing factor is the average distance from any point to the nearest air void. (Ozyildirim) .................................................................56

3-17. Stabilization of air voids by air-entraining admixture molecules (PCA) .....................................57

3-18. One mechanism by which water reducers work is dispersion. (a) Charged cement particles cling together, trapping water. (b) Water reducers separate cement grains, releasing the water and making it available for hydration. .............................................................59

3-19. Curing compounds keep concrete partially saturated near the surface during the curing period. (PCA) ..............................64

Chapter 44-1. Compounds in cement .............................................70

4-2. General hydration curve delineating the five stages ..................................................................70

4-3. A very brief heat spike occurs during mixing. .........................................................................71

4-4. A gel-like substance coats cement compounds, controlling the reactions and heat. .....................................................................71

4-5. The concrete does not generate heat during the dormancy stage. ....................................71

4-6. During dormancy, the water becomes saturated with dissolved ions. ................................71

4-7. Significant heat is generated during the hardening stage. .......................................................72

4-8. Hydration products grow. ........................................72

4-9. Heat energy peaks and then drops during cooling. .......................................................................72

4-10. Hydration products grow during cooling. .............72

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List of Figures, continued

4-11. Very little heat is generated during the final, densification stage. ........................................73

4-12. Hydration products mesh into a dense solid. ............................................................................73

4-13. Concrete characteristics, and implications for workers, during stages of hydration ................74

4-14. Compounds in cement .............................................75

4-15. Composition of portland cement ............................84

4-16. Sample mill certificate (ASTM C 150-04) ..............86

4-17. Reactions of C3A and CS–

, in solution, are responsible for an early heat spike during cement hydration. ........................................89

4-18. Gel-like C-A-S–

-H limits water’s access to cement particles. ..................................................89

4-19. After sulfate is consumed, remaining C3A reacts with ettringite. .......................................89

4-20. Dissolving cement results in calcium ions in solution. .........................................................90

4-21. Alite reactions form C-S-H and CH. .......................90

4-22. Calcium silicate hydrate (C-S-H) and calcium hydroxide (CH) begin to form. ..................90

4-23. Hydration products accumulate and mesh. ...........................................................................91

4-24. Belite reactions begin contributing to strength gain later. ....................................................91

4-25. Calcium silicate hydrate (C-S-H) and calcium hydroxide (CH) mesh into a solid mass. .................................................................91

4-26. Estimates of the relative volumes of cement compounds and products of hydration with increasing hydration (adapted from Tennis and Jennings 2000). ...........93

4-27. Ternary diagram illustrating the basic chemical composition of various SCMs compared to portland cement ................................94

4-28. Effect of pozzolans on cement hydration ..............96

4-29. Early cement reactions are a balance of tricalcium aluminate (C3A) and sulfate (SO4) in solution. Excess of either will cause unexpected setting. (CTLGroup) .................98

4-30. Plot of heat generated by cement hydration of cement pastes containing varying amounts of lignosulfonate-based water-reducing admixture (CTLGroup) .................99

4-31. A typical example of air voids clustered around an aggregate particle ...............................100

4-32. A set of field calorimetry data with three different cements and the same dosage of

water-reducing admixture, including one set showing clear retardation and low heat output consistent with delayed setting and slow strength gain (Knight, Holcim) .............102

Chapter 55-1. Segregated concrete

(Hanson, Iowa DOT) ...............................................111

5-2. Bleeding (Ozyildirim) ..............................................112

5-3. Penetration resistance of mortar sieved from concrete as a function of time, per ASTM C 403. Initial and final setting times are defined as shown. (Dodson 1994) .................115

5-4. Loads on a pavement induce flexural stresses in the concrete slab. (ACPA 1994) .......117

5-5. Cementitious systems that produce high early-strengths (blue line) tend to have lower strengths in the long term when compared to slower hydrating systems (red line). (CTLGroup) .............................................118

5-6. (a) In third-point loading, the entire middle one-third of the beam is stressed uniformly, and thus the beam fails at its weakest point in the middle one-third of the beam. (b) By forcing the beam to fail at the center, the center-point loading flexural test results are somewhat higher than the third-point test results. Typically, center-point results are about 15 percent greater than third-point results and vary widely, depending on many concrete mix properties. (PCA) .....................................................120

5-7. Splitting tensile strength (PCA) ............................120

5-8. Maturity of the field concrete is equivalent to maturity of the laboratory concrete when the area under the curves is the same. (CTLGroup) ...............................................................122

5-9. Generalized stress-strain curve for concrete (PCA) .......................................................123

5-10. Total shrinkage is a sum of all individual shrinkage mechanisms. Minimizing any or all of the mechanisms will reduce the risk of cracking. (CTLGroup) .................................125

5-11. Relationship between total water content and drying shrinkage. Several mixtures with various proportions are represented within the shaded area of the curves. Drying shrinkage increases with increasing water contents. (PCA) ........................126

5-12. A typical field semi-adiabatic temperature monitoring system for mortar (W.R. Grace & Company) ..........................129

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5-13. Measuring the coefficient of thermal expansion ...............................................................130

5-14. Air-entrained concrete (bottom bar) is highly resistant to damage from repeated freeze-thaw cycles. (PCA) ........................................132

5-15. Scaled concrete surface resulting from lack of air entrainment, use of deicers, and poor finishing and curing practices ....................................................................133

5-16. Air voids provide a place for expanding water to move into as it freezes. (CTLGroup) ...............................................................134

5-17. Air bubbles rising in the AVA (FHWA) ................137

5-18. Sulfate attack is a chemical reaction between sulfates and the calcium aluminate (C3A) in cement, resulting in surface softening. (Emmons 1994) .......................139

5-19. Alkali-silica reaction is an expansive reaction of reactive aggregates, alkali hydroxides, and water that may cause cracking in concrete. (Emmons 1994) .................141

5-20. Test apparatus for measuring abrasion resistance of concrete to ASTM C 779 (PCA) .........................................................................147

5-21. Rotating cutter with dressing wheels for the ASTM C 944 abrasion resistance test (Goodspeed 1996) ............................................147

5-22. (a) Cracks generally do not develop in concrete that is free to shrink. (b) In reality, slabs on the ground are restrained by the subbase or other elements, creating tensile stresses and cracks. (ACPA) ........................................................149

5-23. Curling and warping of slabs ................................150

5-24. Exaggerated illustration of pavement curling. The edge of the slab at a joint or a free end lifts off the base, creating a cantilevered section of concrete that can break off under heavy wheel loading. (PCA) .........................................................151

5-25. An eroded base can lead to high tensile stresses, resulting in cracking. ...........................151

5-26. A saw cut that has cracked through as planned (PCA) .........................................................152

5-27. This joint was cut too late, resulting in random transverse cracking. (ACPA) .......................................................................153

5-28. Effect of initial curing on drying shrinkage of portland cement concrete prisms. Concrete with an initial 7-day moist cure at 4°C (40°F) had less shrinkage than

concrete with an initial 7-day moist cure at 23°C (73°F) Similar results were found with concretes containing 25 percent fly ash as part of the cementitious material. ...........154

5-29. A ring shrinkage specimen marked where a crack has occurred (CTLGroup) ...........156

5-30. Typical plastic shrinkage cracks ..........................1585-31. Deep plastic shrinkage cracks .............................1585-32. Map cracking ..........................................................1605-33. Random transverse crack .....................................1615-34. Random longitudinal crack ...................................1635-35. Corner break ............................................................164

Chapter 66.1. Example w/c ratio vs. strength curve

(laboratory mix target w/c = 0.40) ........................1756-2. Modified coarseness factor chart

(Shilstone 1990) .......................................................1776-3. 0.45 power chart for 25 mm (1 in.)

nominal maximum aggregate (Shilstone personal communication) ...................177

6-4. Percent retained chart ...........................................1786-5. Approximate relationship between

compressive strength and w/cm ratio for concrete using 19-mm to 25-mm (3/4 in. to 1 in.) nominal maximum size coarse aggregate. Strength is based on cylinders moist-cured 28 days per ASTM C 31 / AASHTO T 23. (Adapted from ACI 211.1, ACI 211.3, and Hover 1995) ...............................................................180

6-6. Bulk volume of coarse aggregate per unit volume of concrete. Bulk volumes are based on aggregates in a dry-rodded condition (ASTM C 29 / AASHTO T 19). For less workable concrete (slipform paving), the bulk volume may be increased by about 10 percent. (Adapted from ACI 211.1 and Hover 1995) ...........180

6-7. Target total air content requirements for concretes using different sizes of aggregate. The air content in job specifications should be specified to be delivered within –1 to +2 percentage points of the target value for moderate and severe exposure. (Adapted from ACI 211.1 and Hover 1995) .....................................181

6-8. Approximate water requirement for various slumps and crushed aggregate sizes for air-entrained concrete (Adapted from ACI 211.1 and Hover 1995) ............................182

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Chapter 77-1. Effects of two examples of nonuniform

support on concrete slabs on the ground (Farny 2001) ..............................................................192

7-2. Relationship between frost action and hydraulic properties of soils (ACPA 1995) ..........194

7-3. Autograder that references the string line and trims the subgrade material (ACPA) ............195

7-4. Trackline of slipform paving machine (ACPA) .......................................................................199

Chapter 88-1. Portable concrete plant (ACPA) ...........................205

8-2. Loader operation is key to stockpile management. (ACPA) .............................................207

8-3. Typical sequence of adding material in a stationary mix plant (Ayers et al. 2000) ............207

8-4. Typical sequence of adding material in a truck mixer (Ayers et al. 2000) ...........................208

8-5. Depositing concrete in front of the paving machine (ACPA) .........................................209

8-6. A belt placer/spreader ensures a consistent amount of concrete in front of the paver. (ACPA) ...............................................210

8-7. Components of a typical slipform paving machine (ACPA) ......................................................212

8-8. A roller screed (i.e., single-tube finisher) is one type of equipment that can be used to strike off the concrete in fixed-form placements. Others include vibratory screeds, form riders, and bridge deck finishers. (ACPA) .....................................................214

8-9. An array of vibrators under a slipform paver (ACPA) ...........................................................215

8-10. Typical string line setup (ACPA 2003b) ................216

8-11. Two types of edge slump .......................................217

8-12. (a) Staking or (b) pinning dowel cages (ACPA) ...........................................................218

8-13. Dowel bar insertion equipment (ACPA) ..............219

8-14. Artificial turf drag texturing rig, weighted with sand (source) ..................................................222

8-15. A curing machine coats both the top surface and sides of a slipform paving slab. (ACPA) .............................................................224

8-16. A monograph to estimate the rate of evaporation (PCA) ...................................................227

8-17. Plastic sheeting ready for placement to protect the fresh surface from rain (ACPA) .......229

8-18. Typical scaling of concrete pavement due to rain on nondurable paste surface (ACPA 2003a) ...........................................................230

8-19. Edge erosion of freshly placed slab due to rain (ACPA 2003a) .............................................................230

8-20. An example plot reported by HIPERPAV showing a high risk of cracking at about six hours after paving ....................................................232

8-21. Common sawing equipment. (a) For most projects, transverse or longitudinal contraction joints are cut with single-blade, walk-behind saws. (b) For wider paving, contractors may elect to use span-saws that are able to saw transverse joints across the full pavement width in one pass. (c) A newer class of saw, the early- entry dry saw, is a walk-behind saw that allows sawing sooner than with conventional saws (ACPA) ......................................234

8-22. Sawing window (ACPA) ...........................................2348-23. Close-up of different degrees of raveling

caused by joint sawing (ACPA) ..............................2348-24. Different forms of joint sealant (adapted from

ACPA) ..........................................................................238

Chapter 99-1. Example control chart: concrete

unit weight ...............................................................2449-2. Sample coarseness/workability chart ................2549-3. Sample combined aggregate gradation

0.45 power curve ......................................................2549-4. Percent retained chart ............................................2549-5. Heat signature sample plots .................................2599-6. Sample maturity curve ...........................................262

Chapter 1010-1. Early-age cracking ....................................................273

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List of TablesChapter 3

3-1. Portland Cement Classifications (ASTM C 150 / AASHTO M 85) ................................30

3-2. Blended Cement Classifications (ASTM C 595 / AASHTO M 240) ..............................30

3-3. Performance Classifications of Hydraulic Cement (ASTM C 1157) ..........................30

3-4. Cement Types for Common Applications ..............313-5. Specifications for Supplementary

Cementitious Materials ............................................323-6. Chemical Analyses and Selected

Properties of Type I Cement and Several Supplementary Cementitious Materials ....................................................................33

3-7. Effects of Supplementary Cementitious Materials on Fresh Concrete Properties ..............37

3-8. Effects of Supplementary Cementitious Materials on Hardened Concrete Properties ...................................................................37

3-9. Mineral Constituents in Aggregates ......................413-10. Rock Constituents in Aggregates ...........................413-11. Typical CTE Values for Common Portland

Cement Concrete Ingredients .................................483-12. Mineral Constituents of Aggregate That

Are Potentially Alkali-Reactive ...............................483-13. Rock Types Potentially Susceptible to

Alkali-Silica Reactivity .............................................493-14. Acceptance Criteria for Combined

Mixing Water .............................................................533-15. Optional Chemical Limits for Combined

Mixing Water .............................................................533-16. Effect of Recycled Water on Concrete

Properties ...................................................................543-17. Common Chemical Admixture Types for

Paving Applications ..................................................553-18. Admixture Types Defined by

ASTM C 494 / AASHTO M 194 ................................553-19. Effects of Materials and Practices on

Air Entrainment .........................................................573-20. Tiebar Dimensions and Spacings .............................62

Chapter 44-1. Major Compounds in Portland Cement .................854-2. Forms of Calcium Sulfate .........................................874-3. Primary Products of Cement Hydration ................924-4. Comparison of Alite and Belite Reactions ............964-5. Recommended Tests and Their

Applications .............................................................101

Chapter 55-1. Requirements for Uniformity of Concrete

(ASTM C 94 2004) ....................................................1075-2. Factors Affecting Compressive and

Flexural Strength .....................................................1175-3. Chemical Composition and Heat

Evolution of Typical Portland Cements ................1285-4. Requirements for Concrete Exposed to

Sulfates in Soil or Water ........................................1405-5. Test Methods for Potential Alkali-Silica

Ractivity (ASR) and Mitigation Measures ..........1445-6. FHWA High-Performance Concrete

Performance Grades for Abrasion .......................147

Chapter 66-1. Suggested Laboratory Testing Plan .....................1746-2. Suggested Field Trial Batch Testing Plan ...........1756-3. Cementitious Materials Requirements for

Concrete Exposed to Deicing Chemicals ............183

Chapter 77-1. Soil Index Properties and Their Relationship

to Potential for Expansion (Bureau of Reclamation 1998) ...................................................193

7-2. Alternatives for Reducing Friction or Bond between Concrete Pavement and Stabilized Base Materials (ACPA 2002) ..............198

Chapter 88-1. Tests/Tools for Field Verification ..........................2048-2. Concrete Plant Checklist .......................................2068-3. Common Elements of Paving Machines .............2138-4. Description of Various Concrete

Pavement Texture Options ....................................2218-5. Specification Factors that

Influence Pavement Smoothness ........................2238-6. Construction Factors that Influence

Pavement Smoothness ..........................................223

8-7. Factors that Shorten the Sawing Window .........235

Chapter 99-1. Example of Testing Precision ................................2449-2. Example Concrete Unit Weight Test Results ......2459-3. Recommended Batch Tolerances for

Ready-Mixed Concrete (ASTM C 94) ...................2469-4. Suite of QC tests from TPF-5(066) .........................2509-5. Relationship Between Coulombs and

Permeability .............................................................268

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List of Tables, continued

Chapter 1010-1. Problems Observed Before the Concrete

Has Set .....................................................................275

10-2. Problems Observed in the First Days After Placing ........................................................... 282

10-3. Preventing Problems That Are Observed at Some Time After Construction ..............................286

10-4. Assessing the Extent of Damage in Hardened Concrete ................................................290

�Integrated Materials and Construction Practices for Concrete Pavement

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Chapter 1

Introduction

Thismanualprovidesareadyreferenceand/orinstructionguideforanyoneinvolvedindesigningorconstructingconcretepavements.Theemphasisisontheconcretematerialitself—specifically,onoptimizingconcrete’sperformance.Tooptimizeconcrete’sperfor-manceintoday’spavementconstructionenvironment,everyoneinconcretepavingprojectsmustunderstand

concreteasthecentralcomponentofacomplex,integratedpavementsystem.

Readerswillfindsomeoverlapandrepetitionofinformationamongthechapters.Thisisaresultoftheintegratednatureofthevariousstagesandconsider-ationsinvolvedindesigningandconstructingconcretepavementprojects.

Purpose of This Manual 2

Today’s Construction Environment 2

Principles of Concrete Pavement as an Integrated System 2

Optimizing Concrete for Pavements 3

Organization of This Manual 4

Integrated Materials and Construction Practices for Concrete Pavement�

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Purpose of This ManualThepurposeofthismanualistobridgethegap

betweenrecentresearchandcommonpracticerelatedtoproducingconcreteforpavements.Theintendedaudienceisagencyorindustrypersonnelwhoareinterestedinoptimizingconcreteperformanceforeverypavingproject.Readersmayinclude

Engineers.Qualitycontrolpersonnel.Specifiers.Contractors.Materialsandequipmentsuppliers.Technicians.Constructionsupervisors.Tradespeople.

Specifically,thismanualwillhelpreadersdothefollowing:

Understandconcretepavementsascomplex,integratedsystems.Appreciatethatconstructingaconcretepave-mentprojectisacomplexprocessinvolvingseveraldiscretepractices.Thesepracticesinter-relateandaffectoneanotherinvariousways.Implementtechnologies,tests,andbestprac-ticestoidentifymaterials,concreteproperties,andconstructionpracticesthatareknowntooptimizeconcreteperformance.Recognizefactorsleadingtoprematuredistressinconcrete,andlearnhowtoavoidorreducethosefactors.Quicklyaccesshow-toandtroubleshootinginformation.

Today’s Construction EnvironmentIntheearlydaysofroadbuilding,acivilengineer

wouldworkonallaspectsofaproject.Thisincludedsecuringright-of-way,designingtheroadandselectingmaterials,andactingastheresidentengineertohelpthecontractorbuildtheproject.Theengineerkneweverythingabouttheproject,andthiscentralizedknowledgefacilitatedprojectquality.

Asthepavementindustryhasgrownandchanged,processespreviouslyhandledbyasingleengineerhavebeensplitintoseparatespecialtiesordepart-ments.Thisisatleastpartlybecausethevarious

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•

•

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processeshavebecomemorecomplex.Moreingre-dients(likesupplementarycementitiousmaterialsandchemicaladmixtures)havebeenintroducedtotheconcretemix.Newtestingprocedureshavebeendeveloped.Equipmentandplacementtechniqueshavechanged.

Intoday’scomplexroad-buildingenvironment,dividingresponsibilitiesamongdepartmentsiseffec-tiveonlyaslongascommunicationiseffective.Toooften,however,thisisnotthecase.Thematerialsengineerfocusesonmaterials,thedesignengineerfocusesondesigndetails,thecontractorfocusesonconstruction,andrarelydothepartiesthinkaboutorcommunicatewitheachotherabouttheeffectsoftheiractivitiesonotherpartiesinvolvedintheprocess.

Forexample,engineerstryingtoadvanceanewdesignorsolveaspecificproblemmayoverlooktheconcretematerialsandfocusonotherpavementdetails.Likewise,contractorssometimestrytoover-comeconstructabilityissuesassociatedwithapoorconcretemixturebyoverusingtheirequipmentratherthanseekingtocorrectthemixture.

Itisprobablyimpossibletogobacktothedayswhenoneengineerhandledaconcretepavingproj-ectfrombeginningtoend.Therefore,asthenumberofvariablesandspecialtiescontinuestoincrease,allpersonnelinvolvedineverystageofaprojectneedtounderstandhowtheirdecisionsandactivitiesaffect,andareaffectedby,everyotherstageoftheproject.

Inotherwords,today’sroad-buildingprocessmustbeintegratedtobecost-effectiveandreliable.

Principles of Concrete Pavement as an Integrated System

Attheheartofallconcretepavementprojectsistheconcreteitself.Theconcreteaffects,andisaffectedby,everyaspectoftheproject,fromdesignthroughconstruction.

Theconcretematerialitselfisonlyonecompo-

nentofaspecificpavementsystemorproject.(Other

componentsinclude,forexample,thepavement’sstructuralandfunctionaldesign,thesubgrade/base.)

Theconcretematerialisarguablythecentralcomponentoftheconcretepavementsystem.

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Byandlarge,theperformanceofasystemisjudgedbytheperformanceoftheconcrete.Concrete’sperformanceiscriticallyaffectedbymanyvariablesthroughoutthepavementsystemandthroughouttheprocessofbuildingthesystem.Thesevariablesincludethesources,quality,andproportionsofitsingredients;con-structionvariableslikeweather,pavingequip-ment,andpractices,etc.;anddesignparam-eterslikedesignstrengthandclimatefactors(figure1-1).Understandingconcretepavementsasintegrat-edsystems,andpavementconstructionasanintegratedprocess,willhelpreadersoptimizeconcreteperformance.

Optimizing Concrete for Pavements

Optimizingtheperformanceofconcreteforpave-mentsinvolvesunderstandingthevariablesthataffectconcrete’sperformanceandthepropertiesofconcretethatcorrespondtoperformance.

Variables That Affect Concrete Performance

Thestartingpointforachievingagood-qualityconcretepavementistoproportion,make,anduseagood-qualityconcretemixture.Thedefinitionofagood-qualityconcretemixdependsonthespecific

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________________________________________________ Figure 1-1. Concrete pavement construction is an integrated system, with the concrete material at the center.

application.Asmentionedabove,howwellaconcreteperformsforaspecificapplicationisaffectedbymanyfactors,includingthefollowing:

Structuralandfunctionaldesignofthepave-mentsystem.Thepavementmustcarrythedesignloadswithoutexperiencingdistresses.Itshouldprovideasmoothrideandadequatetraction.Duringitsdesignlife,itmayhavetowithstandtherigorsofextremetemperaturecycles.Qualityofandvariabilityinherentinconcrete’sconstituentmaterials.Aggregates,admixtures,cement,andsupplementarycementitiousmate-rialsallvaryintheirpropertiestosomedegreebasedontheirrawmaterialsandmanufacturingprocesses.Thesevariationsinmaterials,andvariationsintheirproportions,affectthedegreeofuniformitythatcanbeachievedwhentheyaremixedinmanyseparateconcretebatchesduringaproject.Constructionfactors,likeweather,equipment,andpersonnel.Environmentalvariationscansignificantlyaffectthepropertiesofplasticandhardeningconcrete.Pavementsarebuiltout-side,whereweatherconditionsduringconcreteplacementcanbeunpredictableandcanvarywidelyfromconditionsassumedduringmixdesignandmaterialsselection.Siteconditions,equipment,andtheconstructionandinspec-tionteamsplayvitalrolesinthedevelopmentofgood-qualityconcrete.

Mix Properties That Correspond to Concrete Performance

Severalpropertiesofconcretemixturescorrelatewithconcreteperformance.A16-StatepooledfundstudyatIowaStateUniversity,“MaterialsandConstructionOptimizationforPreventionofPrematurePavementDistressinPortlandCementConcretePavements”(TPF-5[066]),hasidentifiedmanyoftheseproperties,includingworkability,strengthgain,air-voidsystem,permeability,andthermalmovement(seeallofchapter5).

Duringthevariousstagesofaconcretepavingproject—mixdesign,pre-constructionverification,

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Chapter 2.Basics of Concrete Pavement Design:howconcretepavementdesigninteractswithmaterialsandconstructionrequirements.

Chapter 3. Fundamentals of Materials Used for Concrete Pavements:theingredients thatwehavetoworkwith,andhow theyinfluenceconcreteperformance.

Chapter 4. Transformation of Concrete from Plastic to Solid: howcementchemistryandthe cement’sphysicalchangesduring hydrationarecentraltogood-quality concrete,andhowsupplementary cementitiousmaterialsandchemical admixturesaffectthehydration process.

Chapter 5. Critical Properties of Concrete:freshandhardenedpropertiesofconcrete thatcorrelatewithconcreteperformance.

Chapter 6. Development of Concrete Mixtures:howtoachievetherequiredperformancewiththematerialsthatwehave.

Chapter 7.Preparation for Concrete Placement: howthesubgradeandbaseinfluence theconcrete.

Chapter 8.Construction:howconstruction activitiesandworkmanshipinfluence theconcrete,whattoolsareavailable, andthecurrentbestpracticestoensurehigh-qualitypavement.

Chapter 9.Quality and Testing:abriefdiscussionofqualitysystems,anddescriptionsof sometestmethodsthatcanbeusedtomonitorconcreteperformance.

Chapter 10.Troubleshooting and Prevention: identifyingtheproblemandthefixwhensomethinggoeswrong,andpreventingrecurrence.

andconstruction—theseandotherpropertiescanbemonitoredandnecessaryadjustmentscanbemadetomixtureproportionsortoconstructionpracticestohelpensurethefinalconcreteproductperformsasdesigned.Design,verification,andfieldtestsarecriticalelementsofqualityassuranceandqualitycontrol(QA/QC).

Inadeparturefrompastpractice,laboratorytestsofmixtureproperties,usingmaterialsspecifictoaproject,arenowoftenneededbeforeconstructionstarts.Ideally,full-sizetrialbatchesshouldbedonebeforepavingisbeguntoverifylaboratoryfindingsanddetermineifadjustmentsareneeded.Thisisespeciallyimportantwhenusingunfamiliarmaterials.

Thesetestsprovidedatathatserveasabasisformakingnecessaryadjustmentsinthefield.Theconstructionteam(agencyandcontractor)shouldprepareinadvanceforsituationsthatmayariseduringconstruction,suchaschangesinmaterialssourcesorunexpectedlyhotorcoldweather.Theteamshouldpre-determinehowthesechangeswilllikelyaffecttheconcretemixtureproperties,anddecideinadvancehowtocompensatefortheeffects.

Forexample,willhotweatherleadtodecreasesinairentrainment?Howrapidlywillamixtureloseworkabilityinhotweather?Theprojectteamshouldhaveanswerstothesetypesofquestionswellbeforeconstructionbeginssothatsolutionsareathandifandwhenconditionschange.Trialbatchingattheplantisessentialtoverifythelaboratoryfindings.

Organization of This ManualThemanualisorganizedinto10chapters,listedat

right.Somechaptersaremoredetailedthanothers.Inaddition,eachchapterandsectionbeginwithgeneralinformationandthenbecomemoredetailed.

Critical Details: Chapters 3, 4, 5, 6, 9Theemphasisthroughoutthemanualison

concreteasamaterialandhowitsquality(performance)isaffectedbyallaspectsofapavementproject.Topicscoveredinthechaptershighlightedinblueatright—3(Materials),4(Hydration),5(Properties),6(Mix),and9(QA/QC)—arecentraltothisemphasis;thesechaptersarequitedetailed.


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Completecoverageofthetopicsintheremainingchaptersisbeyondthescopeofthisbook.Thesechaptersprovideoverviewsonly,butagainfromtheperspectiveofoptimizingconcreteperformance.

Quick References: Chapters 4 and 10Somesectionsofthemanualarepresentedas

references:Chapter 10 (Troubleshooting).Fieldpersonnelin

particularwillfindchapter10usefulonsite.Stages of Hydration charts in Chapter 4

(Hydration).ManyreaderswillrefertotheStagesofHydrationcharts(pages76–83)againandagain.

Understandingcementhydrationiscentralto

successfullyintegratingthevariousstagesofconcrete

pavementprojectsforoptimumconcreteperformance.

Thechartsprovideaquickreferencetohelp

readersunderstandtherelationshipsamongcement

chemistry,stagesofhydration,theimplicationsof

hydrationfortheconstructionprocess,andtheeffects

onhydrationwhensupplementarycementitious

materialsandmineraladmixturesareincludedin

themixture.Inaddition,thechartshighlightsome

materialsincompatibilityissuesthatcanarise.

A full-size Stages of Hydration poster.Ifaposterisnot

includedwiththismanualandyouwouldlikeone,

contacttheNationalConcretePavementTechnology

Center(contactinformationisinsidethefrontcover

ontheTechnicalReportDocumentationpage).

Subject index.Themanualendswithanabbreviated

subjectindex.(Seethesidebarbelow.)

The topics addressed in this manual are complex. Concrete pavements are complex, integrated systems, and the process of designing and constructing them is a complex, integrated process. Any thorough discussion about optimizing concrete as the central component of pavement systems cannot be presented in a strictly linear manner.

Readers will therefore discover overlap, repetition, and interaction among the chapters.

Here are two examples:Aggregates. Aggregates compose the largest volume in concrete mixes. Readers will find pertinent information about aggre-gates in several sections of the manual.

Chapter 3 describes the ingredients of concrete mixtures, including aggregates—their role in concrete, and the various types and properties of aggregate that can positively or negatively affect concrete performance. Chapter 6 provides guidelines for proportioning the ingredients, including aggregates, to achieve the desired concrete performance.

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Chapter 8 describes the importance of managing aggregate stockpiles so as not to tip the moisture balance in the mix.

Testing. As mentioned earlier, testing is critical to an effective QA/QC program to ensure concrete performance. Readers will find information about testing in several sections of the manual.

Chapter 3 includes information about testing required or recommended for the ingredients of concrete. Chapter 5 covers tests of concrete properties, both fresh and hardened. Chapter 9 provides suggested procedures for conducting tests identified and conducted in Iowa State’s study (TPF-5[066]).

As a result of this kind of overlap of information, important topics are thoroughly cross-referenced throughout the manual.

In addition, a basic index at the back of the manual lists primary topics and the chapters and pages where each is discussed (see page 323).

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Cross-References and the Subject Index


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Chapter 2

Basics of Concrete Pavement Design

Integrated Pavement Design 8

Design Considerations: What Do We Want? 11

Design Considerations: What Site Factors Do We Have to Accommodate? 19

Design Procedures: Getting What We Want, Given Site Factors 20

Concrete Overlays 23

Toeffectivelyintegratebestmaterialsandconstruc-

tionpractices,readersneedtounderstandthebasics

ofconcretepavementdesign.Thischaptertherefore

providesanintroductiontodesign.Itidentifiesgeneral

materialsandconstructionissuesrelatedtodesign,as

wellassomeofthelatesttechnologicalchangesthat

allowtheassumeddesignvariablestorelatemore

closelytoas-builtresults.

Thechapterbeginswithbriefoverviewsofthephilosophyofintegratedpavementdesignandbasicconcretepavementdesigntypes.Itthendiscussestheelementsofpavementdesign:first,determiningwhatwewant;then,determiningthespecificsitevariablesthatmustbeaccommodated;and,finally,designingpavementsthatgiveuswhatwewantinlightofthevariables.Thechapterendswithabriefdiscussionofconcreteoverlaydesigns.

� Integrated Materials and Construction Practices for Concrete Pavement

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Integrated Pavement Design

Key Points

Theobjectiveofpavementdesignistoselectpavementfeatures,suchasslabthickness,jointdimensions,andreinforcementandloadtransferrequirements,thatwilleconomicallymeettheneedsandconditionsofaspecificpavingproject.

Traditionally,concretepavementdesignhasfocusedonslabthickness.Amoreintegratedapproachtopavementdesignconsidersallcomponentsofthepavementsystemthataffectperformance.

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pavementfeaturesthatwilleconomicallymeetthespecificneedsandconditionsofaparticularproject.

Figure2-1showsthevarietyofbasicfeaturesthatmustbedeterminedwhendesigningaconcretepavement.

Pavementdesignisthedevelopmentandselectionofslabthickness,jointdimensions,reinforcementandloadtransferrequirements,andotherpavementfeatures.Apavementdesigner’sobjectiveistoselect

TheGuide for Mechanistic-Empirical Design of New and Rehabilitated Pavements(M-EPDG)(NCHRP2004)incorporatesanintegratedapproachtoslabthicknessdetermination.

Mostconcretepavementsconstructedtodayareplain(unreinforced)andjointed.Thesedesignsaregenerallycost-effectiveandreliable.

Unjointed,reinforcedconcretepavementdesigns,althoughmoreexpensivetoconstruct,maybewarrantedforcertainhigh-trafficroutes,suchassomeurbanroutes.

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_____________________________________________________________________________________________________ Figure 2-1. Pavement features (ACPA)


Integrated Materials and Construction Practices for Concrete Pavement

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Mechanistic-Empirical Design GuideDuringthedesignprocess,broadassumptionsare

oftenmadeaboutmaterialsandconstructionissuesthathaveaprofoundimpactontheultimateperfor-manceofaspecificpavementdesign,butthatareneververifiedorachievedintheas-builtproject.Itisthereforecriticaltocharacterizematerialsproperlythroughfieldtestsduringmixdesignandverificationandthroughappropriatesubgradeandbaseconstruc-tionactivities.Suchtestsareimportantnotonlytoensurethequalityofeachmaterial,butalsotoensurethattheactualconcreteandbasematerialsreflecttheassumptionsmadewhenthepavementwasdesigned.

Thefocusofconcretepavementdesignhasgenerallybeenondetermininghowthicktheslabshouldbe.Today,agenciesareadoptingamoreintegratedapproachtopavementdesign.Suchanapproachsimultaneouslyconsiderskeypavementfeaturesaswellasdurableconcretemixtures,constructabilityissues,etc.Suchanintegratedapproachisreflectedinthelong-lifepavementconceptsthathavebeenadoptedbymanyhighwayagencies.ThisintegratedapproachcanalsobeobservedinthethicknessdeterminationconceptsincorporatedintotheGuide for Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures(M-EPDG)(NCHRP2004).

Moredetailedinformationonconcretepavementdesignmaybefoundinseveralsources(YoderandWitzak1975;PCA1984;AASHTO1993;AASHTO1998;SmithandHall2001;ACI2002;NCHRP2004).

Basic Concrete Pavement TypesInpavementconstruction,threedifferentconcrete

pavementdesigntypesarecommonlyused:jointedplainconcretepavements(JPCP),jointedreinforcedconcretepavements(JRCP),andcontinuouslyrein-forcedconcretepavements(CRCP)(figure2-2).

Eachofthesedesigntypescanprovidelong-lastingpavementsthatmeetorexceedspecificprojectrequirements.Eachtypeissuitablefornewconstruc-tion,reconstruction,andoverlays(resurfacing)ofexistingroads(seemoreonoverlayslaterinthischapter,ConcreteOverlays,page23).

JPCPBecauseoftheircost-effectivenessandreliability,

thevastmajorityofconcretepavementsconstructedtodayareJPCPdesigns.Theydonotcontainreinforcement.Theyhavetransversejointsgenerally

________________________________________________ Figure 2-2. Concrete pavement types (Adapted from ACPA)



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spacedlessthan5to6.5m(15to20ft)apart.Theymaycontaindowelbarsacrossthetransversejointstotransfertrafficloadsacrossslabsandmaycontaintiebarsacrosslongitudinaljointstopromoteaggregateinterlockbetweenslabs.

JRCPJRCPdesignscontainbothjointsandreinforcement

(e.g.,weldedwirefabric,deformedsteelbars).Jointspacingsarelonger(typicallyabout9to12m[30to40ft]),anddowelbarsandtiebarsareusedatalltransverseandlongitudinaljoints,respectively.

Thereinforcement,distributedthroughouttheslab,composesabout0.15to0.25percentofthecross-sectionalareaandisdesignedtoholdtightlytogetheranytransversecracksthatdevelopintheslab.Itis

difficulttoensurethatjointsarecutwheretherein-forcementhasbeendiscontinued.ThispavementtypeisnotascommonasitoncewasonStatehighways,butitisusedtosomeextentbymunicipalities.

CRCPCRCPdesignshavenotransversejoints,but

containasignificantamountoflongitudinalrein-forcement,typically0.6to0.8percentofthecross-sectionalarea.Transversereinforcementisoftenused.Thehighcontentofreinforcementbothinfluencesthedevelopmentoftransversecrackswithinanacceptablespacing(about0.9to2.5m[3to8ft]apart)andservestoholdcrackstightlytogether.SomeagenciesuseCRCPdesignsforhigh-traffic,urbanroutesbecauseoftheirsuitabilityforhigh-trafficloads.

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Design Considerations: What Do We Want?

Key Points

Foreveryconcretepavementproject,thedesignermustdefinecertainparameters:

Levelofstructuralandfunctionalperformance.

Targetservicelife,ordesignlife.

Levelsofvariousconcreteproperties—durability,strength,rigidity,shrinkage,etc.—required(ortolerated)toachieveoptimumperformanceforthedesignlife.

Pavementsmustperformwellstructurally(i.e.,carryimposedtrafficloads)andfunctionally(i.e.,provideacomfortableride).Performanceisgenerallydescribedintermsofstructuralandfunctionaldistresses.

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Foreachspecificpavementdesign,adesignerdefinesthedesiredpavementperformance,servicelife,andvariousconcreteproperties.

Pavement PerformanceThegoalofallpavementdesignmethodsisto

provideapavementthatperformswell;thatis,thegoalistoprovideaserviceablepavementoverthedesignperiodforthegiventrafficandenvironmentalloadings.Apavement’sdesiredperformanceisgenerallydescribedintermsofstructuralandfunctionalperformance:

Structuralperformanceisapavement’sabilitytocarrytheimposedtrafficloads.Functionalperformanceisapavement’sabilitytoprovideusersacomfortablerideforaspecifiedrangeofspeed.

Bothstructuralandfunctionaldistressesarecon-sideredinassessingoverallpavementperformanceorcondition.Evenwell-designedandwell-constructedpavementstendtodegradeatanexpectedrateof

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Concretestrengthhastraditionallybeenthemostinfluentialfactorindesignandisstillaprimarydesigninputfordeterminingslabthickness.

Concretedurabilityisacriticalaspectoflong-termpavementperformance,butitisnotadirectinputinpavementdesignprocedures.(Concretedurabilityisgovernedthroughappropriatematerialandconstructionspecifications.)

Concretestiffnessanddimensionalstability(dryingshrinkageandthermalmovement)influenceperformance,buthavenottraditionallybeenconsideredindesignprocedures.

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deteriorationasafunctionoftheimposedloadsand/ortime.Poorlydesignedpavements(eveniftheyarewell-constructed)willlikelyexperienceaccelerateddeterioration(figure2-3).

Notethatgoodconcretedurabilitythroughgoodmixdesignandconstructionpracticesisnecessary

____________________________________________ Figure 2-3. Pavement condition as a function of time or traffic (ACPA)



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long-termperformance.Ingeneral,cracksthatdonotseparateandfault(i.e.,undergodifferentialverticalmovementordisplacement)arenottypicallydetrimentaltopavementstructuralperformance.Thetransversecrackshowninfigure2-5isconsideredastructuraldistressbecausefaultingisevident.Suchacrackhassignificantstructuralimplications,sincethedynamicloadingfromaroughpavementreducesstructurallife.

Transversecrackingcanbeduetoanumberoffactors,includingexcessiveearly-ageloading,poorjointloadtransfer,inadequateornonuniformbasesupport,excessiveslabcurlingandwarping(seeCurlingandWarpinginchapter5,page150),insuffi-cientslabthickness,inadequatesawing,andmaterialsdeficiencies.

Figure 2-4. Corner cracking in jointed concrete pavement (ACPA)

Figure 2-5. Transverse crack that represents structural distress (ACPA)

toachievetheinitialexpectedlevelofpavementperformance.Poormixdesignand/orconstructionpracticesmaysignificantlyacceleratefunctionaland/orstructuraldeterioration,leadingtoprematurefailureofthepavement.

Structural PerformanceStructuralperformanceistheabilityofthepavement

tosupportcurrentandfuturetrafficloadingsandtowithstandenvironmentalinfluences.Structuralperfor-manceofconcretepavementsisinfluencedbymanyfactors,includingdesign,materials,andconstruction.Themostinfluentialdesign-relatedvariablesforstructuralperformanceatagivenleveloftrafficareslabthickness,reinforcement,concretestrength,elasticmodulus(characterizationoftheconcrete’sstiffness),andsupportconditions.

Themostprevalenttypeofstructuraldistressisload-relatedcracking,whichmayappearascornercracks,transversecracks,orlongitudinalcracks.

Corner Cracks.Cornercracksorcornerbreaks,asshowninfigure2-4,areusuallycausedbystructuralfailureunderloads,particularlywhenapavementhasagedandrepeatedloadingscreatevoidsunderslabcorners.

Factorsthatcontributetocornercracksincludeexcessivecornerdeflectionsfromheavyloads,inadequateloadtransferacrossthejoint,poorsupportconditions,curling(seeCurlingandWarpinginchapter5,page150),insufficientslabthickness,inadequatecuring,and/orinadequateconcretestrength.Itiscriticalthatuniformsupportbeprovidedtopreventexcessivestressesresultingfromvaryingsupportconditions.Inaddition,theslabisbestabletodistributewheelloadsatthecenteroftheslab,ratherthanattheedges;therefore,longitudinaljointsinthewheeltrackshouldbeavoided.Cornerbreaksarenotcommonwhenrealistictrafficprojectionsareusedinthedesignandwhereeffective,uniformbasesupportandjointloadtransferexist.

Transverse Cracks.Transversecrackingisacommontypeofstructuraldistressinconcretepavements,butnotalltransversecracks(alsocalledmid-panelcracks)areindicativeofstructuralfailure.Moreover,manytransversecracksmayhavelittleornoimpacton


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Longitudinal Cracks.Longitudinalcrackingmayormaynotbeconsideredastructuraldistress,dependingonwhetherthecrackremainstightandnonworking.Figure2-6showsalongitudinalcracktypicalofpoorsupportconditions.Notethatthecrackhassignificantseparationandshowsdifferentialverticalmovement,whichindicatesastructuraldistress.

Longitudinalcrackingisgenerallyassociatedwithpoorornonuniformsupportconditionsrelatedtofrostheave,moisture-inducedshrinkage/swellinginthesubgrade,orpoorsoilcompaction.Longitudinalcrackingmayalsoresultfrominadequateplacementoflongitudinaljoints,over-reinforcingoflongitudinaljoints,ortoo-shallowjointsawcuts.

Shattered Slabs.Shatteredslabsaredividedintothreeormorepiecesbyintersectingcracks(figure2-7).Theseworkingcracksallowfordifferen-tialsettlementoftheslabsectionsatarapidrate.

Thistypeofdistresscanbeattributedtonumerousfactors,themostimportantbeingtoo-heavyloads,inadequateslabthickness,andpoorsupport.

Functional PerformanceMostoften,functionalperformanceisthoughtto

consistofridequalityandsurfacefriction,althoughotherfactorssuchasnoiseandgeometricsmayalsocomeintoplay.Functionaldistressisgenerallyrepresentedbyadegradationofapavement’sdrivingsurfacethatreducesridequality.

Thefunctionalperformanceofaconcretepavementisimpactedbydesign,constructionpractice,concrete

Figure 2-6. Longitudinal crack representing structural distress (ACPA)

Figure 2-7. Shattered slab representing structural distress (ACPA)

materials,andenvironment.Concretepavementthicknessdesignmethodsconsiderstructuralperfor-mancedirectlyandfunctionalperformanceonlyintermsofpavementsmoothnessandfaulting,widen-ing,lanecapacity,etc.Additionalfunctionaldistresstypesaregenerallyconsideredthroughspecificationsgoverningitemslikesurfacefrictioncharacteristics.

Somefunctionaldistresses,suchasalkali-silicareactivity(ASR)andD-cracking,canarisefrommaterials-relatedproblems(seeAggregateDurabilityinchapter3,page47).Thecrackingandspallingresultingfromthesedistresstypesaffectridequality,safety,andthestructuralcapacityofthepavement.However,distresseslikeASRandD-crackingarenotpavementdesignissues;theymustbeaddressedthroughmixdesignandmaterialsselectionandverification(seeAdjustingPropertiesinchapter6,page185).

Surfacefrictionisanimportantfunctionalcharac-teristicofpavements.Frictionisnotconsideredadesignelementperse,buthasimplicationsfornoiseanddry-andwet-weatherskidresistance.

Thefrictionnecessaryforskidresistanceincreaseswithincreasingroughness(texture).Surfacetexturecanbeaffectedbymaterialsintheconcretemixture.Forexample,aggregatesthatpolishundertrafficeventuallyresultinreducedpavementsurfacetexture(seeAbrasionResistanceinchapter5,page146).Aggregatespecificationstargetingaminimumsilicacontentinthefineaggregateportionoftheconcretemixtureareoftenusedtopreventpolishing.



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Surfacetextureisalsoaffectedbyconstructionpractices.Concretepavementsurfacesaregenerallytexturedtoprovideadequatefrictionandskidresis-tance.Avarietyoftexturingtechniquesmaybeusedtopromotegoodfrictioncharacteristics(seeTexturinginchapter8,page221).

Surfacetexturesignificantlyaffectstire-pavementnoise.Ridequalityisconsideredakeyindicatoroffunctionalperformanceandisaffectedbybothinitialconstructionandtheoccurrenceofdistress.Ridequalityisgenerallyassumedtoequatetopavementsmoothness,althoughthisisnotalwaysthecase.

Therearemanymethodstodeterminepavementsmoothness,rangingfromprofilographs(appropriateforconstructionqualitycontrol)tononcontact,high-speedinertialprofilers(usefulforanetwork-levelanalysis).

Functionaldistressisnotalwaysdistinguishablefromstructuraldistressandoftenmayhavethesamerootcause.Structuraldistresswillalmostalwaysresultinareductioninfunctionalperformance,becausestructuraldistressesgenerallyreduceridequality.Faultingisoneexample.Faultingresultsfromacombinationofinadequateloadtransferatthejoints,

Surface Texture: Balancing Friction and Noise

(continued on the following page)

adequate friction on roadways with higher speeds when the concrete mix includes adequate amounts of durable (e.g., siliceous) sand (Wiegand et al. 2006).

Longitudinal Tining. Longitudinally tined textures are created by moving a tining device (commonly a metal rake controlled by hand or attached to a mechani-cal device) across the plastic concrete surface in the direction of the pavement. Although skid trailer friction levels of longitudinally tined textures may not be as high as those of transversely tined surfaces (see below), longitudinal tining provides adequate friction on high-speed roadways while sometimes significantly reducing tire-pavement noise.

Variations in the amount of concrete displaced to the surface during tining appear to affect the level of tire-pavement noise. As a result, there are conflicting reports about the effect of tining depth on noise. Further investigation is needed before optimum dimensions can be recommended.

Transverse Tining. Transversely tined textures are created by moving a tining device across the width of the plastic pavement surface. The tines can be uniformly or randomly spaced, or skewed at an angle.

Transverse tining is an inexpensive method for providing durable, high-friction pavement surfaces. The friction qualities are especially evident on wet pavements.

Work is ongoing to find the optimum means of achieving satisfactory skid resistance while reducing noise effects at the tire/pavement interface (Rasmussen et al. 2004).

Conventional Texturing Techniques In general, various tools or materials are dragged across fresh concrete to produce a surface texture. These tools and materials include moistened burlap, brooms, tining rakes, and artificial turf. Another technique is diamond grinding the hardened concrete surface.

Diamond Grinding. Diamond grinding is a process of removing a thin layer of hardened concrete pavement using closely spaced diamond saw blades. Diamond grinding has traditionally been used to restore smoothness to an existing pavement. However, this process has been shown to significantly reduce tire-pavement noise and increase friction; therefore, diamond grinding has become an effective, if somewhat expensive, option for texturing newly placed concrete pavements after a minimum specified curing time.

Drag Textures. Dragging artificial turf or moistened, coarse burlap across the surface of plastic concrete creates a shallow surface texture. This texturing method is inexpensive, results in relatively quiet pavements, and provides sufficient friction characteristics for many road-ways, particularly those with speeds less than 72 km/hr (45 mph). Iowa has found that drag textures can provide



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Figure 2-8. Faulting in jointed concrete pavement (ACPA)

highcornerdeflectionscausedbyheavytrafficloading,andinadequateorerodiblebasesupportconditions.

Jointfaultingisshowninfigure2-8.Faultingisalsopossiblewherecrackshavedevelopedinthepavement,asdescribedearlier.Faultedcrackshavethesameimpactonthefunctionalandstructuralperformanceofthepavementasfaultedjoints.

Service LifeConcretepavementscanbedesignedforvirtually

anyservicelife,fromaslittleas10yearsto60yearsormore.Theprimaryfactorsinthedesignlifeare

Surface Texture: Balancing Friction and Noise, continued

In Europe, exposed aggregate pavements are regarded as one of the most advantageous methods for reducing tire-pavement noise while providing adequate friction. Smaller aggregate sizes have been reported to provide larger noise reductions, while aggregates with a high polished stone value increase durability.

In trial projects in North America, however, reported noise levels on exposed aggregate surfaces have not been low. Additional research is ongoing.

Pervious Concrete. Large voids are intentionally built into the mix for pervious, or porous, concrete, allowing water and air to flow through the pavement. The voids tend to absorb tire-pavement noise. The sound absorption levels of pervious concrete pavements have been shown to increase with higher porosity levels and smaller aggregate sizes.

However, European trials have found the acoustical performance and friction characteristics of pervious pavements to be poor, and research continues in this country.

Other Techniques. New technologies, including poroelas-tic, euphonic, and precast pavements, have demonstrated ability to reduce noise. However, further research must be conducted to predict the noise and friction performance of these construction methods. Contact the National Concrete Pavement Technology Center for information regarding the most recent research (see page iii).

However, uniform transverse tining has been shown to exhibit undesirable wheel whine noise and should be avoided if possible.

Contrary to earlier studies, recent observations have found that, although randomly spaced and/or skewed transverse tining may reduce the audible whine while providing adequate friction, random and/or skewed transverse tining is not an adequate solution for reducing tire-pavement noise (Wiegand et al. 2006).

If transverse tining is used, it is recommended that a spacing of 12.5 mm (0.5 in.) be used and that care be taken to produce as uniform a texture as possible.

Innovative TechniquesExposed aggregate pavements and pervious pavements are being investigated for their friction and noise levels.

Exposed Aggregate Pavements. Texture is created on exposed aggregate pavements via a two-layer, “wet on wet” paving process. A thin layer of concrete containing fine siliceous sand and high-quality coarse aggregate is placed over a thicker layer of more modest durability. A set-retarding agent is applied to the newly placed top layer. After 24 hours the surface mortar is brushed or washed away, exposing the durable aggregates.

When designed and constructed properly, these pavements have been reported to improve friction and durability while reducing noise.

(continued from previous page)



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Flexuralstrengthtestingisconductedonaconcretebeamundereithercenter-pointorthird-pointload-ingconditions.Thethird-pointloadingconfiguration,describedunderASTMC78/AASHTOT97,ismorecommonlyusedinpavementdesignandprovidesamoreconservativeestimateoftheflexuralstrengththanthecenter-pointtest.

Althoughflexuralstrengthisspecifiedindesign,manyStatetransportationagenciesdonotmandatetheuseofbeamtestsindeterminingthestrengthvalue.Instead,theydevelopcorrelationstoothertests,suchascompressivestrengthorsplittensilestrength,whicharemoreconvenientandlessvariable(Kosmatka,Kerkhoff,andPanarese2002).Eachindividualconcretemixturehasitsowncorrelationofcompressivestrengthtoflexuralstrength.Thatis,thecorrelationismix-specific,canvaryfrommixtomix,andshouldbemeasuredinlaboratorytests.Ingeneral,thecorrelationcanbeapproximatedbytheuseofeitheroftwoequations:

thematerialsqualityandslabthickness.Pavementmixtureswithenhancedstrengthanddurabilitychar-acteristics,combinedwithenhancedstructuraldesignelements,arenecessaryforlonglife-spans(FHWA2002).

Specialhigh-performanceconcretemixturesmaybespecifiedforlong-lifepavements.Ideally,suchmixturesgenerallycontainhigh-quality,durable,andwell-gradedcoarseaggregate;atargetedaircontent(6to8percententrained,withaspacingfactorof<0.20mm[0.008in.])forincreasedfreeze-thawprotection;anappropriateamountofground,granu-latedblast-furnaceslagorflyashforreducedpermeability;andawater-cementitiousmaterialsratioof0.40to0.43.Anotherrequirementmayincludeamaximum28-dayrapidchloridepenetrability(ASTMC1202)of1,500coulombsforconcretepermeability.Propercuringisalsoimportant.Notethatexperiencedpractitioners,usingconsiderablecare,maybeabletoconstructlong-lifepavementswithgap-gradedaggregateand/orhigherwater-cementitiousmaterialsratios.

Concrete PropertiesManyconcretepropertiesarecriticaltotheper-

formanceofconcretepavementsoveragivendesignlife.Someofthesepropertiesareusedasinputstothepavementdesignprocess;othersareassumedindeter-miningconcretethicknessorarenotconsideredinthedesignprocess(seechapter5,CriticalPropertiesofConcrete,page105).

Themostinfluentialconcretepropertiesforcon-creteperformanceincludestrengthandstiffness,dimensionalstability(dryingshrinkageandthermalsensitivity),anddurability,whicharediscussedinthefollowingsections.

Concrete StrengthConcretestrengthisaprimarythicknessdesign

inputinallpavementdesignprocedures.Usually,flexuralstrength(alsocalledthemodulusofrupture)isusedinconcretepavementdesignbecauseitcharacterizesthestrengthunderthetypeofloadingthatthepavementwillexperienceinthefield(bending)(seeStrengthandStrengthGaininchapter5,page116.)

(2.1)(ACPA2000)

(2.2)(Raphael1984)

where,

MR=modulusofrupture(flexuralstrength),inpoundspersquareinch(lb/in2)ormegapascal(MPa).

=compressivestrength,lb/in2orMPa.

a=coefficientrangingfrom7.5to10forlb/in2,or0.62to0.83forMPa(coefficientmustbedeterminedforspecificmixture).

b=2.3forlb/in2,or0.445forMPa(exactcoefficientmustbedeterminedforspecificmix).

Themoreconservativeequationshouldbeused,dependingontherangeofvaluesbeingused(figure2-9).

Maturitytestsarebecomingmorecommoninassessingthein-placestrengthofconcreteforopeningthepavementtotraffic(ASTMC1074;ACPA2003).Maturitytestingprovidesareliabletechniqueforcontinuouslymonitoringconcretestrengthgainduringthefirst72hours.Thetechnol-ogyoffersseveraladvantagesovertraditionaltestingmethods.Mostimportant,maturitytestingallows



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pavementownerstoopenanypavementtotrafficassoonasithasattainedthenecessarystrength,with-outdelay,whetherornotitisafast-trackproject(seeMaturityTestinginchapter5,page121).

Elastic ModulusAnotherconcretepropertyimportantinpave-

mentdesignistheelasticmodulus,E,whichtypicallyrangesfrom20to40gigapascal(GPa)(3,000to6,000ksi).Themodulusofelasticity,orstiffness,ofconcreteisameasureofhowmuchthematerialwilldeflectunderloadandstronglyinfluenceshowtheslabdistributesloads.Thedeterminationofelastic

modulusisdescribedinASTMC469.However,elas-ticmodulusisoftendeterminedfromtheempiricalformularelatedtocompressivestrength(ACI318):

(2.3)

(2.4)

where,

E=moduluselasticity,lb/in2(equation2.3)orMPa(equation2.4).

=compressivestrength,lb/in2(equation2.3)orMPa(equation2.4).

Drying Shrinkage and Thermal Expansion/Contraction

Concreteisaffectedbyitsdryingshrinkageandthermalexpansion/contractionproperties(seeShrinkageandTemperatureEffectsinchapter5,pages125and127,respectively).Theeffectsdependonanumberofinfluentialfactors,includingtotalwatercontent,typesandamountsofcementitiousmaterials,water-cementitiousmaterialsratio,coarseandfineaggregatetypesandquantities,andcuringmethod.

Slabcurlingandwarping(verticaldeflectionsintheslabduetodifferentialtemperaturesandmoisturecontents)arefunctionsofvolumechangesintheconcrete.Excessivecurlingandwarpinghaveasubstantialeffectonlong-termpavementperformanceintermsofcracking,faulting,andsmoothness(seeCurlingandWarpinginchapter5,page150).

Inthepast,concretevolumechangeswerenotadesigninput.However,volumechangesplayanimportantroleintheM-EPDG(NCHRP2004).Accountingfordryingshrinkageandthermalexpan-sion/contractionpropertiesinthicknessdesignevaluationleadstobetterdesignsthatoptimizetheslabthicknessrequiredforthedesigncircumstances.

Jointspacing,anotherimportantelementofconcretepavementdesign,shouldtakeintoaccountpotentialvolumechangesoftheconcretefromtemperatureexpan-sion/contractionandcurling/warping.Reducingjointspacingtypicallycontrolscurlingandwarping.Higherdegreesofdryingshrinkageandconcretecoefficientofthermalexpansioncanbeaccommodatedwithoutincreasingtheriskofcrackingproblems.

________________________________________________ Figure 2-9. Comparison of compressive and flexural strength correlations, based on equations 2.1 and 2.2



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DurabilityDurableconcreteisstableinitsenvironment;

thatis,itexperiencesminimumdeteriorationduetofreeze-thawcycles,adversechemicalreactionsbetweentheconcreteanditssurroundings(forexam-ple,deicingchemicals),orinternalreactionsbetweentheportlandcementandaggregates(seethesidebaronConcreteDurabilityinchapter5,page130).

Concretedurabilityisthemostcriticalaspectoflong-termpavementperformance.However,durabilityisnotcharacterizedbyconcretestrengthandisnotadirectinputindesignprocedures.Instead,durabilityisassumedindesignandgovernedthroughappropri-ate-qualitymaterialsandconstructionspecifications.Forexample,freeze-thawdurabilityisprimarilyaffectedbytheenvironmentandtheair-voidsystemoftheconcrete.Alkali-silicareactivityiscontrolledthroughspecificationsbycontrollingtheamountofalkaliintheconcrete,prohibitingaggregatesthathavedetrimentalreactivity,addinganaggregatethatlimitstheexpansionoftheconcretetoanacceptablelevel,orusinganadequateamountofeffectivepozzolansor

ground,granulatedblast-furnaceslag.D-crackingcanbecontrolledbyspecifyingthequalityormaximumsizeofthecoarseaggregate.

However,apavementdesignermustbeawareofthepotentialimpactofmaterialsselection(madetoimprovedurability)onstructuralandfunctionalperformance.Alistofsomepotentialimpactsislistedhereforillustration:

Smalltop-sizedcoarseaggregates,selectedtoavoidD-cracking,willlikelyreduceloadtransferatjoints.Aircontenttargets(forfreeze-thawdurability)thatexceedsixpercentwilllikelydecreasetheconcretestrengthachievedatgivenlevelsofcement.Gravelorrelativelyhardcoarseaggregatesproduceconcretewitharelativelyhigherelasticmodulus(rigidity)thansofteraggregates.Theelasticmodulusaffectscrackspacingdevel-opmentonCRCPdesignsandjointspacingrequirementsonJPCPdesigns.Higherlevelsofstrengthusedindesignmaydriveincreasedcementcontentintheconcretemixture,leadingtoincreasedpastecontent,decreaseddurability,andahigherriskofcracking.

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Relationship of Concrete Durability to Pavement Design

Durability is not a function of concrete strength. Rather, it is an indication of a particular concrete’s stability in its environment. Durability is determined by a concrete’s susceptibility (preferably, lack of susceptibility) to deterioration due to freeze-thaw cycles, adverse chemical reactions between the concrete and its surroundings (for example, deicing chemicals), or internal reactions between the portland cement and

aggregates. Durability also includes the ability of concrete to protect reinforcing steel from corrosion and to resist abrasion.

Concrete durability is not an input in pavement design but is assumed during the pavement design process. Concrete durability is generally the goal of good mix design and materials selection.


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Design Considerations: What Site Factors Do We Have to Accommodate?

Key Points

Concretepavementsneedadequate,well-draining,uniformsubgrade/basesupport.Uniformityisespeciallyimportant.

Environmentalfactorscanhaveasignificanteffectonpavementperformancebutarenotconsideredinmostdesignprocedures.(ThedesignprocedureintheNCHRPM-EPDGattemptstoincorporateimportantenvironmentalfactors.)

Trafficdata,particularlyregardingtheanticipatednumberoftrucksandtheirloadings,aremajorfactorsinpavementdesign.

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Factorsuniquetothelocationofaparticularpavingprojectthatsignificantlyaffectitsdesignandperfor-mancearesitefactors:subgradesupportconditions,environmentalforces,andanticipatedtrafficloadings.

SupportConcretepavementsdistributewheelloadsovera

largeareathroughtheslab.Poorornonuniformsup-portundertheslab,orsupportdegradingwithtime,causessettlementthatleadstocrackingandfailure.Concretepavementsdonotrequireasmuchsupportasotherpavementmaterials,thusreducingtheextentofnecessarybasepreparation,buttheydorequireuniformsupportforgoodlong-termperformance.

Inconcretepavementdesign,subgradesupportischaracterizedbythemodulusofsubgradereaction(k).Themodulusofsubgradereactioncanbedeter-minedthroughaplateloadtest,back-calculationofdeflectiondata,orcorrelationtootherreadilydeter-minedsoilstrengthparameters.

Thesupportvalueusedindesigngenerallyrepre-sentsaseasonallyadjustedaverageoverthedesignlifeoftheproject.Itisassumedthatthesupportisnonerodibleandrelativelyconstant.

Environmental FactorsEnvironmentalfactorslikeprecipitationand

temperaturescansignificantlyaffectpavementperformance.

Generally,pavementsexposedtosevereclimates(e.g.,higherrainfallormorefreeze-thawcycling)maynotperformaswellaspavementsinmoderateclimates.Pavementsinsimilarclimaticregionsorexposedtosimilarclimaticforcesshouldperforminasimilarmannerifsimilarmaterials,proportions,andconstructionpracticesarefollowed.

Mostdesignprocedures,however,donotincor-porateenvironmentalfactors,ormayconsideronlyafewofthemindirectly.TheM-EPDG(NCHRP2004)attemptstoincorporatesomeoftheimportantenvi-ronmentalfactorsintothepavementdesignprocess.

Dailyandseasonalenvironmentalvariationscaninfluencethebehaviorofconcretepavementinthefollowingways(SmithandHall2001):

Openingandclosingoftransversejointsinresponsetodailyandseasonalvariationinslabtemperature,resultinginfluctuationsinjointloadtransfercapability.Upwardanddownwardcurlingoftheslabduetodailycyclingofthetemperaturegradientthroughtheslabthickness(seeCurlingandWarpinginchapter5,page150).Permanentcurling(usuallyupward),whichmayoccurduringconstructionashighsettem-peraturedissipates.Upwardwarpingoftheslabduetovariationinthemoisturegradientthroughtheslabthickness.Erosionofbaseandfoundationmaterialsduetoinadequatedrainage.Freeze-thawweakeningofsubgradesoils.Freeze-thawdamagetosomecoarseaggregates.Corrosionofsteel,especiallyincoastalenviron-mentsandinareaswheredeicingsaltsareused.

Traffic ConsiderationsTraffictypesandloadingsanticipatedonaroadway

overitsdesignliferepresentamajorfactorinpave-mentdesign.Ofparticularinterestisthenumberoftrucksandtheiraxleloads(axletype,axleweight,numberofaxles,axlespacing,andloadfootprint).

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�0 Integrated Materials and Construction Practices for Concrete Pavement

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Design Procedures: Getting What We Want, Given Site Factors

Key Points

ThedesignprocedureintheNCHRPM-EPDG,whichusesmechanistic-empiricaldesignprinciples,willeventuallyreplacethe1993AASHTOdesignprocedure.

Mechanistic-empiricaldesigncombinesmechanisticprinciples(stress/strain/deflectionanalysisandresultingdamageaccumulation)andempiricaldataforreal-worldvalidationandcalibration.

Thevalidityofanyconcretepavementdesignisonlyasgoodasthedatausedasinputsandthevaluesusedforlocalconditionsinthemodels.

Inaconstructabilityreviewprocess,designerspartnerwithconstructionpersonnelwhohaveextensiveconstructionknowledgetoensure,earlyinthedesignprocess,thataprojectisbuildable,cost-effective,biddable,andmaintainable.

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Twoimportantdevelopmentsareoccurringinconcretepavementdesign:By2010,theM-EPDG(NCHRP2004)mechanistic-empirical(M-E)designprocedurewilllikelyreplacethe1993AASHTOdesignprocedure.Additionally,manyagenciesaredevelopingaconstructabilityreviewprocessfortheearlystagesofconcretepavementdesigntohelpensurethesuccessoftheirprojects.

Mechanistic-Empirical Design Procedure

Currently,mostStateagenciesareusingthe1993AASHTOdesignprocedure.The1993designproce-dureisanempiricalprocedurebasedontheresultsoftheAASHORoadTestconductednearOttawa,Illinois,from1958to1960.

Asofthiswriting,theM-EPDG,whichincludescompanionsoftwareandsupportingtechnicaldocu-mentation,hasbeenreleasedonalimitedbasisforindustryevaluation.TheM-EPDGwasdevelopedinanefforttoimprovethebasicpavementdesignprocess(NCHRP2004).Italsoprovidesameanstodesignselectivepavementrestorationandoverlays,integratingmaterialsfactors.

TheM-EPDGproceduremustbelocallycalibratedsothatappropriatevaluesareusedforlocalcondi-tions.ManyStatesarecurrentlybeginningcalibrationeffortsbydevelopingtherequiredprograminputs.Thevalidityofanyconcretepavementdesignisonlyasgoodasthedatausedasinputsandthevaluesusedforlocalconditionsinthemodels.

M-Edesigncombinesmechanisticprinciples(stress/strain/deflectionanalysisandresultingdamageaccumulation)withfieldverificationandcalibration.TheM-EprocedureintheM-EPDGisconsideredtobeamorescientificallybasedapproachthanthe1993AASHTOprocedure,incorporatingmanynewaspectsofpavementdesignandperformanceprediction,andgenerallyresultsinlessconservativedesigns.Designinputsincludeelasticmodulus,flexuralstrength(modulusofrupture),andsplittingtensilestrength.

TheM-Eapproachisradicallydifferentfrompreviousdesignprocedures.TheM-Eprocedureisbasedontheaccumulationofincrementaldamage,inwhichaloadisplacedonthepavementatacriticallocation,theresultingstresses/strainsanddeflectionsarecalculated,thedamageduetotheloadisassessed,andfinallyatransferfunctionisusedtoestimatethedistressresultingfromtheload.Asimilarprocedureisfollowedforeachload(basedontrafficspectra)undertheconditionsexistinginthepavementatthetimeofloadapplication(consideringloadtransfer,uniformityandlevelofsupport,curlingandwarping,concretematerialproperties,andsoon).Thefinalstepistosumallofthedistressesthataccumulateinthepavementasafunctionoftimeortraffic.

AbenefitofM-Eanalysisisthatitpredictsspecificdistresstypesasafunctionoftimeortraffic.Cracking,faulting,andchangesinsmoothness(basedontheInternationalRoughnessIndex)areestimated.



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Thresholdvaluesforeachdistresstypeareinputbythedesignerbasedonexperience,policy,orrisktolerance.

Constructability IssuesConstructabilityreferstothefeasibilityofcon-

structingtheproposedpavementdesign,includingthematerials,construction,andmaintenanceaspects.Itistheassurancethatthepavementdesigncanbecapa-blyconstructedusingavailablematerialsandmethodsandtheneffectivelymaintainedoveritsservicelife.

Tohelpensurethesuccessoftheirconstruction

projects,manyagenciesareexploringdevelopment

ofaconstructabilityreviewprocess(CRP).Asdefined

bytheAASHTOSubcommitteeonConstruction,

constructabilityreviewis“aprocessthatutilizes

constructionpersonnelwithextensiveconstruc-

tionknowledgeearlyinthedesignstagesofprojects

toensurethattheprojectsarebuildable,whilealso

beingcost-effective,biddable,andmaintainable”

(AASHTO2000).

Input Parameters for M-E Design

The M-E design procedure (NCHRP 2004) is based on the process of adjusting factors that we can control in order to accommodate factors that we cannot control and to achieve the parameters and performance that we want.

1. What do we want?

All of the following are modeled, based on experience and experimentation, to predict the state of each parameter at the end of the selected design life. The model uses inputs from items 2 and 3 below. If the results are unacceptable, then the parameters that can be changed are adjusted and the model recalculated.

Acceptable surface roughness at the end of the life.

Acceptable cracking at the end of the life.

Acceptable faulting at the end of the life.

2. What do we have to accommodate?

These are parameters that vary from location to location and cannot be changed, but must be accounted for in the modeling.

Expected traffic loading.

Type of traffic (classes).

Growth of traffic density with time.

The climate in which the pavement is built.

Water table depth.

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3. What can we adjust?

These are the parameters that can be adjusted in the design process in order to achieve the properties and performance required in item 1.

Pavement type (JPCP, JRCP, or CRCP).Joint details (load transfer, spacing, sealant).Edge support (if any).Drainage.Layer 1: Concrete properties.

Thickness.Strength and modulus of elasticity.Thermal properties (coefficient of thermal expansion, conductivity, heat capacity).Shrinkage.Unit weight.

Layer 2: Stabilized layer properties.Material type.Thickness.Strength.Thermal properties.

Layer 3: Crushed stone properties.Strength.Gradation.

Layer 4: Soil properties .Type (soils classification).Strength.Gradation.

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ImplementingaCRPisexpectedtoofferthefollow-ingadvantages(AASHTO2000):

Enhanceearlyplanning.Minimizescopechanges.Reducedesign-relatedchangeorders.Improvecontractorproductivity.Developconstruction-friendlyspecifications.Enhancequality.Reducedelays.Improvepublicimage.Promotepublic/workzonesafety.Reduceconflicts,disputes,andclaims.Decreaseconstructionandmaintenancecosts.

Constructabilityissuesareinvolvedinallaspectsoftheconcretepavementdesignandconstructionprocess.Furthermore,theissuescanrangefromverygeneraltoverydetailed.Someconstructabilityissuesintheconcretepavementdesignandconstructionprocessatthebroadestlevelincludethefollowing:

Mixdesign.Pavementdesign.Construction.Curingandopeningtotraffic.Maintenance.

Mix DesignArequalitymaterialsavailablefortheproposedmix

design?Doestheproposedmixdesignprovidetherequiredproperties(workability,durability,strength)fortheproposedapplication?Whatspecialcuringrequirementsmaybeneededforthemix?

Pavement DesignWhatdesignprocedurewasusedindeveloping

theslabthicknessdesign?Wasanalternativedesignprocedureusedforverification?Havetheindividual

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designelementsbeendevelopedasapartoftheentirepavementsystem?Hasaneffectivejointingplanbeendevelopedforintersectionsorothercomplexlocations?

ConstructionArecompetentcontractorsavailabletodothe

work?Dothepropertiesofthebaseandsubgradematerialsmeetthedesignassumptions?Doesthedesignhaveanyuniqueconstructionrequirements?Whatarethedurationofconstructionandtheanticipatedweatherconditions?Isthereanextremeweathermanagementplaninplace?Arecontractorsfamiliarwithhot-weather(ACI305R-99)andcold-weather(ACI306R-88)concreteconstruction?Aresafeguardsinplacetopreventraindamageduringconstruction?Whatcuringisrequired?Howwilltrafficbemaintainedorcontrolledduringconstruction?

Curing and Opening to TrafficWhatcuringisrequiredforthemixdesigninorder

tomeetopeningrequirements?Whatsystemsareinplacetomonitorstrengthgain?Whatprotocolsareinplacetodealwithadverseenvironmentalchangesorconditions?

MaintenanceIsthedesignmaintainable?Aretherefeaturesthat

maycreatemaintenanceproblemsinthefuture?Shouldalongerlifedesignbecontemplatedtominimizetrafficdisruptionsforfuturemaintenanceandrehabili-tationactivities?

Again,theserepresentverygeneralitems;manymoredetaileditemsmaybeaddedtothereviewpro-cess.Variouslevelsandfrequenciesofconstructabilityreviewscanbeconducted,dependingonthescopeandcomplexityoftheproject.MoredetailedinformationonconstructabilityreviewsisavailablefromNCHRP(AndersonandFisher1997)andAASHTO(2000).


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Concrete Overlays

Key Points

Concreteoverlaysareusedasmaintenanceandrehabilitationtechniquesonexistingconcrete,asphalt,andcomposite(asphaltonconcrete)pavements.

Themostimportantaspectofoverlaydesignistheinterfacewiththeexistingpavement.

Thefollowingtypesofconcreteoverlaysmaybeconsidered:bondedconcreteoverlaysandunbondedconcreteoverlays.

Ifanexistingconcretepavementiscracked,andistohaveabondedoverlay,thecrackneedstoberepairedoritwillreflectthroughtheoverlay.Iftheoverlayisunbonded,aseparationlayerpreventsthecracksfromreflectingupthroughthenewlayer.

Foroverlaysoverasphalt,theconcretewilldominateandanycracksintheasphaltnormallywillnotreflectthroughtheoverlay.

Iftheexistingpavementsuffersfrommaterials-relateddistress,itmaynotbeagoodcandidateforanoverlay.

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Concreteoverlaysarebeingusedmoreoftenasmaintenanceandrehabilitationtechniquesonexistingconcreteandasphaltpavements.Concreteoverlayscanserveascost-effectivemaintenanceandrehabilita-tionsolutionsforalmostanycombinationofexistingpavementtypeandcondition,desiredservicelife,andanticipatedtrafficloading.Acrossthecountry,mostStateshaveusedatleastonetypeofconcreteoverlaytomaintainorrehabilitateagingpavements.Concreteoverlaysoffermanyimportantbenefits,includingextendedservicelife,increasedload-carryingcapacity,fastconstructiontimes,lowmaintenancerequire-ments,andlowlife-cyclecosts.

Two Families of Concrete OverlaysConcreteoverlaysareclassifiedaccordingtothe

existingpavementtypeandthebondingcondi-tionbetweenlayers(Smith,Yu,andPeshkin2002).Concreteoverlaysformtwofamilies:bondedconcreteoverlaysandunbondedconcreteoverlays(figure2-10andfigure2-11).

Bondedconcreteoverlaysarebondedtoanexist-ingpavementcreatingamonolithicstructure.Theyrequirethattheexistingconcretepavementbeingoodstructuralconditionandtheexistingasphaltpavementbeinfaircondition.Theoverlayeliminatessurfacedistresses,andtheexistingpavementcontinuestocarrytrafficload.

Unbondedconcreteoverlaysaddstructuralcapac-itytotheexistingpavement.Constructedessentiallyasnewpavementsonastablebase,unbondedoverlayprojectsdonotrequirebondingbetweentheoverlayandtheunderlyingpavement.

Bothbondedandunbondedoverlayscanbeplacedonexistingconcretepavements,asphaltpavements,orcompositepavements(originalconcretepavementsthathavebeenresurfacedpreviouslywithasphalt).

Concrete Overlays on Concrete PavementsConcreteoverlaysonconcretepavementsare

designedaseitherbondedorunbondedoverlays.Bonded Concrete Overlays on Concrete Pavements.A

bondedconcreteoverlayisathinconcretesurfacelayer(typically5.1to12.7cm[2to5in.]thick)thatisbondedtoanexistingconcretepavement.Bondedoverlaysareusedtoincreasethestructuralcapacityofanexistingconcretepavementortoimproveitsoverallridequality,skidresistance,andreflectivity.

Bondedoverlaysshouldbeusedonlywheretheunderlyingpavementisfreeofstructuraldistressandinrelativelygoodcondition,withoutexistingcracksthatwillreflectthroughthenewlayer.Toperformwell,thejointsshouldbealignedwithjointsinthesupportlayer.Matchedjointshelpeliminatereflectivecrackingandensurethatthetwolayersofthepave-mentstructuremovetogether.

Unbonded Concrete Overlays on Concrete Pavements.Unbondedoverlayscontainaseparationlayerbetweentheexistingconcretepavementandthenewconcrete


Concrete Overlays

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surfacelayer.Theseparationlayeristypically2.5cm(1in.)ofhotmixasphalt.Itisplacedtoprovideashearplanethathelpspreventcracksfromreflectingupfromtheexistingpavementintothenewoverlay.Inaddition,theseparationlayerpreventsmechanicalinterlockingofthenewpavementwiththeexistingpavement,sobotharefreetomoveindependently.

Unbondedconcreteoverlaysaretypicallycon-structedabout10.2to27.9cm(4to11in.)thick.Becauseoftheseparationlayer,theycanbeplacedonconcretepavementsinpracticallyanycondition.However,overlaysonpavementsinadvancedstagesofdeteriorationorwithsignificantmaterials-relatedproblems(suchasD-crackingoralkali-silicareactiv-ity)shouldbeconsideredcautiously,becauseexpansivematerials-relateddistressescancausecrackinginthenewoverlay.

Somehighwayagenciesspecifythatjointsinunbondedconcreteoverlaysshouldbemismatchedwiththoseintheunderlyingpavementtomaximizethebenefitsofloadtransfer.However,manyStatesdonotintentionallymismatchjointsandhavenotexperiencedanyadverseeffects.

Concrete Overlays on Asphalt PavementsConcreteoverlaysonasphaltpavementsare

designedaseitherbondedorunbondedoverlays.Bonded Concrete Overlays on Asphalt Pavements

(Previously Called Ultrathin Whitetopping, UTW).Bondedconcreteoverlaysmaybeusedasamaintenanceorresurfacingtechniqueonexistingasphaltpavements.ThesethintreatmentshavebeenusedmostoftenonStatehighwaysandsecondaryroutes.

Bondedconcreteoverlaysonasphaltpavementstypicallyinvolvedtheplacementofa5.1-to12.7-cm(2-to5-in.)concretelayeronanexistingasphaltpave-ment.Theexistingasphaltpavementmaybemilledtoenhancethebondbetweentheconcreteoverlayandtheasphaltpavementandincreaseload-carryingcapacity.Smallsquareslabsarealsoused(typicallyfrom10.9to2.4m[3to8ft])toreducetheoverallthermalmovementofeachpanelsotheconcretedoesnotseparatefromtheasphaltandtoreducecurlingandwarpingstresses(seeCurlingandWarpinginchapter5,page150).

Bonded Concrete Overlays on Concrete Pavements–previously called bonded overlay–

Bonded Concrete Overlays on Asphalt Pavements–previously called ultra-thin whitetopping, UTW–

Bonded Concrete Overlays on Composite Pavements

Unbonded Concrete Overlays on Concrete Pavements

–previously called unbonded overlay–

Unbonded Concrete Overlays on Asphalt Pavements

–previously called conventional whitetopping–

Unbonded Concrete Overlays on Composite Pavements

________________________________________________ Figure 2-10. Examples of bonded overlays

________________________________________________ Figure 2-11. Examples of unbonded overlays

Concrete Overlays


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Unbonded Concrete Overlays on Asphalt Pave-ments (Previously Called Conventional Whitetopping).Unbondedconcreteoverlaysmaybeplacedonexist-ingdistressedasphaltpavements.Thistypeofoverlayisgenerallydesignedasanewconcretepavementstructureandconstructed10.2to27.9cm(4to11in.)thick.Bondingbetweentheconcreteoverlayandhotmixasphaltpavementisnotpurposelysoughtorreliedonaspartofthedesignprocedure.Somebond-ing,however,mayincreasetheeffectivethicknessthroughcompositeaction.Itisnotnecessarytotrytopreventbondingforthesetypesofoverlayssincetheconcretewilldominateinitsmovementandthuscanseparatefromtheunderliningasphalt.Thejointingpatternoftheoverlayissimilartonormalconcrete.

Concrete Overlays on Composite PavementsConcreteoverlaysoncompositepavementsare

designedaseitherbondedorunbondedoverlaysandfollowmanyofthesameprinciplesasconcreteover-laysonconcreteandasphaltpavements.

Concrete Overlay Design Considerations

Someoftheimportantaspectsofoverlaydesignistheconditionoftheunderliningpavement,trafficloading,andthetypeofclimaticconditionstheover-laywillbein.

Conventionalconcretepavingmixturesmaybeusedforconcreteoverlays.

Someconcreteoverlays(particularlythethinnerones)requireadditionalmaterialsconsiderations:

Forbondedconcreteoverlaysonconcretepavements,itisimportanttomatchthethermalexpansion/contractioneffectsoftheoverlayconcretematerialtotheexistingconcretesub-strate(pavement).Iftwoconcretelayerswithvastlydifferentcoefficientsofthermalexpan-sionarebonded,thebondlinewillbestressedduringcyclesoftemperaturechange.Materialswithsimilartemperaturesensitivitywillexpandandcontractsimilarlyattheinterface.Ther-malexpansion/contractionproblemsgenerallyresultfromdifferenttypesofcoarseaggregateintheoriginalconcreteslabandtheconcreteoverlay(seeThermalExpansion/Contractioninchapter5,page129).(Thisisnotanimportantconsiderationwhenplacingaconcreteoverlayonasphalt.)Forbondedconcreteoverlaysonasphaltpave-ments,includingfibersintheoverlayconcretemixture(seeMaterialsforDowelBars,Tiebars,andReinforcementinchapter3,page61)canhelpcontrolplasticshrinkagecrackingandprovidepost-crackingintegrity(Smith,Yu,andPeshkin2002).

Afewadditionalissuesneedtobeconsideredspecifictothepreparationforandconstructionofconcreteoverlays(seePreparationforOverlaysinchapter7,page200).

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�� Integrated Materials and Construction Practices for Concrete Pavement

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References

AASHTO standards may be found in Standard Specifications for Transportation Materials and Methods of Sampling and Testing, 24th Edition, and AASHTO Provisional Standards, 2004 Edition, HM-24-M. https://www.transportation.org/publications/bookstore.nsf.

AASHTO.1993.AASHTOGuideforDesignofPavementStructures.Washington,DC:AmericanAssociationofStateHighwayandTransportationOfficials.

———.1998.SupplementtotheAASHTOGuideforDesignofPavementStructures,PartII—RigidPavementDesignandRigidPavementJointDesign.Washington,DC:AmericanAssociationofStateHigh-wayandTransportationOfficials.

———.2000.ConstructibilityReviewBestPracticesGuide.Washington,DC:AmericanAssociationofStateHighwayandTransportationOfficials,SubcommitteeonConstruction.

ASTM standards may be found in Annual Book of ASTM Standards, ASTM International. www.astm.org.

ASTMC78,StandardTestMethodforFlexuralStrengthofConcrete(UsingSimpleBeamwithThird-PointLoading)

ASTMC469,StandardTestMethodforStaticModulusofElasticityandPoisson’sRatioofConcreteinCompression

ASTMC1074,StandardPracticeforEstimatingCon-creteStrengthbytheMaturityMethod

ASTMC1202,StandardTestMethodforElectricalIndicationofConcrete’sAbilitytoResistChlorideIonPenetration

ACI.2002.GuideforDesignofJointedConcretePavementsforStreetsandLocalRoads.ACI325.12R-02.FarmingtonHills,MI:AmericanConcreteInstitute.

———.2002.HotWeatherConcreting.ACI305R-99.FarmingtonHills,MI:AmericanConcreteInstitute.

———.2002.ColdWeatherConcreting.ACI306R-88.FarmingtonHills,MI:AmericanConcreteInstitute.

———.2005.BuildingCodeRequirementsforStructuralConcrete.ACI318-05.FarmingtonHills,MI:AmericanConcreteInstitute.

ACPA.2000.MCO16P.WinPASPavementAnalysisSoftwareUserGuide.MCO16P.Skokie,IL:AmericanConcretePavementAssociation.

Anderson,S.D.andD.J.Fisher.1997.Construct-ibilityReviewProcessforTransportationFacilities.NCHRPReport390.Washington,DC:TransportationResearchBoard.

FHWA.2002.HighPerformanceConcretePavements:ProjectSummary.FHWA-IF-02-026.Washington,DC:FederalHighwayAdministration.

Kosmatka,S.H.,B.Kerkhoff,andW.C.Panarese.2002.DesignandControlofConcreteMixtures.Engi-neeringBulletin001.Skokie,IL:PortlandCementAssociation.

NCHRP.2004.GuideforMechanistic-EmpiricalDesignofNewandRehabilitatedPavements.NCHRPProject1-37a.Washington,DC:TransportationResearchBoard.

PCA.1984.ThicknessDesignforConcreteHighwayandStreetPavements.EB109.01P.Skokie,IL:PortlandCementAssociation.

Raphael,J.M.1984.TensileStrengthofConcrete.ACIJournal81.2:158–165.

Rasmussen,R.O.,Y.A.Resendez,G.K.Chang,andT.R.Ferragut.2004.ConcretePavementSolutionsForReducingTire-PavementNoise.FHWA/ISUCooperativeAgreementDTFH61-01-X-00042.Ames,IA:CenterforTransportationResearchandEduca-tion,IowaStateUniversity.Smith,K.D.andK.T.Hall.2001.ConcretePavementDesignDetailsandConstructionPractices.ReferenceManualforCourse131060.Arlington,VA:NationalHighwayInstitute.

Smith,K.D.,H.T.Yu,andD.G.Peshkin.2002.PortlandCementConcreteOverlays:StateoftheTech-nologySynthesis.FHWA-IF-02-045.Washington,DC:FederalHighwayAdministration.

Wiegand,P.T.Ferragut,D.S.Harrington,R.O.Rasmussen,S.Karamihas,andT.Cackler.2006.EvaluationofU.S.andEuropeanConcretePavementNoiseReductionMethods.FHWA/ISUCooperativeAgreementDTFH61-01-X-00042(Project15).Ames,IA:CenterforTransportationResearchandEducation,IowaStateUniversity.

Yoder,E.J.andM.W.Witzak.1975.PrinciplesofPavementDesign.SecondEdition.NewYork,NY:JohnWileyandSons.


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Chapter 3

Fundamentals of Materials Used for Concrete Pavements

Cementitious Materials 28

Aggregates 39

Water 52

Chemical Admixtures 55

Dowel Bars, Tiebars, and Reinforcement 61

Curing Compounds 64

Atitssimplest,concreteisamixtureofglue(cement,water,andair)bindingtogetherfillers(aggregate)(figure3-1).Butothermaterials,likesupplementarycementitiousmaterials(SCMs)andchemicaladmixtures,areaddedtothemixture.Duringpavementconstruction,dowelbars,tiebars,andreinforcementmaybeaddedtothesystem,and

curingcompoundsareappliedtotheconcretesurface.Allthesematerialsaffectthewayconcretebehavesinbothitsfreshandhardenedstates.

Thischapterdiscusseseachmaterialinturn:whyitisused,howitinfluencesconcrete,andthestandardspecificationsthatgovernitsuse.

9–15% Cement

15–16% Water

25–35% Fine aggregate

30–45% Coarse aggregate

Paste (cement + water)

Mortar (paste + fine aggregate)

Concrete (mortar + coarse aggregate)

_____________________________________________________________________________________________________ Figure 3-1. Concrete is basically a mixture of cement, water/air, and aggregates (percentages are by volume). (PCA)


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Cementitious Materials

Key Points

Cementisthegluethatbindsconcretetogether,anditschemicalcompositioninfluencesconcretebehavior.

Cementitiousmaterialsincludehydrauliccementsandavarietyofpozzolansthatactlikeorcomplementthebehaviorofhydrauliccements.

Hydrauliccementsreactwithwaterinanonreversiblechemicalreaction(hydration)toformhydratedcementpaste,astrong,stiffmaterial.

Portlandcementisatypeofhydrauliccementcommonlyusedinconstruction.

Blendedcementsareamanufacturedblendofportlandcementandoneormoresupplementarycementitiousmaterials(SCMs)and,likeportlandcement,areusedinallaspectsofconcreteconstruction.

SCMscontributetothefreshandhardenedpropertiesofconcrete.Theirbasicchemicalcomponentsaresimilartoportlandcement.

SomeSCMsbehavelikehydrauliccements,otherslikepozzolans,andsomelikeboth.

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Pozzolansreactchemicallywithcalciumhydrate(CH)toformadditionalcalciumsilicatehydrate(C-S-H),abeneficialproductofcementhydrationthatcontributestoconcretestrengthandimpermeability.

FlyashisthemostwidelyusedSCM.Itisusedinabout50percentofready-mixedconcrete.

Flyashandground,granulatedblast-furnace(GGBF)slagcanbebeneficialinmixturesforpavingprojectsbecausetheyprolongthestrength-gainingstageofconcrete.

ItisimportanttotestmixturescontainingSCMstoensuretheyareachievingthedesiredresults,toverifythecorrectdosage,andtodetectanyunintendedeffects.

Hydrauliccementscanbespecifiedusingprescriptive-basedspecifications(ASTMC150/AASHTOM85orASTMC595/AASHTOM240)orbyaperformancespecification(ASTMC1157).FlyashandnaturalpozzolanscanbespecifiedusingASTMC618/AASHTOM295.GGBFslagcanbespecifiedusingASTMC989/AASHTOM302.

Seechapter4fordetailsaboutcementchemistryandhydration.

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Cementpaste(cement,water,andair)isthegluethatbindstogethertheaggregatesinconcrete.Althoughaggregatesaccountformostofthevolumeinconcrete,cementsignificantlyinfluencesthebehavioroffreshandhardenedconcrete.Forexam-ple,thehydrationproductsofcementaremostlikelytobeaffectedbychemicalattackandtochangedimensionallywithachangingenvironment(e.g.,dryingshrinkage).

Portlandcementisthemostcommoncementusedinconcreteforconstruction.Supplementarycementitiousmaterials(SCMs)likeflyashand

ground,granulatedblast-furnace(GGBF)slagaretypicallyusedalongwithportlandcementinconcretemixesforpavements.

Thissectionintroducesthecharacteristicsandbehaviorofallcementitiousmaterials,includinghydrauliccements(portlandcementsandblendedcements)andSCMs.

Forspecificinformationonthechemistryofcementsandcementhydration,seeallofchapter4.(Forspecificinformationonproportioningcementitiousmaterialsinconcretemixtures,seeAbsoluteVolumeMethodinchapter6,page179.)



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Hydraulic CementHydrauliccementisamaterialthatsetsandhard-

enswhenitcomesincontactwithwaterthroughachemicalreactioncalledhydration,andiscapableofdoingsounderwater(ASTMC125-03).Hydra-tionisanonreversiblechemicalreaction.Itresultsinhydratedcementpaste,astrong,stiffmaterial.(Foracompletediscussionofhydrauliccementchemistryandhydration,seechapter4,page69.)

Hydrauliccementsincludeportlandcementandblendedcements.(Othertypesofhydrauliccementsarerapid-settingcalciumsulfo-aluminacementsusedforrepairmaterialsorforpavementswherefastturn-aroundtimesarecritical[Kosmatka,Kerkhoff,andPanarese2002].)

Portland Cement (ASTM C ��0 / AASHTO M ��)Portlandcementisthemostcommonhydraulic

cementusedinconcreteforconstruction.Itiscom-posedprimarilyofcalciumsilicates,withasmallerproportionofcalciumaluminates(seechapter4formoredetailsofthesecompounds).Bydefinition,the


Simple Definitions Cement (hydraulic cement)—material that sets and hardens by a series of nonreversible chemical reactions with water, a process called hydration.

Portland cement—a specific type of hydraulic cement.

Pozzolan—material that reacts with cement and water in ways that improve microstructure.

Cementitious materials—all cements and pozzolans.

Supplementary cementitious materials—cements and pozzolans other than portland cement.

Blended cement—factory mixture of portland cement and one or more SCM.

compositionofportlandcementfallswithinarela-tivelynarrowband.

Portlandcementismadebyheatingcarefullycontrolledamountsoffinelygroundsiliceousmateri-als(shale)andcalcareousmaterials(limestone)totemperaturesabove1,400°C(2,500°F).Theproductofthisburningisaclinker,normallyintheformofhardspheresapproximately25mm(1in.)indiam-eter.Theclinkeristhengroundwithgypsumtoformthegrayorwhitepowderknownasportlandcement.(TheinventorofthefirstportlandcementthoughtitscolorwassimilartothatofrockfoundnearPortland,England;thus,thename.)TheaverageBlainefinenessofmoderncementsrangesfrom300to500m2/kg.

Differenttypesofportlandcementaremanufacturedtomeetphysicalandchemicalrequirementsforspe-cificpurposesandtomeettherequirementsofASTMC150/AASHTOM85orASTMC1157(Johansenetal.2005).Despitetheirbroadsimilarities,therearesomesignificantdifferencesbetweentheAASHTOandASTMrequirements.Whenorderingcements,besuretoinformthemanufacturerwhichspecificationapplies.

Hydraulic cements Pozzolans (or materials with pozzolanic characteristics)

Portland cement Blended cement

Supplementary GGBF slag GGBF slag cementitious materials Class C fly ash Class C fly ash Class F fly ash Natural pozzolans (calcined clay, calcined shale, metakaolin) Silica fume



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ASTMC150/AASHTOM85describetypesofportlandcementusingRomannumeraldesignations(table3-1).Youmightseethesetypedesignationswiththesubscript“A,”whichindicatesthecementcontainsair-entrainingadmixtures.However,air-entrainingcementsarenotcommonlyavailable.

Blended Cements (ASTM C �� / AASHTO M ��0)

Blendedcementsaremanufacturedbygrindingorblendingportlandcement(orportlandcementclinker)togetherwithSCMslikeflyash,GGBFslag,oranotherpozzolan(seeSupplementaryCementitiousMaterialslaterinthischapter,page31).

Blendedcementsareusedinallaspectsofconcreteconstructioninthesamewayasportlandcements.Likeportlandcements,blendedcementscanbetheonlycementitiousmaterialinconcreteortheycanbeusedincombinationwithotherSCMsaddedattheconcreteplant.

BlendedcementsaredefinedbyASTMC595/AASHTOM240,whichareprescriptive-basedspeci-fications(table3-2).Likeportlandcements,blendedcementscanbeusedunderASTMC1157,aperfor-mance-basedspecification.

Performance Specification for Hydraulic Cements (ASTM C ��)

ASTMC1157isarelativelynewspecificationthatclassifieshydrauliccementsbytheirperformance

Table 3-3. Performance Classifications of Hydraulic Cement (ASTM C 1157)

Using Blended Cements

There are advantages to using a manufactured blended cement in a concrete mix instead of adding portland cement and one or more SCMs separately to the mix at the concrete plant: By blending the cement and SCMs at the cement manufacturing plant, the chemical composition of the final product can be carefully and deliberately balanced, thereby reducing the risk of incompatibility problems (see Potential Materials Incompatibilities in chapter 4, page 97). There is also less variability in the properties of a manufactured blended cement compared to SCMs added at the concrete plant.

attributesratherthanbytheirchemicalcomposition.UnderASTMC1157,hydrauliccementsmustmeetphysicalperformancetestrequirements,asopposedtoprescriptiverestrictionsoningredientsorchemistryasfoundinothercementspecifications.

ASTMC1157isdesignedgenericallyforhydrau-liccements,includingportlandcementandblendedcement,andprovidesforsixtypes(table3-3).

Selecting and Specifying Hydraulic CementsWhenspecifyingcementsforaproject,checkthe

localavailabilityofcementtypes;sometypesmaynotbereadilyavailableinallareas.Ifaspecificcement

Table 3-1. Portland Cement Classifications (ASTM C 150 / AASHTO M 85)

Type Description

I NormalII Moderate sulfate resistanceIII High early strengthIV Low heat of hydrationV High sulfate resistance

Table 3-2. Blended Cement Classifications (ASTM C 595 / AASHTO M 240)

Type Blend

IS (X) [A, MS, LH] Portland blast-furnace slag cementIP (X) [A, MS, LH] Portland-pozzolan cement

Where:X = targeted percentage of slag or pozzolanA = optional air entraining cementMS = optional moderate sulfate resistanceLH = optional low heat of hydration

Type Performance

GU General useHE High early strengthMS Moderate sulfate resistanceMH Moderate heat of hydrationLH Low heat of hydration



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typeisnotavailable,youmaybeabletoachievethedesiredconcretepropertiesbycombininganothercementtypewiththeproperamountofcertainSCMs.Forexample,aTypeIcementwithappropriateamountsofflyashmaybeabletoprovidealowerheatofhydration.

Allowflexibilityincementselection.Limitingaprojecttoonlyonecementtype,onebrand,oronestandardcementspecificationcanresultinincreasedcostsand/orprojectdelays,anditmaynotallowforthebestuseoflocalmaterials.

Donotrequirecementswithspecialpropertiesunlessthespecialpropertiesarenecessary.

Table 3-4. Cement Types for Common Applications*

Projectspecificationsshouldfocusontheneedsoftheconcretepavementandallowtheuseofavarietyofmaterialstomeetthoseneeds.Acementmaymeettherequirementsofmorethanonetypeorspecifica-tion.Table3-4listshydrauliccementtypesforvariousapplications.

Supplementary Cementitious Materials

Inatleast60percentofmodernconcretemixturesintheUnitedStates,portlandcementissupplementedorpartiallyreplacedbySCMs(PCA2000).Whenusedinconjunctionwithportlandcement,SCMscon-tributetothepropertiesofconcretethroughhydraulicorpozzolanicactivityorboth.

Hydraulicmaterialswillsetandhardenwhenmixedwithwater.Pozzolanicmaterialsrequireasourceofcalciumhydroxide(CH),usuallysup-pliedbyhydratingportlandcement.GGBFslagsarehydraulicmaterials,andClassFflyashesaretypicallypozzolanic.ClassCflyashhasbothhydraulicandpozzolaniccharacteristics.

Using Unfamiliar Cements

As with other concrete ingredients, if an unfamiliar portland cement or blended cement is to be used, the concrete should be tested for the properties required in the project specifications (PCA 2000; Detwiler, Bhatty, and Bhattacharja 1996).

Cement General Moderate High early Low heat of Moderate High Resistance to specification purpose heat of strength hydration sulfate sulfate alkali-silica hydration (patching) (very resistance resistance reactivity ** (massive massive (in contact (in contact (for use with elements) elements) with sulfate with sulfate reactive soils) soils) aggregates)

ASTM C 150 / I II III IV II V Low alkali option AASHTO M 85 (moderateportland heat option)cements

ASTM C 595 / IS, IP IS, IS(LH), IS(LH), IS(MS), AASHTO M 240 IP, IP(LH) IP(LH) IP(MS) blended hydraulic cements

ASTM C 1157 GU MH HE LH MS HS Option Rhydraulic cements ***

Source: Adapted from Kosmatka, Kerkhoff, and Panarese (2002)* Check the local availability of specific cements, as all cements are not available everywhere.** The option for low reactivity with aggregates can be applied to any cement type in the columns to the left.*** For ASTM C 1157 cements, the nomenclature of hydraulic cement, portland cement, modified portland cement, or

blended hydraulic cement is used with the type designation.



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UseofSCMsinconcretemixtureshasbeengrow-inginNorthAmericasincethe1970s.Therearesimilaritiesamongmanyofthesematerials:

SCMs’basicchemicalcomponentsaresimilartothoseofportlandcement.MostSCMsarebyproductsofotherindustrialprocesses.ThejudicioususeofSCMsisdesirablenotonlyfortheenvironmentandenergyconservation,butalsoforthetechnicalbenefitstheyprovidetoconcrete.

SCMscanbeusedtoimproveaparticularconcreteproperty,likeresistancetoalkali-aggregatereactiv-ity.However,mixturescontainingSCMsshouldbetestedtodeterminewhether(1)theSCMisindeedimprovingtheproperty,(2)thedosageiscorrect(anoverdoseorunderdosecanbeharmfulormaynotachievethedesiredeffect),and(3)thereareanyunin-tendedeffects(forexample,asignificantdelayinearlystrengthgain).ItisalsoimportanttorememberthatSCMsmayreactdifferentlywithdifferentcements(seePotentialMaterialsIncompatibilities,page97).

Traditionally,flyash,GGBFslag,calcinedclay,calcinedshale,andsilicafumehavebeenusedincon-creteindividually.Today,duetoimprovedaccesstothesematerials,concreteproducerscancombinetwoormoreofthesematerialstooptimizeconcreteprop-erties.Mixturesusingthreecementitiousmaterials,calledternarymixtures,arebecomingmorecommon.

Table3-5liststheapplicablespecificationsSCMsshouldmeet.TheuseofthesematerialsinblendedcementsisdiscussedbyDetwiler,Bhatty,andBhattacharja(1996).Table3-6providestypicalchemicalanalysesandselectedpropertiesofseveralpozzolans.

Types of Supplementary Cementitious Materials

Fly Ash.FlyashisthemostwidelyusedSCMinconcrete.Itisusedinabout50percentofready-mixedconcrete(PCA2000).

ClassFandClassCflyashesarepozzolans.SomeClassCashes,whenexposedtowater,willhydrateandharden,meaningthattheymayalsobeconsideredahydraulicmaterial.

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Table 3-5. Specifications for Supplementary Cementitious Materials

Flyashgenerallyaffectsconcreteasfollows:Lesswaterisnormallyrequiredtoachieveworkability.Settingtimemaybedelayed.Earlystrengthsmaybedepressed,butlaterstrengthsareincreased,becauseflyashreactionratesareinitiallyslowerbutcontinuelonger.Heatofhydrationisreduced.Resistancetoalkali-silicareactionandsulfateattackmaybeimprovedwhentheappropriateashsubstitutionrateisused.Permeabilityisreduced;consequently,resis-tancetochlorideionpenetrationisimproved.Incompatibilitywithsomecementsandchemi-caladmixturesmaycauseearlystiffening.

ClassFflyashisgenerallyusedatdosagesof15to25percentbymassofcementitiousmaterial;ClassCflyashisgenerallyusedatdosagesof15to40per-cent.Dosageshouldbebasedonthedesiredeffectsontheconcrete(Helmuth1987,ACI2322003).

Flyashisabyproductofburningfinelygroundcoalinpowerplants.WhenitisusedasanSCMinconcretemixtures,utilitiesdonothavetodisposeofitasawastematerial.Duringcombustionofpulverizedcoal,residualmineralsinthecoalmeltandfuseinsuspensionandthenarecarriedthroughthecombus-tionchamberbytheexhaustgases.Intheprocess,thefusedmaterialcoolsandsolidifiesintosphericalglassyashparticles(figure3-2).Theflyashisthencollectedfromtheexhaustgasesbyelectrostaticprecipitatorsorfabricbagfilters.

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Type of SCM Specifications

Ground, granulated ASTM C 989 / AASHTO M 302blast-furnace slag

Fly ash and natural ASTM C 618 / AASHTO M 295pozzolans

Silica fume ASTM C 1240

Highly reactive AASHTO M 321pozzolans



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Flyashisprimarilysilicateglasscontainingsilica,alumina,calcium,andiron(thesameprimarycom-ponentsofcement).Minorconstituentsaresulfur,sodium,potassium,andcarbon,allofwhichcanaffectconcreteproperties.Crystallinecompoundsshouldbepresentinsmallamountsonly.Therelativedensity(specificgravity)offlyashgenerallyrangesbetween1.9and2.8.Thecolorisgrayortan.Particlesizes

Figure 3-2. Scanning electron micrograph of fly ash particles. Note the characteristic spherical shape that helps improve workability. Average particle size is approximately 10 µm. (PCA)

Table 3-6. Chemical Analyses and Selected Properties of Type I Cement and Several Supplementary Cementitious Materials

varyfromlessthan1µmtomorethan100µm,withthetypicalparticlesizemeasuringunder35µm.Thesurfaceareaistypically300to500m2/kg,similartocement(figure3-3).

Flyashwilllosemasswhenheatedto1,000°C(1,830°F),mainlyduetoorganicvolatilesandcombustionofresidualcarbon.Thismasslossisreferredtoasloss-on-ignition(LOI)andislimitedin

Source: Kosmatka, Kerkhoff, and Panarese (2002)

________________________________________________ Figure 3-3. Typical size distributions of cementitious materials (CTLGroup)



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mostspecificationstolessthan6percent.ClassFflyashestypicallycontainlessthan10percentcalcium(CaO),with5percentLOI.ClassCmaterialsoftencontain18to30percentcalcium(CaO),withlessthan2percentLOI.

ASTMC618/AASHTOM295ClassFandClassCflyashesareusedinmanydifferenttypesofcon-crete.Formoreinformationonflyash,seeACAA(2003)andACI232(2003).

Ground, Granulated Blast-Furnace Slag.GGBFslag,alsocalledslagcement,hasbeenusedasacementi-tiousmaterialinconcretesincethebeginningofthe1900s(Abrams1924).Becauseofitschemistry,GGBFslagbehavesasacementinthatitwillhydratewithoutthepresenceofaddedcalcium,althoughhydrationratesareacceleratedwhenthepHofthesystemisincreased.

Ground,granulatedblast-furnaceslaggenerallyaffectsconcreteasfollows:

Slightlylesswaterisrequiredtoachievethesameworkability.Settingtimemaybedelayed.Earlystrengthsmaybedepressed,butlaterstrengthsareincreased.Resistancetochloridepenetrationissignifi-cantlyimproved.

WhenusedingeneralpurposeconcreteinNorthAmerica,GGBFslagcommonlyconstitutesupto35percentofthecementitiousmaterialinpavingmixes.Higherdosagesmaybeconsideredifrequiredforprovidingresistancetoalkali-silicareactionorforreducingheatofhydration.

GGBFslagisabyproductofironsmeltingfromironore.Itisthemoltenrockmaterialthatseparatesfromtheironinablastfurnace.Itisdrainedfromthefurnaceandgranulatedbypouringitthroughastreamofcoldwater,orpelletizedbycoolingitincoldwaterandspinningitintotheairoutarotarydrum.Theresultingquenchedsolidisaglassymate-rialthatisgroundtoasizesimilartoportlandcement(figure3-4).(Slagthatisnotrapidlycooledorgranu-latedisnotusefulasanSCM,butcanbeusedasacoarseaggregate.)

ThegranulatedmaterialisnormallygroundtoaBlainefinenessofabout400to700m2/kg.Therela-

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tivedensity(specificgravity)forGGBFslagisintherangeof2.85to2.95.Thebulkdensityvariesfrom1,050to1,375kg/m3(66to86lb/ft3).

GGBFslagconsistsessentiallyofsilicatesandaluminosilicatesofcalcium,thesamebasiccom-ponentsinportlandcementorflyash.GGBFslaggenerallydoesnotcontaintricalciumaluminate(C

3A),

whichaffectssetting.ASTMC989/AASHTOM302classifyGGBF

slagbyitslevelofreactivityasGrade80,100,or120(whereGrade120isthemostreactive).ACI233(2003)providesanextensivereviewofGGBFslag.

Natural Pozzolans.Ingeneral,pozzolansareincludedinconcretemixturestohelpconvertcalciumhydroxide(CH),alessdesirableproductofhydration,intothemoredesirablecalciumsilicatehydrate(C-S-H)(seethediscussionofCHtoC-S-HconversioninthesectiononPozzolanicSCMsinchapter4,page94).

Naturalpozzolanshavebeenusedforcenturies.Theterm“pozzolan”comesfromavolcanicashminedatPozzuoli,avillagenearNaples,Italy.However,theuseofvolcanicashandcalcinedclaydatesbackto2,000BCEandearlierinothercultures.ManyoftheRoman,Greek,Indian,andEgyptianpozzolanconcretestructurescanstillbeseentoday,attestingtothedurabilityofthesematerials.

ASTMC618definesClassNpozzolansas“raworcalcinednaturalpozzolans.”ThemostcommonClassNpozzolansusedtodayareprocessedmaterials,

Figure 3-4. Scanning electron micrograph of GGBF slag particles. Note the angular shape. (PCA)



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havingbeenheat-treatedinakilnandthengroundtoafinepowder;theyincludecalcinedclay,calcinedshale,andmetakaolin.

Calcinedclaysareusedingeneralpurposeconcreteconstructioninmuchthesamewayasotherpoz-zolans.Theycanbeusedasapartialreplacementforcement,typicallyintherangeof15to35percent,andcanenhancestrengthdevelopmentandresistancetosulfateattack,controlalkali-silicareactivity,andreducepermeability.Calcinedclayshavearelativedensitybetween2.40and2.61,withBlainefinenessrangingfrom650to1,350m2/kg.

Calcinedshalemaycontainontheorderof5to10percentcalcium,whichresultsinitshavingsomecementingorhydraulicproperties.

Metakaolinisproducedbylow-temperaturecalcinationofhigh-puritykaolinclay.Theproductisgroundtoanaverageparticlesizeofabout1to2µm;thisisabout10timesfinerthancement,butstill10timescoarserthansilicafume.Metakaolinisusedinspecialapplicationswhereverylowpermeabilityorveryhighstrengthisrequired.Intheseapplications,metakaolinisusedmoreasanadditivetotheconcreteratherthanareplacementofcement;typicaladditionsarearound10percentofthecementmass.

NaturalpozzolansareclassifiedasClassNpozzolansbyASTMC618/AASHTOM295.ACI232(2003)providesareviewofnaturalpozzolans.

Silica Fume.Becauseitcanreduceworkabilityandisexpensive,silicafumeistypicallynotusedinpave-mentsexceptforspecialapplicationssuchasthosesubjectedtostuddedtiresorincurbsandgutters.

Silicafume,alsoreferredtoasmicrosilicaorcon-densedsilicafume,isabyproductofthesiliconorferrosiliconeindustries.Theproductisthevaporthatrisesfromelectricarcfurnacesusedtoreducehigh-purityquartzwithcoal.Whenitcools,itcondensesandiscollectedinclothbags,thenprocessedtoremoveimpurities.Theparticlesareextremelysmall,some100timessmallerthancementgrains,andaremainlyglassyspheresofsiliconoxide.

Theloosebulkdensityisverylowandthematerialisdifficulttohandle.Inordertomakeiteasiertohandle,silicafumeisusuallydensifiedby

Effect of Pozzolans in Cement Paste

In very broad terms, the primary reaction in hydrating cement is the following:

water + cement = calcium silicate hydrate (C-S-H) + calcium hydroxide (CH)

Calcium silicate hydrate (C-S-H) is the primary compound that contributes to the strength and impermeability of hydrated cement paste. Calcium hydroxide (CH) is not as strong and is more soluble, so it is somewhat less desirable.

Adding a pozzolan like fly ash, in the presence of water, results in conversion of the calcium hydroxide (CH) to more calcium silicate hydrate (C-S-H):

calcium hydroxide (CH) + pozzolan + water = calcium silicate hydrate (C-S-H)

This conversion is a significant benefit of adding pozzolans like fly ash to the mixture. (See chapter 4, page 69, for a detailed description of cement chemistry and hydration, including the effects of specific SCMs on the hydration process.)

tumblinginanairstreamthatcausestheparticlestoagglomerateintolargergrainsheldtogetherbyelec-trostaticforces.Itisimportantthatconcretemixturescontainingsilicafumearebatchedandmixedinawaythatensuresthattheagglomerationsarebrokenupandthematerialisuniformlydistributedinthemix.

ThematerialisusedasapozzolanandisspecifiedinASTMC1240.Thewaterrequirementofsilicafumemaybehigh,requiringthatsuperplasticizersbeusedinconcretecontainingmorethanfivepercentbymassofcement.Theresultingconcretenormallyexhibitssignificantlyincreasedstrengthandreducedpermeability.Concretecontainingsilicafumeisoftenathigherriskofplasticshrinkagecrackingbecausebleedingismarkedlyreduced.Specificgravityofsilicafumeisintherange2.2to2.6.

Other Pozzolans.Otherindustrialbyproductslikericehuskasharepotentiallyusefulasconcretecon-stituents.AspecificationhasbeenrecentlydevelopedbyAASHTOM321toserveasaspecificationfor



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thesematerials,whichmaynotfallundercategoriescoveredbyotherspecifications.

Effects of Supplementary Cementitious Materials in Concrete

SCMsinconcreteaffectawiderangeoffreshandhardenedconcreteproperties.Someoftheeffectsmaybeconsidereddesirableandarethereasonwhythematerialsareused.Othersideeffectsmaybelessdesirableandhavetobeaccommodated.Anunder-standingofallthepotentialeffectsisessentialtopreventsurprises.

TheeffectsofSCMsonpropertiesoffreshandhardenedconcretearebrieflydiscussedinthefollow-ingsectionsandsummarizedintables3-7and3-8,respectively(seechapter5,page105,foracompletediscussionofconcreteproperties).

Inmostcases,theextentofchangeinconcretebehaviorwilldependontheparticularmaterialused,theamountused,andthepropertiesofotheringredi-entsintheconcretemixture.

Trialbatchingwithunfamiliarmaterialcombi-nationsisessentialtoprovideassuranceofcriticalconcreteproperties.

Fresh Properties.Infreshconcrete,SCMscanaffectworkabilityandsettingtimesinthefollowingways:

WorkabilityisalwayschangedbySCMs.Flyashwillgenerallyincreaseworkability,aswillGGBFslagtoalesserextent.Silicafumemaysignificantlyreduceworkabilityatdosagesabovefivepercent.Therateofslumploss(stiffening)maybeincreasediftherearechemicalincompatibilities(seePotentialMaterialsIncompatibilitiesinchapter4,page97).SettingtimesmaybedelayedandearlystrengthgainslowedifGGBFslagandflyashareincluded.However,thiseffectwilldependontheproductused.

Allofthesefactorscanhaveasignificanteffectonthetimingoffinishingandsawcuttinginpave-ments,makingitimportantthattheperformanceofthecementitioussystembeingselectedforaprojectbetestedintrialbatcheswellbeforetheprojectstarts.Trialbatchesneedtobetestedatthetemperatures

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expectedwhenthepavingoperationwillbeconducted.

Durability/Permeability.SCMsgenerallyimprovepotentialconcretedurabilitybyreducingpermeability.Almostalldurability-relatedfailuremechanismsinvolvethemovementoffluidsthroughtheconcrete.Testsshowthatthepermeabilityofconcretedecreasesasthequantityofhydratedcementitiousmaterialsincreasesandthewater-cementitiousmaterialsratiodecreases.

Withadequatecuring,flyash,GGBFslag,andnaturalpozzolansgenerallyreducethepermeabilityandabsorptionofconcrete.GGBFslagandflyashcanresultinverylowchloridepenetrationtestresultsatlaterages.Silicafumeandmetakaolinareespeciallyeffectiveandcanprovideconcretewithverylowchlo-ridepenetration(Bargeretal.1997).

InorderforSCMstoimprovedurability,theymustbeofadequatequalityandusedinappropriateamounts,andfinishingandcuringpracticesmustbeappropriate.

Alkali-Silica Reactivity Resistance. Alkali-silicareactivity(ASR)ofmostreactiveaggregates(seeAggre-gateDurabilitylaterinthischapter,page47)canbecontrolledwiththeuseofcertainSCMs.Low-calciumClassFflyasheshavereducedreactivityexpansionupto70percentormoreinsomecases.Atoptimumdosage,someClassCflyashescanalsoreducereactiv-ity,butatalowdosageahigh-calciumClassCflyashcanexacerbateASR.

SCMsreduceASR(Bhatty1985,BhattyandGreen-ing1978)by(1)providingadditionalcalciumsilicatehydrates(C-S-H)thatchemicallytieupthealkaliesintheconcrete,(2)dilutingthealkalicontentofthesystem,and(3)reducingpermeability,thusslowingtheingressofwater.

Itisimportanttodeterminetheoptimumdosageforagivensetofmaterialstomaximizethereductioninreactivityandtoavoiddosagesandmaterialsthatcanaggravatereactivity.Dosageratesshouldbeveri-fiedbytests,suchasASTMC1567orASTMC1293.(Descriptionsofaggregatetestingandpreventivemeasurestobetakentopreventdeleteriousalkali-aggregatereactionarediscussedlaterinthischapterunderAggregateDurability,page47.)



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Table 3-7. Effects of Supplementary Cementitious Materials on Fresh Concrete Properties

Table 3-8. Effects of Supplementary Cementitious Materials on Hardened Concrete Properties



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SCMsthatreducealkali-silicareactionswillnot

reducealkali-carbonatereactions,atypeofreaction

involvingcementalkaliesandcertaindolomitic

limestones.

Sulfate Resistance.Withproperproportioningand

materialsselection,silicafume,flyash,naturalpoz-

zolans,andGGBFslagcanimprovetheresistance

ofconcretetoexternalsulfateattack.Thisisdone

primarilybyreducingpermeabilityandbyreducing

theamountofreactiveelements(suchastricalcium

aluminate,C3A)thatcontributetoexpansivesulfate

reactions.

OnestudyshowedthatforaparticularClassFash,

anadequateamountwasapproximately20percentof

thecementitioussystem(Stark1989).Itiseffective

tocontrolpermeabilitythroughmixtureswithlow

water-cementitiousmaterialsratios(seethesectionon

SulfateResistanceinchapter5,page139).

ConcreteswithClassFashesaregenerallymore

sulfateresistantthanthosewithClassCashes.GGBF

slagisgenerallyconsideredbeneficialinsulfateenvi-

ronments.However,onelong-termstudyinavery

severeenvironmentshowedonlyaslightimprovement

insulfateresistanceinconcretecontainingGGBF

slagcomparedtoconcretecontainingonlyportland

cementasthecementingmaterial(Stark1989,1996).

Calcinedclayhasbeendemonstratedtoprovide

sulfateresistancegreaterthanhigh-sulfateresistant

TypeVcement(Bargeretal.1997).

Resistance to Freeze-Thaw Damage and Deicer Scaling.Thereisaperceptionthatconcretecontain-ingSCMsismorepronetofrost-relateddamagethanplainconcrete.Thisispartiallyduetotheseverityofthetestmethodsused(ASTMC666,ASTMC672),butmayalsoberelatedtothechangingbleedratesandfinishingrequirementsforconcreteswithSCMs(Taylor2004).WithorwithoutSCMs,concretethatisexposedtofreezingcyclesmusthavesoundaggregates(seeAggregateDurabilitylaterinthischapter,page47),adequatestrength,aproperair-voidsystem,andpropercuringmethods.

Forconcretesubjecttodeicers,theACI318(2002)buildingcodestatesthatthemaximumdosageofflyash,GGBFslag,andsilicafumeshouldbe25percent,50percent,and10percentbymassofcementitiousmaterials,respectively.TotalSCMcontentshouldnotexceed50percentofthecementitiousmaterial.Con-cretes,includingpavementmixtures,withSCMsatdosageshigherthantheselimitsmaystillbedurable,however.

Selectionofmaterialsanddosagesshouldbebasedonlocalexperience.Durabilityshouldbedemon-stratedbyfieldorlaboratoryperformancewhennewmaterialsanddosagesareintroduced.

Drying Shrinkage.Whenusedinlowtomoderateamounts,theeffectofflyash,GGBFslag,calcinedclay,calcinedshale,andsilicafumeonthedryingshrinkageofconcreteofsimilarstrengthisgenerallysmallandoflittlepracticalsignificance.


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Aggregates

Key Points

Inconcrete,aggregate(rocksandminerals)isthefillerheldtogetherbythecementpaste.Aggregateformsthebulkoftheconcretesystem.

Aggregatesaregenerallychemicallyanddimensionallystable;therefore,itisdesirabletomaximizeaggregatecontentinconcretemixturescomparedtothemorechemicallyreactivecementpaste.

Aggregatestronglyinfluencesconcrete’sfreshproperties(particularlyworkability)andlong-termdurability.

Itiscriticalthataggregatebewell-graded(thatis,thereshouldbeawiderangeofaggregatesizes).Well-gradedaggregatehaslessspacebetweenaggregateparticlesthatwillbefilledwiththemorechemicallyreactivecement

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paste.Italsocontributestoachievingaworkablemixwithaminimumamountofwater.

Fordurableconcretepavements,theaggregateshouldbedurable(ingeneral,notalkalireactive,pronetofrostdamage,orsusceptibletosaltdamage).Manykindsofaggregatecanbeused,butgraniteandlimestonearecommoninconcretepavements.

Alwayspreparetrialbatchesofconcreteusingthespecificprojectaggregatestoestablishthefinalmixturecharacteristicsand,ifnecessary,makeadjustmentstothemix.

Thephysicalanddurabilityrequirementsofaggregateforconcretemixtures,aswellasclassificationsofcoarseandfineaggregates,arecoveredinASTMC33.

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Aggregate—rocksandminerals—isthefillerheldtogetherbycementpaste.Aggregatetypicallyaccountsfor60to75percentofconcretebyvolume.Comparedtocementpaste,aggregatesaregenerallymorechemi-callystableandlesspronetomoisture-relatedvolumechanges.Therefore,inconcretemixturesitisdesirabletomaximizethevolumeofaggregateandreducethevolumeofcementwhilemaintainingdesiredconcreteproperties.

Aggregatesusedinconcretemixturesforpavementsmustbeclean,hard,strong,anddurableandrelativelyfreeofabsorbedchemicals,coatingsofclay,andotherfinematerialsthatcouldaffecthydrationandbondingwiththecementpaste.Aggregatesareoftenwashedandgradedatthepitorplant.Somevariationintype,quality,cleanliness,grading,moisturecontent,andotherpropertiesisexpectedfromloadtoload.

Servicerecordsareinvaluableinevaluatingaggregates,particularlywithrespecttoalkali-silicareactivity.Intheabsenceofaperformancerecord,

aggregatesshouldbetestedbeforetheyareusedinconcrete.Aswiththeintroductionofanymaterialintoaconcretemixdesign,preparetrialbatchesusingthespecificprojectaggregatestoestablishthecharac-teristicsoftheresultantconcretemixtureandidentifyanynecessarymixadjustments.

Thissectiondescribesthetypesofaggregateandthepropertiesofaggregatesthataffectconcretemixesforpavements.

Aggregate TypesAggregatesaresometimesidentifiedbytheirmineral-

ogicalclassification,thatis,bytheirchemistryandhowtheywereformed.Theseclassificationsareimportantbecausetheyprovideameansofpartiallypredictingaspecificaggregate’seffectonplasticandhardenedconcretemixtures.However,differentmaterialsfromthesamegeologicalformationmaybesignificantlydifferent.Beforeusingaggregatefromanewsourceorquarry,verifyitsperformanceinconcretemixtures.


Aggregates

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Naturallyoccurringaggregates,likethosefrompitsorquarries,areamixtureofrocksandminerals(seeASTMC294forbriefdescriptions).Amineralisanaturallyoccurringsolidsubstancewithanorderlyinternalstructureandanarrowchemicalcomposition.Rocksaregenerallycomposedofseveralminerals.

Single-mineralrocksthatmaybeusedforconcreteaggregatesincludedolomiteandmagnetite.Mineralsthatappearinrocksusedforaggregatesincludesilica(e.g.,quartz),silicates(e.g.,feldspar),andcarbonates(e.g.,calcite).Rocktypescomposedofmorethanonemineralincludegranite,gabbro,basalt,quartzite,traprock,limestone,shale,andmarble.

Theselistsarenotexhaustive.Moreinformationisshownintables3-9and3-10.

Rocksareclassifiedaccordingtotheirorigin.Igneousrocksaretheproductofcooledmoltenmagma,sedimentaryrocksaretheproductofsedi-mentdepositssqueezedintolayeredsolids,andmetamorphicrocksaretheproductofigneousorsedimentaryrocksthathavebeentransformedunderheatandpressure.

Coarse-grainedigneousrocks(forexample,granite)andsedimentaryrocksconsistingofcarbonatemateri-alsfromdepositedseashells(forexample,limestone)aretworocktypescommonlyusedasaggregateinconcrete,asdiscussedbelow.Carbonatematerialsareprimarilycomposedofcalciumcompounds,whilesiliceousmaterials(includinggranite)arepredomi-nantlybasedoncompoundsofsilica.

Foradetaileddiscussionofotherrocktypes,seeBarksdale(1991).

Thetypesofaggregateshavedifferenteffectsontheperformanceofconcreteindifferentenvironmentsandtraffic.Forinstance,siliceousmaterialstendtoresistwearingandpolishing.Carbonaterockshavelowcoefficientsofthermalexpansion,whichisbenefi-cialinreducingexpansionandshrinkageinclimateswherelargefluctuationsintemperatureoccur(seethesectiononAggregateCoefficientofThermalExpan-sionlaterinthischapter,page47).

Somerocktypesareunsoundforuseinconcretemixturesbecausetheyexpand,causingcracking.Examplesincludeshaleandsiltstone.

Carbonate RockTherearetwobroadcategoriesofcarbonaterock:

(1)limestone,composedprimarilyofcalcite,and(2)dolomite,composedprimarilyofthemineraldolomite.Mixturesofcalciteanddolomitearecommon(figure3-5).

Carbonaterockhasseveraldifferentmodesoforiginanddisplaysmanytexturalvariations.Twocarbonaterockshavingthesameoriginmaydisplayagreatrangeoftextures.Insomecarbonatedrocks,thetextureissodensethatindividualgrainsarenotvisible.Othersmayhavecoarsegrainswiththecalciteordolomitereadilyrecognizable.Inmanycarbonaterocksfragmentsofseashellsofvariouskindsarepresent.Shellsandsandshellbanks,aswellascoralreefsandcorallinesands,areexamplesofcarbonatedeposits.

Ways to Describe Aggregates

Aggregates—rocks and minerals—can be described by their general composition, source, or origin:

General composition.Mineral: naturally occurring substance with an orderly structure and defined chemistry.Rock: mixture of one or more minerals.

Source.Natural sands and gravels: formed in riverbeds or seabeds and usually dug from a pit, river, lake, or seabed; sands are fine aggregates; gravels are coarser particles.Manufactured aggregate (crushed stone or sand): quarried in large sizes, then crushed and sieved to the required grading; also, crushed boulders, cobbles, or gravel.Recycled: made from crushed concrete.

Origin.Igneous: cooled molten material; includes siliceous materials primarily consisting of compounds of silica (for example, granite).Sedimentary: deposits squeezed into layered solids; includes carbonate materials from deposited sea shells (for example, limestone).

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Aggregates


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Table 3-10. Rock Constituents in Aggregates*

Carbonaterocksdisplayalargerangeofphysi-calandchemicalproperties.Theyvaryincolorfromalmostpurewhitethroughshadesofgraytoblack.Thedarkershadesareusuallycausedbyplant-based(carbonaceous)material.Thepresenceofironoxidescreatesbuffs,browns,andreds.Dolomiteiscommonlylightincolor.Itoftenhasferrousironcompoundsthatmayoxidize,tintingtherockshadesofbuffandbrown.

Thepropertiesoflimestoneanddolomitevarywiththedegreeofconsolidation.Thecompressivestrengthofcommerciallimestonetypicallyvariesfrom70to100MPa(10,000to15,000lb/in2).Theporosityofmostlimestoneanddolomiteisgenerallylow,andtheabsorptionofliquidsiscorrespondinglysmall.

Granite Granitesareigneousrockscomposedpredomi-

nantlyofformsofsilica(forexample,quartz)andsilicates(forexample,feldspar).Thegrainortexturevariesfromfinetocoarse.Granitesmayvarymarkedlyincolorandtexturewithinindividualquarries,butmorecommonlythecolorandtextureareuniformforlargevolumesofrock.

Becauseofitsmineralcompositionandinterlock-ingcrystals,graniteishardandabrasionresistant.Itscompressivestrengthtypicallyrangesfrom50to400MPa(7,000to60,000lb/in2),withthetypi-calvalueof165MPa(24,000lb/in2)inthedrystate.Mostgraniteiscapableofsupportinganyloadto

Table 3-9. Mineral Constituents in Aggregates

Minerals

Silica Quartz Opal Chalcedony Tridymite CristobaliteSilicates Feldspars Ferromagnesian Hornblende Augite Clay Illites Kaolins Chlorites Montmorillonites Mica ZeoliteCarbonate Calcite DolomoteSulfate Gypsum AnhydriteIron sulfide Pyrite Marcasite PyrrhotiteIron oxide Magnetite Hematite Goethite Imenite Limonite

Source: Kosmatka, Kerkoff, and Panarese (2002)

Igneous rocks Sedimentary rocks Metamorphic rocks

Granite Conglomerate Marble Syenite Sandstone Metaquartzite Diorite Quartzite Slate Gabbro (Traprock) Graywacke Phyllite Peridotite Subgraywacke Schist Pegmatite Arkose Amphibolite Volcanic glass Claystone, siltstone, Hornfels Obsidian argillite, and shale Gneiss Pumice Carbonate Serpentinite Tuff Limestone Scoria Dolomite Perlite Marl Pitchstone Chalk Felsite Chert Basalt

Source: Kosmatka, Kerkoff, and Panarese (2002)* Roughly in order of abundance


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whichitmightbesubjectedduringordinarycon-structionuses.Theflexuralstrengthofintactgranite,expressedasamodulusofrupture,variesfromabout9to28MPa(1,300to4,000lb/in2).Themodulusofelasticityofgraniteishigherthanthatofanyoftheotherrocktypesforwhichdataareavailable.

Gravel and SandClosetohalfofthecoarseaggregatesusedin

concreteinNorthAmericaarenaturalgravelsdugordredgedfromapit,river,lake,orseabed.Weather-inganderosionofrocksproduceparticlesofstone,gravel,sand,silt,andclay.Gravel(includingsand)isnaturalmaterialthathasbrokenofffrombedrockandhasbeentransportedbywaterorice.Duringtrans-port,gravelisrubbedsmoothandgradedtovarioussizes.

Gravelandsandareoftenamixtureofseveralmineralsorrocks.Thequality(orsoundness)ofsandandgraveldependsonthebedrockfromwhichtheparticleswerederivedandthemechanismbywhichtheyweretransported.Sandandgravelderivedfromsoundrocks,likemanyigneousandmetamorphicrocks,tendtobesound.Sandandgravelderivedfromrocksrichinshale,siltstone,orotherunsoundmaterialstendtobeunsound.

Sandandgraveldepositedathigherelevationsfromglaciersmaybesuperiortodepositsinlowareas.Thereasonisthattherockhighinanicesheethasbeencarriedfromhigher,moremountainousareas,whichtendtoconsistofhard,soundrocks.Sandandgravelthathavebeensmoothedbyprolongedagitationinwater(figure3-6[b])usuallyareconsideredbetterqualitybecausetheyareharderandhaveamoreroundedshapethanlessabradedsandandgravel.

Manufactured Aggregate Manufacturedaggregate(includingmanufactured

sand)isoftenusedinregionswherenaturalgravelsandsandsareeithernotavailableinsufficientquanti-tiesorareofunsuitablequality(AddisandOwens2001).Itisproducedbycrushingsoundparentrockatstonecrushingplants.

Manufacturedaggregatesdifferfromgravelandsandintheirgrading,shape,andtexture.Becauseofthecrushingoperation,theyhavearoughsurfacetexture,areveryangularinnature(figure3-6[a]),andtendtobecubicalinshape(dependingonthemethodofcrushing)anduniformingrade(size)(Wigumetal.2004).Inthepast,manufacturedsandswereoftenproducedwithoutregardtosizing,butproducerscannowprovidehigh-quality

_____________________________________________________________________________________________________ Figure 3-5. Family of carbonate minerals showing rock and mineral names (CTLGroup)

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materialmeetingspecifiedparticleshapesandgrada-tions(AddisandOwens2001).Inmanycases,theparticleelongationandflakinessofmanufacturedsandscanbereducedthroughappropriatecrushingtechniques.Impactcrushersgeneratebetterparticleshapethancompressioncrushers.

Manyofthecharacteristicsofmanufacturedaggregatesareattributabledirectlytotheinherentpropertiesoftherocktypefromwhichtheywereproduced.Manufacturedaggregatesarelesslikelythangravelandsandtobecontaminatedbydeleteri-oussubstancessuchasclaymineralsororganicmatter(AddisandOwens2001).Somespecificationspermithigherfinescontentinmanufacturedsandsbecauseof

theexpectationoflessclaycontamination(AddisandGoldstein1994).

Thesharpnessandangularityofmanufacturedsandsmayresultina“harsh”mixture:onethatisdif-ficulttoworkandfinish.Suchmixturesalsotypicallyrequiremorewater(QuirogaandFowler2004).Ontheotherhand,theappropriateuseofmanufacturedsandcanimproveedgeslumpcontrolduringslip-formpavingandmayalsoleadtoslightincreasesinconcretestrengthforafixedwatercontent(McCaig2002).Aswiththeintroductionofanymaterialintoaconcretemixdesign,preparetrialbatchesusingthespecificprojectmaterialstoestablishthecharacteris-ticsoftheresultantconcretemixtureandidentifyanynecessarymixadjustments.

Recycled AggregatesRecyclingconcretepavementisarelativelysimple

process.Itinvolvesbreakingandremovingthepave-ment,removingreinforcementandotherembeddeditems,andcrushingtheconcreteintomaterialwithaspecifiedsize.Thecrushingcharacteristicsofhard-enedconcretearesimilartothoseofnaturalrockandarenotsignificantlyaffectedbythegradeorqualityoftheoriginalconcrete.

AccordingtoanFHWAstudy(2002),manyStatesuserecycledaggregateasanaggregatebase.Recycled-aggregatebaseshaveexperiencedsomeleachingofcalciumcarbonateintothesubdrains(Mulligan2003).

Inadditiontousingrecycledaggregateasabase,someStatesuserecycledconcreteinnewportlandcementconcrete.Mostoftheseagenciesspecifyrecyclingtheconcretematerialdirectlybackintotheprojectbeingreconstructed.Whenusedinnewconcrete,recycledaggregateisgenerallycombinedwithvirginaggregate.Recycledmaterialisnotrecom-mendedforuseasfineaggregate,however,becauseofthehighwaterdemand.

Thequalityofconcretemadewithrecycledcoarseaggregatedependsonthequalityoftherecycledaggregate.Typically,recycledcoarseaggregateissofterthannaturalaggregateandmayhaveahigheralkaliorchloridecontentthannaturalaggregate.Recycledaggregatemayalsohavehigherporosity,leadingtohigherabsorption.Therefore,relativelytighterquality

a) Aggregates from crushing

b) River gravel

________________________________________________ Figure 3-6. Aggregates produced by crushing operation (top) have a rougher surface texture and are angular compared to round river gravel (bottom). (PCA)


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aggregategrading.Well-gradedaggregate—thatis,aggregatewithabalancedvarietyofsizes—ispreferred,becausethesmallerparticlesfillthevoidsbetweenthelargerparticles,thusmaximizingtheaggregatevolume(figure3-7).

Inaddition,theamountofmixwaterisoftengovernedbytheaggregateproperties.Inordertobeworkable,concretemixturesmustcontainenoughpastetocoatthesurfaceofeachaggregateparticle.Buttoomuchwaterinthepastereduceslong-termcon-cretedurabilitybyreducingstrengthandincreasingpermeability.So,itisimportanttoachieveoptimumwatercontent—nottoolittle,nottoomuch.This,too,canlargelybeaccomplishedbyselectingtheoptimumaggregatesizeandgrading.Smaller(finer)aggregates

controlsmayberequiredtopreventconstructabilityproblems.Therecycledaggregateshouldbetakenfromasinglepavementthatisknownnottohaveexperiencedmaterials-relatedproblems.Caremustbetakentopreventcontaminationoftherecycledaggre-gatebydirtorothermaterials,suchasasphalt.

Physical Properties of AggregatesThefactorsthatcanbemonitoredinanaggregate

fromagivensourcearethegrading,particleshape,texture,absorption,anddurability.Ingeneral,therequiredphysicalanddurabilitycharacteristicsofaggregatesarecoveredinASTMC33.

Aggregate GradationGradationisameasureofthesizedistributionof

aggregateparticles,determinedbypassingaggregatethroughsievesofdifferentsizes(ASTMC136-04/AASHTOT27).Gradingandgradinglimitsareusu-allyexpressedasthepercentageofmaterialpassing(orretainedon)sieveswithdesignatedholesizes.AggregatesareclassifiedasfineorcoarsematerialsbyASTMC33/AASHTOM6/M80:

Coarse:Aggregateretainedona#4sieve(great-erthan4.75mm[3/16in.]indiameter)(AbramsandWalker1921);consistsofgravel,crushedgravel,crushedstone,air-cooledblast-furnaceslag(nottheGGBFslagusedasasupplemen-tarycementitiousmaterial),orcrushedcon-crete;themaximumsizeofcoarseaggregatesisgenerallyintherangeof9.5to37.5mm(3/8to11/2in.).Fine:Aggregatepassinga#4sieve(lessthan4.75mm[3/16in.]indiameter)(AbramsandWalker1921);consistsofnaturalsand,manu-facturedsand,oracombination.Clay:Verysmall,fineparticleswithhighsur-faceareaandaparticularchemicalform.

Why Well-Graded Aggregate is Critical.Becauseaggre-gatesaregenerallymorechemicallyanddimensionallystablethancementpaste,itisimportanttomaximizetheamountofaggregateinconcretemixtures(withinotherlimits,liketheneedforsufficientpastetocoatalltheaggregateparticlesforworkability).Thiscanbeaccomplishedlargelybyselectingtheoptimum

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Generally Desirable Physical Properties of Aggregate

Although mixtures can be developed to compensate for the lack of some of the following characteristics in the aggregate, in general it is desirable that the aggregate have the following qualities:

Well-graded (for mixtures that require less water than mixtures with gap-graded aggregates, have less shrinkage and permeability, are easier to handle and finish, and are usually the most economical).

Rough surface texture (for bond and interlock).

Low absorption (reduces water requirement variability).

Low alkali-aggregate reactivity (for reduced risk of deleterious alkali-silica reactivity and alkali-carbonate reactivity).

Frost resistant (for durability associated with D-cracking and popouts).

Low salt susceptibility (for durability).

Low coefficient of thermal expansion (for reduced cracking from volume change due to changing temperatures).

Abrasion resistant (for durability and skid resistance).

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requiremorepastebecausetheyhavehighersurface-to-volumeratios.Innaturalsands,itiscommonformuchoftheveryfine(<150µm,#100sieve)particlestobeclay,withextremelyhighsurfaceareas.There-fore,theamountofsuchfinematerialsislimitedinspecifications.(Crushedfineaggregateislesslikelytocontainclayparticles,andconsiderationmaybegiventopermittingslightlyhigherdustcontents.)

Finally,mixturescontainingwell-gradedaggregategenerallywillhavelessshrinkageandpermeability,willbeeasiertohandleandfinish,andwillbethemosteconomical.

Theuseofgap-graded(single-sized)aggregate,ontheotherhand,canresultinmixturesthatsegregateandrequiremorewatertoachieveworkability.Veryfinesandsareoftenuneconomical;theymayincreasewaterdemandinthemixtureandcanmakeentrainingairdifficult.Verycoarsesandsandcoarseaggregatecanproduceharsh,unworkablemixtures.Ingen-eral,aggregatesthatdonothavealargedeficiencyorexcessofanysize(theywillgiveasmoothgradingcurve)willproducethemostsatisfactoryresults.

Fine-Aggregate Grading Requirements.ASTMC33/AASHTOM6permitarelativelywiderangeinfine-aggregategradation.Ingeneral,iftheratioofwatertocementitiousmaterialsiskeptconstantandtheratiooffine-to-coarseaggregateischosencorrectly,awide

rangeingradingcanbeusedwithoutameasurableeffectonstrength.However,itmaybemosteco-nomicaltoadjusttheproportionsoffineandcoarseaggregateaccordingtothegradationoflocalaggre-gates.

Ingeneral,increasingamountsoffinematerialwillincreasethewaterdemandofconcrete.Fine-aggregategradingwithinthelimitsofASTMC33/AASHTOM6isgenerallysatisfactoryformostconcretes.Theamountsoffineaggregatepassingthe300µm(#50)and150µm(#100)sievesaffectwaterdemand,workability,surfacetexture,aircontent,andbleedingofconcrete.Largeamountsoffinematerialmayincreasethewaterdemandandincreasesticki-ness,whileinsufficientfinescouldresultinbleeding.Mostspecificationsallow5to30percenttopassthe300µm(#50)sieve.

OtherrequirementsofASTMC33/AASHTOM6areasfollows:

Thefineaggregatemustnothavemorethan45percentretainedbetweenanytwoconsecu-tivestandardsieves.Thefinenessmodulusmustbenotlessthan2.3,normorethan3.1,norvarymorethan0.2fromthetypicalvalueoftheaggregatesource.Ifthisvalueisexceeded,thefineaggregateshouldberejectedunlesssuitableadjustmentsaremadeintheproportionsoffineandcoarseaggregate.

Thefinenessmodulus(FM)isameasureofthefinenessofanaggregate—thehighertheFM,thecoarsertheaggregate.AccordingtoASTMC125,theFMiscalculatedbyaddingthecumulativepercent-agesbymassretainedoneachofaspecifiedseriesofsievesanddividingthesumby100.Thespeci-fiedsievesfordeterminingFMare150µm(#100),

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Consistent Grading is Critical

Variations in aggregate grading between batches can seriously affect the uniformity of concrete from batch to batch.

________________________________________________ Figure 3-7. The level of liquid in the cylinders, representing voids, is constant for equal absolute volumes of aggregates of uniform (but different) sizes. When different sizes are combined, however, the void content decreases. (PCA)


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developduetoapoorgradation,considerusingalternativeaggregates,blendingaggregates,orcon-ductingaspecialscreeningofexistingaggregates.

Toolsthatcanbeusedtoevaluatethegradingofacombinedsystemareavailable(seeAggregateGradingOptimizationinchapter6,page176;andtheCom-binedGradingtestmethodinchapter9,page253).Theseincludethecoarsenessandworkabilityfactors,alongwiththe0.45powercurve(Shilstone1990).

Aggregate Particle Shape Aggregateparticlesaredescribedasbeingcubic,

flat,orelongated.Aggregateofanyshapecanbeusedinconcretetoobtainhighstrengthanddurability,pro-videdthequantityofmortaratagivenratioofwatertocementitiousmaterialsisadjusted.However,keepinmindsomerulesofthumb:

300µm(#50),600µm(#30),1.18mm(#16),2.36mm(#8),4.75mm(#4),9.5mm(3/8in.),19.0mm(3/4in.),37.5mm(11/2in.),75mm(3in.),and150mm(6in.).

Finenessmodulusisnotauniquedescriptor,anddifferentaggregategradingsmayhavethesameFM.However,theFMoffineaggregatecanbeusedtoestimateproportionsoffineandcoarseaggregatesinconcretemixtures.

Coarse-Aggregate Grading Requirements.Thecoarse-aggregategradingrequirementsofASTMC33/AASHTOM80alsopermitawiderangeingradingandavarietyofgradingsizes.Aslongasthepropor-tionoffineaggregatetototalaggregateproducesconcreteofgoodworkability,thegradingforagivenmaximum-sizecoarseaggregatecanbevariedmoder-atelywithoutappreciablyaffectingamixture’scementandwaterrequirements.Mixtureproportionsshouldbechangedifwidevariationsoccurinthecoarse-aggregategrading.

Themaximumcoarseaggregatesizetobeselectedisnormallylimitedbythefollowing:(1)localavail-ability,(2)amaximumfractionoftheminimumconcretethicknessorreinforcingspacing,and(3)abilityoftheequipmenttohandletheconcrete.

Combined-Aggregate Grading.Themostimportantgradingisthatofthecombinedaggregateinacon-cretemixture(Abrams1924).Well-gradedaggregate,indicatedbyasmoothgradingcurve(figure3-8),willgenerallyprovidebetterperformancethanagap-gradedsystem.Crouch(2000)foundinhisstudiesonair-entrainedconcretethattheratioofwatertocementitiousmaterialscouldbereducedbymorethaneightpercentusingcombinedaggregategradation.

Combinationsofseveralaggregatesinthecorrectproportionswillmakeitpossibletoproduceacom-binedaggregategradingthatisclosetothepreferredenvelope.Sometimesmid-sizedaggregate,aroundthe9.5-mm(3/8-in.)size,isnotavailable,resultinginaconcretewithhighparticleinterference,highwaterdemand,poorworkability,poorplace-ability,andhighshrinkageproperties.

Aperfectgradationdoesnotexistinthefield,butyoucantrytoapproachitbyblendingtheavail-ablematerialsinoptimizedproportions.Ifproblems

No Local Source of Well-Graded Aggregate?

If a well-graded system of aggregates is not available from local sources, satisfactory concrete can still be made and placed, but it will require more attention to detail in the construction phase.

________________________________________________ Figure 3-8. An example of a smooth grading curve that would be preferred (red lines define an acceptable envelope) and an actual aggregate system that was not optimum (blue) (CTLGroup)

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However,theabsorptionvaluesofmostaggregatesrangefrom0.2to4percentbymass.

Usingaggregateswithhighabsorptionvaluesoftenresultsinlargevariationsinconcretequalitybecauseofthedifficultiescontrollingtheaggregates’moisturecontent.AggregatesthatarelessthanSSDwillabsorbwaterfromthepaste,makingtheconcretestiffenandloseworkability.ThiseffectisreducedwhenprobesareusedtomonitorthemoistureoftheaggregatesinstorageandwhenthewaterintheconcretemixisadjustedtoaccommodatethedifferencebetweentheactualmoisturecontentandtheSSDcondition(seeMoisture/AbsorptionCorrectioninchapter6,page183,andStockpileManagementandBatchinginchapter8,pages206–207).

Aggregate Coefficient of Thermal Expansion Amaterial’scoefficientofthermalexpansion(CTE)

isameasureofhowmuchthematerialchangesinlength(orvolume)foragivenchangeintemperature.Typically,anincreaseintemperaturewillresultinlengthening,andadecreasewillresultinshortening.Becauseaggregatesmakeupamajorityofaconcrete’svolume,theCTEoftheaggregateparticleswilldomi-natetheCTEfortheconcreteoverall(AASHTOTP60).

CTEvaluesareconsideredindesigncalculationsforpavements(NCHRP2004)andareusedinther-malmodelingofconcretes.Limestoneandmarblehavethelowest,andthereforemostdesirable,thermalexpansions.Table3-11showssometypicallinearCTEvaluesofseveralaggregates.

Aggregate DurabilityOneofthereasonsthatconcretepavementsare

economicallydesirableisthattheylastlongerthanpavementsmadefromothersystems.Itistherefore

Inpavements,angularparticlesleadtohigherflexuralstrengthsduetoaggregateinterlock.Angular,nonpolishingfineaggregateparticlespromotehighskidresistance,althoughtheyalsoreduceworkability.Angular,flat,orelongatedparticlesmayreduceconcreteworkabilityduetoparticleinterfer-encewhileintheplasticstate.Thisisespeciallytrueofparticlesbetweenthe9.5-mm(3/8-in.)and2.36-mm(#8)sieves.

Aggregate Surface TextureAggregatewithanytexture,varyingfromsmoothto

rough,canbeusedinportlandcementconcrete,pro-videdthemixtureisproperlyproportioned.Aroughtextureisanadvantagewhenbeamsaretestedforqualitycontrolbecausebetterbondandinterlockisprovidedatthejunctionofthemortarandaggregate.

Aggregate AbsorptionAbsorptionisthepenetrationofliquidintoaggre-

gateparticles.Theamountofwateraddedtotheconcretemixtureatthebatchplantmustbeadjustedforthemoistureconditionsoftheaggregatesinordertoaccuratelymeetthewaterrequirementofthemixdesign.Ifnotaccountedfor,dryaggregateparticleswillabsorbwaterfromtheconcretemixture,sowaterwillhavetobeaddedtothemixtoachievethedesiredworkability;thismayultimatelyreducelong-termconcretedurability.

Themoistureconditionsofaggregates(figure3-9)aredesignatedasfollows:

Ovendry—fullyabsorbent.Airdry—dryattheparticlesurfacebutcontain-ingsomeinteriormoisture,thusstillsomewhatabsorbent.Saturatedsurfacedry(SSD)—neitherabsorb-ingwaterfromnorcontributingwatertotheconcretemixture.Damporwet—containinganexcessofmois-tureonthesurface(freewater).

Absorptionvaluesforvarioustypesofaggregaterangefromvirtuallyzerotoover7percentofthedryaggregatemassfornaturalaggregates,andupto10percentforlightweightormanufacturedmaterials.

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________________________________________________ Figure 3-9. Moisture conditions of aggregates (PCA)


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importanttoensurethatnewpavementsareasdura-bleaspossible.Someoftheaggregatemechanismsrelatedtopavementdurabilityincludealkali-aggregatereactivity,frostresistance,coefficientofthermalexpansion,andabrasionresistance.

Aggregate Durability and Alkali-Aggregate Reactivity.Aggregatescontainingcertainminerals(table3-12)canreactwithalkalihydroxidesinconcretetoformagelthatexpandswhenexposedtomoisture.Thisreactionandexpansioneventuallyresultsincrackingintheaggregateandtheconcrete(figure3-10).

Reactivemineralsareoftenfoundinspecifictypesofrocks(table3-12),whichinturnmayormaynotbereactive,dependingontheexactmakeupoftherock(table3-13).Thereactivityispotentiallyharmfulonlywhenitproducessignificantexpansion.

Alkali-aggregatereactivityhastwoforms—alkali-silicareactivity(ASR)andalkali-carbonatereactivity(ACR).Formoreinformation,seeFarnyandKosmatka(1997)andFolliard,Thomas,andKurtis(2004).

ASRismuchmorecommonthanACR.ThebestwaytoavoidASRistotakeappropriateprecau-tionsbeforeconcreteisplaced.Expansiontests(ASTMC1260/AASHTOT303andASTMC1293)andpetrography(ASTMC295)arethemostcom-monlyusedmethodstodeterminethepotentialsusceptibilityofanaggregatetoASRiftheperfor-

Figure 3-10. An aggregate particle that has cracked due to alkali-silica reaction (PCA)

Table 3-11. Typical CTE Values for Common Portland Cement Concrete Ingredients

Table 3-12. Mineral Constituents of Aggregate That Are Potentially Alkali-Reactive

manceoftheaggregateinthefieldisnotavailable.(TheseandothertestsandmethodsforcontrollingASRarediscussedindetailunderAlkali-SilicaReactioninchapter5,page141.)

crack

Coefficient of thermal expansion 10-6/°C 10-6/°F

Aggregate Granite 7 to 9 4 to 5Basalt 6 to 8 3.3 to 4.4Limestone 6 3.3

Dolomite 7 to 10 4 to 5.5Sandstone 11 to 12 6.1 to 6.7Quartzite 11 to 13 6.1 to 7.2Marble 4 to 7 2.2 to 4

Cement paste (saturated)w/c = 0.4 18 to 20 10 to 11w/c = 0.5 18 to 20 10 to 11w/c = 0.6 18 to 20 10 to 11

Steel 11 to 12 6.1 to 6.7

Sources: FHWA (2004b) and PCA (2002)Note: These values are for aggregates from specific

sources, and different aggregate sources may provide values that vary widely from these values.

Alkali-reactive mineral Potentially deleterious amount

Optically strained, More than 5%micro fractured or microcrystalline quartzChert or chalcedony More than 3%Trydimite or cristobalite More than 1%Opal More than 0.5%Natural volcanic glass More than 3.0 %

Source: PCA

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Table 3-13. Rock Types Potentially Susceptible to Alkali-Silica Reactivity

D-crackingisaseriousproblemthatwillcompro-misetheintegrityofconcrete.Itcannotbestoppedorundone;itcanonlybeprevented.Therefore,whendesigningamixtureitiscriticaltoselectaggregatesthatarenotsusceptibletofreeze-thawdeterioration.Ifmarginalaggregatesmustbeused,youmaybeabletoreduceD-crackingsusceptibilitybyreducingthemaximumparticlesizeandbyprovidinggooddrain-ageforcarryingwaterawayfromthepavementbase.

Popouts

Apopoutisusuallyaconicalfragmentthatbreaksoutofthesurfaceofconcrete,leavingashallow,typicallyconical,depression(figure3-12).Gener-ally,afracturedaggregateparticlewillbefoundatthebottomoftheholewiththeotherpartoftheaggregatestilladheringtothepointofthepopoutcone.Mostpopoutsareabout25to50mm(1to2in.)wide;however,popoutscausedbysandparticlesaremuchsmaller,andverylargepopoutsmaybeupto300mm(1ft)indiameter.

Unlesstheyareverylarge,popoutsareonlyacos-meticflawanddonotgenerallyaffecttheservicelifeoftheconcrete.

Figure 3-11. D-cracking (Grove, CP Tech Center)

TopreventACR,avoidusingreactiverock.Ifthisisnotfeasible,thendilutetheaggregatewithnonreactivestone(Ozol1994).

Aggregate Durability and Frost Resistance.ConcretecontainingaggregatesthatarenotfrostresistantmayexperienceD-cracking,popouts,ordeteriorationfromdeicingsalts.

D-CrackingAggregateparticleswithcoarseporestructuremay

besusceptibletofreeze-thawdamage.Whentheseparticlesbecomesaturatedandthewaterfreezes,expandingwatertrappedintheporescannotgetout.Eventually,theaggregateparticlescannotaccom-modatethepressurefromtheexpandingwater;theparticlescrackanddeteriorate.Inconcreteswithappreciableamountsofsusceptibleaggregate,thisfreeze-thawdeteriorationoftheaggregateultimatelyresultsincrackingintheconcreteslab,calledD-cracking.D-crackingisgenerallyaregionalproblemcausedwhenlocallyavailable,susceptibleaggregateisusedinconcrete.

D-cracksareeasytoidentify.Theyarecloselyspacedcracksparalleltotransverseandlongitudinaljoints(wheretheaggregateismostlikelytobecomesaturated).Overtime,thecracksmultiplyoutwardfromthejointstowardthecenterofthepavementslab(figure3-11).

Rocks

AreniteArgilliteArkoseChertFlintGneissGraniteGraywackeHornfelsQuartz-areniteQuartziteSandstoneShaleSilicified carbonateSiltstone

Source: Folliard, Thomas, and Kurtis (2003)


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Salt (Sulfate) Susceptibility

Certainaggregatessuchasferroandolomitesaresusceptibletodamagebysalts.Theseaggregatesmayexhibitexcellentfreeze-thawresistancebutdeterioraterapidlywhendeicingsaltsareused.Inthepresenceofsalts,thecrystallinestructureofsuchaggregatesisdestabilized,thereforeincreasingtherateofdamageunderfreeze-thawconditions.

Aggregate Durability and Abrasion Resistance.Animportantpropertyofpavementsistheirabilitytoprovideanadequateskidresistancebyresistingabrasionorresistingwear.Theabrasionresistanceofconcreteisrelatedtobothaggregatetypeandconcretecompressivestrength(figure3-13).Strongconcretehasmoreresistancetoabrasionthandoesweakcon-crete,whilehardaggregateismorewear-resistantthansoftaggregate.Inotherwords,high-strengthconcrete

Popoutsmayoccurinconcreteswherethesurfacecontainssmallamountsofcoarse(ratherthanfine)aggregateparticleswithhigherporosityvaluesandmedium-sizedpores(0.1to5µm)thatareeasilysaturated.(Largerporesdonotusuallybecomesatu-ratedorcauseconcretedistress,andwaterinveryfineporesdoesnotfreezereadily.)Astheoffendingaggregateabsorbsmoistureandfreezingoccurs,theswellingcreatesenoughinternalpressuretorupturetheconcretesurface.Sometimes,allthatisneededforexpansiontooccurisaseasonofhighhumidity.

Aggregatescontainingappreciableamountsofshale,softandporousmaterials(claylumps,forexample),andcertaintypesofchertmaybepronetopopouts.Theseparticlesareoftenoflighterweightandcanfloattothesurfaceunderexcessivevibration.Thiswillincreasethenumberofpopouts,evenwhentheamountoftheseparticlesissmallorwithinspeci-ficationlimits.Specificationsforconcreteaggregates,suchasASTMC33,allowasmallamountofdeleteri-ousmaterialbecauseitisnoteconomicallypracticaltoremovealloftheoffendingparticles.

Thepresenceofweak,porousparticlesintheaggregatecanbereducedbyjigging,heavy-mediaseparation,orbeltpicking;however,thesemethodsmaynotbeavailableoreconomicalinallareas.

Figure 3-12. A popout at the surface of concrete (PCA)

________________________________________________ Figure 3-13. Effect of compressive strength and aggregate type on the abrasion resistance of concrete (ASTM C 1138-97)

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madewithahardaggregateishighlyresistanttoabra-sion(Liu1981)Theuseofnaturalsandsinconcretemixtureswillimproveabrasionresistanceandcanresultinimprovedabrasionresistanceofconcrete,evenwhensoftcoarseaggregatesareused.

Theabrasionresistanceofanaggregatecanbeusedasageneralindicatorofitsquality.Usingaggre-gatewithlowabrasionresistancemayincreasethequantityoffinesintheconcreteduringmixingandconsequentlymayincreasethewaterrequirement.Ingeneral,siliceousaggregatesaremoreresistantto

abrasionthancalcareous(acid-soluble)materials.Asaresult,somespecificationslimittheamountofcalcareousmaterialinaggregatetoensureadequateabrasionresistance(seeAbrasionResistanceinchapter5,page146).

Coarseaggregateabrasionresistanceisusuallymea-suredusingtheLosAngeles(LA)abrasiontestmethod(ASTMC535andASTMC131).TheMicroDevaltest,developedinFranceduringthe1960s,isunderevaluationasanalternativetotheLAabrasiontest(KandhalandParker1998).


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Water

Key Points

Mixingwatercanconsistofbatchwater,ice,freemoistureonaggregates,waterinadmixtures,andwateraddedafterinitialmixing.

Ideally,waterforconcreteshouldbepotable(fitforhumanconsumption).

Somewaterrecycledfromreturnedconcreteandplantwashingcanbeacceptable.

Questionablemixingwatershouldbetestedforitseffectonconcretestrengthandsettingtime.

ThespecificationformixingwaterinconcretemixturesisASTMC1602.

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Thissectionfocusesonwatersourcesandqual-ity.(Forinformationaboutachievingtheoptimumquantityofwaterinconcretemixtures,seeCalculatingMixtureProportionsinchapter6,page179.Concretemixturesmustcontainenoughpaste[cementandwater]tothoroughlycoattheaggregatesandtomakethemixworkable.Toomuchwaterreduceslong-termconcretedurability.Itiscriticaltoachieveoptimumwatercontentforeachspecificmixandproject.)

Sources of Water in the MixtureWaterdischargedintothemixeristhemainbutnot

theonlysourceofmixingwaterinconcrete:Duringhot-weatherconcreting,icemaybeusedaspartofthemixingwatertocooltheconcrete.Theiceshouldbecompletelymeltedbythetimemixingiscompleted.Aggregatesoftencontainsurfacemoisture,andthiswatercanrepresentasubstantialportionofthetotalmixingwater.Therefore,itisimportantthatthewaterbroughtinbytheaggregateisfreefromharmfulmaterialsandaccountedforwhenbatchingconcreteingredients.

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Mix Water and Aggregate Moisture

To account for mix water properly, you need to know the assumed moisture condition of aggregates used in the mix design. In the trial batch and at the beginning of construction, you may need to adjust water content based on the actual moisture condition of the aggregates used. For more information, see Moisture/Absorption Correction in chapter 6, page 183, and Stockpile Management and Batching in chapter 8, pages 206 and 207, respectively.

Watercontainedinadmixturesalsorepresentspartofthemixingwaterifitaffectsthemix-ture’sratioofwatertocementitiousmaterialsby0.01ormore.

Water QualityThequalityofwaterusedinconcretemixtures

canplayaroleinthequalityofconcrete.Excessiveimpuritiesinmixingwatermayaffectsettingtimeandconcretestrengthandcanresultinsaltdepositsonthepavementsurface,staining,corrosionofreinforcementmaterials,volumeinstability,andreducedpavementdurability.

ManyStatedepartmentsoftransportationhavespecificrequirementsformixingwaterinconcrete(NRMCA2005).Ingeneral,suitablemixingwaterformakingconcreteincludesthefollowing:

Potablewater.Nonpotablewater.Recycledwaterfromconcreteproductionoperations.

Bothnonpotablewaterandrecycledwatermustbetestedtoensuretheydonotcontainimpuritiesthatnegativelyaffectconcretestrength,settime,orotherproperties.Watercontaininglessthan2,000partspermillion(ppm)oftotaldissolvedsolidscangener-allybeusedsatisfactorilyformakingconcrete.Watercontainingorganicmaterialsormorethan2,000ppmofdissolvedsolidsshouldbetestedforitseffectonstrengthandtimeofset(Kosmatka2002).

Waterofquestionablesuitabilitycanbeusedformakingconcreteifconcretecylinders(ASTMC39/AASHTOT22)ormortarcubes(ASTMC109/

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Water


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AASHTOT106)madewithithaveseven-daystrengthsequaltoatleast90percentofcompanionspecimensmadewithpotableordistilledwater.Settingtimetestsshouldbeconductedtoensurethatimpuritiesinthemixingwaterdonotadverselyshortenorextendthesettingtimeofconcrete(ASTMC403/AASHTOT197)orcement(ASTMC191/AASHTOT131).Inaddition,thedensityofthewater(ASTMC1603)hastobetestedifwaterfromconcreteproductionoperationsorwatercombinedfromtwoormoresourcesistobeusedasmixingwater.AcceptancecriteriaforwatertobeusedinconcretearegiveninASTMC1602/AASHTOT26(table3-14).

Optionallimitsmaybesetonchlorides,sulfates,alkalies,andsolidsinthemixingwater(table3-15),orappropriatetestsshouldbeperformedtodeterminetheeffecttheimpurityhasonconcreteproperties.Someimpuritiesmayhavelittleeffectonwater

requirement,strength,andsettingtimewhileadverselyaffectingconcretedurabilityandotherproperties.

Recycled WaterRecycledwaterisprimarilyamixtureofwater,

admixtures,cementitiousmaterials,andaggregatefinesresultingfromprocessingreturnedconcrete.Recycledwatermayalsoincludetruckwashwaterandstormwaterfromtheconcreteplant.Usually,recycledwaterispassedthroughsettlingpondswherethesolidssettleout,leavingclarifiedwater.Insomecases,recycledwaterfromareclaimerunitiskeptagi-tatedtomaintainthesolidsinsuspensionforreuseasaportionofthebatchwaterinconcrete(figure3-14).Solidcontentinrecycledwatervariesfrom2.5to10percentbymass.

Usingrecycledwaterinconcretemixturesmayaffectthewaterrequirement,settingtime,strength,andpermeabilityofconcrete.ASTMC1602/

Table 3-14. Acceptance Criteria for Combined Mixing Water

Table 3-15. Optional Chemical Limits for Combined Mixing Water


Water

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AASHTOM157permittheuseofwashwaterasconcretemixwater,withapprovalfromthepurchaserwhocaninvokechemicallimitsforchlorides,sul-fates,alkalis,andsolids.Themaximumpermittedsolidscontentis50,000ppm,orfivepercent,ofthetotalmixingwater.Thisamountstoabout8.9kg/m3(15lb/yd3)solidsinatypicalconcretemixture.A

solidscontentofapproximately50,000ppmrepre-sentswaterwitharelativedensity(specificgravity)of1.03.ResearchattheNationalReadyMixedCon-creteAssociation(LoboandMullings2003)identifiedseveralimportanteffectsofusingrecycledwateronpropertiesofconcreteincomparisontocontrolcon-cretemixturesmadewithtapwater;seetable3-16.

Table 3-16. Effect of Recycled Water on Concrete Properties

_____________________________________________________________________________________________________ Figure 3-14. Recycled water and reclaimed aggregate at a ready-mixed concrete plant (PCA)


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Chemical Admixtures

Key Points

Admixturesarematerialsaddedtoconcretemixturestomodifyconcretepropertiessuchasaircontent,waterrequirement,andsettingtime.Admixturesshouldcomplement,notsubstitutefor,goodconcreteproportioningandpractice.

Admixturesmayhaveunintendedsideeffects.Therefore,runtrialbatcheswithjobmaterialsandunderjobconditions.

Generally,foreveryonepercententrainedair,concretelosesaboutfivepercentofitscompressivestrength.

Air-entrainingadmixturesarespecifiedbyASTMC260.Water-reducingandset-modifyingadmixturesarespecifiedbyASTMC494/AASHTOM194.

Forinformationaboutpossibleincompatibilitiesofvariouschemicaladmixtures,seethesectiononPotentialMaterialsIncompatibilitiesinchapter4,page97.

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Chemicaladmixturesareaddedtoconcreteduringmixingtomodifyfreshorhardenedconcreteproper-tiessuchasaircontent,workability,orsettingtime.Table3-17listscommonadmixturesusedinconcreteforpavements.Table3-18listsadmixturesspecifiedunderASTMC494/AASHTOM194.

Addingchemicaladmixturesmayhelpconcretedesignersachievedesiredconcretepropertiesmoreefficientlyoreconomicallythanadjustingotheringredientsormixproportions.Admixturesarealsousedtomaintainspecificpropertiesduringconcret-ingoperationsorunderunusualconditions.However,admixturesshouldnotbeusedasasubstituteforgoodconcretepractice.Theyshouldbeusedtoenhancegoodconcrete,nottofixbadconcrete.Chemical

admixturesmayhaveunintendedsideeffectsthatmustbeaccommodated.

Anadmixture’seffectivenessdependsonmanymixfactors,includingcementitiousmaterialsproperties,watercontent,aggregateproperties,concretematerialsproportions,mixingtimeandintensity,andtemperature.

Commonadmixturesareair-entraining,water-reducing,andset-modifyingadmixtures.

Table 3-17. Common Chemical Admixture Types for Paving Applications

Table 3-18. Admixture Types Defined by ASTM C 494 / AASHTO M 194

Class Function

Air-entraining To stabilize microscopic bubbles inadmixture (AEA) concrete, which can provide freeze- thaw resistance and improve resistance to deicer salt scaling.

Water-reducing To reduce the water content admixture (WR) by 5 to 10%, while maintaining slump characteristics.

Mid-range water To reduce the water content by 6 to reducer (MRWR) 12%, while maintaining slump and avoiding retardation.

High-range water To reduce the water content by 12 to reducer (HRWR) 30%, while maintaining slump.

Retarder To decrease the rate of hydration of cement.

Accelerator To increase the rate of hydration of cement.

Class Name

Type A Water reducingType B RetardingType C AcceleratingType D Water reducing and retardingType E Water reducing and acceleratingType F Water reducing, high rangeType G Water reducing, high range, and

retarding

Note: Air-entraining admixtures are specified by ASTM C 260 / AASHTO M 154, and

superplasticizers are specified by ASTM C 1017.


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Air-Entraining AdmixturesAir-entrainingagentsarethemostcommonlyused

chemicaladmixturesinconcrete.Theyaretypicallyderivedfrompinewoodresins,vinsolresins,andothersyntheticdetergents.

Specificationsandmethodsoftestingair-entrainingadmixturesaregiveninASTMC260/AASHTOM154andASTMC233/AASHTOT157.Applicablerequirementsforair-entrainingcementsaregiveninASTMC150/AASHTOM85.

Function of Air Entrainment in ConcreteStabilizedairbubblesinhardenedconcreteare

calledairvoidsorentrainedair.Concretewithproperairentrainmentcontainsauniformdistributionofsmall,stablebubbles,orairvoids,throughoutthecementpaste(figure3-15).Properairentrainmentwilldothefollowing:

Dramaticallyimprovethedurabilityofconcreteexposedtomoistureduringcyclesoffreezingandthawing.Improveconcrete’sresistancetosurfacescalingcausedbychemicaldeicers.Tendtoimprovetheworkabilityofconcretemixtures,reducewaterdemand,anddecreasemixturesegregationandbleeding.

Anair-voidsystemconsistingofmanysmaller,closelyspacedvoidsprovidesthegreatestprotectionagainstfreeze-thawdamage.Theaveragedistancebetweenanypointinthepasteandanairvoidshouldtypicallybelessthan0.20mm(0.008in.)(figure3-16).

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Figure 3-15. Entrained air bubbles in concrete (PCA)

Figure 3-16. Spacing factor is the average distance from any point to the nearest air void. (Ozyildirim)

How Entrained Air Improves Concrete Freeze-Thaw Resistance

Entrained air plays a critical role in improving concrete pavements’ durability by reducing their susceptibility to freeze-thaw damage. Here’s how:

When most of the cement in a concrete mixture has reacted with water in the mixture, some of the remaining water is lost to evaporation, leaving behind capillary pores. Any remaining water can move through these pores. If the temperature drops below freezing, water

in capillary pores turns to ice and expands. As the freezing, expanding water forces its way through the pores, it exerts pressure on the surrounding hardened cement paste. Entrained air voids act as a pressure relief valve, providing space for the water/ice to expand and relieving pressure on the surrounding concrete. Without adequate air voids, the pressure may cause the hardened cement paste to crack, initiating early pavement deterioration.

Chemical Admixtures


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Air-Entraining Agent MechanismsAir-entrainingadmixturesstabilizemillionsoftiny

airbubblesintheconcretemixture(createdbytheconcretemixingaction)bycausingasoap-likecoat-ingtoformaroundtheairbubbles.Moleculesofair-entrainingagentareattractedtowateratoneendandtoairattheotherend,reducingsurfacetensionattheair-waterinterface;theendsthatprotrudeintowaterareattractedtocementparticles(figure3-17).Itiscriticalthatsufficientmixingtimebeallowedfortheairbubblestobegeneratedandstabilized.

Theair-voidsystemwillbeaffectedbythetype/amountofagent,mixingtime,placementmethods,carbonimpurities(seediscussionoflossonignition,orLOI,onpage33)fromSCMsoraggregates,andotherreactivematerialsinthemix,includingwater-reducingadmixtures.Forexample,mixtureswithhigh-rangewater-reducingadmixturesshowashifttowardslargervoidsizes(WhitingandNagi1998).

Table3-19liststrendsoftheeffectsofconcreteingredientsandproductiononairentrainment.Theextentofthechangeswilldependonspecificmateri-als,practices,andequipment.Thistablemaysuggestwaystocorrecthighorlowaircontents.

Ifyoudonothavepriorexperiencewiththespe-cificjobmaterialsandjobequipment,developatrialmixtodeterminetheproperdosageandminimummixingtime.

Figure 3-17. Stabilization of air voids by air-entraining admixture molecules (PCA)

Table 3-19. Effects of Materials and Practices on Air Entrainment

Side Effects of Air-Entraining AdmixturesGenerally,foreveryonepercententrainedair,

concretelosesaboutfivepercentofitscompressivestrength,everythingelsebeingequal(WhitingandNagi1998).However,thislossofstrengthcangener-allybemitigatedbyadjustingthewater-cementitiousmaterialsratioofthemixture.Withnon-vinsolair-entrainingadmixtures,airvoidsmaycoalescearound


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aggregates,leadingtoreducedconcretestrength(Crossetal.2000)(seeAir-VoidSystemIncompatibil-ities,page100).Thisresulttendstooccurinconcretewithhigheraircontentsthathavehadwateraddedafterinitialmixing,andmayalsobeaffectedbythetypeofaggregate.

Water ReducersWaterreducersarethesecondmostcommonly

usedchemicaladmixtures.Asummaryofthevariousclassesisgivenintable3-18.Mid-rangeproductsaregenerallyacombinationofnatural,petroleum-based,and/orpolycarboxylatechemicalcompounds(seethesidebarbelow).TypeFandGwaterreducersarenotnormallyusedinpavementsbecauseoftheirhighcostandbecauseitisdifficulttocontroltheslumprangerequiredforslipformpavingwiththeiruse.

Function of Water ReducersYouwillrememberthatmorewaterthanisneeded

forcementhydrationmustbeaddedtoconcretemixturestomakethemworkable.Toomuchwater,however,canreduceconcretedurabilitybyincreasingporosityandpermeabilityinthehardenedconcrete,allowingaggressivechemicalstopenetratethepave-ment.Waterreducersarethereforeaddedtomixturestoreducetheamountofwaterneededtomaintainadequateworkabilityinplasticconcrete.

Water-reducingadmixturesalsoindirectlyinflu-enceconcretestrength.Forconcretesofequalcementcontent,aircontent,andslump,the28-daystrengthofawater-reducedconcretecanbe10percentgreaterthanconcretewithoutsuchanadmixture.Water-reducingadmixturesarespecifiedbyASTMC494/AASHTOM194.

Theeffectivenessofwaterreducersonconcreteisafunctionoftheirchemicalcomposition,concretetem-perature,cementitiousmaterialcontentandproperties,andthepresenceofotheradmixtures.Typicalrangesofwaterreductionandslumpareshownintable3-17.

High-rangewaterreducers(HRWRs)(TypesFandG,alsocalledsuperplasticizers)canbeusedtoreducenecessarywatercontentby12to30percent.Inolderproducts,however,therateofslumplossisnotreducedandinmostcasesisincreased.Newergenera-tionHRWRsdonotexhibitasrapidarateofslumploss.Rapidslumplossresultsinreducedworkabilityandlesstimetoplaceconcrete.Hightemperaturescanalsoaggravateslumploss.SomemodernTypeFwater-reducingadmixturesmayentrainair,meaningthatlessair-entrainingadmixturewillberequired.

Water-Reducing Agent MechanismsWaterreducersoperatepredominantlybyapplying

surfacechargesoncementparticles,whichbreaksupagglomerationsofparticles.Asthecementparticles

First-generation water-reducing admixtures were primarily derived from natural organic materials such as sugars and lignins (extracted from wood pulp). These products had a limited effectiveness and may have been somewhat variable in performance, depending on the source material. These water reducers would often retard the mixture, particularly when overdosed. They are still used as Type A, B, and D products.

Second-generation (early high-range) water-reducing admixtures were derived from petroleum feedstocks: sulfonated naphthalene or melamine condensates with formaldehyde. These products offer greater water reduction and are less detrimental when overdosed.

Three Generations of Water-Reducing AdmixturesThey often only provide about 20 minutes of effectiveness before the concrete exhibits significant slump loss. These products can be re-dosed after this time to maintain workability, if required.

Third-generation water-reducing admixtures are polycarboxylates, which are copolymers synthesized from carefully selected monomers. These products are currently used in mid- or high-range water-reducing admixtures (Type F and G). These admixtures can be fine-tuned for a given application, including a range of effectiveness and setting times. They may also increase air entrainment.

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breakupandmoveapart,trappedwaterismadeavailableforhydration(figure3-18).Somenewerpolycarboxylate-basedwaterreducersalsouseastericrepulsioneffect.Thesehavepolymerswithlongsidechainsthatattachtothesurfaceofcementgrainstokeepthegrainsapartphysically.Ramachandran(1995)containsmoredetailonadmixturemechanisms.

Side Effects of Water ReducersNewer,polycarboxolatewater-reducingadmix-

tureshavebeenreportedtoincreaseairentrainmentinsomecases.TypesD,E,andGwater-reducingadmixturescansignificantlyaffectsettingbehavior.Overdosesofwaterreducers,particularlynormal-rangeproducts,mayseverelyretardorpreventsetting.

Set-Modifying AdmixturesAdmixturesmaybeusedtocontrol(i.e.,retard

oraccelerate)therateofsettingandstrengthgainofconcretes.Theseeffectscanbeparticularlyimportantforhot-andcold-weatherconcretingoperations.Theycanalsobeusefulforfast-trackconstructionandtocontrolproductioncycles,permitlongerhaultimes,orcompensateforslowerstrengthgainofconcretesmadewithsomesupplementarycementitiousmaterials.

Chemicaladmixturescontainingcalciumchloride(CaCl

2)shouldnotbeusedinreinforcedpavements

becauseoftheriskofcorrosionofsteel.

Figure 3-18. One mechanism by which water reducers work is dispersion. (a) Charged cement particles cling together, trapping water. (b) Water reducers separate cement grains, releasing the water and making it available for hydration.

RetardersaredesignatedasTypeBadmixturesunderASTMC494/AASHTOM194,althoughretardationisassociatedwithTypesDandGaswell(table3-18).

AcceleratorsaredesignatedasTypeCadmixturesunderASTMC494/AASHTOM194.TypeEadmix-turesalsohaveacceleratingproperties.

Primary Function of Set-Modifying AdmixturesRetardingadmixturesareusedtoslowtherate

ofconcretesetting.Theymaybeusedduringhot-weatherpaving,whenhighconcretetemperaturesoftencauseanincreasedrateofstiffeningthatmakesplacingandfinishingdifficult.Retarderschemicallycontrolcementhydration,consequentlyreducingthemaximumtemperature.Retardersmayalsobeusedtodelaysettingunderdifficultorunusualplacementconditions.

Acceleratingadmixturesareusedtoincreasetherateofstrengthdevelopmentofconcreteatanearlyage,includingincoldweather.However,excessaccel-erationmayresultincrackingbeforefinishingand/orsawcuttingcanbecompleted.

Set-Modifying Agent MechanismsThemechanismsbywhichacceleratorsandretard-

ersworkarecomplex.However,someretardersappeartoformacoatingaroundcementgrainsthatpreventsorslowshydration.Eventually,thecoatingisbreachedandthehydrationresumes.

Someacceleratingadmixtures,suchascalciumchloride,appeartopreferentiallyacceleratehydrationofthesilicatephases(alite[C

3S]andbelite[C

2S]),

whileothersappeartoaccelerateotherphases.Formoreinformation,seeRamachandran(1995)andThomasandWilson(2002).

Side Effects of Set-Modifying AdmixturesTheeffectsofset-modifyingadmixturesonother

propertiesofconcrete,likeshrinkage,maynotbepre-dictable.Therefore,acceptancetestsofsetmodifiersshouldbemadewithjobmaterialsunderanticipatedjobconditionstodetectpossiblesideeffects:

Set-Retarding Admixtures’ Side Effects.Effectsmayincludethefollowing:

Manyretardersalsoactaswaterreducers;theyarefrequentlycalledwater-reducingretarders.

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protecttheconcretefromfreezingpriortoachievingsufficientstrength.

Whencalciumchlorideisused,itshouldbeaddedtotheconcretemixinsolutionformaspartofthemixingwater.Ifaddedindryform,theparticlesmaynotcompletelydissolve.Undissolvedlumpsinthemixcancausepopoutsanddarkspotsinhardenedconcrete.Anoverdosecanresultinplacementprob-lemsandcancauserapidstiffening,alargeincreaseindryingshrinkage,corrosionofreinforcement,andlossofstrengthatlaterages.

Other AdmixturesSeveralotheradmixturesmaybeusedinconcrete,

includinglithium-basedmaterialsformitigatingalkali-silicareaction,shrinkage-reducingadmixtures,andcorrosioninhibitors.MoreinformationcanbefoundinThomasandWilson(2002);Kosmatka,Kerkhoff,andPanarese(2002);andRamachandran(1995).

Set-retardingadmixturesmayincreaseaircon-tentinthemixture.Delayingsettingofamixmayincreasetheriskofplasticshrinkagecrackingandmayinterferewiththesaw-cuttingwindow.Ingeneral,useofretardersmaybeaccompa-niedbysomereductioninstrengthatearlyages(onetothreedays),butlaterstrengthsmaybehigher.Withretarders,thebleedingrateandbleedingcapacityofconcreteareincreased.Set-retardingadmixturesareusefulinextend-ingsettingtimesofconcrete,butnotallareeffectiveindecreasingslumplossandextend-ingworkabilitypriortoplacementatelevatedtemperatures(WhitingandDziedzic1992).

Set-Accelerating Admixtures’ Side Effects.Accelera-torslikecalciumchloridearenotantifreezeagents.Precautionsshouldbetakenduringcoldweatherto

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Dowel Bars, Tiebars, and Reinforcement

Key Points

Dowelbarsareplacedacrosstransversejointstoprovideverticalsupportandtotransferloadsacrossjoints.

Tiebarsareplacedacrosslongitudinaljoints(centerlinesorwhereslabsmeet)topreventtheslabsfromseparatingandtotransferloadsacrossthejoints.

Slabreinforcementmaybeusedinconcretepavementstoimprovetheabilityofconcretetocarrytensilestressesandtoholdtightlytogetheranyrandomtransversecracksthatdevelopintheslab.

Epoxy-coatedtiebarsshouldconformtoASTMA775/AASHTOM284,andepoxy-coateddowelbarsshouldconformtoAASHTOM254.

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Dowelbars,tiebars,andreinforcementmaybeusedinconcretepavementstohelptheconcretecarryten-silestresses(i.e.,stressesthatpulltheconcreteapart)and/ortotransferloadsacrossjoints.Collectively,thesematerialsareoftencalled“steel,”althoughtheymaybemadeofothermaterials.

(AdditionalinformationcanbefoundunderDowelBarsandTiebarsinchapter8,page218.)

Dowel Bars (Smooth Bars)Dowelbars(smoothbars),orsimplydowels,are

placedinconcreteacrosstransversejointstoprovideverticalsupportandtotransferloadsacrossjoints.Dowelbarsaretypicallyusedonheavytruckroutes.Dowelbarsreducethepotentialforfaulting,pump-ing,andcornerbreaksinjointedconcretepavements(Smithetal.1990;ACPA1991).

Dowelsaresmoothandroundoroval.Becausetheyaresmooth,theydonotrestrictthehorizontalmovementoftheslabatthejointrelatedtoseasonalexpansionandcontractionoftheslab.

Therecommendeddowelbardiameterisroughlyone-eighthofthepavementthickness.Theminimumdiameteris32mm(11/4in.),exceptatconstruction

joints(orheaders)inpavementslessthan180mm(7in.)thick.Dowelsizesof25mm(1in.)andlessinheavy-trafficpavementshavebeenshowntocausehighbearingstressesontheconcretesurroundingthedowelbar,resultingindamageintheconcretearoundtheplacethedowelemergesfromthesideoftheslab,knownasdowelsocketing.

Typicaldowellengthsare455mm(18in.),althoughsome380-mm(15-in.)barsareused.Thislengthisprimarilyrelatedtotherequiredembedmentlengthoneachsideofthejoint.MostStatesspecify455-mmlong(18-in.long)dowelbarswith150mm(6in.)ofembedmentoneachsideofthejoint.Thisleaves150mm(6in.)inthecenterofthebarasatoleranceforthesawingofthejointandtheformationofthesubsequentcrack.

A305-mm(12-in.)spacingbetweenbarsisstan-dard,althougha380-mm(15-in.)spacingcanbeusedforlighter-trafficfacilities.Dowelsaretypicallyplacedacrosstheentirelengthofthejoint,primar-ilyforconstructabilitypurposes.However,onlythreeorfourdowelsareneededineachwheelpathtosufficientlytransferloadfromoneslabtothenext(ACPA1991);thisiscommonpracticewhendowelsareretrofittedinpavementsoriginallyconstructedwithoutdowels.

Shapesotherthanroundbarshavebeentestedandinstalledinlimitedtrials.Themostpromisingistheellipticaldowel,whichallowsforsmallereffectivediameterbarswiththesamebearingcapacityasroundbars.Thisreducestheweightandcostofthedowelassemblybecauseofthereducedamountofsteelused.Ellipticaldowelshavebeenshowntohavesimilarperformancetorounddowels(Porter2002).

Othermechanicalloadtransfersystemsincludeflat(plate)dowelsandsquaredowels,bothofwhicharenottypicallyusedinroadwayorhighwaypavements.

Tiebars (Deformed Bars)Tiebars(deformedbars),orrebar,areplacedacross

longitudinaljoints(centerlinesorwhereslabsmeet).Tiebarspreventfaultingandlateralmovementoftheslabsandassistwithloadtransferbetweenslabs.Tiebarsarealsousedtoconnectedgefixturessuchascurbsandgutterstothepavement.



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Becausetiebarsaredeformed,theybondtotheconcreteanddonotallowmovement(unlikesmoothdowelbars,whichbydesignallowsuchmovement.)Tiebarsthusminimizelongitudinaljointopeningbetweenslabsandsomaintainaggregateinterlock.

Tiebarsize,spacing,andlengthvarywiththethick-nessofthepavement,tiebarmaterial,andamountofpavementtiedtogether(table3-20).Tiebarsizesandembedmentlengthsshouldreflecttheactualforcesactinginthepavement,notonlytheworkingstrengthofthesteel.Specifiersshouldchoosestandardmanu-facturedtiebarlengths.

Tiebarsshouldnotbeplacedwithin380mm(15in.)oftransversejoints,ortheycaninterferewithjointmovement(ACPA1991),leadingtocracking.Thisspaceshouldincreaseto450mm(18in.)ifthetiebarsarelongerthan800mm(32in.).Nomorethanthreelanesshouldbetiedtogether.

Whilecorrosionoftiebarsisnotacommonproblem,someprotectionshouldbeprovidedifdeicingsaltsaregoingtobeusedonthepavement.ASTMD3963/AASHTOM284provideguidelinesforsuchprotectionintheformofacoating.Thecoat-ingshouldbenothickerthan125to300µm(5to12mil)orthebondingcapacityofthebarisreduced.

ReinforcementReinforcementmaybeusedinconcretepavements

toimprovetheabilityofconcretetocarrytensilestressesandtoholdtightlytogetheranyrandom

transversecracksthatdevelopintheslab.Generally,crackinginjointedconcretepavementsiscontrolledbylimitingthespacingbetweenjoints.Whenthecon-creteslabisreinforced,jointspacingcanbeincreased.

Currentpracticedoesnotgenerallyincludeanydistributedreinforcementinconcreteslabs,withtheexceptionofcontinuouslyreinforcedconcretepavements(seeBasicConcretePavementTypesinchapter2,page9).However,someagenciesreinforceodd-shapedpanelstoholdexpectedcrackstight(ACPA2003a)andusejointless,continuouslyrein-forcedconcreteforhigh-trafficurbanroutes.

Conventionalreinforcementisgenerallyintheformofweldedwirefabricordeformedbarsthatareplacedatspecificlocationsinthepavement.Asanalterna-tive,reinforcingfibersareaddedtofreshconcreteduringthebatchingandmixingprocess.Fibers,whichareavailableinavarietyoflengths,shapes,sizes,andthicknesses,areequallydistributedthroughouttheconcrete,aremuchshorterthanbars,andtakeupamuchsmallercross-sectionalareaofthepavement.

Materials for Dowel Bars, Tiebars, and Reinforcement

Theseitemsareoftencollectivelyreferredtoas“steel”butmaybemadeofothermaterials.

SteelSteelisthemostcommonmaterialfordowels,

tiebars,andreinforcement.Dowelsandtiebarsshould

Table 3-20. Tiebar Dimensions and Spacings

Tiebar spacing, mm (~in.) Slab thickness Tiebar size x Distance to nearest free edge or to nearest joint where movement can occur mm (~in.) length, mm (~in.) 3.0 m (~10ft) 3.7 m (~12 ft) 4.3 m (~14 ft) 7.3 m (~24 ft)

130 (5) 13M x 600 (24) 760 (30) 760 (30) 760 (30) 700 (28)150 (6) 13M x 600 (24) 760 (30) 760 (30) 760 (30) 580 (23)180 (7) 13M x 600 (24) 760 (30) 760 (30) 760 (30) 500 (20)200 (8) 13M x 600 (24) 760 (30) 760 (30) 760 (30) 430 (17)230 (9) 16M x 760 (30) 900 (35) 900 (35) 900 (35) 600 (24)250 (10) 16M x 760 (30) 900 (35) 900 (35) 900 (35) 560 (22)280 (11) 16M x 760 (30) 900 (35) 900 (35) 860 (34) 500 (20)310 (12) 16M x 760 (30) 900 (35) 900 (35) 780 (31) 460 (18)

Source: Adapted from ACI 325.12R

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conformtoASTMA615/AASHTOM31.Theirtypicalyieldstrengthis420MPa(60ksi);280MPa(40ksi)isallowable.TiebarscanbeGrade420(60)or280(40);yieldstrengthdeterminesrequiredlength.

Steelfibersaregenerally12.7to63.5mm(0.5to2.5in.)longand0.45to1.0mm(0.017to0.04in.)indiameter.Theusualamountofsteelfibersrangesfrom0.25to2percentbyvolume,or20to157kg/m3(33to265lb/yd3).Steelfibersaregenerallyusedathighratesforimprovedhardenedproperties.Theben-efitsofsteelfibersincludeupto150percentincreaseinflexuralstrength,reducedpotentialforcrackingduringconcreteshrinkage,andincreasedfatiguestrength(Kosmatka,Kerkhoff,andPanarese2002).

Epoxy-Coated BarsCorrosionresistanceiskeyforembeddedsteel,

particularlyinharshclimateswheredeicingchemicalsareusedonpavements.Themostcommonmethodofresistingordelayingcorrosioniscoatingsteelbarswithepoxy0.2to0.3mm(0.008to0.012in.)thick.Epoxy-coatedtiebarsshouldconformtoASTMA775/AASHTOM284,andepoxy-coateddowelbarsshouldconformtoAASHTOM254.

Epoxy-coatedsteelisstillsusceptibletocorro-sion,particularlywhenincorrecthandlinginthefieldresultsinnicksorscratchesintheepoxycoating.

Epoxy-coateddowelsshouldbecoatedwithabond-breakingmaterialtopreventthemfromlock-ingupthejoint.Thismaterialistypicallyappliedbythemanufacturer.Ifthedowelsareleftexposedtotheelementsforanextendedtime(e.g.,ataprojectsiteoroverthewinter),thebondbreakermustbereap-pliedonatleastoneendofeachbar.Materialsappliedinthefieldaretypicallyform-releaseoilorwhite-pigmentedcuringcompound.Donotusegrease,asitwillformavoidspacearoundthebar,preventingtheconcretefromconsolidatingadequatelyandfullyencasingthedowel.

Stainless Steel BarsSomeagencieshaveexperimentedwithstainless

steel(304and316)reinforcingbarsforlong-lifeconcretepavements.Solidstainlesssteelisveryexpensive,butithasamuchlongerservicelife.Stain-lesssteeldowelsshouldconformtoASTMA955.

Stainless-Clad BarsStainless-cladsteeldowelbarshavea1.8-to

2.3-mmthick(0.070-to0.090-in.thick)stainlesssteelcladdingthatismetallurgicallybondedtoacarbonsteelcore.Theyarequiteexpensivecomparedtoepoxy-coatedbars,buttheycostlessthansolidstainlesssteelwhileofferingitslongservicelife.

Fiber-Reinforced PolymerConsiderableresearchhasbeenconductedinthe

useoffiber-reinforcedpolymerdowelbars.Theyofferpromisebutarestillquiteexpensive.

Synthetic FibersSyntheticreinforcingfibersaremanufacturedfrom

materialslikeacrylic,aramid,carbon,nylon,poly-ester,polyethylene,orpolypropylene.Theirusehasbeenincreasingsteadily,generallyatlowdosageratestoreduceplasticshrinkage.Theirprimaryuseinpavementsisinultrathinwhitetopping,where50to100mm(2to4in.)ofconcreteisbondedtoasphalt(seeConcreteOverlaysinchapter2,page23).

Themostcommonsyntheticfibersinconcretepavementsaremadeoffibrillatedpolypropylene.Theyarenormallyusedatarateofatleast0.1per-centbyvolume.Ultrathinwhitetoppingtypicallyuses1.8kg/m3(3lb/yd3)ofsuchfibers.Thebenefitsofthesefibersincludereducedplasticshrinkageandsubsidencecracking,andincreasedtoughnessorpost-crackintegrity.Infreshconcrete,polypropylenefibersalsoreducesettlementofaggregatefromthepave-mentsurface,resultinginalesspermeableandmoredurable,skid-resistantpavement(ACPA2003b).

Other Materials Stainlesssteeltubesandpipefilledwithconcrete

havealsobeenusedexperimentally(ASCE2004),buttheirlong-termperformancehasnotyetbeendemon-strated.Low-carbonmicrocompositesteelshavebeenshowntobemorecorrosion-resistantthanepoxy-coatedsteelinsometests,andquitecosteffective.Zinc-coateddowelbarsarealsoshowingpromiseaslonger-termperformanceisbecomingmoredesirable.Theseproprietarysteelsarelessresistanttocorrosionthanstainlesssteel,butarealsolesscostly.Plasticcoating,approximately0.5mm(0.02in.)thick,isusedbysomeagenciesondowelbars.


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Curing Compounds

Key Points

Curingcompoundsmustbeappliedthoroughlytoconcretesurfacesaftertexturingtoreducemoisturelossfromthesurface.

Themostcommoncuringcompoundsareliquidmembrane-formingmaterials.

Usepigmentedcuringcompoundssoyoucanseethatcoverageiscompleteanduniform.

Liquidmembrane-formingcuringcompoundsarespecifiedbyASTMC309andASTMC1315.

•

•

•

•

Curingcompoundisappliedtothesurfaceandexposededgesofconcretesoonaftertheconcretehasbeenplacedandtextured.(Somepeopleconsiderthepropertimeforapplyingcuringcompoundtobeapproximatelyatinitialset,whenbleedingiscomplete[Poole2005].)Thepurposeistosealthesurface—thatis,topreventorslowwaterevaporationfromthesurface—sothathydrationcancontinueforpreferablysevenormoredays(figure3-19).

Cementhydrationisaslowchemicalreaction(seeallofchapter4).Iftheconcretesurfaceisallowedtodryprematurely,thereactionstopsanddesirablepropertiesoftheconcrete,suchasdurability,willbe

compromisedatthesurface.Itisimportantnottoletthishappen,becausetheconcretesurfacecarriesthetrafficandisexposedtoaggressiveenvironments.Propercuringwillreducesurfacepermeabilityandincreasesurfaceresistancetowearandweather.

Curingcompoundmustbethoroughlyappliedtofreshconcretebeforethesurfacedries;otherwisethereisnobenefit.

Types of Curing CompoundsLiquidmembrane-formingcuringcompounds

areamongthemostwidelyusedcuringmaterialsforconcretepavements.Theyarecommonlywax-orresin-basedcompounds,emulsifiedinwaterordis-solvedinasolvent.(Thesecompoundsareusedratherthanwatercuringfortheirconvenienceandeconomy.)Aftertheyareappliedtotheconcretesurface,thewaterorsolventevaporatesandthewaxorresinformsamembraneontheconcretesurface.Liquidmembrane-formingcompoundsneedtoconformtotherequirementsofASTMC309/AASHTOM148andASTMC1315,asapplicable.ASTMC156/AASHTOT155specifyamethodfordeterminingtheefficiencyofcuringcompounds,waterproofpaper,andplasticsheeting.

Pigmentedcuringcompoundsarerecommendedbecausetheymakeiteasytoverifyproperapplica-tion.Whenplacingconcreteonsunnydaysandinhotweather,thecuringcompoundshouldcontainawhitepigmenttoreflectthesun’sheat.Translucentcompoundsmaycontainafugitivedyethatmakesiteasiertocheckvisuallyforcompletecoverageoftheconcretesurfacewhenthecompoundisapplied.Thedyefadessoonafterapplication.

Nottobeconfusedwithcuringcompounds,evaporation-retardingmaterialsareappliedimmedi-atelyafterconcreteisplacedandbeforeitisfinished.Evaporationretardersformathin,continuousfilmthatretardslossofbleedwater,reducingtheriskofplasticshrinkagecrackingbeforefinishing/curing,particularlyinhot,dryweather.Ifused,evapora-tion-retardingmaterialsshouldbeappliedassoonaspossibleafterplacement.Theyshouldnotbeusedasafinishingaidbutcanbeworkedintothesurfaceduringfinishingwithoutdamagingtheconcrete.

________________________________________________ Figure 3-19. Curing compounds keep concrete partially saturated near the surface during the curing period. (PCA)


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ReferencesAASHTO standards may be found in Standard Specifications for Transportation Materials and Methods of Sampling and Testing, 24th Edition, and AASHTO Provisional Standards, 2004 Edition, HM-24-M. https://www.transportation.org/publications/bookstore.nsf.

AASHTOM6,FineAggregatesforPortlandCementConcrete

AASHTOM31,StandardSpecificationforDeformedandPlainCarbonSteelBarsforConcreteReinforcement

AASHTOM80,CoarseAggregatesforPortlandCementConcrete

AASHTOM85,PortlandCement

AASHTOM148,LiquidMembrane-FormingCom-poundsforCuringConcrete

AASHTOM154,Air-EntrainingAdmixturesforConcrete

AASHTOM194,ChemicalAdmixturesforConcrete

AASHTOM240,BlendedHydraulicCement

AASHTOM254,SpecificationforCorrosionResistantCoatedDowelBars

AASHTOM284,StandardSpecificationforEpoxy-CoatedReinforcingBars:MaterialsandCoatingRequirements

AASHTOM295,CoalFlyAshandRaworCalcinedNaturalPozzolanforUseasaMineralAdmixtureinConcrete

AASHTOM302,GroundGranulatedBlastFurnaceSlagforUseinConcreteandMortars

AASHTOM321,High-ReactivityPozzolansforUseinHydraulic-CementConcrete,Mortar,andGrout

AASHTOT22,CompressiveStrengthofCylindricalConcreteSpecimens

AASHTOT26,QualityofWatertoBeUsedinConcrete

AASHTOT27,SieveAnalysisofFineandCoarseAggregates

AASHTOT106,CompressiveStrengthofHydrau-licCementMortar(Using50-mmor2-in.CubeSpecimens)

AASHTOT131,TimeofSettingofHydraulicCementbyVicatNeedle

AASHTOT155,WaterRetentionbyConcreteCuringMaterials

AASHTOT157,Air-EntrainingAdmixturesforConcrete

AASHTOT197,TimeofSettingofConcreteMixturesbyPenetrationResistance

AASHTOT303,AcceleratedDetectionofPotentiallyDeleteriousExpansionofMortarBarsDuetoAlkali-SilicaReaction

AASHTOTP60,StandardMethodofTestforCoef-ficientofThermalExpansionofHydraulicCementConcrete


ASTMA615/A615M-04b,StandardSpecificationforDeformedandPlainBillet-SteelBarsforConcreteReinforcement

ASTMA775,SpecificationforEpoxy-CoatedSteelReinforcingBars

ASTMA955,SpecificationforDeformedandPlainStainlessSteelBarsforConcreteReinforcement

ASTMC33/AASHTOM6,SpecificationforConcreteAggregates

ASTMC39/AASHTOT22,StandardTestMethodforCompressiveStrengthofCylindricalConcreteSpecimens

ASTMC109/AASHTOT106CompressiveStrengthofHydraulicCementMortar(Using50-mmor2-in.CubeSpecimens)

ASTMC125-03StandardTerminologyRelatingtoConcreteandConcreteAggregates

ASTMC131,TestMethodforResistancetoDegrada-tionofSmall-SizeCoarseAggregatebyAbrasionandImpactintheLosAngelesMachine

ASTMC136-04/AASHTOT27StandardTestMethodforSieveAnalysisofFineandCoarseAggregates

ASTMC150/AASHTOM85,SpecificationforPort-landCement

ASTMC156/AASHTOT155,TestMethodforWaterRetentionbyConcreteCuringMaterials,

ASTMC191/AASHTOT131TestMethodforTimeofSettingofHydraulicCementbyVicatNeedle


References

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ASTMC294-04StandardDescriptiveNomenclatureforConstituentsofConcreteAggregates

ASTMC295,StandardGuideforPetrographicExami-nationofAggregatesforConcrete

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ASTMC403/AASHTOT197,TestMethodforTimeofSettingofConcreteMixturesbyPenetrationResistance

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ASTMC1157,PerformanceSpecificationforHydrau-licCements

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ASTMC1315,SpecificationforLiquidMembrane-FormingCompoundsHavingSpecialPropertiesforCuringandSealingConcrete

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Whiting,D.andW.Dziedzic.1992.EffectsofConventionalandHigh-RangeWaterReducersonConcreteProperties.RD107.Skokie,IL:PortlandCementAssociation.

Whiting,D.andM.A.Nagi.1998.ManualontheControlofAirContentinConcrete.EB116.NationalReadyMixedConcreteAssociationandPortlandCementAssociation.

Wigum,B.J.,P.Holmgeirsdottir,S.W.Danielsen,andO.V.Anderson.2004.ProductionandUtilisationofManufacturedSandforConcretePurposes.JournalNo.128-03.Reykjavik,Iceland:Hönnun.

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Chapter 4Transformation of Concrete from Plastic to Solid

Stages of Hydration: Overview 70

Stages of Hydration: Details 75

Hydration Compounds 84

Portland Cement Hydration 88

Reactions of Supplementary Cementitious Materials 94

Potential Materials Incompatibilities 97

Attheconcreteplant,theingredientsdiscussedinchapter3—cementitiousmaterials,water,aggregates,andchemicaladmixtures—aremixedtogether.Duringthenextfewhours,themixturechangesfromaplasticmixturetoasolidconcreteslab.Centraltothistrans-formationisacomplexprocesscalledhydration—anirreversibleseriesofchemicalreactionsbetweenwaterandcement.Hydrationisamysterytomanypeopleinvolvedindesigningandconstructingconcretepavements.Thegoalofchapter4istodemystifythisprocess,focusingonthepracticalimplicationsfordesignersandconstructionpersonnel.

Whyshouldreaderscareabouthydration?Thechemicalreactionstakingplaceduringthefirstfewhoursinfluencehowtheconcretemixturebehaveswhenitisbeingplacedandfinished.Laterreactionsgovernhowstronganddurablethehardenedconcretebecomes.Withageneralunderstandingofthesereac-tions,readerscanhelppreventorcorrectproblemsandensurethattheconcreteperformsasitwasdesigned.

Notestohelpyouusechapter4:• Chapter4takestwo,overlappingapproaches

todescribinghydration.Thefirstapproachfocusesonfivestagesofhydration.(TheStagesofHydrationchartonpages4-8through4-15isreplicatedinafull-sized,foldoutposteraccompanyingthismanual.)Thesecondapproachprovidesamorein-depthdiscussion

ofthechemicalprocessesinvolvedinhydration,focusingonthecompoundsincementandthecompoundscreatedduringhydration.

• Thereissomerepetitionofinformationthroughoutthechapter,asthetextmovesfromabasicdiscus-siontoamoretechnicalanddetaileddiscussion.

• Thegeneralinformationabouthydrationisbasedonanominalconcretepavementmixturewithalowwater-cementitiousmaterials(w/cm)ratio.Manyfactors,likeactualw/cmratio,cementfineness,aggregategradation,consolidation,curing,andenvironment,allstronglyinfluencetheperformanceofapavement.Thesefactorsarediscussedinmoredetailinotherchapters,especiallychapters3,6,and8.

• Thetime/heatcurveusedthroughoutthischapterrepresentsasmall,insulatedconcretesample.Itillustratesonlychangesinheatgenerallyassoci-atedwithcementhydration.Itdoesnotrepresentanactualtime/heatcurveofafull-depthconcreteslab,whichwouldlikelyreflectheatcontributedfromexternalsourceslikesunlight,andslowercoolingduetoinsulationplusthethermalmassofafull-depthslab.

• Chapter4usescementchemists’shorthandnota-tiontodescribecompounds:A=Al

2O

3,C=CaO,

F=Fe2O

3,H=H

2O,M=MgO,S=SiO

2,andS=SO

3.

Thus,tricalciumsilicateisC3S.

–

�0

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Stages of Hydration: Overview

Hydrationbeginsassoonaswaterandcementcomeintocontact.Thecementparticlespartiallydissolve,andthevariousdissolvedcomponentsstarttoreactatvariousrates.Duringthereactions,heatisgeneratedandnewcompoundsareproduced.Thenewcompoundscausethecementpastetoharden,bondtotheaggregateintheconcretemixture,andbecomestronganddense.

Portlandcementismanufacturedbymixinglimestone,clay,andshaletogetherataveryhightemperature,thengrindingtheresultingclinkerwithgypsum.Thisprocessresultsinthreeprimaryingredientsincementthatreactduringhydration(figure4-1):

Calciumsilicates(referredtoassilicates).Calciumaluminates(referredtoasaluminates).Calciumsulfates(referredtoasgypsum).

Silicatescontainsilicon(asinglass).Aluminatescontainaluminum(asinpotteryclay,andthesandinsandpaper).Gypsumcontainssulfur(asinrotteneggs).Allthreecompoundscontaincalcium(asinbonesandteeth)andoxygen.

Thereactionsofthesecompoundswithwatercanbedescribedinfivestages—mixing,dormancy,hardening,cooling,anddensification.Thereactionscanalsobe

•••

trackedbymonitoringheatateachstage(figure4-2):abriefheatspikeduringmixing;noheatgeneratedduringdormancy;asignificant,steadyriseinheatduringhardening;apeakandthencontinuousdropinheatduringcooling;and,finally,relativelylittleheatgeneratedduringdensification.

Thebasicexplanationofthefivestagesonpages71–73describeshowconcretegenerallyreactswhenthesystemisbalancedandperformingasdesigned.

Figure 4-2. General hydration curve delineating the five stages

Figure 4-1. Compounds in cement

Calcium silicatesGypsum particles

Calciumaluminates

Clinker particle

Hea

t

Time

CementWater

Aggregate

Mixing

Dormancy

Hardening

Cooling

Densification

Stage 3Stage 2Stage 1 Stage 4 Stage 5

Initial set

Final set

Stages of hydration: Overview


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MixingIngeneral,duringhydrationthereactionsbetweensilicatesandwaterproducetheprimarycom-poundsthatmakeconcretestronganddurable.Butsilicatesdissolveveryslowlyanddonothaveanimmediateeffect.

Aluminatesandgypsum,ontheotherhand,dis-solveandreactwithinminutesofbeingmixedwithwater,withimmediateeffect.Togetherwithwater,thedissolvedaluminatebeginsdevelop-ingnewcompounds,generatingsignificantheat.Unchecked,thesereactionswouldcauseirrevers-ible,flashsetorstiffeningoftheconcrete.

Fortunately,thefast-dissolvinggypsumreactswiththedissolvedaluminateandwatertocreateagel-likesubstancethatcoatsthecementcompounds(figure4-4).Thecoatingslowsthealuminatereactionsalmostassoonastheystart,reducingtheamountofheatgeneratedandthepotentialforflashset(figure4-3).

DormancyAluminatereactionsaregenerallycontrolledforabouttwotofourhours,thedormantperiod.Duringthistime,theconcreteisplasticanddoesnotgenerateheat(figure4-5).Thedormantstagegivestheconstructioncrewtimetotransport,place,andfinishtheconcretewhileitisworkable.

Duringdormancy,itlooksasthoughnothingishappeninginthemixture.However,thecementiscontinuingtodissolve,andthewaterisbecomingsaturatedwithdissolvedcalciumandOH(hydroxyl)ions(figure4-6).

Gel-like substance

Figure 4-3. A very brief heat spike occurs during mixing.

Figure 4-4. A gel-like substance coats cement compounds, controlling the reactions and heat.

Figure 4-5. The concrete does not generate heat during the dormancy stage.

Figure 4-6. During dormancy, the water becomes saturated with dissolved ions.

Ions

Time

Mixing

Hea

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Time

Dormancy

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HardeningWhenthewaterbecomessupersaturatedwithdissolvedcalciumions,newcompoundsbeginforming,heatisgenerated,andthemixturebeginsstiffening.Thisisthebeginningofthehardeningstage(figure4-7).

Initialsetoccurssoonafterthemixturebeginsstiffening.Afterinitialset,constructionworkersshouldnotwork,vibrate,orfinishtheconcrete(anysegregationofmaterialsduringthisstagewillbepermanent).Workersshouldapplycuringcom-poundassoonaspossibleafterfinishingtocontrolevaporationfromtheconcretesurface.

Duringthehardeningstage,newcompounds(productsofhydration)continuegrowingandheatcontinuestobegenerated.Someofthecompoundsarefinger-like,fibrousgrowths;othersaremorecrystalline(figure4-8).Thesecompoundsinter-weaveandmeshtogetheraroundtheaggregates,causingtheconcretetobecomestifferandeven-tuallybecomeasolid.Atfinalset,theconcretehasbecomestrongenoughtowalkon.Still,theconcretecannotcarrytraffic.

CoolingDuetochangesintemperatureandmoisturecontent,theconcreteshrinks.Frictionbetweentheshrinkingconcreteandthebaselayerunderthepavementcausesstressthatwilleventuallycausetheconcretetoseparateorcrack.Torelievethestressandpreventconcretefromcrackingrandomly,workersmustsawjoints.(Jointssimplycontrolthecracklocations.)

Thereisonlyabriefperiodoftime,perhapstwotofourhours,tosawjointssuccessfully.Thisperiodiscalledthesawingwindow.Thesawingwindowbeginswhentheconcretecanbesawedwithoutexcessiveraveling,andendsbeforetheconcretebeginstocrackrandomly.

Approximatelyfourtosixhoursafterinitialset,hydrationslowsandtheamountofheatgeneratedbeginstodrop(figure4-9).

Thisslowdowniscausedbythebuildupofhydra-tionproducts,whichhavebeguntointerferewithcontactbetweentheremainingwaterandcementintheconcrete(figure4-10).

Figure 4-7. Significant heat is generated during the hardening stage.

Figure 4-8. Hydration products grow.

Figure 4-9. Heat energy peaks and then drops during cooling.

Figure 4-10. Hydration products grow during cooling.

Products of hydration

Hardening

Time

Hea

t

Cooling

Time

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DensificationDuringthefinalstageofhydration,thereactionscontinueslowly,generatinglittleheat(figure4-11).

Continuedgrowthandmeshingofhydrationprod-uctsresultsinastrong,solidmass(figure4-12).Longaftertheconcreteisstrongenoughtocarrytraffic,thisprocesswillcontinue—aslongascementandwaterarepresentintheconcrete—increasingtheslab’sstrengthandreducingitspermeability.

Constructioncrewscandotwofinalthingstohelpensurelong-termpavementperformance:

Aftersawingjoints,insulatethepavementifairtemperaturesareexpectedtodroprapidlyandsignificantly.(Ifthepavementsurfacecoolstooquickly,theslabmaycrack.)

Keeptrafficandconstructionequipmentoffthepavementaslongaspossible(preferably,atleast72hours)toprotectthecuringcompound.Curingcompoundreducesmoisturelossfromtheconcreteandthusenablescontinuedhydration,increasingconcretestrengthandreducingitspermeability.

Figure 4-11. Very little heat is generated during the final, densification stage.

Figure 4-12. Hydration products mesh into a dense solid.Figure 4-8. Calcium silicate hydrate (C-S-H) and calcium hydroxide (CH) mesh into a solid mass.

Densification

Time

Hea

t

SummaryAsyoucansee,hydrationistheunderlyingand

unifyingprocessthatmustbeunderstoodandman-

agedormitigatedthroughoutaconcretepavement

project,frommixdesignandmaterialsselectionto

construction.

Thepreviousoverviewcoversonlythemostbasic

informationregardingthestagesofcementhydration.

Asomewhatmoredetaileddescriptionisgivenin

thenextsection,whichincludesStagesofHydration

chartsbeginningonpage76(andreproducedinthe

enclosedposter).

Youcanfindmoreinformationaboutseveral

relatedtopicsthroughoutthismanual:

EffectsofSCMsonconcretemixtures,see

SupplementaryCementitiousMaterialsin

chapter3,page31.

Effectsoffinecementsonconcretemixtures,

seeCementFinenessAffectsHydrationand

ConcreteProperties,page87.

Effectsofadmixturesonconcretemixtures,see

ChemicalAdmixturesinchapter3,page55.

Possibleproblemswithearlyorlatestiffening

duringhydration,seePotentialMaterials

Incompatibilitieslaterinthischapter,

page97;ChemicalAdmixturesinchapter3,

page55;FactorsAffectingSettinginchapter

5,page114;andStiffeningandSettingin

chapter6,page186.

Hydrationconsiderationsduringconcrete

pavementconstruction,seeallofchapter8,

beginningonpage203.

Maximizingthewater-cementitiousmaterials

ratioforoptimumhydration,seeStep2:Water-

CementitiousMaterialsRatioinchapter6,

page179.

Cementhydrationandconcretestrength,

seeStrengthandStrengthGaininchapter5,

page116;andStrengthinchapter6,page187.

Cementhydrationandconcretecracking,see

ConcreteStrengthGain,TensileStress,and

theSawingWindowlaterinthischapter,page

93;ShrinkageandTemperatureEffectsin

chapter5,pages125and127,respectively;and

Early-AgeCrackinginchapter5,page148.

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Optimizinghydrationforreducedpermeability,seeFactorsAffectingPermeabilityinchapter5,page131;andAdjustingPropertiesinchapter6,page185.

•

Figure 4-13. Concrete characteristics, and implications for workers, during stages of hydration

Conventional sawing window

Earl

y sa

win

g w

indo

w

Mixing Dormancy Hardening Cooling Densification

Hea

t

Time

Lasts about 15 minutes

Lasts about 2–4 hours

Lasts about 2–4 hours

Continues for years

Mixture is plastic, workable, and not generating significant heat.

Transport, place, and finish the concrete while plastic and workable.

High heat is generated immediately, followed by rapid cooling.

Ensure adequate mixing.

• Hydration generates significant heat. • Mixture sets, begins to harden, and gains strength.

• Stress begins developing in the concrete.

• Begin curing as soon as possible.

• Thoroughly apply curing compound.

Stress development will exceed strength development if stress is not relieved.

Saw joints to relieve stress and control cracking.

Slab continues to become stronger and less permeable.

• Insulate the slab if the air temperature is expected to cool rapidly and significantly.

• Protect curing compound as long as possible.

What engineers, supervisors, and workers must do to ensure durable pavement

Characteristics of concrete mixture

Stage 1 Stage 2 Stage 3 Stage 4 Stage 5

Initial set

Final set

CementWater

Aggregate

Check for early sawing

Check for conventional sawing

− − − − − Hydration acceleration − −

− − − − − − − − − − − Strength/Stress development − − −>

Hydration deceleration − − − − − − > >

Figure4-13brieflydescribesthecharacteristicsofconcreteandtheimplicationsforworkersateachstageofhydration.

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Stages of Hydration: Details

Primary Hydration ProductsTheprimaryproductsofhydrationare

Calciumsilicatehydrate(C-S-H).Calciumhydroxide(CH).Ettringite(C-A-S

–-H).

Monosulfate(C-A-S–

-H).Thechartsonthefollowingpagesfocusonthe

formationofC-S-HandCH.Thesecompoundsarecentraltoconcretestrengthanddurability.

••••

Figure 4-14. Compounds in cement

Alites(C3S)

Gypsum (sulfate)(CS)

Ferrite(C4AF)

Aluminate(C3A)

Belites(C2S)

_

TheStagesofHydrationcharts(pages76through83)providemoredetailsabouthydrationandthetransformationofconcretefromaplasticmixturetoasolidslab.

Thechartsillustrateandexplainthefollowing:

Specificchemicalreactionsoccurringbetweencementandwateratvariousstagesofhydration.

Howthesereactions

Areobservedinchangesinheat.

Resultinphysicalchangesinthemixture.

Influenceconstructionpractice.

Howsupplementarycementitiousmaterialschangethesystem.

Howchemicaladmixtureschangethesystem.

Howincompatibilitiesmayoccur.

Issuesrelatedtocracking.

Issuesrelatedtotheair-voidsystem.

Thesetopicsarecoveredingreaterdepthlaterinthesecondhalfofthischapter,beginningonpage84,whichfocusesmoreonthecompoundsinvolvedincementhydrationthanonthestages.

Primary Compounds in Unhydrated Cement

Thethreeingredientsincement—silicates,alu-minates,andsulfates—consistofseveralprimarycompounds:

Silicates.Alite(C

3S).

Belite(C2S).

Aluminates.Tricalciumaluminate(C

3A).

Ferrite(C4AF).

Sulfates(CS–

).Gypsum(dihydrate).Plaster(hemihydrate).Anhydrite.

TheStagesofHydrationchartsonthefollowingpagesfocusonalites(C

3S),belites(C

2S),thealumi-

nateC3A,andsulfates(figure4-14).Thesearethe

compoundsincementthathavethemostsignificantinfluenceontheformationofconcrete.

•

•

◦◦◦

•

•

•

•

•

•◦◦

•◦◦

•◦◦◦

Full-Size Hydration Poster

If a full-size Stages of Hydration poster is not included with this manual and you would like one, contact the National Concrete Pavement Technology Center. Contact information is inside the front cover on the Technical Report Documentation page.

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Withinminutesofmixingcementandwater,thealuminatesstarttodissolveandreact,withthefollowingresults:

Aluminate*reactswithwaterandsulfate,formingagel-likematerial(C-A-S

–-H).Thisreaction

releasesheat.

TheC-A-S–-Hgelbuildsup

aroundthegrains,limitingwater’saccesstothegrainsandthuscontrollingtherateofaluminatereaction.Thisoccursafteraninitialpeakofrapidhydrationandheatgeneration.

•

•

Forabouttwotofourhoursaftermixing,thereisadormantperiod,duringwhichthefollowingeventsoccur:

TheC-A-S–-Hgeliscontrolling

aluminate*reactions.Littleheatisgenerated,andlittlephysicalchangeoccursintheconcrete.Theconcreteisplastic.Duringdormancy,assilicates(alite[C

3S]andbelite[C

2S])

slowlydissolve,calciumionsandhydroxyl(OH)ionsaccumulateinsolution.

•

•

Stages of HydrationBasics

Silicates Alite (C3S) Belite (C2S)

Aluminates* Tricalcium aluminate (C3A) Ferrite (C4AF)

Sulfates (CS) Gypsum (dihydrate) Plaster (hemihydrate) Anhydrite

Calcium silicate hydrate (C-S-H)

Calcium hydroxide (CH)

Ettringite (C-A-S-H)

Monosulfate (C-A-S-H)

* In the Stages of Hydration chart, “aluminate” refers generically to tricalcium aluminate (C3A). Ferrite (C4AF) hydration does not contribute significantly to concrete properties.

–

–

–

Compounds key

Gel-like substance

Ions

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Calcium silicate hydrate(C-S-H)


Calcium silicate hydrate(C-S-H) Calcium hydroxide

(CH)

Afterfinalset,therateofalite(C3S)

reactionsbeginstoslow,andtheamountofheatgeneratedpeaksandbeginstodrop.ThisoccursbecausethebuildupofC-S-HandCHinterfereswithcontactbetweenremainingwaterandundissolvedcementgrains.

Duringthisstage,severalthingsareoccurring:

Theconcreteisgainingstrength,astheamountofC-S-H(andCH)increases.However,theconcreteisstillsomewhatporousandshouldcarryonlylightconstructiontraffic.Tensilestressesmaybebuildingfasterthantensilestrength.Atsomepoint,stresswillexceedstrength,causingtheconcretetocrack.Unlessjointsaresawedtocontrolcracklocation,randomcrackingwilloccur.Sometimeafterthetemperaturepeaks,sulfate,whichhascontinuedreactingwithaluminate*(seestages1and2onthepreviouspage)willbedepleted.Anyremainingaluminate*nowreactswithettringitetoformmonosulfate,whichmaybeassociatedwithabriefincreaseinheat.(Monosulfatedoesnotsignificantlyaffectconcreteproperties.)

•

•

•

Thisstageiscriticalforcontinueddevelopmentofconcretestrengthandreductionofconcretepermeability.(Whenconcretehaslowpermeability,substanceslikewateranddissolvedsaltscannotreadilypenetrateitanditislesssusceptibletofreeze-thawdamage.)Theconcretemustbekeptmoistaslongaspossible.Here’swhy:

Aslongasalite(C3S)remains

andthereiswaterintheconcrete,thealitewillcontinuetohydrate.Asthevolumeofhydrationproductsgrows,concreteporosity(andpermeability)decreases,andtheconcretegainsstrength.Eventually,theproducts—particularlyC-S-H—willcombineintoasolidmass.Belite(C

2S),whichreactsmore

slowlythanalite(C3S),also

producesC-S-H.Afterseveraldays,inthepresenceofwater,mostofthealitehasreactedandtherateofbelitehydrationbeginstobenoticeable.Itisimportanttomaintainsufficientmoisturelongenoughforbelitereactionstooccur.Hydrationproductswillcontinuetodevelop,permeabilitywillcontinuetodecrease,andstrengthwillcontinuetoincreaseslowlyfordays,weeks,evenyears,aslongascementitiousmaterialandwaterarepresent.Thisprocessisaffectedbyfactorslikecementtypeandfineness.

•

•

•

Thisstageisdominatedbyalite(C3S)

hydrationandtheresultingformationofC-S-HandCHcrystals:

Whenthesolutionbecomessuper-saturatedwithcalciumions(fromdissolvingalite[C

3S]primarily),

fiber-likeC-S-HandcrystallineCHstarttoform.Thisgeneratesheat.MeshingofC-S-Hwithothersolidscausesthemixturetostiffenandset.Theincreasingheatandstiffeningofthecementpastemarkthebeginningofhydrationacceleration,whichlastsseveralhours.Initialsetoccursearlyinthisstage.Accelerationischaracterizedbyarapidrateofhydration,significantheat,continuedhardening,andstrengthdevelopment.

Theratesofreactionarefasterforfinercementitiousmaterialsandforsystemswithhigheralkalicontents.Slowerreactingsystemswillreactlongerandwillgenerallyprovideabettermicrostructureinthelongrun.Duringacceleration,aluminate*andsulfatecontinuetoreact,and

needle-likeettringite(C-A-S–-H)

crystalsform.Finalset—aboutwhentheconcreteishardenoughtowalkon—occursbeforeheatenergypeaks(beforealite[C

3S]reactionsbegintoslow).

Afterfinalset,tensilestressesstarttodevelopduetotemperatureanddryingeffects,themixture’sincreas-ingstiffness,andtheslab’sfrictionwiththepavementbase.

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Likeportlandcement,duringdormancythesilicatesinSCMsareslowlydissolvingandreleasingcalciumionsandhydroxyl(OH)ions.

ThemagnitudeoftheheatpeakisoftenreducedinsystemscontainingSCMsduetoslowerhydrationrate.Thisgenerallyresultsinlessshrinkagelaterand,thus,potentiallylessstress.AsecondpeakmaybeobservedastheSCMshydrate.

Generallyasaresultofreducingthehydrationrateandheat,SCMsinfluencethedurationandtimingofthesaw-cuttingwindow.Theinfluencedependsonthesystemchemistryandtheenvironment.IfSCMsarebeingusedforthefirsttimeorifsourceschange,thencloseattentionisrequiredtopreventrandomcracking.

SilicatesintheSCMsreactwiththeCHfromthecementreactionstoformadditionalC-S-H,thusreducingporosityofthesystemandincreasingstrengthanddurability.Thesereactionsareslowand,whiletheystartinthisstage,theymayonlybenoticeableinstage5.Theywillcontinueforalongtime,andgenerallyleadtohigherlong-termconcretestrengths.

InmixtureswithSCMs,Settingtimemaybedelayed,andworkingtimemaybeextended.Heatandrateofhydrationareoftenreduced,andthedurationofhydrationextended.Earlystrengthsmaybedepressed.Theseeffectsmaybeespeciallysignificantforconstructionincoldweather.

•

•

•

Effects of Supplementary Cementitious Materials (SCMs)SCMs,likeflyashandground,granulatedblast-furnace(GGBF)slag,areincludedinmorethan65percentofconcretemixturesintheUnitedStates.Ingeneral,theyconsistofthesamebasicelements—silicon,aluminum,andcalcium—andperformbasicallythesamefunction

ascement.(Pozzolansrequireasourceofcalcium,usuallyprovidedbyhydratingportlandcement,tohydrate.)SCMsareusedinconcretetotakeadvantageofavailablematerialsandachievedesiredworkability,strengthgain,anddurability.

SeeSupplementaryCementitiousMaterialsinchapter3,page31.SeeReactionsofSupplementaryCementitiousMaterialslaterinthischapter,page94.

Stages of Hydration

Inthelongterm,silicatesinSCMschemicallycombinewithCHfromcementhydrationtoformadditionalC-S-H.

Strengthdevelopmentmaybeslowerinitiallybutcontinueslonger,normallyleadingtohigherlong-termstrength.Permeabilityisoftensignificantlyreduced,thusimprovingpotentialdurability.

SystemscontainingGGBFslagandflyasharereportedlymorepronetofrostdamage.Theextentof,andmechanismsbehind,thisperceptionarestillunderinvestigation.

Low-calciumflyashandGGBFslagareeffectiveinreducingalkalireactivityofmixturesforthreereasons.TheseSCMs

Reducethemixture’salkalicontent.Reduceconcretepermeability.Reducethesystem’scalcium/silicaratio.

High-calciumflyashesmayhavethesameeffect,andshouldbetestedusingthematerialsunderconsideration.

•

•

•









–

–

–

Compounds key

IfSCMscontainlargeamountsofcalcium(forinstance,ClassCflyash),thecalciummaybeintheformofaluminate,*whichwillincreasetheriskofflashsetifthereisinsufficientsulfateinsolution,asdiscussedintheIncompatibilitieschart(page81).

Flyasheswithhighloss-on-ignition(LOI)(masslosswhenheatedto1,000°C[1,830°F])willlikelyinterferewithdevelopmentoftheair-voidsystem.ThatisbecausehighLOIflyashescontainunburnedcarbonthatabsorbsair-entrainingadmixture.Higherandmorevariabledosagesofair-entrainingagentsmayberequired.

MixtureswithSCMsmayrequirelesswatertoachieveworkability.

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Retarders.Retardersworkbyformingalayeraroundcementgrains,whichcausesthecementtodissolvemoreslowly.Thisdelaysinitialsetandthestartoftheaccelerationperiod.

Theamountofheatgeneratedmaybeslightlylaterandlower,butheatgenerationmaybeextendedlonger.

Mixturescontainingretarderstendtohaveafiner,lesspermeablemicrostructure,leadingtobetterlong-termstrengthanddurability.

Accelerators.Acceleratorsshortenthedormantperiod,leadingtoearliersetting,andoftenresultinahighertemperaturepeak.Themechanismbehindtheaccelerationisnotfullyunderstood,althoughsilicatehydrationisfaster.

Chloride-basedacceleratorsincreasetheriskofcorrosionofanysteelembeddedintheconcrete.

Effects of Chemical AdmixturesSeeChemicalAdmixturesinchapter3,page55.

Water reducers.Waterreducersworkbydispersingclustersofcementgrainsandreleasingwatertrappedintheclusters,makingmorewateravailableforworkability.Waterreducersmayincreaseinitialworkabilitybutmaynotslowslumplosswithtime.Polycarboxolatewaterreducersmayincreaseairentrainment.

Ingeneral,TypeAwaterreducersincreasetherateofaluminate*hydration(thus,theriskofflashset)andslowtherateofalite(C

3S)hydration

(slowingstrengthgain).High-range(TypeF)waterreducersarelesspronetoincompatibility.

Air entrainers.Air-entrainingadmixturesworkbystabilizingsmallairbubblesinthepaste.Severaldifferentchemicalformsofairentrainersareavailable,allwithdifferingstabilityanddifferingeffectsonbubblesize.Ingeneral,thegreatertheslump,theeasieritisforairtobeentrained.

Retarders.Set-retardingadmixturesmayincreaseaircontent.

Stages of Hydration

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Ifusingdumptrucksoragitatortrucks,mixmaterialsandplacethemixtureintothetransportvehicle.Lookoutforstiffeningduringtransportation(seeearlystiffeningunderIncompatibilitiesonthenextpage).

Ifusingready-mixtrucks,placematerialsintotruckandmixduringtransport.

Inbothcases,ensureadequatemixingisprovided.

Transport,place,finish,andtexturetheconcreteduringthedormantperiod,beforeinitialset,whiletheconcreteiscool,plastic,andworkable.

Useappropriatetransportmethodsandequipmenttopreventsegregation.

Ifvisiblebleedingoccurs,finishthesurfaceafterbleedinghasstoppedandbleedwaterhasevaporated.Donotworkbleedwaterbackintothe

surface,becauseitincreasesporosityandpermeabilityofthesurfacelayer.(Inhotweather,bleedingmaybehelpfulinreducingplasticshrinkagecracking.)

Ifnecessary(e.g.,inhot,windyweather)applya(polymeric)evaporationretarderbeforefinishing.

Keepconcretethoroughlycoveredandprotectedwithcuringcompoundaslongaspossible,atleastforthefirst72hoursaftermixing.

Thelongerthecuringcompoundremainsinplace(thatis,protectedfromequipmentandconstructiontraffic),themoremoisturewillberetainedintheconcreteforhydrationandthusfordevelopmentofstrengthandreductionofpermeability.

Afterstiffeningbegins,donotwork,vibrate,orconsolidatetheconcrete.Segregationoftheingredientsatthispointwillbepermanent.

Thoroughlyapplycuringcompoundtotheconcretesurfaceandedgesassoonaspossibleafterfinishing,toreducetherateofwaterevaporationfromtheconcrete.Protectingtheconcretewithcuringcompoundiscriticalbecauseitresultsinstronger,lesspermeableconcrete.(Whenwaterevaporatesfromtheconcrete,morecementremainsunhydratedandfewerproductsform.Also,waterevaporationleavescapillaryporesbehind,reducingconcrete’sdurabilitybecauseitismoresusceptibletofreeze-thawdamageandattackbyaggressivefluids.)

Duringthisstage,preparejoint-sawingequipment.Beginningatfinalset,startcheckingconcreteforreadinessforsawcutting.

Implications of Cement Hydration for Construction PracticesSeechapter8,beginningonpage203.

Stages of Hydration

Topreventrandomcrackingduetobuildupoftensilestresses,sawjointsduringabriefsawingwindow:

Thesawingwindowbeginswhenconcreteisstrongenoughnottoravelwhensawed,andendsbeforetheconcretecracksinfrontofthesaw—typically,nolaterthan24hoursafterplacement.Conventionalsawingwindowsgenerallybeginafterfinalsetbutbeforetheheatpeaks,andendaftertheheatpeaks;howlongafterdependsonthespecificmixture,cement,SCMs,ambienttemperatures,etc.Forearly-agesaws,thewindowmaybeginatorslightlybeforefinalset(stage3)andwillendearlierthanforconventionalsawing.

Covertheslab,especiallyiftemperatureswillcoolsignificantlyduringthefirstnight.Insulationpreventsextremetemp-eratureandmoisturevariationswithintheslabthatcanleadtocurlingandwarpingandrelatedcracking.Insulationalsohelpskeeptheslabmoistandwarm.

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Incompatibilities: Early Stiffening/RetardationTheriskofincompatibilitiesoccurringishigher

Whenusingfinercementitiousmaterials.Atlowwater-cementitiousmaterialsratios.Athightemperatures.

•••

SeePotentialMaterialsIncompatibilitieslaterinthischapter,page97.SeeChemicalAdmixturesinchapter3,page55.

IfcalciumisconsumedbypoorlycontrolledaluminatereactionsearlierinStage1,thensupersaturationofcalciumionswillbeslowedandalitehydrationretarded.Thisretardationcanpotentiallycontinueforseveraldays,severelydelayingorevenpreventingsetting.Itispossibletohaveamixturethatexhibitsfalseset,followedbysevereretardation.

Alite(C3S)hydrationisacceleratedbyhigh

temperatures,highalkalicontents(fromcementitiousmaterials),andhighcementfineness.Thisacceleratessetting,whichcanacceleratethestartof,andshortenthedurationof,thesaw-cuttingwindow.

AlitehydrationisretardedbysomeTypeAwater-reducingadmixturesandlowtemperatures,slowingsettingandthusdelayingthebeginningofthesaw-cuttingwindow.

Ifinsufficientsulfateisinsolutionfortheamountofaluminate*(fromcementandflyash),uncontrolledaluminatehydrationmaycauserapid,permanentstiffen-ingorflashset.Thisischaracterizedbyatemperaturerise.AluminatehydrationisacceleratedbysomeTypeAwater-reducingadmixturesandhightemperature;moresulfatemaybeneededtomaintainanadequateshellaroundthealuminate*particlestocontrolflashset.

Excesssulfateinsolutionresultsingypsumcrystalsbeingdepositedout,prematurelystiffeningthesystem,resultingin(temporary)falseset.Thegypsumeventuallydissolvesasthemixtureismixed,whichiswhyfalsesetistemporary.Thereisnoeffectonotherhydrationprocesses.

Theamountofsulfateinsolutioniscontrolledbytheamount/formofsulfateinthecement.Gypsumdissolvesslowly(increasingtheriskofflashset).Plasterdissolvesrapidly(increasingtheriskoffalseset).Cementmanufacturersnormallybalancethesematerials.

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Becausechemicalreactionsarefasterathighertemperatures,anincreaseininitialmixingtemperaturesignificantlyincreasestheamountofheatgenerated(andcorrespondingstressdevelopment)instage3.

Afterconcretehasset,ittendstodryandcoolmorequicklyatthetopsurface,settingupdifferentialstressesthroughthethicknessoftheslab.Thiswillcausethetopandbottomsurfacestoexpandorshrinkdifferentamounts,resultingincurvatureknownaswarpingandcurling.Dependingonthesupportconditionsandtheextentofthecurvature,stresses(selfweightortrafficloadings)onthecurvedslabmayresultincracking.

Whenconcretesetsathightemperature,increasedstressescandevelopbecausetheconcretecools(andsoshrinks)morethanconcretethatsetsatlowertemperatures.Theincreasedstressesmayincreasethepotentialforrandomcracking.

Implications of Cement Hydration for CrackingMaterialstendtoexpandastheygetwarmerandshrinkwhentheygetcooler.Cementpastetendstomovemorewithsuchvolumechangesthandoesaggregate.Cementpastealsoshrinksasitdries.Objectsthatarerestrainedwhentheymove(shrinkorexpand)willbestressed,leadingtocrackingifthestressesexceedthestrength.(Restraintcomesfromanyconnectionwithadjacentobjects,suchasfrictionwiththesubgrade.)Itisthereforedesirabletoreducepastecontentwithinagivenmix,whilestillachievingworkabilityandfillingallthevoidsbetweenaggregateparticles.

Drying,andconsequentshrinkage,anytimebeforefinalsetmayresultinso-calledplasticshrinkagecracking.

Thevolumeofaggregateissignificantlylargerthanthevolumeofpaste,andtendstocontroltheamountofthermalmovementofconcrete.Ifaggregatewithalowcoefficientofthermalexpansionisused,theriskofproblemswilldecrease.SeeAggregateCoefficientofThermalExpansioninchapter3,page47.Concretewithhighpastecontentandhighfinescontent,duetoimproperaggregategradation,willbeathigherriskofcracking.SeeEarly-AgeCrackinginchapter5,page148,andCrackPredictionwithHIPERPAVinchapter8,page231.

Stages of Hydration

Drying,andconsequentrestrainedshrinkage,beforesufficientstrengthgainmayresultinrandomcracking.

Ifsettingisdelayed,concretemaycrackbecauseitdrieswhiletheconcreteisstilltoosofttostartsawcutting.

Fastersettingmaymeantheconcretewillcrackbeforesawingcanbecompletedbecausethesawingwindowisshorterthanthetimerequiredforsawing.

Cementitioussystemswithhighalkalicontent,aluminate*content,andfinenessmayshrinkmorethanothersystems,thereforeincreasingtheriskofrandomcracking.

ModelingprogramslikeHIPERPAVcanbeusedtopredictthesawingwindowmoreaccuratelyforagivensetofcircumstances,helpingtoreducerandomcracking.









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Theair-voidsystemdevelopsduringmixing.Itishardertoentrainairinsystemsathightemperature,withlowslump,andwithveryfinesupplementarycementitiousmaterialsthathavehighloss-on-ignition(LOI)(masslosswhenheatedto1,000°C[1,830°F])andlowalkalicontents.Additionalair-entrainingadmixturemayberequiredforsuchsystems.Set-retardingadmixturesmayincreaseaircontent.

Thestabilityoftheair-voidsystem(i.e.,theabilitytopreventbubblesfrombreakingduringhandling)dependsonthechemistryoftheair-entrainingadmixture.Someairentrainersaremoresensitivethanotherstothepresenceordosageofotherchemicaladmixturesorsupplementarycementitiousmaterials.(Methodsarebeingdevelopedtotesttheoverallsensitivityofapastesystem.)

Increasedhandling(e.g.,transportation,placing,vibration,finishing)ofunstablesystemsmayreducetheaircontentandaffectthequalityofthein-placeair-voidsystem.Aircontentofconcreteshouldbetestedatthedeliverypointandafterplacingtoassessthestabilityoftheconcretemix.

Implications of Cement Hydration for the Air-Void SystemAgoodair-voidsystem—thatis,auniformdistributionofsmall,stablebubbles—inthefinishedconcreteisnecessaryforconcretedurability.SeeAir-EntrainingAdmixturesinchapter3,page56;FrostResistanceinchapter5,page132;andEffectsofChemicalAdmixturesonpage79.

Theair-voidsystemhasbeenformedatthisstageandisunlikelytochange.

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Hydration Compounds

Portlandcementprimarilyconsistsofclinkergroundwithgypsum.

Clinkerconsistsoffourprimarycompounds:twosilicatesandtwoaluminates.

Silicatescomposeabout75percentofcement.Alite(C

3S)contributestoconcrete’s

initialsetandearlystrength;belite(C

2S)contributestostrengthgainafter

approximatelyoneweek.

Tricalciumaluminate(C3A)hydrates

immediatelywhenexposedtowater;calciumsulfate(generically,gypsum)isaddedtoportlandcementtocontroltricalciumaluminate’shydrationrate.(Theotheraluminate,tetracalciumaluminoferrite[C

4AF],contributeslittletocementother

thanitsgreycolor.)

Thefinenessofportlandcementaffectshydrationratesandconcreteproperties.Finercementsgenerallyhavehigherinitialhydrationrates.

•

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Therestofchapter4describescementchemistryandhydrationinmoredetail,focusingoncompoundsincementandconcrete.First,thespecificcompoundsinportlandcementaredescribed.Then,theroleofeachofthesecompoundsinhydrationisoutlined,followedbytheresultsofhydration:newcompoundsproduced,theporesystem,variousstressesonthesystem,andthedevelopmentofconcretestrengthandlowpermeability.Theeffectsofusingsupplementarycementitiousmaterials(SCMs)inthemixturearediscussedand,finally,potentialincompatibilitiesthatcanarisethroughvariouscombinationsofSCMsandadmixturesinthemixaredescribed.

Portland Cement CompoundsPortlandcementismanufacturedbyheating

limestone,clay,andshaleinakilntoabove1,400°C(2,500°F),thengrindingtheresultingclinkerwithgypsum.Thefinalproductconsistsofsilicates,alumi-nates,andsulfates.Otherelementsandcompoundsincementareinsmallerproportions,butcancontributeto,orinterferewith,thecomplexchemicalreactionsthatoccurduringcementhydration.Theprimarycompoundsinportlandcementarelistedintable4-1andshowninfigure4-15.

Chemicalanalysesofcementitiousmaterialsbyx-rayfluorescencereporttheelementsasoxides.(Thisisforconvenienceonly,astheseelementsarenotalllikelytobepresentaspureoxides.)Thesamecon-ventionisfollowedintheremainingsectionsofthischapter.Anexampleofamillcertificateisshowninfigure4-16.Amillcertificateincludesspecifiedlimits,andtestresults,forvariouscompoundsinaspecifictypeofportlandcement.

Thenextsectionsdiscussportlandcement’spri-marycomponents:clinkerandgypsum.

Portland Cement Clinker Clinkeristhemajoringredientinportlandcement.

Clinkerisprimarilycomposedoftwosilicatesandtwoaluminates,thefirstfourcompoundslistedintable4-1.

Calcium silicatesGypsum particles

Calciumaluminates

Clinker particle

Figure 4-15. Composition of portland cement

Key Points

Hydration Compounds


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Table 4-1. Major Compounds in Portland Cement

Silicates.Silicanormallycomposesabout20per-centofcementbymass,whilecalciumoxidenormallycontributes60to65percent.Thesecombinetoformthesilicatesinclinker:alite(C

3S)andbelite(C

2S).

Portlandcementscurrentlycontainapproximately55percentaliteand20percentbelite(Johansenetal.2005).

Duringcementhydration,alitecontributestothesettingandearlystrengthdevelopmentofconcrete,normallybeginningafewhoursaftermixing.Beliteistheprimarycompoundthatcontributestoconcrete’slaterstrengthdevelopment.Itseffectsbecomenotice-ableaboutaweekaftermixing.

Aluminates.Aluminaisincludedinthemixtureinacementkilnbecauseithelpsreducetheburn-ingtemperaturesrequiredtomakecement.Aluminacombineswithcalciumandironoxidetoformtwocalciumaluminatecompoundsinclinker:tricalciumaluminate(C

3A)andtetracalciumaluminoferrite

(C4AF)orferrite.Typicalportlandcementscontain

approximately10percenttricalciumaluminateand10percentferrite.

Duringcementhydration,tricalciumaluminate(C

3A)reactsveryrapidlywhenmixedwithwater

unlesscontrolledbythepresenceofsulfate.(Uncon-trolledhydrationoftricalciumaluminatecanleadtoflashset.)(SeeAluminateandSulfatereactionslaterinthischapter,page88.)Ferritereactionsdonotcontributesignificantlytothepropertiesofconcreteexceptforthegraycolor.

GypsumWhenclinkerisbeinggroundtoapowder,gypsum

(CS–

H2)isaddedataboutafivepercentdosage.The

primarypurposeforincludinggypsuminportlandcementistoprovidesulfate,whichcontrolsthetri-calciumaluminate(C

3A)reactionsduringhydration.

Thesulfatedosageiscarefullyoptimizedbecausethestrengthofacementisinfluencedbytheamountofsulfate;incompatibility(includinguncontrolledstiffen-ingandsetting)canoccuriftheamountsofsulfateandtricalciumaluminateareoutofbalance(seePotentialMaterialsIncompatibilitiesinthischapter,page97).

Gypsumisoneofthreeformsofcalciumsulfatenormallypresentinportlandcement.Thethreeforms

Hydration Compounds


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Figure 4-16. Sample mill certificate (ASTM C 150-04)

Hydration Compounds


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aredeterminedbytheamountofwatertiedtothesulfatecompounds,asshownintable4-2.(SeeFormofSulfatelaterinthischapter,page98.)

Note:Donotconfusethesulfateinportlandcementwithsulfatesthatmayenterconcretefromgroundwateraftertheconcretehasset.Theseout-

Table 4-2. Forms of Calcium Sulfate

sidesulfateswillreactwiththehydrationproductsoftricalciumaluminate(C

3A),formingexpansive

compoundsthatdamagetheconcrete.Thisisknownassulfateattackandiswhylimitsontricalciumalumi-natecontentareimposedonsulfate-resistantcement(seeHydraulicCementinchapter3,page29).

Cement Fineness Affects Hydration and Concrete Properties

When clinker leaves the kiln and is cooled, it is pulverized in a grinding mill to reduce its size from 25- to 50-mm (1- to 2-in.) particles to a powder. The fineness of the cement is controlled by the manufacturer to achieve some performance characteristics. Approximately 95 percent of cement particles are smaller than 45 micrometers, with the average particle around 15 micrometers.

With increasing fineness, cement exhibits the following characteristics:

Increasing rate of hydration initially, leading to increased early strengths. Longer-term strength development is not as marked with finer cements.

•

Increased heat of hydration.

Reduced workability.

Reduced bleeding.

Possible reduced air entrainment.

Increased risk of incompatibility.

Fineness is usually measured by the Blaine air-permeability test (ASTM C 204 / AASHTO T 153), which indirectly measures the surface area of cement particles per unit mass. The surface area of finer cements is higher than that of coarser materials. Typical Type 1 cements are in the range of 300 to 400 m2/kg. Type 3 cements are generally finer than Type 1 cements.

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Portland Cement Hydration

Key Points

Hydrationofportlandcementiscomplex.

Chemicalreactionsduringhydrationreleaseheat.

Tricalciumaluminate(C3A)reacts

immediatelywhenexposedtowaterandcancauseearlystiffeningorflashset.Thisreactioniscontrolledbysulfatesinsolution.

Silicatereactionsbeginmoreslowly,butultimatelydominatehydration,contributingthebulkofconcreteproperties.Alite(C

3S)

contributestoearlystrengthgain.Belite(C

2S)contributestolong-termstrengthgain

andlowpermeability.

Calciumsilicatehydrate(C-S-H)istheprimaryproductofsilicatereactionsthatcontributestoconcrete’sstrengthanddensity.

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Hydrationisaseriesofnonreversiblechemicalreactionsbetweenhydrauliccement,suchasportlandcement,andwater.Duringhydration,thecement-waterpastesetsandhardens.Hydrationbeginsassoonascementcomesincontactwithwater.Thecementparticlespartiallydissolve,andthevariouscomponentsstarttoreactatvariousrates,generatingheat(aprocessthatmayraisetemperature)andresult-inginvariousreactionproducts.

Theheatgeneratedbyhydrationdoesnotreachzeroforalongtime,indicatingthatreactionscontinueslowly.Thereactionscancontinueforyears,aslongastheconcretecontainswaterandunreactedcement,resultingincontinueddevelopmentofstrengthandotherdesirablecharacteristicslikelowpermeability.

Thehydrationprocessiscomplexandisthesubjectofextensiveresearch.However,abasicunderstandingoftheprimaryreactionsandhydrationproductscanhelpeveryoneinvolvedinconcretepavementprojectspreventorcorrectproblems.

Chemicalreactionsatvariousstagesofhydrationhaveimportantimplicationsforpavementconstructionpractices,includingplacement,curing,sawingjoints,andkeepingtrafficoffnewconcrete.

Ideally,forconcretewithlowpermeability,hydrationproductswilleventuallyfillmostofthespaceoriginallyoccupiedbywaterinthemix.Thisrequiresanappropriatewater-cementitiousmaterialsratio:enoughwatertomakethemixworkablebutnotsomuchthatconcretedurabilityisreduced.

Hydrationwillcontinueforalongtime,aslongaswaterandunhydratedcementgrainsareavailable.Whilehydrationcontinues,concretestrengthincreasesandpermeabilitydecreases.Curing—primarily,protectingtheconcretefrommoistureloss—isthereforeessentialforstrong,durableconcrete.

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Therestofthissectiononhydrationprovidesgeneral,simplifieddescriptionsofthefollowing:

Theprimarychemicalreactionsofaluminates,sulfates,andsilicates.Theproductsofthosereactions.Thedevelopmentofstrengthandstresseswithinconcreteduringhydration.Theimportanceofthedevelopmentofpastedensityduringhydration.Theimplicationsofvariousstagesofhydra-tiononconstructionpracticestoensurestrong,durableconcretepavements.

Aluminate and Sulfate ReactionsThealuminateandsulfatereactionsdominatethe

first15minutesofhydration.Whenmixedinwater,tricalciumaluminate(C

3A)immediatelydissolves

andreactswithwaterandcalciumhydroxidetoformcalciumaluminatehydratecrystals.Thisreactiongeneratesalargeamountofheatand,ifuncontrolled

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bysulfate,willcausefast,permanenthardening.Thiseffect—knownasflashset—isveryundesirable.

Sulfate,however,controlsthetricalciumaluminate(C

3A)reaction.Combinedwithsulfateinsolution

(CS–

),tricalciumaluminateformscomplexcompoundsthateventuallycrystallizeoutasneedle-likeettringite(C-A-S

–-H)(figure4-17).

Duringthefirstfewminutes,theinitialhydrationproduct(agel-likematerial,whichcanalsoberoughlyannotatedasC-A-S

–-H)surroundsthetricalcium

aluminate,limitingwater’saccesstothetricalciumaluminateandtherebyslowingreactions(figure4-18).

Theperiodoftimeduringwhichreactionsareslowed,littleheatisgenerated,andlittlephysicalchangeintheconcreteisobservedisknownasthedormantperiod.Thedormantperiodlaststwotofourhours.Thisisimportantinaconcretepavingprojectbecauseitprovidestimetotransport,place,andfinishtheconcretemixturewhileitiscoolandplastic.

Theimportanceofthedormantperiodmakesitcriticalforsufficientsulfatetobeincludedintheport-landcement(seePotentialMaterialsIncompatibilitieslaterinthischapter,page97).

Afterthedormantperiod,otherchemicalreactionsdominatecementhydration.However,thetricalciumaluminate(C

3A)andsulfate(CS

–)continuetoreact

andformettringite(C-A-S–

-H),whichcontributessomewhattoconcrete’searlystrengthdevelopment.

Thetricalciumaluminate(C3A)andsulfate(CS

–)

reactionscontinueuntilallthesulfateisconsumed,generallywithin24hours.Then,anyremainingtrical-ciumaluminatewillreactwiththeettringitetoformmonosulfate(figure4-19).Thisreactioncontinuesaslongascalciumaluminate,ettringite,andwaterareavailable.Ithaslittleeffectonthephysicalcharacteris-ticsofconcrete.

Silicate (Alite and Belite) ReactionsThesilicatesconsistofalites(C

3S)andbelites(C

2S).

Alitereactsmorequickly,givingconcreteitsearlystrength.Belitereactsmoreslowly,contributingtolong-termstrengthgainandreductionofpermeabil-ity.Thehydrationproductsofsilicatesandwaterarelooselyknownascalciumsilicatehydrate(C-S-H)andcalciumhydroxide(CH),orlime.

Figure 4-18. Gel-like C-A-S–

-H limits water’s access to cement particles.

________________________________________________Figure 4-19. After sulfate is consumed, remaining C3A reacts with ettringite.

Gel-like substance

________________________________________________ Figure 4-17. Reactions of C3A and CS

–, in solution, are re-

sponsible for an early heat spike during cement hydration.



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Alite ReactionsAlite(C

3S)dissolvesandreactswithwatermore

slowlythantricalciumaluminate(C3A).During

dormancy,alitebeginsdissolving,resultingincal-ciumions(andhydroxyl[OH]ions)insolution(figure4-20).(Somecalciumionsinsolutionalsoresultfromthedissolutionoftricalciumaluminate,butaliteistheprimarysource.)

Thecalciumionsaccumulatefortwotofourhours,untilthesolutionbecomessupersaturated.Whenthesolutionissupersaturated,thecalciumionsreactwiththesilicaandwatertoformcalciumsilicatehydrate(C-S-H)andcalciumhydroxide(CH)(figures4-21and4-22).

Soonafterthebeginningofproductionofcalciumsilicatehydrateandcalciumhydroxide,initialsetoccurs.Alargeamountofheatenergyisreleased,causingthetemperaturetobeginrisingquickly,andthemixturebeginsstiffening.Ashydrationaccelerates,thebuildupofcalciumsilicatehydrateandcalciumhydroxideresultsinprogressivestiffening,hardening,andstrengthdevelopment.

Calciumsilicatehydrate(C-S-H)isthesomewhatmoredesirableproduct.Theseparticlesareintheformoffibrousgrowthsthatgraduallyspread,merge,andadheretoaggregates,givingconcreteitsstrengthandreducingitspermeability.Withsufficienthydra-tion,C-S-Hformsasolidmass.

Calciumhydroxide(CH)particlestendtobecrys-talline,hexagonal,andsmooth,andmayprovideaplaneofweaknesswhentheconcreteisstressed.Also,calciumhydroxideisreadilysolubleinwaterandmaybeattackediftheconcreteisexposedtosoftwateroracid.However,calciumhydroxideisnecessaryformaintainingahighpHandforstabilizingthecalciumsilicatehydrate(C-S-H).

Initially,thealitereactionsarerapid.Afterafewhours,however,thehydrationproducts(C-S-HandCH)accumulatetoapointatwhichtheyinterferewithcontactbetweentheundissolvedcementparticlesandwater,slowingthereactionsandthusreducingtheheatofhydration(figure4-23).

Sometimebeforetherateofheatevolutionpeaks,finalsetoccurs.Finalsetisroughlyassociatedwiththetimewhentheconcretehasbecomehardenough

Figure 4-20. Dissolving cement results in calcium ions in solution.

_______________________________________________Figure 4-21. Alite reactions form C-S-H and CH.

Figure 4-22. Calcium silicate hydrate (C-S-H) and calcium hydroxide (CH) begin to form.

Calcium silicate hydrate(C-S-H)


Ions



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towalkonortodemoldandtestacylinder.(FinalsetisdefinedbyASTMpasteandmortarpressuretestsasapointwhenthepastehasacquiredacertaindegreeofhardness;however,thevaluesselectedaresome-whatarbitraryanddonotnecessarilyrelatedirectlytoaphysicalphenomenonintheconcrete.)

Alitereactionswillcontinueslowlyaslongasunhy-dratedcementandwaterarepresentandaccessible.

Belite ReactionsTheothersilicatecompound,belite(C

2S),mimics

thereactionsofalite(C3S)butataslowerpace.When

mixedwithwater,belitedissolvesandreleasescal-ciumionsveryslowly.Onlyafterseveraldaysdobelitereactions(producingcalciumsilicatehydrate[C-S-H]andcalciumhydroxide[CH]crystals)startcontributingtostrength,butthereactionscontinueforalongtime(figure4-24).

Belitereactionsarecriticaltothelong-termdevel-opmentofstrengthandreductionofpermeability.

Aslongasalitesandbelitesremainandthereiswaterintheconcrete,thesilicateswillcontinuetohydrate.Asthevolumeofhydrationproductsgrows,theconcreteporosity(andpermeability)decreasesandtheconcretegainsstrength.Eventually,thehydrationproductswillcombineintoasolidmass(figure4-25).

Primary Products of HydrationTheprimaryproductsofcementhydrationand

theirroleinconcretearedescribedintable4-3.Thechemicalreactionsincementhydrationresultinchangingvolumesofcementcompoundsandhydra-tionproducts(figure4-26).

Factors Affecting Hydration RatesTherateofstiffeningofconcretefromplastic

tosolid,andthesubsequentincreaseinstrength,dependsontheratesofthechemicalreactionsinvolvedincementhydration.Theseinturnareinflu-encedbythefollowing:

Compositionofthecement(e.g.,increasingalkalicontentsacceleratehydration).Cementfineness(finercementshydratefaster).Mixproportions(lowerwater-cementitiousmaterialsratiosacceleratesetting).

•

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Figure 4-23. Hydration products accumulate and mesh.

_______________________________________________Figure 4-24. Belite reactions begin contributing to strength gain later.

Figure 4-25. Calcium silicate hydrate (C-S-H) and calcium hydroxide (CH) mesh into a solid mass.

Calcium silicate hydrate(C-S-H) Calcium hydroxide

(CH)



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Table 4-3. Primary Products of Cement Hydration

Temperature(highertemperaturesacceleratehydration).Admixtures(waterreducersretardsomereactionsandaccelerateothers;seePotentialMaterialsIncompatibilitieslaterinthischapter,page97).

PoresIngeneral,waterthatdoesnotreactwiththe

cementitiousmaterialswilleitherremainintheconcreteorevaporateoutwhentheconcretedries.

•

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Thespaceoccupiedbyunreactedwateristhecapillaryporesystem.Thevolumeofcapillaryporesstronglyinfluencesconcretestrengthanddurabilitybecausetheporesprovideazoneofweaknessforcrackgrowthandarouteforaggressivesolutions.

Ideally,mostspaceoccupiedbywaterinthemixwillbefilledwithhydrationproducts.Thisrequiressufficientcement(lowwater-cementitiousmaterialsratio)andprotectionfrommoisturelossuntilsuffi-cienthydrationhasoccurred(goodcuringpractices).



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____________________________________________________________________________________________________Figure 4-26. Estimates of the relative volumes of cement compounds and products of hydration with increasing hydration (adapted from Tennis and Jennings 2000).Note: These estimates are for a 0.50 water-cementitious materials ratio; decreasing the ratio will decrease the capillary porosity.

Concrete Strength Gain, Tensile Stress, and the Sawing Window

Duringcementhydration,asthehydrationprod-uctscalciumsilicatehydrate(C-S-H)andcalciumhydroxide(CH)accumulate,theconcretedevelopsitsstrength.Compressivestrengthistheabilitytoresistforcesthatpushtheconcretetogether(compression);tensilestrengthistheabilitytoresistforcesthatpullitapart(tension).

Concretedevelopssignificantcompressivestrength,whichmakesitanidealmaterialforpavementsthatsupportheavyloads.Concrete’stensilestrength,how-ever,isonlyaboutone-tenthitscompressivestrength.Withchanginginternalmoistureandtemperature,concreteexperiencesvolumechanges,butthepave-mentbase(amongotherthings)restrainstheconcretemovement.Thisrestrainedmovementsetsuptensilestresses,ortension,intheconcrete.

Sometimeafterfinalset,thegrowingtensilestresseswilllikelyexceedconcrete’stensilestrength,causingtheconcretetocrack.Sawingjointsrelievestensilestressesandpreventsrandomcrackingbyreducingthepanelsizes,thusreducingtheamountofrestraint.Jointsmustbecutduringthecriticalperiodwhentheconcretehashardenedenoughnottoravelalongthesawcut(forconventionalsaws,thisisgenerallysoon

afterfinalset;forearly-agesaws,theperiodmaybeginatorslightlybeforefinalset),butbeforeithasbeguntocrackrandomly.Thatperiodisthesawingwindow.

Hydration and Concrete PermeabilityOneofthecharacteristicsofdurableconcreteis

lowpermeability.Saltsandotherharmfulsubstancescannoteasilypenetratesuchconcrete,anditislesssusceptibletofreeze-thawdamage.

Toensurethatnewconcretepavementsdeveloplowpermeability,itisimportanttoprovidetherightcondi-tionsforhydrationtocontinuesufficiently.Providingtherightconditionsincludesretainingmixwater(i.e.,losingaslittleaspossibletoevaporation)andprotect-ingtheconcretefromextremetemperatures,especiallykeepingtheconcretewarmduringcoolweather.

Reducemoisturelossfromtheconcretebythor-oughlyapplyingcuringcompound(orwetcoverings)afterfinishing(oratthetimeofinitialset,depend-ingontherateofbleeding),andthenprotectingthecuringcompoundfromtrafficforaslongaspossible.Inhotweather,evaporationretardersmayalsobeappliedbetweeninitialplacingandfinishingtofurtherreducemoistureloss.Incoldweather,protectthecon-cretefromextremetemperaturechangesbycoveringitwithinsulatingblankets.

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Hydraulic SCMsHydrauliccementshydrate,set,andhardenwhen

mixedwithwater.Ground,granulatedblast-furnace(GGBF)slagandsomeClassCflyashesareSCMswithhydraulicproperties.

HydrationofsystemscontaininghydraulicSCMsisgenerallyslowerthanportlandcement-onlymixtures.

Pozzolanic SCMsPozzolansincludeClassFflyash,calcinedclay,

calcinedshale,metakaolin,and(rarelyinpavingproj-ects)silicafume.ClassCflyashalsohaspozzolaniccharacteristics.

Pozzolanicmaterialsrequireasourceofcalciumhydroxide(CH)tohydrate.Whenpozzolansareincludedinconcretemixtures,theyhelpconvertcal-ciumhydroxide(CH)(aproductofsilicatereactions)tocalciumsilicatehydrate(C-S-H)(figure4-28).Thus,pozzolanscanhaveapositiveeffectonstrengthgainandconcretepermeability.

Pozzolanicreactionsaresomewhatslowerthancementhydration,sosettingtimesmayberetardedandstrengthdevelopmentisoftenslower.However,slowerhydrationcanreducetheriskofcrackinginsomecases.Inaddition,pozzolanicreactionscontinuelonger,leadingtogreaterlong-termconcretestrength.

Thebasicchemicalcomponentsofsupplementarycementitiousmaterials(SCMs)aresimilartothoseofportlandcement.

Ingeneral,SCMstendtoretardhydrationratesandextendthedurationofhydration.

Pozzolansconvertcalciumhydroxide(CH)tocalciumsilicatehydrate(C-S-H),withapositiveeffectonlater-agestrengthgainandlowpermeability.

TheuseofSCMstocomplementportlandcementshasbecomeincreasinglycommoninconcretemixturesforpavements.

ItisimportanttotestmixturescontainingSCMstoensuretheyareachievingthedesiredresults,verifythecorrectdosage,anddetectanyunintendedeffects.

•

•

•

•

•

________________________________________________Figure 4-27. Ternary diagram illustrating the basic chemical composition of various SCMs compared to portland cement

Supplementarycementitiousmaterials(SCMs)arealmostalwaysincludedintoday’sconcretemixtures.Theyareusedtosubstituteforsomeoftheportlandcementand/ortoaffectconcretepropertiesinspecificways(seeSupplementaryCementitiousMaterialsinchapter3,page31).SCMsarecomposedofgenerallythesameelementsasportlandcement(figure4-27).

MixturescontainingSCMsshouldbetestedtodeterminewhether

TheSCMisachievingthedesiredresult.Thedosageiscorrect(anoverdoseorunder-dosecanbeharmfulormaynotachievethedesiredeffect).Anyunintendedeffectisoccurring(forexam-ple,asignificantdelayinearlystrengthgain).

ItisalsoimportanttorememberthatSCMsmayreactdifferentlywithdifferentcements.

••

•

Key Points

Reactions of Supplementary Cementitious Materials



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Cements for Concrete Pavements: A Durable Tradition

Durability is a hallmark of concrete pavements. In fact, some concrete pavements constructed in the early 1900s are still in service today.

Over the years, cements for concrete have been continuously changed and improved to meet the various needs of the construction and pavement industries. In addition, the construction industry has made use of supplementary cementitious materials that enhance the performance of the concrete.

Changes in Portland Cement In response to the construction industry’s desire to shorten construction times by accelerating concrete strength gain, cement manufacturers have made appropriate changes in portland cement chemistry. Over several decades, cement manufacturers increased cements’ alite (C3S) content and reduced cements’ belite (C2S) content. In the early 1900s the proportions were about the same. Today, the amount of alite is about three times the amount of belite in Type I cement.

As shown in table 4-4, alite reactions begin sooner than belite reactions and are primarily responsible for concrete’s early strength gain. Belite reactions, which are somewhat slower, are primarily responsible for concrete’s long-term strength gain and for continuing reduction in concrete permeability.

Today’s cements are also generally finer than they were in the early 1900s. Smaller particles have greater surface area exposed to water and, therefore, hydrate and gain strength more quickly. Finer particles also result in greater heat of hydration. In warm or hot weather, such fine systems may need to be retarded to achieve the required finished product.

Today’s higher early-strength cements deliver the performance required for most building and commercial uses. They also help the concrete paving industry meet motorists’ demands to open new pavements to traffic more quickly. Concrete pavement strengths after 7 and 14 days are much greater than they were years ago; after 28 days, gains in concrete strength and reductions in permeability can be somewhat smaller.

Supplementary Cementitious MaterialsIn the history of concrete pavements, supplementary cementitious materials (SCMs) are relative newcomers. Primarily byproducts of manufacturing processes, SCMs are generally plentiful and economical. When used properly, SCMs are useful complements to portland cement in concrete for paving applications.

Pozzolanic reactions are beneficial because (among other reasons) they consume calcium hydroxide (CH) (a hydration product of cement) to produce additional calcium silicate hydrate (C-S-H). Calcium hydroxide (CH) contributes relatively little to concrete strength, while calcium silicate hydrate (C-S-H) is the primary contributor to concrete strength and impermeability. Pozzolanic reactions also generally slow hydration initially and reduce the early heat of hydration. Pozzolanic reactions continue for a longer time, however, adding significantly to long-term strength gain and impermeability.

For example, a pavement built in Iowa in 2003 with a ternary mix of cement; ground, granulated blast-furnace slag; and Class C fly ash developed chloride penetration numbers less than 1,000 coulombs in less than a year. (A range of 1,000 to 2,000 coulombs is generally considered low permeability.)

Good PracticesRegardless of the cementitious system used, a sufficiently low water-cementitious materials ratio is critical to achieving the strength and durability needed for concrete pavements.

In addition, solid curing practices—including thoroughly covering the concrete surface with curing compound (or wet coverings) and protecting the compound from equipment and traffic as long as possible—are more important than ever because SCMs are often sensitive to poor curing. Good curing practices help ensure the continuation of cement hydration reactions that add to concrete’s long-term strength and reduce its permeability.



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Table 4-4. Comparison of Alite and Belite Reactions

____________________________________________________________________________________________________Figure 4-28. Effect of pozzolans on cement hydration

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Potential Materials Incompatibilities

Key Points

Somecombinationsofnormallyacceptablematerialsmaybeincompatible.Thatis,theyreactwitheachotherinwaysthatcauseunexpectedchangesinstiffeningorsettingthatultimatelymaycompromisetheconcretesystem.

Incompatibilityisnormallytheresultofacomplexchemicalinteractionbetweenthecementitiousmaterialsandchemicaladmixtures.

Theamountandformofcalciumsulfateareimportantforanappropriatebalancewithtricalciumaluminate(C

3A)toprevent

settingandstiffeningproblems.

Changingthesourceordosageofoneofthereactiveingredientsmaystoptheproblemfromrecurring.

Someincompatibilityproblemscanbeexacerbatedwithincreasingtemperatures.

Testingthemixtureattheexpectedtemperatureisstronglyrecommended.

•

•

•

•

•

•

Somecombinationsofmaterialsmaybepronetoproblemswithsetting,stiffening,orotherissues.Suchproblemscanoccurevenifallmaterialsmeettheirspecificationsandperformwellwhenusedaloneorwithothermaterials.Thisphenomenonisgenerallyknownasincompatibility.

Incompatibilityisimportantbecausesmallchangesinthechemistryofmaterials,orevenintempera-ture,canmakeanacceptablemixtureinonebatchofconcretebehaveinanunacceptablewayinthenextbatch,causingproblemsinplacing,compacting,andfinishingthatareoftenperceivedtobeunpredictableanduncontrollable.

Althoughincompatibilityisnotnew,practitioners

oftensay,“We’veneverseenthisbefore.”Incompatibil-

ityislikelyoccurringbecauseweareusingincreasingly

complexcombinationsofcementitiousmaterials,

chemicaladmixtures,andothermaterialswhileasking

moreoftheconcrete.Thesectionsarethinner,placing

ratesarehigher,turnaroundtimesarefaster,strengths

arehigher,andtheconstructionseasonisstartingearlier

andendinglatersothatconcreteisbeingplacedin

moreextremeweatherconditions.Itisalsobecoming

commonformaterialssourcestobechangedonagiven

projectwithoutrunningtrialmixesormaterialstests.

Thereisnosinglemechanismbehindthewide

rangeofeffectsthatareoccurring.Manyofthemecha-

nismsarecomplexandinteractiveandmayrequire

expertevaluationiftheyoccurinthefield.

Typicalresultsofincompatibilitymayincludeone

ormoreofthefollowing:

Theconcretestiffensmuchtooquickly,pre-

ventingproperconsolidationorfinishingwork.

Theconcretesetsandgainsstrengthbefore

jointscanbecut.

Theconcretedoesnotsetinareasonabletime,

increasingtheriskofplasticshrinkagecracking

andlatesawing.

Theconcretecracksrandomlydespitenormal

effortstopreventit.

Theair-voidsystemisadverselyaffected,

compromisingtheconcrete’sresistancetosalt

scalingandfreeze-thawdamageordecreasing

concretestrength.

Forexample,amixturecontainsClassCflyash,

portlandcement,andchemicaladmixtures,andthe

combinedchemistryofthesystemcausesaccelerated

stiffeningandsetting.Whenoneofthematerials—fly

ash,cement,oradmixture—ischanged,theconcrete

settingbehaviorisnormal.(Actualcaseshavebeen

reportedwherethemixtureissatisfactoryat21°C

[70°F]butcannotbecompactedinthepaverat27°C

[80°F].)

Followingarebriefdiscussionsofsomeofthe

incompatibilityproblemsthatcanoccur.

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Stiffening and SettingIncompatibilityissuesrelatedtostiffeningand

settingaregenerallytheresultofasulfateimbalance,althoughotherfactorscancontribute.

Sulfate-Related Setting and Stiffening Cementhydrationinthefirst15minutesisadeli-

catebalancebetweenthetricalciumaluminate(C3A)

andsulfateinsolution.Ifthebalanceisright,thesulfatecontrolsthehydrationrateoftricalciumalumi-nate.Ifthebalanceisnotright,stiffeningandsettingproblemscanoccur(figure4-29).

_______________________________________________Figure 4-29. Early cement reactions are a balance of trical-cium aluminate (C3A) and sulfate (SO4) in solution. Excess of either will cause unexpected setting. (CTLGroup)

Too Much or Too Little Sulfate.Theamountofsulfateinsolutioninhydratingcementiscritical.Flash Set

Ifthereisinsufficientsulfateinsolution,thetri-calciumaluminatereactsquicklywithwatertoformcalciumaluminatehydrate(CAH)(seetable4-3).Thisreactiongeneratesalargeamountofheat,andthefastbuildupofcalciumaluminatehydrateresultsinflashset:immediateandpermanenthardeningofthemix.False Set

Toomuchsulfateinsolutionmayprecipitateoutassolidgypsum,causingtemporarystiffeningofthemix,orfalseset.Ifmixingcontinues,thegypsumwillredissolveandthestiffeningwilldisappear.

Form of Sulfate.Theformofcalciumsulfate(thatis,gypsum[CS

–H

2],plaster[CS

–H

½],oranhydrite[CS

–])

inthecementisalsocritical,becauseitaffectstheamountofsulfateionsinsolution.Plasterdissolvesfasterthangypsumandisthereforeusefulinprevent-ingflashset;however,cementcontainingtoomuch(ortoolittle)plasterwillresultintoomuchsulfateinsolution,upsettingthebalanceandresultinginfalseset,asdiscussedabove.Thepresenceofanhydritemayalsoupsetthedelicatebalancerequiredbecauseitdissolvesslowlyandmayprovideinsufficientsulfateinsolutiontocontrolcementswithhightricalciumaluminate(C

3A)contents.

Other Stiffening and Setting Incompatibilities Otherfactorsthatcanaffectstiffeningandsetting

incompatibilitiesincludesilicatereactions,supple-mentarycementitiousmaterials,waterreducers,cementfineness,temperature,andwater-cementitiousmaterialsratio.

Silicate Reactions.Mostofthestrengthdevelop-mentinconcreteisduetohydrationofthesilicatesincementitiousmaterialstoformcalciumsilicatehydrate(C-S-H).Ingeneral,thesereactionsstartwhencalciumionsaresupersaturatedinthesolu-tion.Ifcalciumhasbeenconsumedduringtheinitialstagesofhydration(suchasinuncontrolledtricalciumaluminate[C

3A]hydration),thenitispossiblethatthe

silicatereactions(andsosetting)maybesignificantlyretarded.Silicatereactionscanalsobeaffectedbythepresenceofothermaterialsinthemixtureasdiscussedinthefollowingparagraphs.



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Supplementary Cementitious Materials Incompatibili-ties.Ingeneral,supplementarycementitiousmaterials(SCMs)tendtoretardsilicatehydrationrates,par-tiallyduetodilutionandpartiallyduetochangesinthechemicalbalancesofthesystem.Hydrationisnormallyextended,however,leadingtostrengthgainbeginningmoreslowlybutcontinuinglonger.

AnSCMcontainingadditionaltricalciumaluminate(C

3A)(typicallyhigh-calciumflyash)cancompromise

thealuminate-sulfatebalance,causingorexacerbat-ingthestiffeningandsettingproblemsdiscussedpreviously.Itmaythereforebedesirabletousefactory-blendedcementsratherthansite-blendedcementsbecausethemanufacturercanoptimizethesulfateformandcontentforthewholecementitioussystem.

Water Reducer Incompatibilities.Somewater-reducingadmixtureswillinterferewiththehydrationratesofcementcompounds.Lignin-,sugar-,andtriethanol-amine(TEA)-basedproducts(normallyTypeAorB)havethecombinedeffectofacceleratingaluminatereactionsandretardingsilicatereactions.Theuseofsuchanadmixturemaytipamarginallybalancedcementitioussystemintoincompatibility.

Some(TypeA)waterreducersacceleratealuminatereactions.Asystemthathasjustenoughsulfateto

controlnormalaluminatereactions,canthereforebethrownoutofbalance.Aluminatereactionsarethenuncontrolledandworkabilityisreduced.Addingmoreadmixturewiththemixingwatertoboostworkabilitylikelyexacerbatestheproblem,possiblyleadingtoanoverdosewithitsattendantproblems(suchasretarda-tionofthesilicatereactions).

Onesolutionistodelayaddingthewater-reduc-ingadmixtureuntiltheearlyaluminatereactionsareundercontrol.Thelengthofthedelaywillhavetobedeterminedforthespecificmixture.

Inaddition,thesamewater-reducingadmixturesretardthesilicatereactions,delayingsettingandslow-ingstrengthgain(figure4-30).

Itisthereforefeasiblethatasystemcontainingcertainwaterreducerswillexhibitclassicearlystiffen-ingbecauseofuncontrolledaluminatehydration,followedbysevereretardationoffinalsetbecauseofslowedsilicatereactions.Thishasbeenobservedinthelaboratoryandinthefield.

Cement Fineness.Cementfinenessinfluencesreac-tionrates.Thefinerthecement,thegreatertheratesofreactionandthegreatertheriskofanunbalancedsystem.Finercementsrequireahighersulfatecontentandperhapsahigherplaster-to-gypsumratio.

_______________________________________________Figure 4-30. Plot of heat generated by cement hydration of cement pastes containing varying amounts of lignosulfo-nate-based water-reducing admixture (CTLGroup)

Why There are Various Forms of Calcium Sulfate in Cement

Gypsum (CS–

H2) is added to clinker while it is being ground to form cement. During grinding, the temperature is carefully controlled so that some gypsum dehydrates to plaster (CS

–H½) (but not too

much). Trace amounts of sulfate in the form of anhydrite (CS

–) may also be present in the clinker.

The primary difference among these types of calcium sulfate is the amount of water molecules attached to them, which accounts for their different rates of dissolution in water.

The proper balance of calcium sulfate forms is important. Cement manufacturers will normally target a 50/50 balance of plaster and gypsum and will optimize the total sulfate content of their products.



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Temperature.Thesolubilityandreactivityofallofthecompoundsarestronglyinfluencedbytempera-ture,withhighertemperaturesgenerallyincreasingsolubility(exceptcalcium)andacceleratingreactionrates.Increasingtemperature,ontheotherhand,decreasesthesolubilityofcalciumsulfate,thusreducingtheamountofsulfateinsolutionavailabletocontroltheacceleratedaluminatereactionsandtherebypotentiallymakingthesystemunbalanced.Achangeofaslittleas6°C(10°F)cantipamixturefrombeingworkabletoexhibitingearlystiffening.Inwarmerweather,moresulfate(plaster)isneededtocontrolrapidtricalciumaluminate(C

3A)reactions.

Water-Cementitious Materials Ratio.Theseverityoftheseeffectsisalsorelatedtotheratioofwatertocementitiousmaterials(w/cm).Lowerwatercontentseffectivelymeanthatthecementgrainsareclosertogether;therefore,theearlyhydrationproductshavetofilllessspacebeforestiffeningresults,soearlystiffeningmayoccur.Thesamemixtureatahigherwater-cementitiousmaterialsratio(e.g.,0.42),withgreaterparticlespacing,maynotexhibitthesameproblemsasamixturewithaloww/cmratio(e.g.,0.37).Aw/cmratiobelowabout0.40isnotrecom-mendedforslipformpaving,althoughsomeStateshavehadsuccesswithratiosaslowas0.37.

Air-Void System IncompatibilitiesTherehavealsobeenreportsofaccumulationsof

airvoidsformingaroundaggregateparticles,knownasclustering(figure4-31).Thisresultsinreducedstrengthandincreasedpermeability.

Researchhasindicated(Kozikowski2005)thatthisismostlikelytooccurwiththeuseofnon-vinsolair-entrainingadmixturesandwhenwaterisaddedtothemixtureafterinitialmixing.Extendedmixingwillexacerbatetheproblem.

Toavoidthisproblem,checktheair-voidsystemofhardenedconcreteintrialmixespreparedunderfieldconditions,mimickingexpectedhaultimes.

Testing for and Prevention of Incompatibilities

Themostreliablewaytodetectwhetheramixtureislikelytobeproblematicistoconductaseriesof

Figure 4-31. A typical example of air voids clustered around an aggregate particle

testsandtrialbatchesonthematerialsatthefieldtemperature.Laboratorytestsmustberunsometimebeforeconstructionbeginstoprequalifymaterialsplannedfortheproject.Fieldtests(materialsaccep-tancetests)shouldberunasmaterialsaredeliveredtothesitetoensurethatsitematerialsaresimilartoprequalifiedmaterials.Preferably,materials-accep-tancetestsshouldberunthedaybeforebatchingisplanned,althoughattheheightofconstruction,resultsmaybeneededwithinafewhours.

What to Test.Asuggestedtestprotocolissumma-rizedintable4-5(Taylor2005).

Inmanycases.thereisnosinglepass/faillimitbecausewhatisacceptableinonesystemorenviron-mentisnotacceptableinanother.Itisrecommendedthattestresultsbetrackedovertime,andasignificantchangeinatestresultwillindicatepotentialprob-lems.Itisrecommendedthatasmanyofthesetestsaspracticalbeconductedattheprequalificationstagesothatapointofreferenceisavailableforcomparisonwithtestsconductedduringconstruction.Atestresultthatisoutoftheordinarymaybeaproblemwiththetestingorwiththematerial.Suchdatashouldthere-forebereviewedwithanunderstandingoflimitationsandpotentialerrorsintesting.Interpretingtheresultscanbecomplexandmayneedexpertinput.

Thedecisionabouthowmanyoftheseteststoconductwillbebasedontheeconomicsoftheproject,includingthevalueoftheproject,probabilityoffailure(i.e.,isthecementitioussystemstable?),costoftesting,

Air voids


�0�Integrated Materials and Construction Practices for Concrete Pavement

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andcostoffailure.Manyproblemswillbeavoidedbyregularlymonitoringslumploss,unitweight,settime,andadmixturedosages.Significantchangesinanyoftheseparameterswillindicatetheneedformoreinten-siveexaminationofthematerialsathand.

Central Laboratory (Prequalification) Tests.Factorstomonitorgenerallyincludeminislump,temperaturerise,shearstress,rateofstiffening,andearlycracking.

Minislump Theminislumpconetestmonitorstheareaofsmall

slumpconesamplesmadeusingpasteatselectedtimeintervals.Thetestiseffectiveatidentifyingsystemspronetoearlyhydrationproblems(Kantro1981).Thereproducibilitybetweenlabsisreportedlylow.

Isothermal Calorimetry

Theenergyrequiredtomaintainahydratingpastemixtureismonitoredinanisothermalcalorimeter.Changesinthetimingormagnitudeofthetempera-turerise,ortheshapeoftheheat-energy-versus-timeplot,willflagpotentialproblemsinthesilicatereac-tions(Wadso2004).ASTMisdevelopingapracticeforthisapplication.

Shear Stress Increase

Measurementoftheshearstressincreasewithtimeinaparallelplaterheometerusingpasteisshowingpromiseasamethodtomonitorsilicatehydrationprocesses.

Rate of Stiffening

TheultrasonicP-wavetestallowsmeasurementoftherateofstiffeningofamixtureinthelabandthefield.

Table 4-5. Recommended Tests and Their Applications

Early Stiffening

ThemethoddescribedinASTMC359/AASHTOT185isbeingusedinsomelaboratoriestoindicatepotentialproblemsintheearlyaluminatereactions(seePenetrationResistance(FalseSet)inchapter9,page255).Interpretationoftheresultsmustbeundertakenwithcareandunderstanding.ASTMisdevelopingapracticeforthisapplication.

Ring Rest

Theringtestisameasureofwhenafullyrestrainedsampleofconcretecracks.Earliercrackingmaybeconsideredanindicatorofhigherriskofcrackinginthefield.

Field Laboratory (Monitoring) Tests.Factorstomoni-torgenerallyincludethemanufacturer’smilltestx-raydata,semi-adiabatictemperature,concreteslumplossandsettingtime,uniformityoftheair-voidsystem,andair-voidclustering.

X-Ray Data from Manufacturer’s Mill Test Report or

Mill Certificate (ASTM C 114, ASTM C 1365)

Thistestmonitorsthechemistryofthecementandflyash.Changesintotalcalcium,tricalciumaluminate(C

3A),sulfate(SO

3),alite(C

3S)orbelite(C

2S)may

indicatepotentialproblems.Semi-Adiabatic Temperature Measurement

Thistestmonitorsthetemperatureofpaste,mortar,orconcretemixturesinsealedcontainers(Dewarflasksorinsulatedcups).Changesinthetimingormagnitudeofthetemperaturerise,ortheshapeoftheheat-versus-timeplot,willflagpotentialproblemsin


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thesilicatereactions(figure4-32)(seeHeatSignature[AdiabaticCalorimetryTest]inchapter9,page259).

Concrete

Forthistest,makeconcretebatchesandmoni-torslumplosswithtimeaswellassettingtime(ASTMC143).

Foam Index (Dodson 1990), Foam Drainage (Cross

2000), and Air-Void Analyzer (AASHTO 2003) Tests Thesetestsprovideguidanceontheuniformityand

stabilityoftheair-voidsystem(seeAir-VoidSysteminchapter5,page133;andAir-VoidAnalyzerinchap-ter9,page265).Correlationwithfieldconcretestillhastobeestablished.

Clustering Index Test (Kozikowski 2005)

Thisisameasureoftheseverityofair-voidcluster-ingaroundcoarseaggregateparticles.Highvaluesareassociatedwithareductionofstrength.

Potential Solutions to Incompatibilities

Ifproblemsareobservedinthetestsorinthefield,thenoneormoreofthefollowingactionsmayresolvethem:

Reducetheconcretetemperaturebycoolingthematerialsand/orworkingatnight.Seekaflyashwithlowercalciumcontent.Reduceflyashdosage.

•

••

________________________________________________Figure 4-32. A set of field calorimetry data with three different cements and the same dosage of water-reducing admixture, including one set showing clear retardation and low heat output consistent with delayed setting and slow strength gain (Knight, Holcim)

Delayadmixtureaddition.Changethetypeofchemicaladmixture.Changethesourceofcement.Increasemixingtime.

Seekexpertadvicetoestablishwhattherootcauseoftheproblemissothatthecorrectremedialactioncanbetaken.

••••

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References


ASTMC114-04a/AASHTOT105,StandardTestMethodsforChemicalAnalysisofHydraulicCement

ASTMC138,StandardTestMethodforDensity(UnitWeight),Yield,andAirContent(Gravimetric)ofConcrete

ASTMC143,StandardTestMethodforSlumpofHydraulic-CementConcrete

ASTMC150/AASHTOM85,SpecificationforPortlandCement

ASTMC191/AASHTOT131,TestMethodforTimeofSettingofHydraulicCementbyVicatNeedle

ASTMC231,StandardTestMethodforAirContentofFreshlyMixedConcretebythePressureMethod

ASTMC359,StandardTestMethodforEarlyStiffen-ingofHydraulicCement(MortarMethod)

ASTMC403/AASHTOT197,TestMethodforTimeofSettingofConcreteMixturesbyPenetrationResistance

ASTMC457,StandardTestMethodforMicroscopicalDeterminationofParametersoftheAir-VoidSysteminHardenedConcrete

ASTMC1365,StandardTestMethodforDetermina-tionoftheProportionofPhasesinPortlandCementandPortland-CementClinkerUsingX-RayPowderDiffractionAnalysis

AAASHTO,TechnicalInformationGrouponAirVoidAnalyzer.2003.FreshConcreteAirVoidAnalyzer:ATechnicalBackgroundPaper.http://www.aashtotig.org/focus_technologies/ava/related_documents/tech_paper.stm.

Cross,W.,E.Duke,J.Kellar,andD.Johnston.2000.InvestigationofLowCompressiveStrengthsofConcretePaving,PrecastandStructuralCon-crete.SD98-03-F.SouthDakotaDepartmentofTransportation.

Dodson,V.1990.Chapter6:Air-EntrainingAdmix-tures.ConcreteAdmixtures.NewYork,NY:VanNostrandReinhold.129–158.

Johansen,V.C.,P.C.Taylor,andP.D.Tennis.2005.EffectofCementCharacteristicsonConcreteProper-ties.EB226.Skokie,IL:PortlandCementAssociation.

Kantro,D.L.1981.InfluenceofWater-ReducingAdmixturesonthePropertiesofCementPaste–AMiniatureSlumpTest.PCAR&DBulletinRD079.01T.Skokie,IL:PortlandCementAssociation.

Kozikowski,R.L.,D.B.Vollmer,P.C.Taylor,andS.H.Gebler.2005.FactorsAffectingtheOriginofAir-VoidClustering.PCAR&DSerialNo.2789.Skokie,IL:PortlandCementAssociation.

TaylorP.C.,V.C.Johansen,L.A.Graf,R.L.Kozikowski,J.Z.Zemajtis,andC.F.Ferraris.2006.TestsorStan-dardstoIdentifyCompatibleCombinationsofIndividuallyAcceptableConcreteMaterials.Washington,DC:FederalHighwayAdministration.Inreview.

Tennis,P.D,andH.M.Jennings.2000.AmodeloftwotypesofC-S-Hinthemicrostructureofportlandcementpastes.CementConcreteRes30.6:855–863.

Wadso,L.2004.UnthermostatedMultichannnalHeatConductionCalorimeter.Cement,Concrete,andAggregates26.2:1–7.

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Chapter 5

Critical Properties of Concrete

Thischapterdiscussescriticalpropertiesofconcretethatareneededtobeabletomix,transport,place,finish,andmaintainhigh-qualitypavement.Someproperties,likeworkabilityandmaterialssegrega-tion,aremanifestedonlyinthefreshorplasticstageofconcrete,beforeithashardened.Others,likefrostresistance,aremostimportantinthehardenedcon-crete.Stillothers,likestrengthgain,beginearlyinthehydrationprocess,remaincriticalduringthefirstfewdays,andplayaroleinconcreteformanymonthsorevenyears.Ingeneral,thecontractorisconcernedaboutthefreshproperties(howeasyitistogetthecon-creteinplace),andtheownerisconcernedaboutthelong-termhardenedproperties(howlongitwilllast).

Notethat“durability”isnotdiscussedasaproperty.Instead,thevariouspropertiesthatcontributetodura-bility—permeability,frostresistance,sulfateresistance,alkali-silicareactivity,abrasionresistance,andearly-agecracking—arecovered.

Attimes,requirementsfordifferentpropertiesinaspecificmixmaybemutuallyexclusive,meaningthatcompromisesmayneedtobemade.Forexample,highconcretestrengthsareoftenachievedbyincreas-ingcementcontent,whichinturnincreasesshrinkagethatcancausecracking.Itisincreasinglyimportantforthedesigner,readymixprovider,contractor,andownertounderstandhowtheirdecisionsaffecttheotherpartiesandtocommunicateamongthemselvesabouttheirdecisions.Thisisthebasisoftheneedforapavementtobetreatedasanintegratedsystem,notjustaseriesofindependentactivitiesandmaterials.

Foreachconcreteproperty,thischapterdiscussestheproperty’ssignificance,factorsthataffectit,andtestsusedtomeasureit.(Chapter6discusseshowtherequiredpropertiescanbeachievedinthefinalcon-cretesystem.Chapter9discussesqualityassuranceandqualitycontroltestinganddescribesmethodsforasuiteofspecifictests.)

Uniformity of Mixture 106

Workability 108

Segregation of Concrete Materials 111

Bleeding 112

Setting 114

Strength and Strength Gain 116

Modulus of Elasticity and Poisson’s Ratio 123

Shrinkage 125

Temperature Effects 127

Permeability 131

Frost Resistance 132

Sulfate Resistance 139

Alkali-Silica Reaction 141

Abrasion Resistance 146

Early-Age Cracking 148

Summary of Preventable Early-Age Cracks: 158Plastic Shrinkage Cracks 158Map Cracking (Crazing) 160Random Transverse Cracks 161Random Longitudinal Cracks 163Corner Breaks 164

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Uniformity of Mixture

Key Points

Theoverarchinggoalistomakeuniformconcrete,eventhoughthematerialsusedtomaketheconcretemaybevariable.

Materialsandtheenvironmentatthebatchplantareconstantlychanging.Despitethesechanges,itisimportantthatthemixturepropertiesbeuniformfrombatchtobatchtopreventproblemsinthepaverandinthefinalpavement.

Materialspropertiesmustbemonitoredregularlyandproportionsadjustedaccordingly.

Batchingshouldbedonebymassratherthanbyvolume.

Specifications:ASTMC94/AASHTOM157,ASTMC172/AASHTOT141.

•

•

•

•

•

Simple DefinitionInuniformconcrete,theconcretepropertiesare

consistentlythesamefrombatchtobatch,eventhoughthematerialsusedtomaketheconcretemaybevariable.

SignificanceUndereconomicorotherpressures,experienced

personnelcanmakeawiderangeofless-than-desirablematerialsworkwell.Itismuchhardertoaccommodatechangesinconcretepropertiesandper-formance(Scanlon1994)thatresultfromchangesinmaterialsfrombatchtobatch.Ittakestimetoadjustbatchingandpavingequipmenttoallowforchangingmaterialsorenvironment.Whensuchadjustmentsareregularlyrequiredbetweenbatches,thereisamuchhigherriskofunsatisfactoryfinishes,lowstrengths,cracks,anddurabilityproblems.Itisthereforeofgreatimportanceforconcreteanditsmaterialstobeuniformfrombatchtobatch.(Chapter9discusses

someoftheimplicationsofuniformityonqualitymanagementsystemsandtesting.)

Factors Affecting UniformityDifferentaspectsofconcreteproduction—

includingthevariabilityofrawmaterials(specifically,theaggregatesandcement),batchingoperations,andmixingoperations—canaffecttheuniformityofthefinalproduct.

Raw Materials Aggregatesshouldbetestedforgrading,density,

andmoisturecontent,andshouldbestoredandhandledinwaysthatminimizesegregation.Preferably,cementshouldbefromasinglesourceandlot,oratleastdeliveredinlargeenoughquantitiesforseveraldaysofwork.

Batching Operations Batchingistheprocessofmeasuringconcretemix

ingredientsbyeithermassorvolumeandintroducingthemintothemixer(Kosmatka,Kerkhoff,andPana-rese2002).Mostspecificationsrequirethatbatchingbedonebymassratherthanbyvolume(ASTMC94/AASHTOM157),withthefollowingtypicallevelsofaccuracy(seeBatchinginchapter8,page207):

Cementitiousmaterials:+1percentAggregates:+2percentWater:+1percentAdmixtures:+3percent

Mixing Operations Thoroughmixingisrequiredtodistributeallmix-

tureingredientsevenlyuntilthemixisuniformandtostabilizeentrainedbubbles(seeMixingConcreteinchapter8,page209,forinformationaboutmixingtime).Mixersshouldnotbeloadedabovetheirratedcapacitiesandshouldbeoperatedatthemixingspeedrecommendedbythemanufacturer.Mixingbladesshouldberoutinelyinspectedforwearorcoatingwithhardenedconcrete,bothofwhichcandetractfromeffectivemixing.ASTMC94providesdetailsofrequirementsformixing.

Uniformity TestingOvertheyears,avarietyoftestshavebeenused

toassessconcreteuniformity(table5-1).Commonly,

••••

Uniformity of Mixture


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unitweight(seeUnitWeightinchapter9,page258),aircontent,slump,compressivestrength,andcoarseaggregatecontentareusedasindicatorsofconcreteuniformity(Scanlon1994).ASTMC94/AASHTOM157setouttherequirementsformonitoringand

Table 5-1. Requirements for Uniformity of Concrete (ASTM C 94 2004)

acceptingtheuniformityofconcrete.

Theneedforrepresentativesamplesiscriticalforassessingconcreteuniformity.SamplesshouldbeobtainedinaccordancewithASTMC172/AASHTOT141.

Test Requirement, expressed as maximum Applicable permissible difference in results of testing tests of samples taken from two standard locations in the concrete batch

Unit weight of fresh concrete, calculated 1.0 ASTM C 138to an air-free basis, lb/ft3

Air content, volume % of concrete 1.0 ASTM C 138, ASTM C 173, or ASTM C 231

Slump ASTM C 143 If average slump is 4 in. (10 cm) or less, in 1.0 If average slump is 4 to 6 in. (10 to 15 cm), in 1.5

Coarse aggregate content, portion by weight of 6.0 Washout test (see ACI 1992)each sample retained on # 4 sieve, %

Average compressive strength at 7 days for each 7.5 ASTM C 39 sample, based on average strength of all comparative test specimens, %

Source: Bureau of Reclamation 1963

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Workability

Key Points

Workabilityisanindicationoftheeasewithwhichconcretecanbeplacedandcompacted.

Goodworkabilitybenefitsnotonlyfreshconcretebuthardenedconcreteaswell,especiallyinitsfinaldensity.

Freshconcretecharacteristicsthataffectworkabilityincludeconsistency,mobility,pumpability,compactability,finishability,andharshness.

Concreteshouldhavetherightworkabilityforthetoolsbeingusedtoplaceit.

•

•

•

•

Simple DefinitionAsgiveninACI116R,workabilityisdefinedas

thatpropertyoffreshlymixedconcreteormortarthatdeterminestheeaseandhomogeneitywithwhichitcanbemixed,placed,compacted,andfinished(ACI2000).

Significance of WorkabilityWorkabilityisanimportantpropertyoffresh

concretebecauseitaffectsthequalityofseveralaspectsoftheconstructionprocess,includingfinishing.Workabilityisalsoameasureofuniformity.Thebenefitsofgoodworkabilityareoftenassociatedonlywithfreshconcrete,intermsoftheamountofmechanicalworkrequiredtoplaceandconsolidatetheconcretewithoutsegregation.However,goodwork-abilityprovidesindirectbenefitstothehardenedconcreteaswell,becausefullconsolidation(density)iseasiertoachieve.Workablemixturesthatallowadequateconsolidationgreatlyreducethepresenceoflargevoidsintheconcrete,whichcanotherwisesignificantlyreduceconcretestrength(Neville1996).Also,poorworkabilitycanmakefinishingdifficult,causingtearingofthesurfacethatcanleadtocrack-ing.Requirementsforworkabilitywillvaryaccordingtothetechniquesbeingusedinthefield.Therefore,

Changesinworkabilityindicatethattherawmaterials,proportions,ortheenvironmentarechanging.

Watershouldnotbeaddedtoconcreteatthepaverunlessthiscanbedonewithoutexceedingthewater-cementitiousmaterialsratioofthemixdesign.

Testingspecifications:

Slump:ASTMC143/AASHTOT119.

Penetration:ASTMC360.

•

•

•

◦

◦

Do Not Add Water to Improve Workability

When paving crews experience problems with workability, they are often tempted to add water to the mix. However, water should not be added to concrete at the paver unless this can be done without exceeding the water-cementitious materials ratio of the approved mix design. (For a discussion of the water-cementitious materials ratio, see step 2 under the Absolute Volume Method in chapter 6, page 179.) Adding excess water will always lead to other problems, which may include reduced strength and increased permeability.

contractorsshouldbeallowedtoselecttheirownworkabilityrequirementsandtoimposevariabilitylimitsforqualityassurancepurposes.

Thedegreeofworkabilityrequiredforproperplacementandconsolidationofconcreteisgovernedbythetypeofmixingequipment,sizeandtypeofplacingequipment,methodofconsolidation,andtypeofconcrete(Scanlon1994).

Factors Affecting WorkabilityIngeneral,workabilitycanbecontrolledthrough

theproperproportioningofthemixturematerials.However,workabilitydependsonseveralfactors,

Workability


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includingthephysicalandchemicalpropertiesoftheindividualcomponents(Scanlon1994).

Water Content Theprimaryfactoraffectingtheworkabilityofa

concretemixisthewatercontent.Increasingthewatercontentwillincreasetheflowandcompactabilityofthemix,butcanalsoreducestrengthwhileincreasingsegregation,bleeding,andpermeability(MindessandYoung1981).

Aggregates Severalfactorsrelatedtotheaggregatesinamix

haveasignificanteffectonworkability.Thefirstistheamountofaggregate;increasesintheaggregate-cementratioresultinadecreaseinworkabilityforafixedwater-cementitiousmaterialsratio(MindessandYoung1981).

Aggregategradingiscritical.Adeficiencyoffineaggregatecanleadtoaharshmixthatisdifficulttowork,whileanexcessofveryfinematerialwillmakethemixsticky(Scanlon1994).Amoreuniformgrad-ingofaggregateparticlesmayimproveworkabilitybyfillingthevoidsbetweenlargerparticlesandreducingtheamountoflockingbetweenparticles(seeAggregateGradationinchapter3,page44).

Finally,thepropertiesoftheaggregateitself,includingparticleshape,texture,andporosity,areimportant.Moresphericalparticlesgenerallyproducemoreworkablemixturesthansharp,elongated,angu-larones,whilemoreabsorptive,dryaggregatesmayreduceworkabilitybyremovingwaterfromthepaste.

Entrained Air Entrainedairincreasesthepastevolumewhile

actingasalubricanttoimprovetheworkabilityofconcrete(Scanlon1994).Entrainedairisparticularlyeffectiveinimprovingtheworkabilityoflean(lowcementcontent)mixturesthatotherwisemightbeharshanddifficulttoworkandofmixtureswithangu-larandpoorlygradedaggregates(Kosmatka,Kerkhoff,andPanarese2002).However,excessiveamountsofentrainedaircanmakeamixturestickyanddifficulttofinishandmayreduceconcretestrength.

Time and Temperature Workabilitydecreaseswithtimeaswaterfromthe

mixtureevaporates,isabsorbedbytheaggregate,

andreactswithcementduringtheinitialchemicalreactions(Wongetal.2000).Increasesinambienttemperatureswillacceleratetheseeffectsbecausehighertemperaturesincreaseboththeevaporationandhydrationrates(MindessandYoung1981).

Cement Althoughlessimportantthanothercomponents’

properties,thecharacteristicsofthecementmayalsoaffectworkability.Forexample,theincreasedcementfineness(therefore,theincreasedspecificsurfacearea)ofTypeIIIcementsmeanstheywillhavealowerworkabilityatagivenwater-cementitiousmaterialsratiothanaTypeIcement(MindessandYoung1981).

Supplementary Cementitious Materials Flyashandgroundgranulatedblastfurnace

(GGBF)slaghavegenerallybeenfoundtoimproveconcreteworkabilitybecauseofthefinesphericalnatureofflyashandtheglassysurfaceofGGBFslagparticles.Inhotweather,someflyashesmaycauseearlystiffeningandlossofworkabilityofthemixture(seeStiffeningandSettinginchapter6,page186.)

Silicafumewillmarkedlyincreasethewaterrequirementandstickinessatdosagesabovefivepercentbymassofcementbecauseofthehighsurfacearea.Lessthanfivepercentsilicafumemayimproveworkabilitybecausethesilicafumeparticlestendtobesphericalandassistwithseparatingcementgrains.(Silicafumeisnottypicallyusedinconcreteforpave-ments;seeSupplementaryCementitiousMaterialsinchapter3,page31.)

AdmixturesWater-reducingadmixturesareusedtoincrease

workability,althoughtherateofslumplossmaynotbereduced,dependingonthechemistryoftheadmixture(Kosmatka,Kerkhoff,andPanarese2002).Set-retardingadmixturesreducetheearlyrateofhard-eningandpermitconcretetobehandledandvibratedforalongerperiodoftime(Scanlon1994).

Workability TestingSeveralcharacteristicsoffreshconcretearerelatedto

workability.Someofthesecharacteristicsincludethefollowing(MindessandYoung1981;Scanlon1994):

Consistencyorfluidity.Mobility.

••

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Pumpability.Compactability.Finishability.Harshness(amountofcoarseaggregate).

Unfortunately,notestmethodforassessingworkabilitycomprehensivelymeasuresalloftheseproperties.Andthereisnosingletestthatdirectlymeasuresworkabilityintermsofthefundamen-talpropertiesoffreshconcrete(Neville1996).Inaddition,comparisonscannotbemadeamongthedif-ferenttestsbecauseeachmeasuresadifferentaspectofconcretebehavior.Thestudyofrheology(loosely,flowbehavior)isongoingtoaddressthisconcern.

Asaresult,themeasurementofworkabilityisdeterminedtoalargeextentbyjudgmentandengi-neeringexperience(Scanlon1994).

However,commonlyavailabletestsareusefulasqualitycontrolmeasures(KoehlerandFowler2003;MindessandYoung1981).Thefollowingsectionssummarizecommontestsformeasuringworkability,withmoredetailedinformationprovidedbyMindessandYoung(1981),Scanlon(1994),Neville(1996),andWongetal.(2000).(Seealsochapter9,page250.)

Slump Test

Theslumptest(ASTMC143/AASHTOT119)measurestheconsistencyofthemix,thatis,theabil-ityoffreshconcretetoflow.Duringflow,twoeventstakeplace;onestartsthemovementandtheothercontinuesthemovement.Theslumptestisanindica-toroftheformer.

Theslumptestisvalidforarangeofresultsfrom12to225mm(0.5to9in.)usingcoarseaggregatelessthan38mm(1.5in.).Theslumptestmeasuresconcreteslumpundertheself-weightoftheconcreteonly,andassuchdoesnotreflectthebehaviorandworkabilityofthemixtureunderdynamicconditionslikevibrationorfinishing(Neville1996).

Withdifferentaggregatesormixproperties,thesameslumpcanbemeasuredforconcretemixturesthatexhibitverydifferentworkability.Neverthe-less,theslumptestisveryusefulasaqualitycontrolmeasure,withsignificantchangesintheslumponagivenjobindicatingthatchangeshaveoccurredinthecharacteristicsofmaterials,mixtureproportions,

••••

watercontent,mixing,timeoftest,orthetestingitself(Kosmatka,Kerkhoff,andPanarese2002).

Compacting Factor TestThecompactingfactortestisusedtomeasure

thecompactabilityofconcretemixturesforagivenamountofwork.Thetestinvolvesdroppingfreshconcretethroughmultipleheightsandmeasuringthedegreetowhichitcompacts(Wongetal.2000).

Thedegreeofcompaction,calledthecompactingfactor,isexpressedintermsoftheratioofthedensityactuallyachievedinthetesttothedensityofthesameconcretefullycompacted(Neville1996).Thetestismoresensitiveatthelow-workabilityendofthescaleandismostappropriateformixtureswithamaximumaggregatesizelessthan38mm(1.5in.).(ThetestisstandardizedinEuropeasEN12350-4.)

Vebe TestTheVebetestwasdevelopedinthe1940sandis

particularlysuitablefordeterminingdifferencesintheconsistencyofverydrymixes(MindessandYoung1981).Inthistest,asampleofconcreteismoldedwiththeslumpconeinsidealargercylinder.Theslumpmoldisremoved,atransparentdiskisplacedontopoftheconcrete,andthesampleisthenvibratedatacontrolledfrequencyandamplitudeuntilthelowersurfaceofthediskiscompletelycoveredwithgrout(MindessandYoung1981).ThetimeinsecondsforthedisktobecomecoveredistheVebetime,andcancommonlyrangefrom5to30seconds.

AlthoughthereisnoASTMstandardforthistest,thereisaEuropeanstandardtestthatiswidelyusedthroughoutEurope(EN12350-3).

Penetration TestsPenetration-typetestscanbeusedtoassesscon-

creteworkability.Thesetestsmeasurethepenetrationofsometypeofindenterintoconcreteunderavarietyofconditions.

TheKellyballpenetrationtestisdescribedinASTMC360,whichhasbeendiscontinued.Inthistest,alarge,ball-shaped,steelprobeisgentlyplacedontheconcretesurfaceandthedepthofpenetrationismeasured.Thistestcanbeconductedveryquicklyandcanevenbeappliedtoconcreteintheform(Neville1996).

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Segregation of Concrete Materials

Key Points

Segregationofcoarseaggregatefromthemortarresultsinconcretewithlowerstrengthandpoordurability.

Theprimarywaytopreventsegregationistousewell-gradedaggregate.

Ifcoarseaggregatesadvanceinfrontoforbehindthefineparticlesandmortarwhentheconcreteisbeingplaced,themixtureissegregating.

•

•

•

Simple DefinitionSegregationisthetendencyforcoarseaggregate

toseparatefromthemortarinaconcretemixture,particularlywhenthemixtureisbeingtransportedorcompacted.

Significance of SegregationSegregationresultsinpartofthebatchhavingtoo

littlecoarseaggregateandtheremainderhavingtoomuch.Theformerislikelytoshrinkmoreandcrackandhavepoorresistancetoabrasion,whilethelattermaybetooharshforfullconsolidationandfinishing.Theresultisthateffectivelyanumberofdifferentcon-cretemixturesarebeingplaced(figure5-1).

Segregationisespeciallyharmfulinplacingcon-creteforpavement,asitresultsinproblemssuchasstrengthloss,edgeslump,spalling,blistering,andscaling.

Factors Affecting SegregationSegregationisprimarilypreventedbyusinga

well-gradedaggregatesystem.Agap-gradedaggregatesystemismorelikelytosegregate.

Segregationtendstodecreasewithincreasingamountsoffinematerials,includingcementandsupplementarycementitiousmaterials,inthesystem.Attheotherextreme,poormixtureproportioningwithexcessivepastecanalsoleadtosegregation.

Segregation TestingManyspecificationsrequirethatsegregationbepre-

vented,althoughthereisnostandardtesttomeasureit.However,segregationisreadilyvisibleandcanberecognizedeasilywhentheconcreteisbeingdis-chargedfromachuteorapump.Ifthecoarseparticlesadvanceinfrontoforbehindthefineparticlesandmortar,themixtureissegregating.Thebatchplantshouldthenbecontactedtoadjustthemixproportions.

Figure 5-1. Segregated concrete (Hanson, Iowa DOT)

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Bleeding

Key Points

Bleedingistheappearanceofwateratthesurfaceofnewlyplaced,plasticconcreteduetosettlementoftheheavierparticles.

Somebleedinghelpsavoidplasticcrackinginthesurface,whichcanoccurwhenwaterevaporatingfromthesurfacecausesthesurfacetodryquickly.

Excessivebleedingcanresultinvoidsunderlargeaggregateparticlesandtheformationofchannelsthroughtheconcrete.Excessivebleedingwillalsoincreasetheeffectivewater-cementitiousmaterialsratioatthesurfaceandweakenthesurface.

Ideally,finishingandcuringshouldoccurwhenbleedinghasfinishedandbleedwaterhasevaporated.

Bleedingisreducedwithincreasingfinescontentandwithair-entrainingand/orwater-reducingadmixtures.

Specifications:ASTMC232/AASHTOT158.

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Simple DefinitionBleedingistheappearanceofalayerofwateratthe

toporthesurfaceoffreshlyplacedconcreteafterithasbeenconsolidatedandstruckoff,butbeforeithasset(MindessandYoung1981).Bleedingmayalsobereferredtoaswatergain,weeping,orsweating.

Significance of BleedingBleedingiscausedbythesettlementofsolidparticles

(cementandaggregate)inthemixtureandthesimulta-neousupwardmigrationofwater(Kosmatka,Kerkhoff,andPanarese2002).Asmallamountofbleedingisnormalandexpectedinfreshlyplacedconcrete.

Infact,somebleedingisactuallyhelpfulincontrol-lingthedevelopmentofplasticshrinkagecracking.If

therateofmoistureevaporationatthesurfaceexceedsthebleedingrate(Kosmatka1994;Poole2005),thesurfacewilldryandcrack.Alackofbleedwatercanalsoleadtoadrysurfacethatcanbeverydifficulttofinish(Kosmatka1994).

However,excessivebleedingreducesconcretestrengthanddurabilitynearthesurface.Therisingwatercancarrywithitaconsiderableamountoffinecementparticles,formingalayerofweakandnondu-

________________________________________________Figure 5-2. Bleeding (Ozyildirim)

Bleeding


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rablematerial,calledlaitance,atthesurface(Neville1996).

Excessivebleedingalsomaydelaythefinishingprocess,whichingeneralshouldnotproceeduntilthebleedwaterhasevaporatedfromthesurface(Kosmatka1994).Ifthesurfaceisfinishedwithbleedwaterpresent,athinandweaklayeriscreatedonthesurfacethatissusceptibletoscalinganddelamina-tion.Insomecases,ifthefreshconcretesurfaceisprematurelysealedbytrowelingwhiletheunderlyingconcreteisstillreleasingbleedwater,blisters(smallhollowbumpsbeneaththeconcretesurface)canform.

Bleedwatercanalsoaccumulatewithinthecon-cretemixitself,underlargeaggregateparticlesorreinforcingbars(MindessandYoung1981).Theformerresultsinreducedconcretestrength(duetodecreasedaggregate-pastebond).Thelattermayreducethepaste-steelbond,possiblypromotingthecorrosionofsteelbecausethesteelisnotincontactwiththecorrosion-resistivepaste(Kosmatka1994).

Factors Affecting BleedingTheinitialbleedingprocessgenerallybeginsafter

agitationoftheconcretemixends,andbleedingcon-tinuesuntilthecementpastehasstiffenedsufficientlytoresistthesettlementofthesolidparticles(Neville1996).Thedurationofbleedingdependsonthethicknessoftheconcretesectionaswellasthesettingpropertiesofthecementitiousmaterials,withthin-nersectionsorfastersettingconcretesexhibitinglessbleeding(Kosmatka1994).

Anumberofdifferentconcretemixconstituentscanaffectthedevelopmentofbleeding.

Water Content and Water-Cementitious Materials (W/CM) Ratio

Anyincreaseintheamountofwaterorinthew/cmratioresultsinmorewateravailableforbleeding(Kos-matka1994).

Cement Asthefinenessofcementincreases,theamountof

bleedingdecreases,possiblybecausefinerparticleshydrateearlierandalsobecausetheirrateofsedimen-tation(settlement)islower(Neville1996).Increasingcementcontentalsoreducesbleeding(Kosmatka1994).Cementwithahighalkalicontentorahigh

calciumaluminate(C3A)contentwillexhibitless

bleeding(Neville1996).

Supplementary Cementitious Materials Concretecontainingflyashgenerallyexhibitsa

lowerbleedingrate,butduetoretardedsettingthetotalbleedvolumemaybesimilarorgreaterthanportlandcement-onlyconcrete.Ground,granulatedblast-furnaceslagshavelittleeffectonbleedingrates(WainwrightandRey2000).Silicafumecangreatlyreduce,oroftenstop,bleeding,largelybecauseoftheextremefinenessoftheparticles(Neville1996).

AggregateOrdinaryvariationsinaggregategradinghavelittle

effectonthebleedingofconcretemixtures,providedthatthereisnoappreciablevariationinmaterialsmallerthan75µm.However,concretemixturescon-tainingaggregateswithahighamountofsilt,clay,orothermaterialpassingthe75-µm(#200)sievecansig-nificantlyreducebleeding(Kosmatka1994),althoughtheremaybeotheradverseeffectsontheconcrete,likeincreasedwaterrequirementandshrinkage.

Chemical AdmixturesAir-entrainingagentshavebeenshowntosignifi-

cantlyreducebleedinginconcrete,largelybecausetheairbubblesappeartokeepthesolidparticlesinsuspension(Neville1996).Waterreducersalsoreducetheamountofbleedingbecausetheyreleasetrappedwaterinamixture.

Testing for Assessing BleedingWhenrequired,ASTMC232/AASHTOT158

includetwotestmethodsforassessingbleeding.Thefirsttestmethodinvolveshandconsolidatingasampleofconcretebyroddinginacontainerofstandarddimensionsandthencoveringthesampleandleav-ingitundisturbed.Thebleedwaterisdrawnoffthesurfaceevery10minutesduringthefirst40minutes,andevery30minutesthereafter,untilbleedingstops.Thetotalbleedingandtherateofbleedingmaythenbedetermined.

ThesecondtestmethodinASTMC232usesasampleoffreshconcreteconsolidatedbyvibrationand,aftercoveringthesample,intermittentlyvibratingitforaperiodofonehourtodeterminethetotalvolumeofbleedwater.

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Setting

Key Points

Settingiswhenconcretelosesitsworkabilityandbecomeshard;itisinfluencedbythechemistryofthecementitioussystem.

ClassFflyashandground,granulatedblast-furnaceslagwillgenerallyretardsettingtime.

Settingisacceleratedwhentheconcretetemperatureincreases.

Set-acceleratingandretardingchemicaladmixturesinthemixturecancontrolsettime.

Settingaffectsthetimeavailableforplacingandfinishing,aswellasthesawingwindowwhenjointscanbesawed.

Testingspecifications:ASTMC191/AASHTOT131,ASTMC266/AASHTOT154(rare),ASTMC451/AASHTOT186,ASTMC403/AASHTOT197.

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Simple DefinitionSettingisthestagewhenconcretechangesfrom

plastictosolid.

Significance of SettingOvertime,thechemicalreactionsofportland

cementandwaterleadtostiffening,setting,andhard-ening—acontinuousprocess.Thetimewhensettingoccursisimportantbecauseitinfluencesthetimeavailabletoplaceandfinishtheconcrete.Thesaw-cuttingwindowisalsoindirectlyrelatedtosetting.Thesettingtimeisanimportantcharacteristicbecausevariationbetweenbatchesmayindicateissueswithconstructionschedulingormaterialscompatibility.

Typically,initialsetoccursbetweentwoandfourhoursafterbatching;finalsetoccursbetweenfourandeighthoursafterbatching.Initialsetandfinalset

arearbitrarilydeterminedbasedonthetestmethodsavailable,butinitialsetgenerallyoccursshortlyafterinitiationofthesilicatephaseofcementhydration(seePortlandCementHydrationinchapter4,page88.)

Falsesetandflashsetarethestiffeningofthemixtureinthefirstfewminutesaftermixingandareduetouncontrolledreactionsofthealuminate/sulfatecompoundsinthecement(seePotentialMaterialsIncompatibilitiesinchapter4,page97).Falsesetistemporaryandcanbeworkedthroughwithcontinuedmixing,butflashsetmeansthatthemixturewillhavetobediscarded.

Factors Affecting SettingAnumberoffactorsaffectconcretesettingtime.

Temperature/Weather Increasingtemperaturereducessettime.Decreasing

temperatureincreasessettime.Hydrationwillstopwhenthetemperatureiscloseto0°C(32°F).Expo-suretosunlightandwindyconditionsalsoinfluencesetting,especiallyatthesurface,largelyduetotheeffectsofheatingandevaporativecooling.

Water-Cementitious Materials (W/CM) Ratio Alowerw/cmratioreducessettime.

Cement Content Increasingcementcontentreducessettime.

Cement Type Cementchemistrywillstronglyinfluencesettime

(seeReactionsofSupplementaryCementitiousMateri-alsandPotentialMaterialsIncompatibilitiesinchapter4,pages94and97.)

Chemical Admixtures Acceleratingandretardingadmixturesareused

deliberatelytocontrolthesettingtime(Dodson1994).Overdoseofsomewaterreducersmayresultinsetretardation(seePotentialMaterialsIncompatibili-tiesinchapter4,page97.)

Timing of Addition of AdmixturesDelayedadditionofsomewaterreducersmaypre-

ventearlystiffeningorretardation.

MixingImprovedmixinginfluenceshydrationbyimprov-

Setting


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ingthehomogeneityanddispersionofthereactantsand,thus,alsoacceleratessetting.

Supplementary Cementitious Materials ClassFflyashandground,granulatedblast-fur-

naceslagwillgenerallyretardthesettingtimeofconcrete.ClassCflyashmayaccelerateorretardsetting,dependingonthechemistryoftheflyashandtheothercomponentsinthesystem.

Testing for Setting TimeSettingtimeisbothacharacteristicofcement

(testedasastandardpaste)andofconcrete(testedasamortarextractedfromtheconcrete).

CementspecificationstypicallyplacelimitsonsettingtimeusingtheVicatapparatus(ASTMC191/AASHTOT131).Thistestisrunonpastemadetoastandardconsistencyandmeasuresthetimeatwhichdifferentindicatorspenetratethesurface.AnoptionaltestusingtheGillmoreneedle(ASTMC266/AASHTOT154)israrelyused.

Cementsaretestedforearlystiffening(flashset/falseset)usingASTMC451/AASHTOT186(pastemethod)andASTMC359/AASHTOT185(mortarmethod),whichusethepenetrationtechniquesofthe

Vicatapparatus.Thetestrecordsthedepthofpenetra-tionafterremixingatfixedintervals.

ConcretesettingisdeterminedusingASTMC403/AASHTOT197.Mortarisseparatedfromtheconcretethrougha4.75mm(#4)sieve,andthepenetrationresistanceisrecordedasafunctionoftime.Theinitialandfinalsettingtimesaredefinedasthetimethemortarachievesapenetrationresistanceof3.4MPa(500lb/in2)and27.6MPa(4,000lb/in2),respectively(figure5-3).

_______________________________________________Figure 5-3. Penetration resistance of mortar sieved from concrete as a function of time, per ASTM C 403. Initial and final setting times are defined as shown. (Dodson 1994)

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Strength and Strength Gain

Key Points

Concretestrengthisoftenusedtomeasureconcretequality,althoughthiscanbeafalseassumption.

Sufficientconcretestrengthisneededtocarrytheloads.

Strengthincreaseswithadecreasingw/cmratio.

Strengthgainisacceleratedathighertemperaturesanddeceleratedatlowertemperatures.

Strengthdecreasesasaircontentincreases.

Strengthismeasuredincompressionorinflexure(bending).Ingeneral,concretehassignificantlymorecompressivestrengththanflexuralstrength.

Earlystrengthcanbeassessedinthefieldusingmaturity(timeandtemperature)measurements.

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Simple DefinitionStrengthisameasureoftheabilityofconcreteto

resiststressesorforcesatagivenage.

Significance of Strength and Strength Gain

Strengthisthemostcommonlymeasuredpropertyofconcreteandisoftenusedasthebasisforassess-ingconcretequality.Thisispartlybecausestrengthmeasurementsgiveadirectindicationofconcrete’sabilitytoresistloadsandpartlybecausestrengthtestsarerelativelyeasytoconduct.Theageatwhichagivenstrengthisrequiredwillvarydependingontheneed.Contractorsmaywantearlystrength(rapidstrengthgain)inordertoputconstructiontrafficonthepavement,whiletheownermaybeinterestedonlyinthestrengthatalaterage.Therateofstrength

developmentwillalsoinfluencetheriskofcracking(seeEarly-AgeCrackinginthischapter,page148.)

Somepeoplerelyonstrengthmeasurementsasindicatorsforotherconcreteproperties,suchasabrasionresistanceorpotentialdurability.Awordofcaution:Thiscorrelationbetweenstrengthandotherpropertiesisnotnecessarilyaccurate.Inmanycases,strengthaloneisnotsufficienttodeterminethesuit-abilityofaconcreteforaspecificapplication.Specificpropertiesshouldbetestedaccordingtorelevantstandards,notcorrelatedtostrengthmeasurements.Forinstance,highearlystrengthachievedbychemi-caladmixturesorheatingwilloftenresultinapoorermicrostructure,higherpermeability,andlossofpoten-tialdurability.

Concreteisgenerallystrongincompression,meaningitcanresistheavyloadspushingittogether.However,concreteismuchweakerintermsoften-sion,meaningthattheconcreteisrelativelyweakinresistingforcespullingitapart.

Mostloadsonpavements(seefigure5-4)resultinbending(orflexure),whichintroducescompres-sivestressesononefaceofthepavementandtensilestressesontheother.Theconcrete’scompressivestrengthistypicallymuchgreaterthanthecompres-sivestressescausedbytheloadontheslab.However,thetensilestrengthisonlyabout10percentofthecompressivestrength.Mostslabfailuresareinflexureratherthanincompression.Consequently,theflexuralstressandtheflexuralstrength(modulusofrup-ture)oftheconcreteareusedinpavementdesigntodeterminerequiredslabthickness.Slabfailuresafterthefirstfewweeksareoftenduetoalossofsupportundertheslab,whichresultsinexcessivestresses.

Anotherkeystrengthparameterofconcreteforpavementsisfatigue,ortheconcrete’sabilitytocarryrepeatedloadingandunloading.Underfatigue,asmallcrackdevelopsintheconcretethatthengrowswitheverycycle.

Factors Affecting Strength and Strength Gain

Fundamentally,strengthisafunctionofthevolumeofvoidsintheconcrete.Voidsprovideashortcutforcracksgrowingthroughthesystemtopropagate.



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Thesevoidsmaybelarge,suchasairvoidsduetopoorconsolidation,orsmall,suchascapillaryvoidsleftafterexcesswaterhasevaporatedfromthepaste.

Theprimaryfactorsthatinfluencestrengthinwell-compactedconcrete,therefore,arethewater-cementitiousmaterialsratio(adirectinfluenceoncapillaryvolume)andtheextenttowhichhydrationhasprogressed.Theseandotherfactorsarediscussedbelow(table5-2).

Water-Cementitious Materials (W/CM) Ratio Strengthincreasesasthew/cmratiodecreases

becausethecapillaryporositydecreases.Thisobserva-tionholdsfortheentirerangeofcuringconditions,ages,andtypesofcementsconsidered.Remember,however,thatalthoughthereisadirectrelationshipbetweenw/cmratioandstrength,concreteswiththesamew/cmratiobutdifferentingredientsareexpectedtohavedifferentstrengths.

Degree of Hydration Hydrationbeginsassoonascementcomesin

contactwithwaterandcontinuesaslongasfavorable

_______________________________________________Figure 5-4. Loads on a pavement induce flexural stresses in the concrete slab. (ACPA 1994)

Table 5-2. Factors Affecting Compressive and Flexural Strength

Areas Factor Pavement impact Testing impact

Construction Change in w/cm ratio Higher w/cm=lower strength Higher w/cm= lower strength Poor consolidation Low strength Lower strength if cores are tested Excessive vibration Segregation may result in lower strength Lower strength if cores are tested Improper curing Lower strength Lower strength if cores are tested Reduced air content Higher strength Higher strength if cores are tested Increased air content Lower strength Lower strength if cores are tested

Sampling Nonrepresentative, Lower strength Lower strength segregated sample

Specimen casting Poor consolidation Low strength and high permeability Lower strength Poor handling Low strength and high permeability Lower strength

Specimen curing Specimen freezes Higher strength than indicated Lower strength or dries out

Specimen breaking Rapid rate Lower strength than indicated Higher strength

Poor end preparation Higher strength than indicated Lower strength



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moistureandtemperatureconditionsexistandspaceforhydrationproductsisavailable(althoughhydra-tionslowssignificantlyafterafewdays).Ashydrationcontinues,concretebecomesharderandstronger.Hydrationproceedsatamuchslowerratewhentheconcretetemperatureislowandaccelerateswhenthetemperaturerises.Hydration(andthusstrengthgain)stopswhenthereisinsufficientwaterinthesystem.

Cement Concretestrengthisinfluencedbythecomposi-

tionandfineness,andperhapsbytheamount,ofthecement.Thesilicatealite(C

3S)hydratesmorerapidly

thanbelite(C2S)andcontributestoearlystrength.

Belite(C2S)hydratesmoreslowlybutcontinues

hydratinglonger,thusincreasinglaterstrengths(seePortlandCementHydrationinchapter4,page88).Finercementshydratefasterthancoarsercementsandtendtohavealimitedlaterstrengthdevelopmentbecauseofapoorerqualitymicrostructure(figure5-5).

Supplementary Cementitious Materials (SCMs) SCMscontributetothestrengthgainofconcrete.

However,theamountorrateofthiscontributionwilldependonthechemistry,fineness,andamountoftheSCM.Generally,withClassFflyashandground,gran-ulatedblast-furnaceslag,earlystrengthsarelowerthanthoseofsimilarmixtureswithportlandcementonly,andultimatestrengthsarehigher.TheeffectofClassCflyashwillgoeitherway,dependingonthespecificflyashused.Silicafumenormallyincreasesstrengthsatbothearlyandlaterages(althoughsilicafumeisnotgenerallyusedinconcreteforpavements).Mixpropor-tionscanbeselectedtoachievetherequiredstrengthswithorwithoutthepresenceofSCMs,butthemajor-ityofconcretemixturesincludeoneormoreSCMs.

Air Content Asaircontentisincreased,agivenstrengthgener-

allycanbemaintainedbyholdingtheratioofvoids(airandwater)tocementconstant.Toaccomplishthis,thecementcontentmayneedtobeincreasedorthew/cmratioreduced.Air-entrainedaswellasnon-air-entrainedconcretecanreadilybeproportionedtoprovidemoderatestrengths.

Aggregate Bond Roughandangular(includingcrushed)aggregate

_______________________________________________Figure 5-5. Cementitious systems that produce high early-strengths (blue line) tend to have lower strengths in the long term when compared to slower hydrating systems (red line). (CTLGroup)

particlesexhibitagreaterbondwithcementpastethanaggregateswithsmoothandroundedsurfaces.Largeaggregatesgenerallyprovideimprovedaggregateinter-lockatjointsandcracks,increasingflexuralstrength.

Handling and Placing Impropermixing,handling,andplacingwillaffect

concretestrengthandstrengthgain.Thelateradditionofwatercanmarkedlyreducestrength(seeWater-CementitiousMaterials(w/cm)Ratio,onthepreviouspage).Concretemustbethoroughlycompactedinordertoreducethevoidcontentofthemixture.

High Early-Strength ConcreteTheperiodinwhichaspecifiedconcretestrength

isachievedmayrangefromafewhourstoseveraldays.Highearly-strengthconcrete,usedinfast-trackconcrete,achievesitsspecifiedstrengthatanearlieragethannormalconcrete.(Itisnotablethatconcreteconsiderednormaltodayperformsthesameashighearly-strengthconcreteofonlyafewyearsago.)

Highearly-strengthcanbeobtainedbyusingoneormoreofthefollowing:

TypeIIIorHEhigh-early-strengthcement.Highcementcontent(400to500kg/m3or675to850lb/yd3).

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Lowwater-cementitiousmaterialsratio(0.37to0.45bymass).Higherfreshlymixedconcretetemperature.Highercuringtemperature.Acceleratingadmixtures.Insulationtoretainheatofhydration.Hydraulicnon-portlandcementsthathaverapidstrengthgain.

Highearly-strengthconcretecanbeusedforseveralapplications,includingcold-weatherconstruction,rapidrepairofpavementstoreducetrafficdowntime,fast-trackpaving,andseveralothers.Infast-trackpaving,usinghighearly-strengthmixturesallowstraf-fictoopenwithinafewhoursafterconcreteisplaced.SeeKosmatka,Kerkhoff,andPanarese(2002)foranexampleofafast-trackconcretemixtureusedforabondedconcretehighwayoverlay.

Whendesigningearly-strengthmixtures,strengthdevelopmentisnottheonlycriteriathatshouldbeevaluated;durability,earlystiffening,autogenousshrinkage,dryingshrinkage,temperaturerise,andotherpropertiesshouldalsobeevaluatedforcompat-ibilitywiththeproject.Formoreinformationonhighearly-strengthconcrete,consultACPA(1994).

Strength Testing: Mix Design StageConcretestrengthisnotanabsoluteproperty.

Resultsobtainedfromanygivenconcretemixturewilldependonspecimengeometryandsize,preparation,andloadingmethod(Carrasquillo1994).Specifica-tionsthereforeneedtosetoutthetestmethodsaswellasthevaluetobeachievedatagivenage.Variabilityisinherentinthetestmethodsandmaterials.Therefore,trialmixesshouldyieldstrengthssomewhathigherthantheminimumspecified(seeSequenceofDevel-opmentinchapter6,page172;QualityControlinchapter9,page242;andACI214[2002]).

Strengthisgenerallyexpressedinmegapascals(MPa)orpoundspersquareinch(lb/in2)ataspeci-fiedage.Seven-daystrengthsareoftenestimatedtobeabout75percentofthe28-daystrength,and56-dayand90-daystrengthsareabout10to15percentgreater,respectively,than28-daystrengths.

Compressivestrengthisnormallymeasuredbyloadingacylinderofconcreteatitsends(ASTMC39).

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Testingthetensilestrengthofconcreteisdifficult;itisnormallydeterminedindirectlybyusingabendingtestonabeam(ASTMC78)orbyusingasplittensiletest(ASTMC496),inwhichalineloadisappliedonoppositesidesofacylinder.

Fatigueisnormallytestedbycyclicallyloadingaconcretebeamwithamaximumloadsomewhatlessthantheconcrete’sultimatecapacity.Thenumberofcyclesbeforethebeamfailsthenindicatesthespeci-men’sfatigue.

Mix Design Testing: Flexural StrengthTheprimaryparameterusedinassessingpavement

strengthisflexuralstrength,ormodulusofrupture(MOR),becausethatisthecriticalmodeofloading.FlexuralstrengthisdeterminedinaccordancewithASTMC78/AASHTOT97(third-pointloading)(figure5-6a).Someagenciesusethecenter-pointflexuralstrengthtest(ASTMC293/AASHTOT177)(figure5-6b),particularlytoassesswhetherthepave-mentcanbeopenedtotraffic.

ForAASHTOthicknessdesign,itisimportantthatthethird-pointloading,28-dayflexuralstrengthbeusedintheAASHTOequation.Ifstrengthvaluesaremeasuredusingsomeothertestmethod,theresultsmustbeconvertedtothethird-pointloading,28-daystrength.(Seewww.pavement.com/PavTech/Tech/FATQ/fatq-strengthtests.htmlforstrengthcorrelationsandconversions.Publishedconversionfactorsarenecessarilygeneric,anditisadvisabletodeveloptherelationshipsforaspecificmixiftheyaretobeusedinacontract.)

Specimensforflexuralstrengthtestingarelargeandsomewhatdifficulttohandle.Measurementsofflexuralstrengtharemoresensitivethanmeasure-mentsofcompressivestrengthtovariationsintestspecimensandprocedures,especiallythemoistureconditionduringtesting.Forthesereasons,arelation-shipshouldbedevelopedbetweenflexuralstrengthandcompressivestrengthbylaboratorytestingforagivenmix.

Mix Design Testing: Compressive StrengthThecompressivestrengthofconcretecylindersor

cores(ASTMC39/AASHTOT22)canbeusedasanalternativetoflexuralstrengthtestingifsospecified.

www.pavement.com/PavTech/Tech/FATQ/fatq-strengthtests.html

www.pavement.com/PavTech/Tech/FATQ/fatq-strengthtests.html



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_______________________________________________Figure 5-6. (a) In third-point loading, the entire middle one-third of the beam is stressed uniformly, and thus the beam fails at its weakest point in the middle one-third of the beam. (b) By forcing the beam to fail at the center, the center-point loading flexural test results are somewhat higher than the third-point test results. Typically, center-point results are about 15 percent greater than third-point results and vary widely, depending on many concrete mix properties. (PCA)

Usually,acorrelationisestablishedbetweenflexuralandcompressivestrengths.Compressivestrengthistheparameternormallyusedinstructuralconcretespecifications.

Mix Design Testing: Splitting Tensile Strength Tests

Thesplittingtensiletestinvolvescompressingacylinderorcoreonitsside(ASTMC496)untilacrackformsdownthemiddle,causingfailureofthespecimen(figure5-7).Theloadinginducestensilestressesontheplanecontainingtheappliedload,causingthecylindertosplit.

Strength Testing: Field TestsStrengthtestsofhardenedconcretecanbeper-

formedonthefollowing:CuredspecimensmoldedinaccordancewithASTMC31orC192/AASHTOT23andT126fromsamplesoffreshlymixedconcrete.Insituspecimenscoredorsawedfromhard-enedconcreteinaccordancewithASTMC42/AASHTOT24.

Samplespreparedandcuredinthefieldarelikelytoshowadifferent(oftenlower)strengththansam-plespreparedandcuredinthelaboratorybecauseofthegreatervariabilityinthefield.Eventhen,theresultingstrengthreportedisthespecimenstrengthandnotthatofthein-placepavement.

Laboratory-curedspecimensareusedtodeterminethespecifiedconcretestrengthwithinadesignatedtimeperiod.Testspecimenscastinthefieldatthetimeofconcreteplacementarenormallyusedtodeterminewhentheconcretehasgainedenoughstrengthtobeopenedtotraffic.Insitucorestakenafterthepavementhashardenedareoftenusedtoverifythein-placeconcretestrength.

Cylindersizecanbe150x300mm(6x12in.)or100x200mm(4x8in.),providedthatthediameterofthemoldisatleastthreetimesthemaximumsizeaggregate.

Testsresultsaregreatlyinfluencedbythecondi-tionsofthecylinderendsandcores.Therefore,forcompressiontesting,specimensshouldbegroundorcapped.ASTMC617/AASHTOT231outlinemeth-odsforusingsulfurmortarcapping.ASTMC1231

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Figure 5-7. Splitting tensile strength (PCA)



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describestheuseofunbondedneoprenecapsthatarenotadheredorbondedtotheendsofthespecimen.

Testingofspecimensshouldbeperformedinaccor-dancewiththefollowingspecifications:

ASTMC39/AASHTOT22forcompressivestrength.ASTMC78/AASHTOT97forflexuralstrengthusingthird-pointloading.ASTMC293/AASHTOT177forflexuralstrengthusingcenter-pointloading.ASTMC496/AASHTOT198forsplittingtensilestrength.

Themoisturecontentofthetestspecimenhasacon-siderableeffectontheresultingstrength.Testingshouldbeperformedonatestspecimenthathasbeenmain-tainedinamoistcondition(thesurfacehasnotbeenallowedtodry).Beamsforflexuraltestsareespeciallyvulnerabletomoisturegradienteffects.Asaturatedcyl-inderspecimenwillshowlowercompressivestrengthandhigherflexuralstrengththanthoseforcompanionspecimenstesteddry.Thisisimportanttoconsiderwhencorestakenfromhardenedconcreteinservicearecomparedtomoldedspecimenstestedastakenfromthemoist-curingroomorwaterstoragetank.

Maturity Testing Inrecentyears,maturitymethodshavebeen

usedextensivelytopredictthein-placestrengthandstrengthgainofconcrete.Thebasisforthismethodissimple:Thestrengthandmodulusofelasticity(seeModulusofElasticityinthischapter,page123)ofaconcretespecimenaredirectlyrelatedtothequantityofheatdevelopedfromthehydratingcement.

Thepracticalbenefitofthismethodisthatfieldevaluationsofin-placestrengthcanbepredictedfromsimplemeasurementsoftheconcretetemperatureovertime.Maturitytestingcanbeeffectivewhenmakingdecisionsaboutopeningpavementstotraffic.

Thetheoreticalbenefitistheabilitytoaccuratelypredictboththestrengthandmodulusofelasticityofconcreteoverawiderangeofconditionsbasedsimplyonthetemperaturedevelopmentinthemod-eledpavement.

Maturitytestingismostusefulinestimatingthein-placepropertiesofconcrete.However,itisalsouseful

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indevelopingconcretemixes.Wheninsufficienttimeisavailabletoadequatelytestaconcretemixbeforeitisusedonaproject,maturitytestscanbeusedtopredictlater-agestrengths.Inthisregard,concretespecimenscanbecuredatelevatedtemperaturesand,usinganassumedorempiricallydeterminedmaturityconstant,themeasuredstrengthdevelopmentcanbecorrelatedbacktothestrengthdevelopmentiftheconcretewerecuredatstandardconditions.

Thebasisofmaturitytestingisthateachconcretemixhasauniquestrength-timerelationship.There-fore,amixwillhavethesamestrengthatagivenmaturitynomatterwhatconditions(timeortem-perature)occurbeforemeasurement.Maturitytestingentailsdevelopingamaturitycurvethatcorrelatesthedevelopmentofparticularconcretepropertiesforaspecificconcretemixtobothtimeandtemperature.Afterthematuritycurveisdeveloped,developmentoftheconcretepropertycanbeestimatedfromameasuredtime-temperaturerecordoftheconcrete.Thematurityfunctionisamathematicalexpressiontoaccountforthecombinedeffectsoftimeandtempera-tureonthestrengthdevelopmentofconcrete.Thekeyfeatureofamaturityfunctionistherepresenta-tionofthewaytemperatureaffectstherateofstrengthdevelopment.

ASTMC918usesthematuritymethodofmonitor-ingthetemperatureofcylinderscuredinaccordancewithstandardmethodsoutlinedinASTMC31/AASHTOT23.Cylindersaretestedatearlyagesbeyond24hours,andtheconcretetemperaturehis-toryisusedtocomputethematurityindexatthetimeoftest.Usinghistoricaldata,apredictionequationisdevelopedtoprojectthestrengthatlateragesbasedonthematurityindexandearly-agestrengthtests.SeeCarino(1994)formoreinformation.

ASTMC1074providesproceduresforusingthemeasuredin-placematurityindextoestimatein-placestrength(figure5-8)(seechapter9,page261).Thispracticedescribestwomaturityfunctions.Thefirst,andmostpopularforusewithconcretepavements,istheNurse-Saulfunction.Theother(preferred)matu-rityequationistheArrheniusfunction.Thisfunctionpresentsmaturityintermsoftheequivalentageofcuringatstandardlaboratoryconditions.Althoughthe



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_______________________________________________Figure 5-8. Maturity of the field concrete is equivalent to maturity of the laboratory concrete when the area under the curves is the same. (CTLGroup)

equivalent-agematurityfunctionpresentsresultsinamoreunderstandableformat(theequivalentage),thecomplexityofitsequationislikelywhythismethodislesspopularthanthetime-temperaturefactormethod.

Equivalentageisexpectedtoprovidemoreaccurateresultswhenlargetemperaturechangesoccurinthefield.However,thetime-temperaturemethodiseasiertoapplyandhasbeensuccessfullyusedforestimatingthein-placestrengthofpavingconcrete.Eachofthesefunctionsrequirespreliminarytestingtorelatetothestrengthofconcrete.Eachmethodhasaconstantthatcanbeassumedor,foraccuracy,determinedforthespecificmix(ASTMC1074).Inthetime-temperaturefactor,adatumtemperaturebelowwhichhydrationisminimalcanbedeterminedexperimentally.Intheequivalent-agemethod,anactivationenergyiscalculated.

Maturitycanbedeterminedwithcommerciallyavailabletestequipmentorwithstandardtemperatureloggingequipmentandanunderstandingofmaturity(seeConcreteMaturityinchapter9,page261).Ineithercase,temperaturesensorsmustbeplacedatthecriticallocationsintheconcrete.Formostpavements,thiswillbeeitheratthetoporbottomsurface.Whenindoubt,usethelowerofthematuritiesfromsensorsatthetopandbottomsurfaces.

Commerciallyavailablematuritymeterslogtheconcretetemperatureasafunctionoftimeandpres-entthecurrentmaturity(time-temperaturefactororequivalentage)oftheconcrete.Therearetwoprimarytypesofmaturitymeters.Thefirsttypeusestempera-turesensorsintheconcretewithloggingequipmentoutsidetheconcrete.Thesecondtypecombinesthetemperaturesensorandloggingequipmentinasinglepackage,whichisembeddedintheconcrete.Wiresextendfromthesensoroutsidetheconcrete.Thesewiresmustbeperiodicallyconnectedtoahand-held

readersothatthematuritydatafromthesensorcanberead.Wirelessequipmentisalsoavailable.

Maturitycanalsobecalculatedfromtemperaturesensors(likethermocouples,thermometers,ortherm-istors)embeddedintheconcrete.Thetimeintervalshouldbeselectedtoadequatelyresolvetemperaturechangesintheconcrete.SomeStatesrequiretwicedailyreadings,althoughmorefrequentintervalswouldimproveaccuracy.

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Modulus of Elasticity and Poisson’s Ratio

Key Points

Concrete’smodulusofelasticitygenerallycorrelateswiththestrengthoftheconcreteandthetypeandamountoftheaggregate.

Modulusofelasticityisusedinstructuraldesignofconcretepavementandformodelingtheriskofcrackinginconcretepavement.

Poisson’sratioisnormallyabout0.2inhardenedconcrete.

Similartothepredictionofconcretestrength,maturitymethodscanbeusedtopredictthemodulusofelasticity.

Testingspecifications:ASTMC469(dynamicmodulus:ASTMC215)

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Simple DefinitionThemodulusofelasticity(E),orstiffness,of

concreteisameasureofhowmuchthematerialwilldeflectunderloadandindicatesriskofcracking.Poisson’sratioisameasureofdeflectionthatisper-pendiculartotheload.

SignificanceThemodulusofelasticity(E)parameterisusedin

thestructuraldesignofthepavementandformodel-ingtheriskofcracking.Strictlydefined,themodulusofelasticityistheratioofstresstocorrespondingstrainforloadsuptoabout40percentoftheulti-matestrength(figure5-9).(Dynamicmodulusistheresponseofconcretetodynamicratherthanstaticloading.Thedynamicmodulusofconcreteisnormallyabout10percenthigherthanthestaticmodulus.)

Normal-densityconcretehasamodulusofelastic-ityof14,000to41,000MPa(2,000,000to6,000,000lb/in2),dependingonfactorslikecompressivestrengthandaggregatetype.Fornormal-densityconcretewithcompressivestrengthsbetween20and35MPa(3,000and5,000lb/in2),themodulusofelasticity

canbeestimatedas5,000timesthesquarerootofstrength(57,000timesthesquarerootofstrengthinlb/in2).Severalformulashavebeensuggestedforhigh-strengthconcrete(FarnyandPanarese1993).Likeotherstrengthrelationships,therelationshipofmodu-lusofelasticitytocompressivestrengthisspecifictomixingredientsandshouldbeverifiedinalaboratory(Wood1992).

Poisson’sratioisameasureofthedeflectionthatisperpendiculartothedirectionoftheload.Acommonvalueusedforconcreteis0.20to0.21,butthevaluemayvaryfrom0.15to0.25dependingontheaggre-gate,moisturecontent,concreteage,andcompressivestrength.Poisson’sratioisgenerallyoflittleconcerntothedesigner;however,itdoesplayaroleinpredictingearly-ageconcretepavementbehaviorandisrequiredinsomenumericalmodelsforconcreteperformance.ItisadesigninputintheGuide for Mechanistic- Empirical Design of New and Rehabilitated Pavements(M-EPDG)(NCHRP2004).

Factors Affecting ElasticityModulusofelasticityincreaseswithanincreasein

compressivestrengthandisinfluencedbythesamefactorsthatinfluencestrength.

Itisprimarilyaffectedbythemodulusofelasticityoftheaggregateandbythevolumetricproportionsoftheaggregateinconcrete(Baalbakietal.1991).Ifan

_______________________________________________Figure 5-9. Generalized stress-strain curve for concrete (PCA)

Modulus of Elasticity and Poisson’s Ratio


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aggregatehastheabilitytoproduceahighmodulus,thenthehighestmodulusinconcretecanbeobtainedbyusingasmuchofthisaggregateaspracticalwhilestillmeetingworkabilityandcohesivenessrequirements.

Testing for Modulus of Elasticity and Poisson’s Ratio

StaticmodulusofelasticityandPoisson’sratioofconcreteincompressionaretestedaccordingtoASTMC469.Specimenscanbeconcretecylindersorcores.IntheASTMC469testprocedure,thecylinderisloadedto40percentoftheconcretecompressive

strength.Inordertodeterminethe40percentload,itisfirstnecessarytodeterminetheconcretecompres-sivestrengthoncompanionspecimens.Thisrequiresamachinewithadequatecapacitytobreakthecylinders.

DynamicmoduluscanbedeterminedbyusingASTMC215,inwhichthefundamentalresonantfrequencyisrecordedinasamplestruckwithasmallhammer.Thedynamicmodulusiscalculatedfromtherecordedfrequency.

Similartothepredictionofconcretestrength,matu-ritymethodscanbeusedtopredicttheactualmodulusofelasticity.

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Shrinkage

Key Points

Concreteshrinkageoccursduetoanumberofmechanismsstartingsoonaftermixingandcontinuingforalongtime.

Shrinkageprimarilyincreaseswithincreasingwater(paste)contentintheconcrete.

Testingspecifications:ASTMC157/AASHTOT160.

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Simple Definitions of Shrinkage Shrinkageisadecreaseinlengthorvolumeofthe

concreteduetomoistureloss.

SignificanceStartingsoonaftermixingandcontinuingforalong

time,concreteshrinksduetoseveralmechanisms.Becauseconcreteshrinkageisgenerallyrestrainedinsomeway,concretealmostalwayscracks.Uncon-trolledcracksthatformatearlyagesarelikelytogrowduetomechanicalandenvironmentalstressesthatwouldotherwisebeofnoconcern.Therefore,minimizinguncontrolledearlyshrinkagecrackingcanprolongtheservicelifeoftheconcrete.

Factors Affecting ShrinkageSoonafterconcreteisplaced,itshrinksdueto

chemicalchangesandlossofmoisture:Autogenousshrinkageistheamountofchemicalshrinkagethatcanbemeasuredinasample.(Chemicalshrinkageoccursbecausethevolumeofthehydrationproductsofcementoccupieslessspacethantheoriginalmaterials.)Autogenousshrinkageisnormallyinsignificantinconcretewithahighwater-cementitiousmaterials(w/cm)ratio,butitbecomesimportantwhenthew/cmratioisbelow0.40.Thedifferenceisobservedasmicrocrackinginthematrix.(Aw/cmratio

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belowabout0.40isnotrecommendedforslipformpaving,althoughsomeStateshavehadsuccesswithratiosaslowas0.37.)Concreteshrinksasmoistureislostfromthesystem,largelyduetoevaporation.Plasticshrinkageoccursbeforetheconcretesetsandisprimarilyduetolossofmoisturefromthesurfaceoffreshconcrete.Itcanresultinplasticcrackinginthesurface.Dryingshrinkageoccursaftertheconcretehasset.Iftheshrinkageisrestrained,dryingshrinkagecrackingwilloccur(seeEarly-AgeCrackinginthischapter,page148.)

Inaddition,concreteshrinkssomewhatasitsettlesandcontractsasitcools(seeTemperatureEffectsinthischapter,page127).Alloftheseshrinkageeffectsareadditive;therefore,reducinganyoneofthemwillreducetheriskofcracking(figure5-10).

Themostimportantcontrollablefactoraffect-ingdryingshrinkageistheamountofwaterperunitvolumeofconcrete(figure5-11).Totalshrinkagecanbeminimizedbykeepingthewater(orpaste)contentofconcreteaslowaspossible.Thehigherthecementcontentofamixture,thegreaterthemagnitudeoflikelydryingshrinkage.Thepastecontentcanbemini-mizedbykeepingthetotalcoarseaggregatecontentoftheconcreteashighaspossiblewhileachievingwork-abilityandminimizingsegregation.

Dryingshrinkagemayalsobereducedbyavoidingaggregatesthatcontainexcessiveamountsofclayin

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________________________________________________Figure 5-10. Total shrinkage is a sum of all individual shrinkage mechanisms. Minimizing any or all of the mechanisms will reduce the risk of cracking. (CTLGroup)

Shrinkage


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_______________________________________________Figure 5-11. Relationship between total water content and drying shrinkage. Several mixtures with various proportions are represented within the shaded area of the curves. Drying shrinkage increases with increasing water contents. (PCA)

theirfines.Quartz,granite,feldspar,limestone,anddolomiteaggregatesgenerallyproduceconcreteswithlowdryingshrinkages(ACICommittee2001).

Weather—airtemperature,wind,relativehumid-ity,andsunlight—influencesconcretehydrationandshrinkage.Thesefactorsmaydrawmoisturefromexposedconcretesurfaceswithresultantslowingofhydrationandincreasedwarping(seeCurlingandWarping:AVariationofVolumeChangeinthischap-ter,page150).

Cementtypesandsupplementarycementitiousmaterialsusuallyhavelittledirecteffectonshrinkage.

Testing for Shrinkage Limitstochangesinvolumeorlengtharesome-

timesspecifiedforconcretepavements.Volumechangeshouldalsobetestedwhenanewingredientisaddedtoconcretetomakesurethattherearenosignificantadverseeffects.

ASTMC157/AASHTOT160arecommonlyusedtodeterminelengthchangeinunrestrainedconcreteduetodryingshrinkage.Sincetherateandmagnitudeofshrinkageareinfluencedbyspecimensize,anyspecificationbasedonASTMC157mustincludethespecimensize.Thestandardprocedureistotakeaninitiallengthmeasurementat24hours.The

specimensarethenstoredinlime-saturatedwaterfor27days,whenanotherlengthmeasurementistaken.Allspecimensarethenstoredinairataconstanttemperatureuntil64weeks.(Sincemostprojectscannotwait64weeks,analternativesetofinitialread-ing,dryingage,andfinalreadingagearesometimesspecified.)

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Temperature Effects

Key Points

Concretehydrationgeneratesheat.

Theriskofcrackingisincreasedwithincreasingplacementtemperature.

Concreteexpandswithincreasingtemperatureandcontracts(shrinks)withdecreasingtemperature.Theamountofthisexpansionandcontractionisgovernedprimarilybytheaggregatetype(seeAggregatesinchapter3,page39.)

Thehydrationrateofcementitiousmaterialsisacceleratedwithincreasingtemperatureandwithincreasingcementfineness.Thiscanacceleratesettingtimeandreducethesawingwindow.

Supplementarycementitiousmaterialstypicallyhavelowerheatofhydration.

Incold-weatherconcreting,thechallengeistomaintaintheconcretetemperaturetopreventfreezingwhilegainingstrength.

Testingspecifications:ASTMC186,ASTMC1064/AASHTOT309,AASHTOTP60.

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Simple DefinitionThereactionofcementwithwater(hydration)in

concretemixturesgeneratesheat(seethetime-heatcurveinfigure4-13,page74).Monitoringthetem-peratureisausefulmeansofestimatingthedegreeofhydrationofthesystem.Inturn,thetemperatureoftheconcretewillinfluencetherateofhydration(strengthdevelopment)andtheriskofcracking.

Significance of Thermal PropertiesAnoptimaltemperatureforfreshlyplacedcon-

creteisintherangeof10to15°C(50to60°F),anditshouldnotexceed30to33°C(85to90°F)(Mindessetal.2003).Problemsassociatedwithhighconcrete

temperaturesincludeincreasedwaterdemandtomaintainworkability,decreasedsettingtime,increaseddangerofplasticshrinkagecracking,reductionintheeffectivenessoftheair-voidsystem,increasedriskofincompatibility(seePotentialMaterialsIncompat-ibilitiesinchapter4,page97),andlowerultimatestrength.Duringthewinter,theprimarydangeristhatlowtemperaturesmayslowhydration,andthusstrengthgain,andinextremecasesmaypermanentlydamagetheconcreteifitfreezesearlyinitslife(Mindessetal.2003).

Otherthermaleffectsthatmaybeofinterestincludesolarreflectance,specificheat,andthermaldiffusivity.Thesepropertiesaffecttheamountofsolarenergyabsorbedbyconcrete,thecorrespondingtem-peraturechangeoftheconcrete,andtherapiditywithwhichtheconcretedissipatesthistemperaturetoitssurroundings.

Effects on HydrationTherateofhydrationofconcreteissignificantly

acceleratedwithincreasingtemperaturesandslowedatlowertemperatures,affectingplacementandconsol-idationduetoearlystiffening,aswellasthetimingofsawcutting.Theearlystrengthofaconcretemixturewillbehigherwithanelevatedtemperature,butthestrengthmaybeloweratlateragesthanthesamemixkeptatalowertemperature.Allchemicalreactionsarefasterathighertemperatures;therefore,settingtimeswillbereducedasthetemperatureoftheconcreterises.Withincreasingtemperature,thepotentialforanimbalanceinthecementitiouspastesystemwillbeexacerbated,possiblyleadingtoproblemswithunex-pectedstiffeningofthemixturebeforethemixturecanbeconsolidated.Ithasbeenobservedthatwatermaybeaddedtosuchamixtorestoreworkability,butwiththeeffectofreducingstrengthanddurability.Watershouldnotbeaddedtothemixinexcessofthespecifiedmaximumwater-cementratio.

Effects on CrackingConcreteexpandsastemperaturerisesand

contractsastemperaturefalls.Thesemovementscancontributesignificantlytotheriskofcrackinginconcrete,particularlywithinthefirst24hours.Iftheconcretesetswhenitishot,thenitwill

Temperature Effects


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contractasignificantamountwhenitcoolslater,thussignificantlyincreasingtheriskofcracking.Whenacoldfrontpassesoverconcretethathasbeenplacedinthemiddleofahotday,theriskofcrackingisgreaterbecausethetemperaturedropislarge(seeEarly-AgeCrackinginthischapter,page148).

Factors Affecting Thermal PropertiesPrimaryfactorsthataffectaconcretepavement’s

thermalpropertiesarethefollowing:

Heat of Hydration of Cementitious Materials TypeIIIcements,whicharetypicallyusedinappli-

cationswhereearlystrengthisagoal,generatemoreheatandatafasterratethanTypeIcements.Table5-3showsthetypicalrangesofchemicalcompoundsincement,andtheheatevolutionassociatedwiththesecompounds.Thistableisusefulinpredictingtherela-tiveheatgenerationofsimilarcements.

Cementfinenessaffectstherateofheatgenera-tion.Finercements(smallerparticlesizes)hydratefasterandgenerateheatatafasterrate.Thetotalheatevolutionisnotaffectedbythefineness,butthepeaktemperatureoftheconcretemaybehigher,astheheat

isevolvedmorequickly.Pozzolansalsogenerateheatduringhydration

but,generally,lessthanportlandcement.(Theben-efitofalowerheatofhydrationistoreducethermalshrinkageandthepossibilityofresultantcracking.Whenblendedwithcement,ClassFflyashhasaheatofhydrationthatistypically50percentofcement.ClassCflyashgenerallyhasaheatofhydrationintherangeof70to90percentofthatofcement.Theheatofhydrationofbothsilicafumeandmetakaolinareapproximately125percentofcement.Theheatofhydrationinconcreteisaffectedbytheamountandgradeofground,granulatedblastfurnace(GGBF)slag.Theheatofhydrationofgrade100GGBFslagistypically80percent.

Initial Temperature Asmentionedpreviously,multiplestrategiescan

beemployedduringconstructiontoreducetheinitialtemperatureheat.Acommonpracticeistoconductpavingonlyatnightand/ortousepre-cooledmate-rialsinthebatch.Pre-coolingcanbeachievedbyshadingandwettingtheaggregatesandusingchilledwateroriceinthemixture.Awhite-pigmentedcuring

Table 5-3. Chemical Composition and Heat Evolution of Typical Portland Cements

Potential phase composition*, %

Cement type C3S (%) C2S (%) C3A (%) C4AF (%) Blaine fineness, m2/kg

Type I Range 45–65 6–21 6–12 6–11 333–431 Mean 57 15 9 8 384

Type II Range 48–68 8–25 4–8 8–13 305–461 Mean 56 17 7 10 377

Type III Range 48–66 8–27 2–12 4–13 387–711 Mean 56 16 8 9 556

Type V Range 47–64 12–27 0–5 10–18 312–541 Mean 58 18 4 12 389

Heat evolution**, a b c dkJ/kg

(kilojoules/kg) 3 d 243 50 887 289 7 d 222 42 1556 494

13 yr 510 247 1356 427

* Source: Tennis and Bhatty (2006)** Cement paste with water/cement ratio 0.4 at 21˚C (70˚F).

Values represent coefficients in the equation: Heat evolution (kJ/kg) = a(C3S) + b(C2S) + c(C3A) + d(C4AF) (Taylor 1997)

Temperature Effects


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compoundshouldalwaysbeappliedimmediatelyafterfinalfinishingwhilethesurfaceisstilldamptoalleviateevaporationandreduceheatbuildupfromsolarradiation(Kosmatkaetal.2003).

Environmental Factors Therateofheatlosstotheenvironmentis

influencedbythethicknessoftheconcrete,thetemperatureoftheenvironment,andthedegreeofinsulationprovidedbythematerialssurroundingtheconcrete.Thinnerconcretesectionswillnotgetashotasthickersections.

Incold-weatherconcreting,thechallengeistomaintaintheconcretetemperaturetopreventfreez-ingwhiletheconcretegainsstrength.Strategiesthatmaybeemployedincludeheatingoneormoreofthematerials,warmingthejobsiteenvironment,and/orcoveringtheslabwithinsulationtoholdintheheatproducedasthecementhydrates.

Thermal Expansion/ContractionThermalexpansionandcontractionofconcrete

(thatis,expansionandcontractionrelatedtotempera-turechange)varywithfactorssuchasaggregatetype,cementcontent,water-cementitiousmaterialsratio,temperaturerange,concreteage,andrelativehumidity.Ofthese,aggregatetypehasthegreatestinfluence,asaggre-gatesaccountforabout60to75percentofconcretebyvolume.

Amaterial’scoefficientofthermalexpansion(CTE)isameasureofhowmuchitchangesinlength(orvolume)foragivenchangeintemperature.Typically,anincreaseintemperaturewillresultinlengthening(expansion)andadecreasewillresultinshortening(contraction).Becauseaggregatesmakeupamajorityofaconcrete’svolume,theCTEoftheaggregatepar-ticleswilldominatetheCTEforaconcrete.Table3-12inchapter3,page48,showssometypicalvaluesofthelinearCTEofseveralaggregates,aswellasthoseforconcreteandotherconcreteingredients.Typically,limestoneaggregateshavelowercoefficientsthansili-ceousaggregates,andconcretesmadewiththemhavelowervalues.CTEvaluesareconsideredindesigncalculationsforpavementsandareusedindurabilitymodelingofconcretes.

AnaveragevaluefortheCTEofconcreteisabout10×10-6/°C(5.5×10-6/°F),althoughvaluesrangingfrom6to13×10-6/°C(3.2to7.0×10-6/°F)havebeenobserved.Inpracticalterms,thisamountstoalengthchangeofaround5mmfor10mofconcrete(2/3in,for100ftofconcrete)subjectedtoariseorfallof50°C(90°F).Temperaturechangesmaybecausedbyenvironmentalconditionsorbytheheatofhydration.

Testing for Thermal PropertiesIfspecificinformationisneededabouttheactual

heatofhydrationofthecementitiousmaterials,theheatofhydrationshouldbemeasured.Testingshouldbeconductedusingtheproportionsthatwillbeusedintheconcretemix,ratherthanusingtheindividualcomponents.

TheheatofhydrationcanbemeasuredinaccordancewithASTMC186orinaconductioncalorimeter.Heatevolutionoftheconcretecanalsobemeasureddirectlybyacalibratedcalorimeter.Severalsuchinstrumentsarecommerciallyavailable.Theyconsistofacalibratedinsulatedcontainerthatmea-surestheheatflowoutofacylinderoffreshconcrete.

Theadiabatic(withoutlossorgainofheatfromthesurroundings)temperatureriseoftheconcretecanbemeasureddirectlybyArmyCorpsMethodCRD-C38(figure5-12).Itshouldbenotedthattheactualtem-peratureriseofaconcretepavementisonlyafractionofthatoftheadiabatictemperaturerisebecausethe

Figure 5-12. A typical field semi-adiabatic temperature monitoring system for mortar (W.R. Grace & Company)

Temperature Effects


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largesurface-to-volumeratioofapavementallowsheattoescapealmostasrapidlyasitisgenerated.

PavementtemperaturesandtheassociatedriskofcrackingcanbeestimatedwithfiniteelementsoftwareorbytheSchmidtmethod(afinitedif-ferencemethod)describedinACI207.FHWA’sHIPERPAVsoftware(www.hiperpav.com)isahelpfulanalysistooltodetermineearly-agepropertiesandthepotentialforcracking.Pavementtemperaturesmeasuredwithembeddedsensors(ASTMC1064/AASHTOT309-99)andenvironmentalconditionsmonitoredwithaportableweatherstation(seeCon-creteTemperature,SubgradeTemperature,andProjectEnvironmentalConditionsinchaper9,page260)areHIPERPAVdatainputs.

Figure 5-13. Measuring the coefficient of thermal expansion

Concrete Durability is Affected by Many Concrete Properties

The rest of this chapter discusses concrete properties that can affect concrete durability.

Durability is concrete’s ability to resist chemical and environmental attack while in service. It is a critical characteristic of long-life pavements but, in itself, is not generally considered a property of concrete.

Durability is environment-specific; that is, a concrete pavement that is durable in a snow environment is probably not going to be durable in the desert. Therefore, concrete properties that contribute to durable concrete may include any (but not necessarily all) of the following:

Low permeability. Frost resistance. Sulfate resistance. Low alkali-silica sensitivity. Abrasion resistance.

Cracking also affects concrete permeability. Note, however, that concrete strength is not necessarily a good measure of potential durability.

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Coefficientofthermalexpansioncanbemea-suredusingthemethoddescribedinAASHTOTP60(figure5-13)(seeCoefficientofThermalExpansioninchapter9,page269).Thetestinvolvesmeasuringthelengthchangeofaspecimenobservedduetoachangeintemperatureof40°C(72°F)controlledusingawaterbath.

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Permeability

Key Points

Concretedurabilityisimprovedbyimprovingconcrete’sabilitytopreventfluidsfrompenetrating,thatis,byreducingconcretepermeability.

Permeabilityisreducedbyreducingcracking,reducingthenumberofconnectedporesinthepastesystem,andimprovingtheinterfacebetweenthepasteandaggregate.

Therefore,permeabilitycanbereducedbyreducingthewater-cementitiousmaterialsratio,increasingthedegreeofhydration,usingsupplementarycementitiousmaterials,andusinggoodcuringpractices.

Strengthisnotnecessarilyagoodmeasureofpotentialpermeability.

TestingspecificationsincludeASTMC1202/AASHTOT277,AASHTOTP64,ASTMC1543and1556,andASTMC1585.

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Simple DefinitionPermeabilityistheeasewithwhichfluidscanpene-

trateconcrete.(Permeabilityisdifferentfromporosity,whichisameasureofthenumberof[possiblydiscon-nected]voidsinconcrete.)

SignificanceAlmostalldurability-relateddistressesinpave-

mentscanbeslowedorstoppedbyreducingtheconcrete’spermeability.Thisisbecausemostdurabil-ity-relateddistressmechanismsinvolvethetransportofharmfulsubstancesintotheconcrete:

Waterthatexpandsonfreezing,leachescal-ciumhydroxide,and/orcarriesdissolvedionsthatattacktheconcrete.Saltsthatcrystallizeonwettinganddryingorexertosmoticpressureduringfreezingandthawing,causingsurfacedamage.

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Alkalisthatreleasehydroxylsthatreactwithalkali-reactiveaggregates.Sulfatesthatattackthealuminatecompounds.Carbondioxidethatreducesthealkalinity(pH).Oxygenandmoisturethatcontributetothecorrosionofsteelbarsorreinforcement.Chloridesthatpromotecorrosionofsteelbars.

Factors Affecting PermeabilityPermeabilityisprimarilycontrolledbythepaste

systeminconcreteandthequalityoftheinterfacialzonebetweenthepasteandtheaggregate.Iftherearealargenumberofporesandtheyareconnected(percolated),thentheconcretewillbepermeable.Reducingtheporosityandthelikelihoodthatporeswillbeconnectediskeytoachievinglowpermeability(Detwiler2005).Thefollowingactivitieswillhelp:

Reducingthewater-cementitiousmaterialsratioaslowasisconsistentwithotherrequirementsoftheconcrete,includingcracking.Usingappropriatesupplementarycementitiousmaterialsfortheenvironment.Ensuringadequateconsolidationandcuring.Minimizingcracking.

TestingSomespecificationsusecompressivestrengthas

anindicatoroflowpermeability.Thisapproachisnotalwaysvalid,whichiswhytestsarebeingdevelopedtodirectlyassessconcretepermeability.

Theso-called“rapidchloride”test(ASTMC1202/AASHTOT277)ismostcommonlyused(seeChlorideIonPenetrationinchapter9,page268).However,thetestdoesnotmeasurepermeabilitydirectlybutmeasuresconductivity,whichmayormaynotberelatedtopermeability.Thedatacanbeusedforcomparisonpurposesbetweenconcretemixtures.

AASHTOTP64isanalternativemethodtoassesstherateofchloridepenetrationintoconcrete.ASTMC1543and1556aretwonewpondingtestsusefulindeterminingthechlorideresistanceofconcrete.Anothernewmethod,ASTMC1585,deter-minesthecapillaryabsorptionofconcreteandoffersausefultoolforassessingtherelativeperformanceofdifferentconcretesystems.

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Frost Resistance

Key Points

Problemsrelatedtolowfrostresistanceincludefreeze-thawdamage,saltscaling,D-cracking,andpopouts.

Concretefrostresistancecanbeimprovedthroughseveralgoodpractices:

Maintainingaproperair-voidsystemtoprovidespaceforfreezingandexpandingwatertomoveinto.

Attainingadequatestrength.

Usingsoundaggregate.

Payingcarefulattentiontogoodmixing,placement,andcuringpractices.

Protectingconcretesothatitgainssufficientstrengthanddriesbeforeitisexposedtofreezingtemperaturesordeicingsalts.

Specificationsincludethefollowing:

Aggregates:ASTMC666/AASHTOT161,ASTMC88/AASHTOT104,IowaPoreIndexTest.

Air-voidsystem:ASTMC231/AASHTOT152,ASTMC173/AASHTOT196,ASTMC138/AASHTOT121,ASTMC457.

Freeze-thaw:ASTMC666.

Saltscaling:ASTMC672.

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Simple DefinitionFrostresistanceisaconcrete’sabilitytoresist

damageduringwinterweatherconditions.

SignificanceConcretethatisexposedtocoldweathercanexpe-

rienceseveralkindsofdamage:freeze-thawdamage,saltscaling,D-cracking,orpopouts.

Freeze-thawdamageisduetotheexpansionofwaterinthecapillariesasitfreezes,causingcrackingthatcanbeasdeepasseveralinchesintotheconcrete.Cyclesoffreezingandthaw-ingcaneventuallycauseseveresurfaceloss(figure5-14).Deicingsaltscanaggravatefreeze-thawdamageandcausecracking,scaling,anddisintegration.Whenaggregateswithacriticalporesizearesaturated,theyexpandandfracturewhenthewaterfreezes,causingD-crackingand/orpopouts.

Freezing and Thawing DamageWaterexpandsaboutninepercentwhenitfreezes.

Ifthecapillariesandvoidsinaconcretearesaturated(filledwithwater),thisexpansioncansetuppressuresgreaterthanthestrengthoftheconcrete(NewlonandMitchell1994;JanssenandSnyder1994;Cordon1966),andtheconcretewillcrack,increasingcon-cretepermeability.

Forresistancetofreezingandthawingdamage,water-saturatedconcretesmusthaveaproperair-voidsystem,soundaggregates,andastrengthofatleast28MPa(4,000lb/in2)tobeabletoresistthestressesthataresetup.Concretethatwillbeexposedtofreez-ingconditionsshouldbeallowedtodrybeforeitisexposedtothefirstfreeze,thusreducingthedegreeofsaturation(VerbeckandKlieger1957).

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Figure 5-14. Air-entrained concrete (bottom bar) is highly resistant to damage from repeated freeze-thaw cycles. (PCA)

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Salt ScalingScaling(figure5-15)isaphysicaldeterioration

mechanismaggravatedbytheuseofdeicingsaltsandfreezingandthawing.Saltsthatareusedtomeltsnowandicegointosolutionandpenetrateconcrete’sporestructure,aggravatinghydraulicpressureswhenthesolutionfreezes.Inaddition,asthewaterfreezestoice,thesaltsareconcentratedatthefreezingsite.Unfrozenwatermigratestowardthesiteduetoosmo-sis(thetendencyforimbalancesinsaltconcentrationstoseekbalance).Theseosmoticpressuresalsocausecracking,scaling,anddisintegration.

Inadditiontohydraulicandosmoticpressures,whicharetheprimarycauseofdeicerscaling,saltsmayalsocrystallizeupondrying,creatingexpansivepressures.

Researchhasshownthatrelativelylowconcentra-tionsofsodiumchloride(twotofourpercent)causegreaterdamagethangreaterconcentrationsofsodiumchloride(Klieger1957).

Thebestwaytocontroldeicerscalingistoensurethattheconcretehasaproperair-voidsystem,soundaggregates,andastrengthofatleast28MPa(4,000lb/in2),andhasbeenallowedtodrybeforeitisexposedtodeicingsalt.

Figure 5-15. Scaled concrete surface resulting from lack of air entrainment, use of deicers, and poor finishing and curing practices

D-CrackingD-crackingisdamagethatoccursinconcretedue

toexpansivefreezingofwaterinsomeaggregatepar-ticles(seeAggregateDurabilityinchapter3,page47).Thedamagenormallystartsnearjointstoformachar-acteristicD-shapedcrack(seechapter3,figure3-11,page49).

Thisproblemcanbereducedeitherbyselectingaggregatesthatarelesssusceptibletofreeze-thawdeteriorationor,wheremarginalaggregatesmustbeused,byreducingthemaximumaggregatesize.Also,providingdrainageforcarryingwaterawayfromthebasemaypreventsaturationofthepavement.

PopoutsApopoutisaconicalfragmentthatbreaksout

ofthesurfaceofconcrete,leavingashallow,typi-callyconical,depression(seechapter3,figure3-12,page50).Generally,afracturedaggregateparticlewillbefoundatthebottomofthehole.Unlessnumerous,popoutsareconsideredacosmeticdetractionanddonotgenerallyaffecttheservicelifeoftheconcrete.

Aggregatescontainingappreciableamountsofshaleorothershalyrocks,softandporousmateri-als(claylumpsforexample),andcertaintypesofcherthavelowresistancetoweatheringandshouldbeavoided(seeAggregateDurabilityinchapter3,page47).Weak,porousparticlesintheaggregatecanbereducedbyvariousmethodsofaggregatebeneficia-tionsuchasjigging,heavy-mediaseparation,orbeltpicking.

Factors Affecting Frost ResistanceManyfactorsinfluencetheresistanceofconcrete

tofrost-relateddamage,eachofwhichisdiscussedbrieflybelow.Themostimportantmeansofprevent-ingdamageincriticallysaturatedconcreteistoensurethatithasanadequateair-voidsystemandadequatestrengthtoresistthestresses.Somehigh-strengthmixturesmaynotneedasmuch(orany)airtobefrostresistant,buttheadditionofairwillprovideaddi-tionalprotectionforrelativelylittlecost.

Air-Void SystemConcrete’ssusceptibilitytofreeze-thawdamage

maybereducedbyentraininganumberofsmall,

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closelyspacedairbubblesinthepaste(seeFunctionofAirEntrainmentinConcreteinchapter3,page56).Theairvoidsprovidespaceforfreezing,expand-ingwaterintheporestomoveinto,thusrelievingthepressure(Mielenzetal.1958).Aproperair-voidsystemcontainssmallairbubblesuniformlydispersedthroughoutthecementpaste(figure5-16).

Powers(1949)introducedtheconceptofaspac-ingfactor,whichindicatesthedistancewatermusttraveltoreachanairvoid.Itisgenerallyacceptedthataspacingfactorof0.20mm(0.008in.)orlesswillbeadequatetoprotectconcrete(Mielenzetal.1958;ACI201.2R2001;ACI3182002).Increasingthetotalaircontentwillreducethespacingfactor,whichiswhymanyspecificationsplacealimitontheminimumamountofairinamixture.Foragivenaircontent,small,closelyspacedairvoidsprovidebetterprotectionthanlarger,moredistantvoids(Klieger1994).Forequalprotection,largerbubbleswouldrequirealargervolumeofair.Thisisundesirable,

_______________________________________________Figure 5-16. Air voids provide a place for expanding water to move into as it freezes. (CTLGroup)

sinceanincreaseinaircontentcanresultinadecreaseinstrength.Optimally,theaircontentofthemortarshouldbeaboutninepercent.

Theamountofair,aswellasthesizeandspacingofthevoids,areinfluencedbythefollowingfactors(WhitingandNagi1998):

Type and Dosage of Air Entrainer.Newer,non-vinsoladmixturesproducesmallerairbubblesthanolderproducts.Air-entrainingadmixturesarealsoavail-ableformulatedspecificallyforlowslumpmixtureslikethoseusedforpaving.Increasingthedosagewillincreasethetotalaircontentofamixture.

Grading and Amount of Aggregate.Airismosteasilyentrainedinconcretethatcontainsincreas-ingamountsofsandinthe150to600µm(#100to#30sieve)range.Increasingthetotalaggregateintheconcretemeansthatthereislesspastethathastobeprotectedwithentrainedair.

Chemical Composition of the Cementitious Materials.Morealkalisinthecementwillresultinmoreentrainedair.Highloss-on-ignition(LOI)flyash(thatis,lossofmasswhenheatedto1,000°C[1,830°F])willrequiregreaterdosagesofadmixturetoachievethesameaircontent.VariableLOIinaflyashsourcemaycauselargejumpsintheconcreteaircontent.Suchaflyashshouldbemonitoredusingthefoamindextestwitheverydeliverytopreventproblemsinthebatchplant.

Fineness and Amount of Cementitious Materials.Mix-tureswithfinercementandincreasingcementcontentwillrequireahigherdosageofadmixturetoachievethesameaircontent.

Chemical Characteristics of Water; Water-Reducing or Retarding Admixtures.Otheradmixturesmayinterferewiththeair-entrainingadmixture.Possibleincompat-ibilitiesshouldbeevaluatedintrialbatchesearlyintheselectionofmaterialsandproportions.

Water-Cementitious Materials (W/CM) Ratio and Slump of the Concrete.Ahigherw/cmratiomeansthatthereismorefreewaterinthemixture,whichassistsairentrainment.Itiseasiertoentrainairinahighslumpmixturethaninalowslumpmixture.

Type of Plant and Production Procedures.Thisincludesthesequenceofmaterialsaddition,mixercapacity,andmixingtimeandspeed.Thelatertheair-entrain-ingadmixtureisaddedtothemixture,theless

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entrainedairtherewillbe.Dependingonthetypeofmixture,thenominalsuggestedmixtimeisapproxi-mately60seconds(CableandMcDaniel1998).Somemixturesloseairduringtransportfromtheplanttothesiteandduringplacement.Therefore,itisrecommendedthatairbecheckedbeforeandafterplacementtoensurethatthetargetaircontentismet.

Temperature of the Concrete; Construction Practices.Theseincludetransport,delivery,andretempering;andplacement,consolidation,andfinishing.Highertemperaturesrequiremoreair-entrainingadmixturetoachievethesameaircontent.Lateadditionofwatertoamixwillcausetheaircontenttoincreaseandpos-siblyexacerbateair-voidclustering.Handlingoftheconcreteafterinitialmixingwilltendtoreducetheaircontent.

StrengthAccordingtoACI201.2R,dry,air-entrained

concreteneedsacompressivestrengthof3.5MPa(500lb/in2)towithstandtheeffectsoffreezing.Forrepeatedcyclesoffreezingandthawingundersatu-ratedconditions,aminimumstrengthof28MPa(4,000lb/in2)isrecommended.

AggregatesAggregatesshouldbeselectedthatareresistantto

freeze-thawdamage.Thiscanbedeterminedfromrecordsoftheperformanceoftheaggregateinthepastorbytesting,asdiscussedbelow.Ifdamage-proneaggregatehastobeused,thesmallestpossiblesizeshouldbeused,andpreferablyitshouldbedilutedwithsoundmaterials.

Cementitious MaterialsAmoderateamountofcementitiousmaterialis

neededtoachievethestrengthsdiscussedabove.Aminimumof335kg/m3(564lb/yd3)isrecommended(Kosmatkaetal.2002)unlesstestingandfieldexpe-rienceindicateotherwise.Scalingresistancemaybereducedwithincreasingamountsofsupplementarycementitiousmaterials(SCMs);however,concretecontainingSCMscanbeexpectedtoexhibitgoodscalingresistanceiftheconcreteisproperlydesigned,placed,andcured.ACI318limitstheamountofpozzolansorground,granulatedblast-furnaceslag(maximum25percentflyash,50percentslag,and

10percentsilicafume)inconcretetominimizedeicerscaling.HigherdosagesofSCMscanbeusedifadequatedurabilityisdemonstratedbytestingand/orfieldperformance.

FinishingIngeneral,concretesurfacesshouldbefinished

afterthedisappearanceofthebleedwater(Kosmatka1994).Workingbleedwaterintoconcreteweakensthetopsurfaceandcancauseacrusttoformwithwateraccumulationunderneath,whichcouldeasilyscaleoff.Over-finishingshouldbeavoidedbecauseitmaycauselossofairatthesurface.

CuringGoodcuringpracticesensurethathydration

reactionsprogressandthatcrackingisminimized,enablingthedesiredpropertiesandperformance(ACI308R).Liquid-membranecuringcompoundsarewidelyandsuccessfullyusedonpavements,pro-videdthatthepropercuringmaterialisappliedintherightamountasthewatersheendisappears.Incoldweather,insulatingblankets,multiplelayersofburlaporstrawcanbeusedtomaintainthefavorabletem-peratureforhydrationandtominimizetemperaturedifferentials.

TestingD-Cracking

Theperformanceofaggregatesunderexposuretofreezingandthawingcanbeevaluatedintwoways:pastperformanceinthefieldandlaboratoryfreeze-thawtestsofconcretespecimens.Ifaggregatesfromthesamesourcehavepreviouslygivensatisfactoryser-vicewhenusedinconcreteundersimilarconditions,theygenerallymaybeconsideredacceptable.

Aggregatesnothavingaservicerecordcanbetestedinfreezingandthawingtests,suchasASTMC1646.Inthesetests,air-entrainedconcretespecimensmadewiththeaggregateinquestionarealternatelyfrozenandthawedinwater(procedureA)orfrozeninairandthawedinwater(procedureB).Deteriorationismeasuredby(1)thereductioninthedynamicmodulusofelasticity,(2)linearexpansion,and(3)weightlossofthespecimens.Anexpansionfailurecriterionof0.035percentin350freeze-thawcycles

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orlessisusedbyanumberofStatedepartmentsoftransportationtohelpindicatewhetheranaggre-gateissusceptibletoD-cracking.Differentaggregatetypesmayrequiredifferentcriteria(VoglerandGrove1989).

AnadditionaltestthatevaluatesaggregatesforpotentialD-crackingistherapidpressurereleasemethod.Anaggregatesampleisplacedinapres-surizedchamber.Thepressureisrapidlyreleased,causingaggregateswithquestionableporesystemstofracture(JanssenandSnyder1994).TheamountoffracturingrelatestothepotentialforD-cracking.

Anothertestfordeterminingthefreeze-thawpoten-tialofcarbonateaggregatesistheIowaporeindextest.Theapparatusmeasuresthelargeorprimaryporesandthesecondaryorcapillaryporesystem.Aggregateswithlargeprimaryporescorrelatetogooddurability,whilethosewithlargesecondaryporescorrelatetopoordurability.StudiesthatcorrelateporeindexresultswithdurabilitytestsindicatethattheIowaporeindextestcanquicklyandaccuratelydeter-mineaggregatedurability.

Specificationsmayrequirethatresistancetoweathering,orfreeze-thawdurability,bedemon-stratedbyasodiumsulfateormagnesiumsulfatetest,ASTMC88/AASHTOT104.Thistestconsistsofimmersingasampleoftheaggregateinasulfatesolutionforanumberofcycles.Pressureiscreatedthroughsaltcrystalgrowthintheaggregateporessimilartothatproducedbyfreezingwater.Thesampleisthenovendried,andthepercentageofweightlossiscalculated.Unfortunately,thistestissometimesmisleading,dueatleastinparttothefactthatthemechanismsofattackarenotthesameasinfreezingandthawinginconcrete.

Air-Void SystemThemostcommontestproceduretodetermine

theaircontentisthepressuremethod,ASTMC231/AASHTOT152(seeAirContent[PlasticConcrete,PressureMethod]inchapter9,page266).Airinthefreshconcreteandtheaggregatesismeasured.Itisapplicabletodense,normal-weightaggregateswiththeappropriateaggregatecorrectionfactor.Anothertestmethodapplicabletodense,cellular,

orlightweightaggregateisthevolumetricmethod(Rollameter),ASTMC173/AASHTOT196.Thevolumetricmethodtakeslongertocompleteandismorephysicallydemanding;therefore,itisnotwidelyusedforconcretewithnormal-weightaggregates.Theaircontentofthefreshlymixedconcretecanalsobedeterminedbythegravimetricmethod,ASTMC138/AASHTOT121.

Thesemethodsdeterminethetotalaircontent.However,forsatisfactoryfrostresistance,anair-voidsystemwithcloselyspacedsmallbubblesisneeded.Theaveragesizeofthebubblesandtheirspacingischaracterizedbythespecificsurfaceandthespac-ingfactor.Theseparametersmustbedeterminedonhardenedconcrete(ASTMC457),althoughtheair-voidanalyzershowspromiseindeterminingspacingfactorsforfreshconcrete.

Thegravimetricmethodfordeterminingunitweight(ASTMC138/AASHTOT121)alsogivesanindicationofaircontent,providedthespecificgravi-tiesofotheringredientsareknown.Themethodisespeciallyhelpfulfordeterminingwhensomethinghasgonewrongwiththeairsystem,becausesignifi-cantchangesintheunitweightwouldsignalachangeinthemixture’saircontent.

Theair-voidanalyzer(AVA)(figure5-17)wasdevelopedinEuropeinthelate1980s.Itmeasuresthesizeandspacingofentrainedairbubblesinfreshconcrete.TheAVAhastheadvantageofprovidingtheair-voidparametersoffreshconcreteinlessthan30minutes,sothatadjustmentscanbemadeduringconstruction(seeAir-VoidAnalyzerinchapter9,page265).

WiththeAVA,amortarsamplecontainingairvoidsisintroducedintoaliquidofacertainviscosityandplacedatthebottomofarisercolumnfilledwithwater.Immediatelyafterinjection,amagneticstirringrodbreaksupthemortarsample,releasingairbubblesintotheanalysisliquid.Theproperselectionoftheliquidenablestheairvoidstoretaintheirquantityandsizeduringthetransfer.Airvoidsriseinthecolumntothesurface,accordingtoStokes’law.Thelargebubblesrisefasterthanthesmallbubbles.Theairbubblesrisingintheliquidarecollectedunderabowlattachedtoabalance.Thebuoyancyofthebowl

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isrecordedovertime,fromwhichtheaircontent,specificsurface,andspacingfactoroffreshlymixedconcretearedetermined.

Sincetheentrappedairvoidsarenotmeasured,AVAaircontentisapproximately1.5to2percentlessthantheaircontentasdeterminedbythepres-suremeterandbyASTMC457.ThespacingfactorisaboutthesameforAVAandASTMC457(Magura1996).AnAVAissensitivetovibrationandusesasmallsample;therefore,itshouldbeusedwithcare.

Theaircontentofconcreteisgenerallymeasuredbeforeitreachesthepaver.However,measurementoftheair-voidsystemafterplacementandvibrationwouldprovidemoreusefulinformation.Somereduc-tioninairisexpectedasitisvibrated;however,thislossdoesnotnecessarilyimplythatfreeze-thawpro-tectioniscompromised.

ASTMC457hastwomicroscopicalproceduresfordeterminingtheair-voidparametersinlappedsec-tionsofhardenedconcretesamples:thelineartraversemethodandthemodifiedpointcountmethod,the

Figure 5-17. Air bubbles rising in the AVA (FHWA)

formerbeingthemostwidelyused(seeAirCon-tent[HardenedConcrete]inchapter9,page267).SomeStatesareusingimageanalysistechniquestodetermineaircontentoflappedsectionsofhardenedconcretesamples;see,forexample,thePortlandCementAssociation’sCT021.Thereis,asyet,nostan-dardmethodforsuchanapproach.

Rapid Freezing and ThawingResistancetofreezingandthawingofaconcreteis

normallydeterminedusingtheprocedures(AorB)describedinASTMC666.ProcedureAinvolvesrapidfreezingandthawingofsamplesofconcreteinwater.ProcedureBrequiresrapidfreezinginairandthawinginwater.

Thefundamentaltransversefrequencyofthesam-plesismeasuredperiodicallyandusedtodeterminetherelativedynamicmodulusofelasticity,fromwhichadurabilityfactoriscalculated(ASTMC666).Asdamageincreasesinthesample,thedynamicmoduluswilldecrease.Foradequateperformance,specimenstestedto300cyclesareexpectedtoexhibitadurabil-ityfactorof60ormore.Lengthchangeandmasslosscanalsobemonitored.

ProcedureAisthemoreseveretest.Concretesperformingwellinthistesthavedonewellinfieldapplications.Concretesfailingthetestmayalsohavesatisfactoryfieldperformance,butsuchperfor-mancemustbeproveninthefield.ProcedureBisnotassevereasprocedureA,sincethespecimensareallowedtodryduringtesting.

InVirginia,ASTMC666procedureAisconductedwithspecimensimmersedintwopercentNaClsolu-tion(Newlon1978).Priortoexposure,specimensaremoist-curedfor14daysandair-driedfor7days.

Salt ScalingResistancetosaltscalingisdeterminedusingthe

ASTMC672test.Specimensaremoist-curedfor14daysandair-driedforanadditional14days.Adikeisplacedaroundthetopsurfaceofthespeci-mens,andthesurfaceoftheconcreteiscoveredwithfourpercentcalciumchloridesolution.Thespecimensareplacedinafreezingenvironmentfor16to18hours,thenthawedinlaboratoryairfor6to8hours.Thesurfaceisflushedattheendofeachfivecycles

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andavisualexaminationismade.Thesolutionisreplacedandthetestrepeatedfor50cycles.Concreteswitharatingofthreeorlessareconsideredsatisfac-tory.(Aratingofthreeindicatesmoderatescalingwithsomecoarseaggregateshowing.)SomeStatesalsomonitorthemasslossofthesamplesandrequireavalueoflessthan0.3lb/ft2after50cycles.

Thistestisalsoconsideredtobesevere,withconcretesperformingsatisfactorilyinthefielddespitefailingASTMC672.TheBureauDeNormalisationduQuébec haspublishedanalternativemethodthatisreportedlylesssevere(BNQNQ2621-900).Thismethodsetsoutslightlydifferentfinishingandcuringrequirementsforthesamples.

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Sulfate Resistance

Key Points

Sulfatesdamageconcretebyreactingwithproductsofhydratedtricalciumaluminate(C

3A)inhardenedcementpasteand

byinfiltratinganddepositingsalts.Theresultingexpansivecrystalsdamagethecementpaste,causingcracking.

Sulfateproblemsaremoreseverewhereconcreteisexposedtowetanddrycyclesthanwhereitiscontinuouslywet.

Concretecanbedesignedtoresistsulfateattack:

Achievelowpermeability(seePermeabilityinthischapter,page131).

Useamixturewithalowwater-cementitiousmaterialsratio,sulfateresistantcements,andproperproportionsofsuitablesupplementarycementitiousmaterials.

Testingspecifications:ASTMC452andASTMC1012.

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Simple DefinitionSulfateresistanceisconcrete’sabilitytoresistattack

by,anddamagefrom,sulfatespenetratingfromout-sidetheconcretesystem.

SignificanceExcessiveamountsofsulfatesinsoilorwatercan,

overaperiodofyears,attackanddestroyconcretepavementsandotherstructures(figure5-18).

Sulfatesdamageconcretebyreactingwithhydratedtricalciumaluminate(C

3A)compoundsinthehard-

enedcementpasteandbyinfiltratinganddepositingsalts.Duetocrystalgrowthpressure,theseexpansivereactionscandisruptthecementpaste,resultingincrackinganddisintegrationoftheconcrete.

Preventivemeasuresarewellaccepted,andcon-cretecanbedesignedtoresistsulfateattack.Thisisdoneprimarilybyreducingpermeabilityandreduc-ingtheamountofreactiveelementsintheconcretesystemneededforexpansivesulfatereactions.

Factors Affecting Sulfate AttackPreventionofsulfateattackdependsonprotecting

theconcretefrominfiltrationbysulfateions.Forthebestdefenseagainstexternalsulfateattack,considerthefollowing:

Designconcretewithalowwater-cementitiousmaterials(w/cm)ratio(around0.4orlower).Usecementsspeciallyformulatedforsulfateenvironments,suchasASTMC150/AASHTOM85TypeIIorVcements,ASTMC595/AASHTOM240moderatesul-fate-resistantcements,orASTMC1157TypesMSorHS.

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_______________________________________________Figure 5-18. Sulfate attack is a chemical reaction between sulfates and the calcium aluminate (C3A) in cement, result-ing in surface softening. (Emmons 1994)

Sulfate Resistance


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Notethat,althoughtheuseofsulfate-resistant

cementsisbeneficial,itmaynotbesufficient

toprotectconcreteagainstsulfateattack.Table

5-4providesrecommendationsforconcretein

varioussulfateexposures.Itisimportanttouse

thetestmethoddescribedinthetabletoassess

thesulfatecontenttopreventmisleadingdata.

Usesupplementarycementitiousmaterials

(SCMs)withproperproportioningandmaterial

selection.SCMsimprovesulfateresistanceby

reducingpermeabilityoftheconcreteandby

dilutingtheC3Acontentofthesystem.Con-

creteswithClassFashesaregenerallymore

sulfateresistantthanthosemadewithClassC

ashes.SomeClassCasheshavebeenshownto

reducesulfateresistanceatnormaldosagerates.

Specifyaminimumcementitiousmaterialscon-

tent(hydrauliccementplusSCM)of335kg/m3

(564lb/yd3)andonlyenoughmixingwaterto

achievethedesiredconsistencywithoutexceed-

ingthemaximumw/cmratioshownintable

5-4.Themaximumw/cmratioshouldnot

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Table 5-4. Requirements for Concrete Exposed to Sulfates in Soil or Water

exceed0.40andmayneedtobelowerinmoreseveresulfateexposures.Extendwetcuringtoincreaseconcrete’sstrengthdevelopmentanddecreaseitsperme-ability,thusimprovingitssulfateresistance.

Sulfateattackismoresevereatlocationswheretheconcreteisexposedtowettinganddryingcyclesthanatlocationswherethereiscontinuouslywetexpo-sureduetosaltdeposition.However,ifthesulfateexposureissevereenough,evencontinuouslymoistsectionscanbeattackedbysulfateswithtime.

TestingTestsonthesulfateresistanceofcementsareusu-

allyperformedonmortars:ASTMC452isusedforportlandcementandASTMC1012isusedforhydrauliccements,includingblendedcements.

Therearecurrentlynowell-acceptedtestsforsulfateresistanceofconcreteintheUnitedStates.Mostlaboratoryinvestigationsuseconcreteprismsimmersedinsulfatesolutionsandtryvariousmethodstoevaluatedamage.Moreseveretestsincorporateawetting/dryingregimeintothetesting.

•

Sulfate exposure Sulfate (SO4) Sulfate (SO4) Cement type ** Maximum Minimum in soil, in water w/c ratio, strength f’ % by mass* ppm* by mass MPa (lb/in2)

Negligible Less than 0.10 Less than 150 No special _ _ type required

Moderate*** 0.10 to 0.20 150 to 1,500 II, MS, IP(MS), 0.50 28 (4,000) IS(MS), P(MS), I(PM)(MS) , I(SM)(MS)

Severe 0.20 to 2.00 1,500 to 10,000 V, HS 0.45 31 (4,500)

Very severe Over 2.00 Over 10,000 V, HS 0.40 35 (5,000)

Source: Adapted from ACI 318 (2002)* Tested in accordance with ASTM 1580, the method for determining the quantity of soluble sulfate in solid (soil and

rock) and water samples, Denver, Colorado, 1977.** Cement types II and V are in ASTM C 150 / AASHTO M 85, types MS and HS are in ASTM C 1157, and the remaining

types are in ASTM C 595 / AASHTO M 240. Pozzolans or GGBF slags that have been determined by test or service record to improve sulfate resistance may also be used.

*** Sea water.

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Alkali-Silica Reaction

Key Points

Thepresenceofalkali-silicareaction(ASR)geldoesnotalwayscoincidewithconcretedistress.

DamagingexpansionandcrackingduetoASRcanbecontrolledinseveralways:

Usingnonreactiveaggregates.

Blendingsufficientnonreactiveaggregatewithreactiveaggregate.

Usingsupplementarycementitiousmaterialsinproperproportions.

Usinglow-alkalicements.

Usingblendedcements.

Usinglithiumcompounds.

Testingspecifications:

Aggregatesandconcrete:Seetable5-5onpage144.

Mitigationmeasures:ASTMC1293,ASTMC1567,ASTMC441

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Simple DefinitionAlkali-silicareaction(ASR)isapotentiallyharmful

conditioninconcreteresultingfromachemicalreac-tionbetweensomeaggregateminerals(seeAggregateDurabilityandAlkali-SilicaReactivityinchapter3,pages47and48,includingtables3-12and3-13)andthehighalkaline(pH)poresolutionsfoundinconcrete.Overtime,theproductofthesechemicalreactions,agelatinousalkali-silicatereferredtoasASRgel,canabsorbwaterandexpand,leadingtoconcretecrackingandreducedservicelife(figure5-19).

_______________________________________________Figure 5-19. Alkali-silica reaction is an expansive reaction of reactive aggregates, alkali hydroxides, and water that may cause cracking in concrete. (Emmons 1994)

SignificanceTheamountofgelformedintheconcretedepends

ontheamountandtypeofsilicaintheaggregateandthealkalihydroxideconcentrationintheconcreteporesolution.Thepresenceofgeldoesnotalwayscoincidewithdistress.Thereactivityispotentiallyharmfulonlywhenitproducessignificantexpan-sion.Formoreinformation,seeFarnyandKosmatka(1997)andFolliard,Thomas,andKurtis(2003).

TypicalindicatorsofdeleteriousASRincludeanet-workofcracksthatareperpendiculartojoints,closedorspalledjoints,orrelativedisplacementsofadjacentslabs.BecauseASRisslow,deteriorationoftentakes



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severalyearstodevelop.Alkali-silicareactionscancauseserviceabilityproblemsandcanexacerbateotherdeteriorationmechanisms,suchasthosethatoccurinfrost,deicer,orsulfateexposures.

Formostreactiveaggregates,thereactioncanbemitigatedorcontrolledthroughproperconcretemate-rialsselectionorothermeans.Infact,eventhoughpotentiallyreactiveaggregatesexistthroughoutNorthAmerica,ASRdistressinconcreteisnolongeracommonproblem.Thereareanumberofreasonsforthis:

Manyaggregatesarenotreactive.Knownreactiveaggregatesareavoided.SomeformsofASRdonotproducesignificantdeleteriousexpansion.TheappropriateuseofcertainpozzolansorslagscontrolsASR.Inmanyconcretemixtures,thealkalicontentoftheconcreteislowenoughtolimitthereaction.Theconcreteinserviceisdryenoughtolimitthereaction.

Factors Affecting Alkali-Silica Reaction

ForASRtooccur,threeconditionsmustbepresent:Reactiveformsofsilicaintheaggregate.High-alkali(pH)poresolution(waterinthepastepores).Sufficientmoisture.

Ifanyoneoftheseconditionsisabsent,ASRgelcannotformanddeleteriousexpansionfromASRcannotoccur.Therefore,thebestwaytoavoidASRisthroughgoodmixdesignandmaterialsselection.

DesignmixesspecificallytocontrolASR,prefer-ablyusinglocallyavailablematerials.Usenonreactiveaggregates,ifpossible,oramixofnonreactiveandactiveaggregate(seetables3-12and3-13inchap-ter3,pages48and49,respectively).

Ifyoumustusesomereactiveaggregate,usesupplementarycementitiousmaterialsorblendedcementsprovenbytestingtocontrolASRorlimitthealkalicontentoftheconcrete.Whereapplicable,dif-ferentamountsofpozzolanorslagshouldbetestedtodeterminetheoptimumdosage.Toolowadosage

•••

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1.2.

3.

offlyashmayexacerbatetheproblem.Low-calcium(typicallyClassF)flyashesaregenerallybetteratmitigatingASRthanhigh-calcium(typicallyClassC)flyashes.Ground,granulatedblast-furnaceslagandnaturalpozzolansalsogenerallyeffectiveinmitigatingASRwhenusedintheproperdosages.Expansionusuallydecreasesasthedosageofthepozzolanorslagincreases.

Usingportlandcementwithanalkalicontentofnotmorethan0.60percent(equivalentsodiumoxide)canoften(butnotalways)controlASR.Itsusehasbeensuccessfulwithslightlyreactivetomoderatelyreac-tiveaggregates.However,low-alkalicementsarenotavailableinallareasandarenoteffectiveagainstallreactiveaggregates.

Lithiumcompoundsmayalsobeusefulinreduc-ingorpreventingexpansion(Folliard,Thomas,andKurtis2003).Typically,asolutionoflithium-bearingcompound(usuallylithiumnitrate)isaddedwhenbatchingconcrete.Cautioniswarranted,asthistechnologyisnewandfieldperformancedataarestillbeingevaluated.Anadditionalconcernisthatnostandardizedtestmethodisabletoevaluatetheabil-ityoflithiumcompoundstopreventASR,althoughguidancehasbeenprovidedbyFolliard,Thomas,andKurtis(2003)inusingASTMC1293andaversionofASTMC1260,inwhichlithiumisaddedtothestor-agesolution.

TestingAggregateshouldbeevaluatedforitssusceptibil-

itytoASR.WhenASR-susceptibleaggregatesmustbeused,mitigationmeasuresshouldbeevaluated.TherearealsotestforhardenedconcretetodeterminewhetherASRisacauseofdistressinconcrete.Seetable5-5.

Aggregate TestingFieldperformancehistoryisthebestmethodof

evaluatingthesusceptibilityofanaggregatetoASR.Forthemostdefinitiveevaluation,theexistingcon-cretemusthavebeeninserviceforatleast15years.Comparisonsshouldbemadebetweentheexistingandproposedconcretesformixtureproportions,ingredients,andserviceenvironments.Thisprocess



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willindicatewhetherspecialrequirementsortestingoftheaggregateisneeded.Theuseofquickmethodsprovidesforinitialscreening.Whereuncertaintiesarise,lengthiertestscanbeusedtoconfirmresults.Iftheaggregateisnotreactivebyhistoricalidentificationortesting,nospecialrequirementsareneeded.

ASTMC295(petrographicanalysis)andASTMC1260(acceleratedmortar-bartest,similartoAASHTOT303)arethequickestindicatorsofthepotentialreactivityofanaggregate.ASTMC1260isanextremelyseveretest,andisintendedtobeanaggregatescreeningtest.Ifthequickmethodsindicatepotentialreactivity,ASTMC1293(concreteprismtest)canberunovertheperiodofayear(twoyearswithSCMsintheconcrete)tohelpresolveuncer-taintiesaboutresultsfromthescreeningtests.Whenreactivityisindicatedbyeitherfieldhistoryortesting,specialrequirementsmustbeusedtocontrolASR.Whenthesemethodsindicatenopotentialforreactiv-ity,nospecialrequirementsareneeded.

UseofASTMC289(chemicalmethod)isnotrec-ommended,asitmayfailaggregateswithgoodfieldperformancehistory.Likewise,useofASTMC227isnotconsideredreliable,asitmaypassreactiveaggre-gatesormeasureexpansionthatmaynotbeduetoASR.

Mitigation Measures TestingWhenreactiveaggregatesareused,theefficacyof

mitigationmeasuressuchastheuseofSCMs,blendedcements,orlow-alkalicementsshouldbeevaluatedinpreventingdeleteriousexpansionduetoASR.ASTMC1293istherecommendedprocedure.How-ever,thistest,althoughanacceleratedtest,requiresuptotwoyearstoobtainresults.

ASTMC1567issimilartoASTMC1260butallowscombinationsofcementitiousmaterialstobetestedsothattheefficacyofmitigatingASRcanbeassessed.Thismethodappearstocorrelatewellwith

concreteprismtesting(Thomasetal.2005;ThomasandInnis1999;Bérubé,Duchesne,andChouinard1995)andiswidelyusedforthispurpose.

ASTMC441usesareactivecrushedglasstomakea“mortar”andhasbeenusedtocomparetheeffec-tivenessofSCMsorblendedcementsforpreventingexpansion.However,noabsolutelimitingvaluesonexpansionareprovided;mitigationisgaugedbyexpansionsofsimilarmortarbarsmadewithlow-alkaliportlandcements.Someaggregatesaretooreactivetobecontrolledbylow-alkaliportlandcement;therefore,thistestshouldbeusedwithcaution.

Concrete TestingApetrographicexamination(ASTMC856)isthe

mostpositivemethodforidentifyingASRgelincon-crete.AnetworkofinternalcracksconnectingreactedaggregateparticlesisagoodindicationthatASRisresponsibleforcracking.Preparedsectionsofconcreteareexaminedunderamicroscopebyanexperiencedpetrographertodeterminethepresenceandlocationofreactiveaggregatesandgel.Petrographycancon-firmthepresenceofreactionproductsandverifyASRasanunderlyingcauseofdeterioration.Visualindica-torsofASRcanbefoundinStark(1991);however,petrographicexaminationisrequiredforconfirma-tion,asdistresscausedbyASRcanlooksimilartothatcausedbyotherdeteriorationmechanisms.

ThepresenceofASRgelcanalsobedetectedbytheuranylacetatetest(AASHTOT299,alsofoundintheannextoASTMC856);however,smallamountsofradioactivematerialsareused,whichrequirespecialhandling.Arelativelynewtechnique,theLosAlamosstainingmethod(Powers1999;GuthrieandCarey1999),providesconfirmationofthepresenceofASRgel.BoththeUranylAcetateTestandtheLosAlamosstainingtestonlyprovideindicationsofASRgel’spres-enceanddonotprovideconclusionsastowhetherASRisthecauseofconcretedeterioration.



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Table 5-5. Test Methods for Potential Alkali-Silica Reactivity (ASR) and Mitigation Measures



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Table 5-5. Test Methods for Potential Alkali-Silica Reactivity (ASR) and Mitigation Measures, continued

Source: Adaped from Farny and Kosmatka, (1997)* Powers (1999)

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Abrasion Resistance

Key Points

Abrasionresistanceisimportantformaintainingadequatetextureandskidresistanceontheconcretepavementsurface.

Forsatisfactoryabrasionresistance,considerthefollowingprocedures:

Selectstrongandhardaggregates.

Usehighcompressivestrengthconcrete.

Provideproperfinishingandcuring.

Testingspecifications:ASTMC418,ASTMC779,ASTMC944,ASTMC1138.

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Simple DefinitionAbrasionresistanceisconcrete’sabilitytoresist

surfacewear.

SignificancePavementsurfacesmustmaintainadequatetexture

andskidresistanceforpropervehicularcontrol.Itisthereforeimportantforconcretepavementstohaveahighabrasionresistance.Abrasionresistanceisgener-allyrelatedtoconcrete’scompressivestrengthandtothetypeofaggregateintheconcrete;harderaggre-gatesresistwearbetterthansofteraggregates.

Skidresistanceisaffectedbyboththeconcrete’smicrotexture(providedbythetypeandhardnessoffineaggregateparticles)andthemacrotexture(pri-marilyprovidedbygroovesformedonfreshlymixedconcreteorcutinthehardenedconcrete[ACI3251988]).Wearofpavementsurfacesoccursduetotherubbingactionfromthewheelsofvehiculartraffic.Therubbingactionisgreatlyincreasedbythepresenceofabrasiveparticles,suchasfineaggregatesmixedwithdeicingchemicals(ACI201.2R2001;Liu1994).

Wearisusuallyminimalwithconcretepavementsunlessvehicleswithstuds,chains,ormetalwheelstravelonthepavementorunlesspooraggregatesandconcreteareused.Evenwithminimalwear,polishedsurfacescanoccurunlessproperaggregatesareselected.Withmorewear,lossoftextureandlossofthicknesscanoccur.

Factors Affecting Abrasion ResistanceThemainfactorsaffectingabrasionresistanceare

thetypeofaggregate,compressivestrength,surfacefinishing,andcuringmethods(ACI201.2R2001;Liu1994).

Aggregate Type Coarseaggregatehasalargeinfluenceonabra-

sionresistance,andtheuseofhardaggregates,suchasgraniteandtraprock,ispreferred(Laplanteetal.1991).Softlimestonehaspoorabrasionresistance,butdolomiticlimestonemayhaveverygoodresistance.

Toprovidegoodskidresistanceonpavements,thesiliceousparticlecontentofthefineaggregateshouldbeatleast25percent.Certainmanufacturedsandsproduceslipperypavementsurfacesandshouldbeinvestigatedforacceptancebeforeuse.

Compressive StrengthAhighconcretecompressivestrengthimproves

abrasionresistance.Highstrengthisachievedbyhavingstrongpasteandstrongbondbetweentheaggregateandpaste,thuspreventingdislodgingofaggregatesoutofthepaste.Thewater-cementitiousmaterialsratiohasalargeeffectontheabrasionresis-tance,sinceitrelatestothecompressivestrength.Thecompressivestrengthatthesurfaceisimportantandisimprovedbyavoidingsegregation,eliminatingbleed-ing,andusingproperfinishingandcuringprocedures(ACI201.2R2001).Supplementarycementitiousmaterialsdonotaffecttheabrasionresistance,pro-videdthatstrengthlevelsaremaintained.

Surface FinishingTechniquesthatdensifythesurfacewillincrease

abrasionresistance(KettleandSadegzadeh1987).

Curing Methods Increasedcuringpromotescementhydration,thus

improvingabrasionresistance.

TestingThemostcommontestforabrasionresistanceof

aggregateistheLosAngeles(LA)abrasiontest(rattlermethod)performedinaccordancewithASTMC131orASTMC535/AASHTOT96.Inthistest,aspeci-fiedquantityofaggregateisplacedinasteeldrumcontainingsteelballs,thedrumisrotated,andthe

Abrasion Resistance


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percentageofmaterialwornawayismeasured.Speci-ficationsoftensetanupperlimitonthisweightloss.However,acomparisonoftheresultsofaggregateabrasiontestswiththeabrasionresistanceofconcretemadewiththesameaggregatedonotshowaclearcorrelation.TheMicroDevaltest,developedinFranceduringthe1960s,isunderevaluationasanalternativetotheLAabrasiontest(KandhalandParker1998).

Thewearresistanceofconcreteisdeterminedmoreaccuratelybyabrasiontestsoftheconcreteitself.Fourstandardtestsmeasuretheabrasionresistanceofcon-creteundervariousconditions.

ASTMC418subjectstheconcretesurfacetoair-drivensilicasand,andthelossofvolumeofconcreteisdetermined.

InASTMC779,threeproceduressimulatedifferentabrasionconditions:

ProcedureA:slidingandscuffingofrevolvingsteeldisksinconjunctionwithabrasivegrit(figure5-20).ProcedureB:impactandslidingfrictionofsteeldressingwheelsridinginacircularpath.ProcedureC:arapidlyrotatingball-bearingun-derloadonawetconcretesurface,causinghighcontactstresses,impact,andslidingfriction.

Inallthreeprocedures,thedepthofwearofthespecimenisusedasameasureofabrasion.

InASTMC944,arotatingcutterabradesthesur-faceoftheconcreteunderload.Lossofmassordepthofwearismeasured.ThistestissimilartoprocedureBoftestmethodASTMC779(figure5-21);however,ASTMC944canbeperformedoncores.ASTMC944isincludedintheFHWA’sdefinitionofhigh-perfor-manceconcreteforhighwaystructures(table5-6).

InASTMC1138,concreteissubjectedtowater-borneparticles,whichsimulatestheconditionofhydraulicstructures.

•

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Figure 5-20. Test apparatus for measuring abrasion resis-tance of concrete to ASTM C 779 (PCA)

Figure 5-21. Rotating cutter with dressing wheels for the ASTM C 944 abrasion resistance test (Goodspeed 1996)

Table 5-6. FHWA High-Performance Concrete Performance Grades for Abrasion

Grade 1 Grade 2 Grade 3

Average daily traffic (ADT) < 50,000 50,000 to 100,000 > 100,000

Specified depth of wear* 1 and 2 mm (0.04 to 0.08 in.) 0.5 to 1 mm (0.02 to 0.04 in.) < 0.5 mm (0.02 in.)(studded tires allowed)

Source: Goodspeed et al. (1996)* As tested in ASTM C 944.

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Early-Age Cracking

Key Points

Concreteshrinksandexpandsduetomoistureandtemperaturechangesandwillcrackwhenthestressfromrestrainedshrinkageexceedstheconcrete’sstrength.

Thenumberandlocationofshrinkagecrackscanbecontrolledbythetimelyconstructionofjointsatoptimumspacinganddepth.

Goodpracticesbydesignersandcontractorswillhelpensurethatcracksdeveloponlyatjointsandthatlittle,ifany,randomcrackingoccurs.

Goodcuringpracticescanreduceorpreventrandomcrackingbecausecuringhelpsdelaytheonsetofstressesuntiltheconcreteisstrongenoughtoavoidcracking.

Goodsupportisessentialforpreventingcrackingduetoloads.

Ifconcretesetsatahightemperature,thestressesandriskofcrackingareincreased.

StandardtestmethodstodetermineshrinkagepotentialincludeASTMC1581/AASHTOPP34,ASTM157.

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Simple DefinitionBasically,concretecrackswhentensilestresses

exceedtensilestrength.Forthepurposesofthismanual,early-agecracksaredefinedasthosethatoccurbeforetheconcretepavementisopenedtopublictraffic.

Later-agecracksarecausedbyanumberofmecha-nisms,includingtrafficloadingandmaterials-relateddistress(seeFrostResistance,SulfateResistance,D-Cracking,andAlkali-SilicaReactioninthischapter,pages132,139,135,and141,respectively).

SignificanceWhilecrackingisnotstrictlyapropertyofcon-

crete,concretealwayscracks.Acertainamountofearly-age,full-depthcrackingtorelievetensilestressesisinevitableandnormallydoesnotposeaproblem.However,thechallengeistocontrolthenumberandlocationofthesecracks,generallybyconstructingjointsintheconcrete,usinggoodcuringpractices,and/orreinforcingtheconcrete.Ideally,theconcretewillcrackalongthejointsdownthroughthedepthoftheslab.

Controlledcracksatjointsareaestheticallyaccept-ableandcanbesealedtopreventtheingressofaggressivefluidsandincompressiblematerials.Jointscanalsohelpdelineatelanes.Undesirableslabmove-mentatjointscanbecontrolledbyinstallingdowelsacrosstransversejoints(topreventverticalmovement)ortiebarsacrosslongitudinaljoints(topreventhori-zontalmovement).

Random(uncontrolled)cracks,however,areundesirable.Theycanreduceridequalityandleadtoreduceddurability.Althoughsomerandomcracksareatfirstonlycosmeticflawsorareeveninvisibletothenakedeye,thesecrackscangrowandbecomeprob-lematicduetomechanicalandenvironmentalstressesthatwouldotherwisebeofnoconcern.

Early-agecrackingoccursduringthefirstfewdaysofapavement’slife.Thiscrackingisgenerallyduetoacombinationofseveralfactors,primarilythermalcontractionanddryingshrinkage(seethesectionsonTemperatureEffectsandonShrinkageearlierinthischapter).Thermal-relatedcracksarenormallyobservedinthefirstday,whiledrying-relatedcracksmayappearoveralongerperiod.

Tocontrolearly-agecracksandpreventrandomcracks,designersandconstructionpersonnelneedtounderstandthemechanismsthatcausethem.

Factors Affecting Early-Age CrackingTheprimaryfactorsaffectingearly-ageconcrete

pavementcrackingincludethefollowing:Volumechange(dryingshrinkage,thermalcontraction)andrestraint.Curlingandwarping.Strengthandstiffness.

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Baseconditionandsupport.Earlyloadings(includingtheweightoftheconcreteitself).

Thesefactorsareaffectedbydecisionsmadeduringpavementdesign,mixturedesignandproportioning,andconstruction.Crackingrarelyoccursasaresultofjustoneofthesefactors,butratherisgenerallyacumulativeeffectofseveralfactors.Inaddition,weatherconditionscangreatlyaffectthemagnitudeand/orimpactofeachfactor.

Volume Change and RestraintConcretealwayschangesinvolume(expandsor

shrinks)duetotheeffectsofchangingtemperatureandmoisture(seeShrinkageandTemperatureEffectsinthischapter,pages125and127,respectively).

Volumechangecanoccurduetomoisturelossbeforeoraftersetting(plasticshrinkageanddryingshrinkage,respectively),orduetoatemperaturedropaftertheconcretehasset(thermalcontraction).Ther-malcontractioncanbe140to350microstrains(0.17to0.42in./100ft)fora22°C(40°F)temperaturechange,dependingontheaggregatetype(seeTem-peratureEffectsinthischapter,page127;AggregateDurabilityandCoefficientofThermalExpansioninchapter3,page47).

Dryingshrinkagecanbesignificantlygreater(400to800microstrains,or0.48to0.96in./100ft),butwilltakeseveralmonthstoachievetheselevels.

Ingeneral,anobjectcanshrinkorcontractfreely,withoutanystresstothesystem,aslongisitisnotrestrained.Theoretically,inhomogeneous,elastic,unrestrainedconcretesystems(forexample,inspacewherethereisnogravityorfriction),shrinkageisnotaproblem;thewholeslabsimplygetssmaller(figure5-22a).

Iftheshrinkingconcreteisrestrained,however,tensilestressesdevelopintheconcreteinproportiontotheconcretestiffnessandthedegreeofrestraint(figure5-22b).Asnotedearlierinthischapter,con-crete’stensilestrength,itsabilitytoresistbeingpulledapart,ismuchlessthanitscompressivestrength.Whenthestressfromrestraintofshrinkageexceedstheconcrete’stensilestrength,acrackwillform.Thisisgenerallycalledshrinkagecracking.

••

Bothinternalandexternalrestraintcancausestress.Internalrestraintcanarise,forexample,iftheouterconcreteshrinkswhilethecoredoesnot.Externalrestraintcanarisefrombondingorfrictionbetweenaslabandthebase,fromabruptchangesintheslab’scrosssection,orfromanexistingslabtowhichthenewpavementistied.

Tying New Lanes to Old? Take Care

Tying a new concrete pavement to an existing pavement can result in significant stress on the new slab from restraint. This can be especially true if the new slab is placed on a warm sunny day following a cool night, as may occur in the fall. In the morning the existing slab is cool. As the day becomes sunny and warm, the existing slab warms and expands. If new concrete is placed and tied to the existing slab in the morning, it will reach the cooling stage of hydration and begin to shrink (see figure 4-2, page 70) during the warmest part of day, when the existing slab’s expansion is peaking. Because they are tied together, each slab’s movement is restrained by the other’s. The new, weaker pavement is likely to crack.

In this situation, minimize the potential for cracking by postponing concrete placement until afternoon. The new slab will reach the cooling stage later in the day when the existing slab also begins to cool.

_______________________________________________Figure 5-22. (a) Cracks generally do not develop in concrete that is free to shrink. (b) In reality, slabs on the ground are restrained by the subbase or other elements, creating tensile stresses and cracks. (ACPA)

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Wetter

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Wet weather

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Temperature curling

Temperature curling

Moisture warping

Moisture warping

Temperature curling

Hot days(curling

counteracts warping)

Moisture warping

Temperature curling

Moisture warping

Cool nights(curling

compounds warping)

_______________________________________________Figure 5-23. Curling and warping of slabs

Curling and Warping: A Variation of Volume Change

Volumechangecanalsocausecurlingand/orwarp-ing.(Thetermcurlingissometimesused,incorrectly,forbothphenomena.)

Curling.Curlingiscausedbydifferencesintempera-turebetweenthetopandbottomoftheslab.

Duringcoolerweather(e.g.,atnightorwhenacoldfrontcomesthrough),thetopsurfaceoftheslabcoolsmorequicklythanthebottomoftheslab,whichisinsulatedbythesoil.Thetoppartoftheslabshrinksmorequicklythanthebottom,causingtheslabtocurlupattheedges(figure5-23a).

Duringhotweatherconditions(typically,duringthedaytime),thetopoftheslabmaybewarmerthanthebottom,resultingincurlingintheoppositedirec-tion(figure5-23b).

Warping.Warpingofconcretepavementsiscausedbydifferencesinmoisturecontentbetweenthetopandbottomoftheslab.

Duringcool,moistweather(e.g.,atnight),thebottomoftheslabmaybedrierthanthetopsurface.Thebottomoftheslabthereforeshrinksmorequicklythanthetop,causingtheslabedgestowarpdown(figure5-23c).Duringwarm,dryweather(typically,duringthedaytime),thetopoftheslabdriesandshrinkswhilethebottomremainsmoist,resultinginwarpingintheoppositedirection(figure5-23d).

Curlingandwarpingactionsmayoffsetoraug-menteachother.Duringsummerdays,curlingmaybecounteractedbywarping(figure5-23e).Duringsummernights,curlingmaycompoundwarpingmovement(figure5-23f).

Alongthejoints,thepavementedgestendtocurlupwardwhenthesurfaceoftheconcreteisdrierandcoolerthanthebottom.Thecurlededgesthenbecomeacantilever.Trafficpassingoverthejoints

Comparing Stresses of Volume Changes

The magnitude of drying shrinkage stresses (and the width of resulting cracks) is likely to be several times larger than thermal contraction stresses.

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causesarepetitiveverticaldeflectionthatcreatesalargepotentialforfatiguecrackingintheconcrete(figure5-24).

Strength and StiffnessTheabilityofconcretetoresiststressesintroduced

byvolumechangedependsonthemixture’sstrengthandthestiffness.

Strength.Thegreatertheconcretestrength,thegreaterthestresstheconcretewillbeabletocarry.Thisiswhymostearly-agecrackingoccurssoonaftertheconcretehasbeenplaced,whenstrengthisstilldevelopingbutstressesrelatedtothermalcontrac-tionanddryingshrinkagearehigh(seeStrengthandStrengthGaininthischapter,page116).

Stiffness.Thestiffertheconcrete(asindicatedbymodulusofelasticity),thegreaterthestressesresultingfromvolumechange.Unfortunately,stiffnessincreasesfasterthanstrengthforthefirstfewhoursafterset-ting,increasingtheriskofcrackingiftheconcreteisallowedtoexperiencesignificanttemperaturevaria-tionsormoistureloss(seeModulusofElasticityinthischapter,page123).

_______________________________________________Figure 5-24. Exaggerated illustration of pavement curling. The edge of the slab at a joint or a free end lifts off the base, creating a cantilevered section of concrete that can break off under heavy wheel loading. (PCA)

_______________________________________________Figure 5-25. An eroded base can lead to high tensile stresses, resulting in cracking.

Strength and Cracking

The solution to cracking is not necessarily to focus on increasing concrete strength. Design strength must be balanced with other factors that affect cracking, such as volume changes, curling and warping, loading, weather, and level of support.

Lack of SupportIftheconcreteslabisnotsupportedonacon-

tinuous,uniformbase,thenveryhighbending(andthereforetensile)stresseswillbeintroduced,resultingincracking.Thelossofsupportmaybeduetothebasematerial’slackofstabilityanduniformityortheerosionofthematerialovertime(figure5-25).

Early LoadingLoadisdistributedthroughaconcreteslabovera

widearea,meaningthatthebaselayerdoesnothavetobeparticularlystrongorstiff.However,attheedgesandcornersofaslab,thereislessareatocarrytheload,resultinginhigherloadsanddeflectionsinthebase.Thisindicatesthattheedgesandcornersofaslabareparticularlysensitivetoloading(i.e.,

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Figure 5-26. A saw cut that has cracked through as planned (PCA)

susceptibletocracking)beforetheconcretehasgainedsufficientstrength.

Itisthereforerecommendedthatconstructionequipmentbekeptawayfromtheedgesofafreshslabuntilaminimumstrengthof3MPa(450lb/in2)isachieved.Whenconstructionequipment(e.g.,asealingtruck)isallowedontheslab,protecttheslabedgeswithconesorothertemporarybarricades.

Controlling Early-Age CracksAsnotedearlier,concretecracksbecausestresses

developintheconcretemorequicklythanthecon-crete’stensilestrengthdevelops.Thegoalistocontrolthenumberandlocationofcracksasmuchaspossibleandtopreventrandomcracks.Thisrequiresusinggooddesignandconstructionpractices.

Controlling Early-Age Cracks with JointsInjointedpavements(themajorityofconcrete

pavements),thenumber,location,andsizeofearly-agecracksiscontrolledbyconstructing(formingorsawing)contractionjoints.Thejointscreateplanesofweaknesswherecracksform(figure5-26)(seeJointSawinginchapter8,page233).

Tocontrolcrackingadequately,jointsmustbeconstructedcorrectly.Inthecaseofsawedjoints,thismeansthatjointsmustbesawedatthecorrecttime,atthecorrectspacing,andtothecorrectdepth.

Several Mechanisms Cause Cracking

Random cracks often get blamed on a single mechanism, when in fact they generally result from a combination of several stresses.

For example, two new pavements using the same materials and mix design are placed on the same day only a few miles apart. On the day after placement, one pavement experiences random cracking; the other does not. What is the difference?

The first pavement was placed early in the morning. Its peak heat of hydration coincided with the hottest part of a summer day, resulting in a peak temperature of 49°C (120°F).

The second pavement was placed late in the afternoon. Its peak heat of hydration occurred during the cooler night, resulting in a peak temperature of 32°C (90°F). As a result of its higher set temperature and peak temperature, the first pavement experienced significantly more stress during the following hours due to cooling and thermal contraction.

In addition, a spotty summer storm passed through the region during the first night. A cold rain fell on the first pavement, but not on the second.

The rain cooled the first pavement surface quickly, increasing the temperature differential throughout the slab. Thus, the first pavement experienced more early curling-induced stress than the second pavement experienced.

Together, the increased contraction-related stress and the increased curling-related stress caused the first pavement to crack randomly.

Preventing random cracking, therefore, requires being aware of all cracking-related variables during pavement design, mix design, materials selection, and construction, and then balancing and/or mitigating the variables’ effects.

Sometimes, compensating for the effects of one variable may exacerbate the effects of another. It is important to balance all cracking-related variables.

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Why are Controlled Cracks at Contraction Joints Preferable to Random Cracks?

Properly constructed contraction joints have many benefits:

Joints are more aesthetically pleasing than random transverse and longitudinal cracks.

Joints can be sealed more efficiently to limit infiltration of harmful materials.

Joints prevent the slab from randomly cracking into small, weak pieces.

Joints can be constructed with dowel bars and tiebars to prevent slab deflection at the joints and to allow proper transfer of vehicle loads between pavement sections (panels).

Joints help designate lanes.

Joints generally provide a smoother ride than random cracks.

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Figure 5-27. This joint was cut too late, resulting in random transverse cracking. (ACPA)

Timing: The Sawing Window.Jointsareusuallycon-structedbysaw-cuttingtheconcreteafewhoursafterplacing.Theoptimumperiodoftimetosawcontrac-tionjointsisknownasthesawingwindow.

Thesawingwindowforconventionalsawsgener-allybeginswhenconcreteisstrongenoughnottoravelexcessivelyalongthesawcut.Thewindowendswhensignificantshrinkageoccursthatinducesuncontrolledcracking(Voigt2000)(figure5-27).Goodpracticegenerallydictatesthat,iftheslabbeginscrackinginfrontofthesaw,sawingshouldbestoppedatthatjoint.

Thebeginning,duration,andendofthesawingwindowforanygivenconcretesystemareuniquetothatsystem.Thesawingwindowisgovernedbytherateofhydrationofthesystemandtheenvironmenttowhichitisexposed.

Higherearly-strengthconcretewillbeabletowithstandmoretensilestresswhenitfirstcoolsandundergoestemperaturedifferentials.However,thesawingwindowforconcretemixturesthatgainstrengthrapidlymaybeginsoonerandbeshorterthanfornormalmixtures.

Theuseoflightweight,early-entrysawsallowsjointstobesawedearlierwithoutraveling,withinanhourortwoofpaving,andmayreducetheriskofrandomcracking(seeImplicationsofCementHydrationforConstructionPracticesintheStagesofHydrationchartinchapter4,page80;andSawTiminginchapter8,page233).

Forearly-entrysaws,thesawingwindowbeginsatfinalsetandendswhentheshallowerearly-entrycutnolongercreatesaneffectiveplaneofweakness.

Itcanbedifficulttoknowwhenshallowcutsarenolongereffectivebecausetheresultingrandomcracksaregenerallynotvisibleuntilmuchlater.Gettingagood“feel”forthedurationofearly-entrysawingwindowscomeswithexperience.

Joint Spacing.Whentransversejointsaretoofarapart,theconcretemaystillcrackrandomlyatloca-tionsotherthanthejoints.

Anoptimaljointspacingexistsforeachspecificproject,dependingontheslabthickness,basestiff-ness,andconcretestrength.Moststateagenciesspecifytransversecontractionjointsinplain

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(unreinforced)pavementatintervalsbetween4.5and6.1m(15and20ft).

Forconcretepavementsplacedongranularsub-base,theAmericanConcretePavementAssociationrecommendsamaximumspacingof24d,where“d”isthepavementdepth;forconcretepavementsplacedonstabilizedmaterials,amaximumspacingof21disrecommended.

Joint Depth.Thedesigndepthofsawcutsistheminimumdepthrequiredtocreateaproperlyfunc-tioningjoint.Cutsthataretooshallowmaynotrelievestressesadequately,allowingrandomcrackstooccur.Cutsthatareunnecessarilydeeprequireadditionaleffort(requiremoretime),causeunnecessaryequip-mentwear,andreduceaggregateinterlock.

Ingeneral,thedepthofconventionalsawcutsisone-thirdofthepavementthickness.However,unlessaStatespecifiesotherwise,early-entrycutscangener-allybeapproximately25mm(1in.)deep,regardlessofpavementthickness.Becauseearly-entrysawcutsaremadebeforetheconcretehasdevelopedsignificantstrengthorstresses,thisshallowercutwillcreateaneffectiveplaneofweaknesswhereacrackshouldform(seeSawTiminginchapter8,page233).

Controlling Early-Age Cracks with CuringTohelpensurethatcracksformonlyatthejoints,

usethemoisturecontrolandtemperaturecontroltechniquesdiscussedonthefollowingpages.Curinghelpsdelaytheonsetofshrinkagestressesuntiltheconcreteisstronger.

Note:Inplain(unjointed)continuouslyreinforcedconcretepavements,thereinforcingmaterialcausescrackstodevelopwithinanacceptablespacing.Thereinforcingmaterialalsoholdsthecrackstightlytogether,limitingtheinfiltrationofaggressivefluidsandothermaterialsthatcanreducedurability.

Preventing Early-Age CracksThefollowingcracksare,atleasttheoretically,

preventable:Randomtransverseand/orlongitudinalcracks.Plasticshrinkagecracks(short,shallowsurfacecracksthatoccurwhenthesurfaceofthecon-cretedriestooquickly).

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Mapcracks(crazing).Theseoccurprimarilyinhandpours.Cornerandedgecracks.

Thesecracksaredescribedinmoredetailbeginningonpage158.Theycanbepreventedbyconstructingjoints,bycontrollingdryingshrinkageandthermalcontractionthroughgoodpracticesandawell-designedmix,andbyprovidingauniform,stablebase.

Using Joints to Prevent Uncontrolled Early-Age Cracks

Undernormalconditions,constructinganadequatesystemofjointsinunreinforcedconcretepavementwillpreventtheformationofrandomtransverseand/orlongitudinalcracks.

Selecting Materials to Prevent Early-Age Cracks

Anothermechanismforcontrollingshrinkageiscare-fulmixdesign/materialsselection.

Minimizethemixwatercontentrequiredforworkabilitybyavoidingtheexcessiveuseof

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_______________________________________________Figure 5-28. Effect of initial curing on drying shrinkage of portland cement concrete prisms. Concrete with an initial 7-day moist cure at 4°C (40°F) had less shrinkage than con-crete with an initial 7-day moist cure at 23°C (73°F). Similar results were found with concretes containing 25 percent fly ash as part of the cementitious material.

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cementitiousmaterials,optimizingthesizeandamountofcoarseaggregate,andminimizingtheuseofdustyaggregate.Considerusingawater-reducingadmixturetoreducedryingshrinkage(seefigure5-28).Avoidcalciumchlorideadmixtures,whichcansignificantlyincreasedryingshrinkage.Reducetheaggregateabsorptionofmixwater(seeStockpileManagementandBatchinginchapter8,pages206and207,respectively).

Note:Somebleedingoftheconcretemayhelpreducetherateofsurfacemoistureloss,butamixtureshouldnotbedesignedtobleedexcessivelysimplytoavoidplasticshrinkagecracking.

Preventing Early-Age Cracks with Moisture Control

Iftheconcretesurfacedriestoorapidlybeforeinitialset,thesurfacewillexperienceplasticshrink-agecracking.Significantdryingshrinkageaftertheconcretehardensmaycontributetomapcrackingandrandomtransverseorlongitudinalcrackingifthecon-creteisnotstrongenoughforthestresses.

Evenifistheconcreteisstrongenough,earlydryingwillexacerbatewarpingandthuscontributetofatiguecracking(seeShrinkageearlierinthischapter,page125).Therefore,controllingmoistureisessential.

Acriticalmechanismforcontrollingmoisture(andthusplasticshrinkage,dryingshrinkage,andwarping)isgoodcuring(FHWA2005).

Applycuringcompoundassoonaspossibleafterfinishing.Applycuringcompoundearlyifnoadditionalfinishingistobeconducted.Iftheevaporationrateishigh,applyanevapo-rationretardertothesurfaceoftheconcretebeforethefinalfinishingactivities.(Evaporationretardersarenotcuringcom-poundsandshouldnotbeusedasfinishingaids,althoughtheycanbereworkedintothesurfaceoftheconcreteifappliedinaccordancewiththemanufacture’srecommendations.)Inhot,dry,and/orwindyconditionswhenevaporationishigh,usewetcuringmethods,suchasfogspraytoincreasetherelativehumid-

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ityoftheairabovethesurface,orcoveringthesurfacewithwetburlap.

Preventing Early-Age Cracks with Temperature Control

Changesinconcretetemperaturewillresultinther-malcontractionand/orcurling,whichcancontributetorandomtransverseorlongitudinalcracking(seeTemperatureEffectsearlierinthischapter,page127).

Therefore,itisimportanttocontrolchangesintemperature.

First,reducesettemperature.Generally,thisinvolvescontrollingtheheatofhydrationthroughthefollowingactivities:

Usesupplementarycementitiousmaterialsinthemixthatreducetheheatandrateofhydration.Avoidorlimittheuseoffinecementitiousmaterials.Considerusingaset-retardingadmixtureinthemix.Reducethetemperatureoffreshconcretebyusingchilledwater,ice,orcooledaggregates.Avoidplacingconcreteduringthehottestpartoftheday.

Second,usegoodcuringpractices:Applycuringcompoundassoonaspossibleafterfinishing.Protecttheconcretefromsignificantchangesinambienttemperatures.Forexample,ifadropintemperatureislikelysoonafterplacement(e.g.,ifacoldfrontisexpected,oratnight),insulatetheconcretewithblanketsorpolyeth-ylenesheets.Thiswillreduceheatlossfromtheconcretesurface.Itwillalsomoderatediffer-encesintemperaturethroughoutthedepthoftheslab,reducingtheriskofcurling.

Preventing Early-Age Cracks with Uniform Support

Edgeandcornerbreaksduetolossofbasesupportcanbepreventedbygooddesignanddetailingandbycarefulpreparationofthesubgradeandbase.Inparticular,thebasemustbeuniformandstable(seeStructuralPerformanceinchapter2,page12;Sub-gradesandBasesinchapter7,pages192and196,respectively).

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Figure 5-29. A ring shrinkage specimen marked where a crack has occurred (CTLGroup)

Testing for Cracking RiskTherearenoteststhatassesstheriskofcracking

foragivenconcretemixtureinagivenenvironment.However,theriskofcrackinginagivenconstructionsitecanbeestimatedusingmodelingtoolssuchasHIPERPAV(seeCrackPredictionwithHIPERPAVinchapter8,page231).Inaddition,twotypesofstan-dardtestsinNorthAmericacanbeusedtocomparetherelativerisksofdifferentmixturecombinationsduringprequalificationandmixdesign.

Onetypeoftest,ASTMC1581/AASHTOPP34,usesrestrainttodeterminetheeffectsofconcretevariationsoncrackingtendency.Concreteiscastaroundasteelring(figure5-29),andstraingagesontheringallowstressmeasurementstobeginassoon

Summary of Tips to Reduce Early-Age Concrete Cracking

Random cracking in concrete can be reduced significantly or eliminated by observing the following practices:

Prepare the subgrade and base properly to provide stable, uniform support with adequate moisture content.Use a mix with a low water-cementitious materials ratio, optimize the size and amount of coarse aggregate, and use low-shrinkage aggregate to minimize shrinkage that may cause cracking. Consider using a water-reducing admixture to reduce paste content.Use appropriate supplementary cementi-tious materials and minimize the amount of fine cementitious materials to reduce the set temperature and the temperature peak due to hydration. This will in turn reduce thermal shrinkage. Consider using a set-retarding admixture to reduce thermal shrinkage.Avoid calcium chloride admixtures, which can significantly increase drying shrinkage. Time concrete placement so that the tempera-ture peak due to hydration does not coincide with the hottest time of day.

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Prevent rapid loss of surface moisture while the concrete is still plastic. For example, con-sider using spray-applied evaporation retard-ers. If the weather is sunny, hot, and/or dry, slow evaporation from the surface with wet curing methods, such as fog curing or covering with wet burlap.Limit tensile stress from external restraint by making sure there are no abrupt changes in the slab’s cross section.Construct contraction joints at appropriate intervals, at the appropriate depth, and at the appropriate time to relieve tensile stresses.Construct isolation joints to prevent restraint on the concrete from the adjoining elements of a structure.If the ambient temperature is likely to drop sig-nificantly, slow heat loss and prevent extreme differentials in temperature through the slab by covering the surface with blankets.Properly place, consolidate, finish, and cure the concrete and protect the curing compound from equipment and traffic for a minimum of 72 hours.

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What about Later-Age Cracks? (After the Pavement is Opened)

Several mechanisms can contribute to cracking problems for years after the pavement is place:

Load-associated issues, including loss of support.

Expansive chemical reactions, such as alkali-silica reaction, salt crystallization, and freezing and thawing, all of which may cause durability problems later in a concrete pavement’s life.

Long-term drying shrinkage.

For more information about these mechanisms, see the section on Shrinkage, as well as the discussion of properties related to concrete durability (specifically, Frost Resistance, Sulfate Resistance, Alkali-Silica Reactions, and Shrinkage), earlier in this chapter.

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asthesamplesarecast.Thismethodisusefulfor(andlimitedto)determiningtherelativesusceptibilityofdifferentconcretemixturestoearly-agecrackingduetoearlydryingshrinkage.Withthistest,dryingcanbestartedatanyrequiredage.Thisisanadvantageovertheothertypeoftest(ASTMC157),inwhichthefirstreadingistakenafter24hours.

ASTMC157usesanunreinforcedconcreteprismwithpinscastintheends.Thesamplesarekeptinwaterforafixedperiod(28daysinthestandardmethod),thenallowedtodryforsometime(64weeksinthestandardmethod).Severalstateauthoritiesusedifferenttimeintervalsforwetstorage(7daysistypical)andfortakingreadings(28or56days).Thechangeinlengthofthesampleisthenrecordedasitdries.

Thesampleisunrestrainedduringdrying,butisnotatruly“unrestrained”testbecausethesampleiskeptinthemoldfor24hoursaftercasting.Thistestisusefulforcomparingtheriskofcrackingduetothelong-termdryingshrinkagebehaviorofdifferentconcretemixtures.

ASTMC1581andASTMC157measuredifferentpropertiesoftheconcrete,anddatafrombothmeth-

odscanbeusefulinassessingthepotentialriskofcrackinginagivenmixture.

TheJapanConcreteInstitutehasreportedanautog-enousshrinkagetest(Tazawa1999),andarestrainedtestwasdescribedbySpringenschmidt(1995).BotharebeinginvestigatedintheUnitedStates.

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Time of Occurrence

Beforeset,whiletheconcreteisplastic.

Description

Onetothreeftapart(figures5-30and5-31).Usuallyshort(afewin.toseveralftlong).Relativelyshallow(onetotwoin.deep).Discontinuous.Generallyperpendiculartowinddirection.Mayoccurthroughoutthepanelsurface.Rarelyintersecttheslabperimeter.

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Cause

Plasticshrinkagecracksarecausedwhenrapidevaporationofmoisturefromtheconcretesurfacecausesthesurfacetodrywhiletheconcreteisstillplastic(beforeinitialset).Asthesurfacedries,itbeginstoformacrust.Shrinkageofthefast-dryingsurfacecrustisrestrainedbytheplasticconcreteunderneathit,resultinginsurfacecracks.

Weatherconditionssuchasthefollowingincreasesurfaceevaporationandthustheformationofplasticshrinkagecracks:

Lowrelativehumidityduringconcreteplacement.Highconcreteorairtemperatures.Windblowingacrossthepavementsurface(atanytemperature).Asuddencoldfrontorrain.

Notethatexcessiveevaporationcanoccurincoolbutwindyweatherconditions.

Combinationsoftheseweatherconditionswillincreasesurfacemoistureevaporationandthelikeli-hoodofplasticshrinkagecracking.

Plasticshrinkagecanalsobeexacerbatedbythefollowing:

Delayedsetoftheconcrete.Dryaggregate,whichabsorbsmoistureandthusreducestheavailablebleedwater.Gap-gradedaggregate,whichrequiresmorepastethanotheraggregatetypes.Too-finesand,whichreducesbleeding.Delayedfinishing,whichpreventsapplicationofprotectivecuringmethods.

Plasticshrinkagecracksshouldnotbeconfusedwithpre-hardeningcrackscausedbyconcretesettlementoneithersideofreinforcingsteelorthemovementofconcreteforms.

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Figure 5-30. Typical plastic shrinkage cracks

Figure 5-31. Deep plastic shrinkage cracks

wind direction

Summary of Preventable Early-Age Cracks:

Plastic Shrinkage Cracks

wind direction

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Effect

Plasticshrinkagecracksaresomewhatunsightly,buttheygenerallydonotposeastructuralproblemunlesstheyareverydeep.However,plasticshrinkagecracksdopermitwaterandchemicalstoentertheconcrete,soextensiveplasticshrinkagecrackingcanresultinlong-termdurabilityproblems.

Prevention

Whenevaporationisexpectedtobehigh,i.e.,whenconditionsarehot,windy,sunny,and/orthehumidityislow,takeappropriateprecautionstopre-ventplasticshrinkagecracking:

Perhapsmostimportant,protectthenewcon-cretesurfacewithadequatecuring.Possibleprotectivemeasuresincludetheuseofevapo-rationretarders,liquidmembrane-formingcuringcompounds,foggingtheareaoverthenewlyplacedconcrete,andwetcuringwithwetburlap(seeCuringCompoundsinchapter3,page64;Curinginchapter8,page224).Ifpossible,plantoplaceconcreteduringcon-ditionsthatarelesslikelytocauseshrinkagecracking.Forexample,duringthehotsummerplaceconcreteinthelateafternoon,earlyevening,oratnightwhenambienttem-peraturesarecoolerandrelativehumidityishigher.Cooltheconcretemixture.Reduceaggregateabsorptionofmixwater(seeStockpileManagementandBatchinginchapter8,pages206and207,respectively).

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Repair

Iftheconcreteisstillplastic,plasticshrinkagecrackscanbeclosedbyrevibratingtheconcrete(ACI305R).Tight,hairlinecracksdonotrequirerepair.Deeporverywidecracksshouldbesealed.

Excessive Evaporation Rates

When the evaporation rate approaches values between 0.5 and 1.0 kg/m2/hr (0.1 and 0.2 lb/ft2/hr), some specifications require action to prevent plastic shrinkage cracking. The evaporation rate depends on air temperature, concrete temperature, relative humidity, and wind velocity. The likely rate of evaporation can be estimated from a monograph published by the Portland Cement Association (2002) (see Weather Considerations in chapter 8, specifically figure 8-16 on page 227).

Dampendry,absorptivesubgrade.Considerapplyingevaporation-retardingmaterialbeforefinishing.Finishtheconcretesurfacepromptly,thenapplycuringearly.Somebleedingoftheconcretemayhelpreducetherateofsurfacemoistureloss,butdonotdesignamixturetobleedexcessivelysimplytoavoidplasticshrinkagecracking.

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Time of Occurrence

Afterconcretehasset;usuallyapparentthedayafterplacementorbytheendofthefirstweek.

Description

Networkoffinefissuresontheconcretesurfacethatenclosesmall(12to20mm[1/2to3/8in.])andirregularhexagonalareas(figure5-32).Shallow;oftenonly3mm(1/8in.)deep.Mayoccurthroughoutthepanelsurface.

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••

Cause

Mapcrackinggenerallyoccursonhandpourswithsignificanthandfinishingand/orinadequatecuring.Thistypeofcrackingiscausedbyrestraineddryingshrinkageofthesurfacelayerafterset.Itisoftenassociatedwiththefollowing:

Overfinishingthenewsurfaceorfinishingwhilethereisbleedwateronthesurface.Mixeswithhighwater-cementitiousmaterialsratios(mixesthataretoowet).Lateorinadequatecuring.Sprayingwateronthesurfaceduringfinishing.Sprinklingcementonthesurfacetodrybleedwater.

•

•

•••

Effect

Somemapcrackingcannotbeseenunlessthepavementsurfaceiswet.Visiblecrazingissomewhatunsightlybutgenerallydoesnotposeastructuralproblem.However,mapcracksdopermitwaterandchemicalstoentertheconcretesurface,soextensivemapcrackingmayresultinlong-termdurabilityproblems.

Figure 5-32. Map cracking

Prevention

Takeappropriateprecautionstopreventmapcracking:

Usemoderateslumpmixtures(generally,withalowwater-cementitiousmaterialsratio).Higherslumpmixturesmaybeacceptableiftheydonotproduceexcessivebleeding.Bleedingcanbecontrolledbyusingwater-re-ducingadmixturesinthemix.Donotspraywaterontheslabtofacilitatefinishing.Donotfinishthesurfacewhilebleedwaterispresent,anddonotsprinkledrycementonthesurfacetodrythebleedwater.Donotoverworkoroverfinishthesurface.Begincuringassoonaspossible.Coverthesurfacethoroughlywithcuringcompound.Usewetcuringmethods,i.e.,fogsprayingand/orwetburlap,andkeepthesurfacemoistforatleastthreedays.

1.

2.

3.

4.5.

6.

Repair

Mapcracksgenerallydonotrequirerepair.Toimproveaesthetics,theymayberemovedbydia-mondgrindingthesurface.


Map Cracking (Crazing)

Early-Age Cracking


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Time of Occurrence

Afterconcretehassetbutbeforethepavementisopenedtotraffic.

Description

Perpendiculartocenterline(figure5-33).Usuallyevenlyspacedandcontinuousfromthecenterlinetotheedgeofthepavement.Mayforkintoa“Y”shape.Extendintothefulldepthofslab.Maybehairline(andnearlyinvisible)oropen.Normallyappearwithinthefirst72hours.

••

••••

Cause

Randomtransversecrackingoccursaftertheconcretesetsbutbeforeithasgainedenoughstrengthtoresisttensilestresses.Thestressesaregenerallycausedbyrestrained,cumulativeshrinkageoftheslabandbycurlingandwarping(seeShrinkageandTemperatureEffectsinthischapter.pages125and127,respectively).Stressmaybeincreasedbythefollowingfactors:

Earlydrying.High-shrinkagemixes.Highsettingtemperature,whichincreasestheamountofcoolingafterset.Dryaggregate,whichabsorbsmoistureandthusincreasesshrinkage.Gap-gradedaggregate,whichrequiresmorepaste.Earlyloadsfromconstructionequipment.Changeinweatherthatincreasesshrinkage.

•••

•

•••

Effect

Injointedpavementswithoutcontinuousreinforcement(themajorityofconcretepavements),randomtransversecrackscanpermitwaterandchemicalstoentertheconcrete,resultinginlong-termdurabilityproblems.“Working”transversecracks(thatis,cracksinwhichverticalmovementordisplacementalongthecracksisdetectable)cancausestructuralfailure.

Randomtransversecracksdonotcauseproblemsinplain(unjointed),continuouslyreinforcedconcretepavements.Thereinforcingmaterialcausescrackstodevelopwithinanacceptablespacingandholdsthecrackstightlytogether.Thesetightcracksdonotdiminishthepavement’sinitialstructuralperformanceanddonotallowtheinfiltrationofaggressivefluidsandothermaterials.

Figure 5-33. Random transverse crack


Random Transverse Cracks

Early-Age Cracking


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Random Transverse Cracks continued

Prevention

Topreventrandomtransversecrackinginunrein-forcedconcrete,usethefollowinggoodpractices:

Minimizedryingshrinkagebyusingamixwithalowpastecontent.Maximizethesizeandamountofcoarseaggregatewhileleavingaworkablemix.Keepaggregatepilesmoist.Wetthegradepriortopaving.Minimizethetemperatureatwhichconcretesetstominimizetheamountofcooling(andthermalcontraction)afterfinalset.Generally,thisin-volvescontrollingtheheatofhydrationthroughcarefulselectionofcementitiousmaterials,usingpre-cooledmaterialsinthemixture,etc.(seeTemperatureEffectsinthischapter,page127).Makesurethejointsareconstructedatthepropertime,spacing,anddepth.Protecttheconcretesurfacefromsuddentem-peraturechanges,frommoistureloss,andfromexcessivecurlingandwarpingthroughpropercuring(seeCuringCompoundsinchapter3,page64;andCuringinchapter8,page224).Keepconstructiontrafficoffthepavementaslongaspossible.

1.

2.

3.4.5.

6.

7.

8.

Repair

Randomtransversecracksthatarenotworking(thatis,cracksinwhichthereisnodetectableverticalmovementordisplacementalongthecracks)shouldbesawedandsealed.However,randomtransversecracksthatareworkingornearajointoranothercrackgenerallyrequireafull-depthrepairtopreventstructuralfailure.

Control

Acertainamountoffull-depthcrackingtorelievetensilestressesisinevitableinconcretepavements.Inunreinforcedconcrete,thenumberandlocationoftheserandomcrackscanbecontrolledwithcontractionjoints.

Constructcontractionjointstocreateplanesofweaknesswherecrackswillformandthusrelievestresses.Tocompletelycontrolthesecracks,jointsmustbecutatthepropertime,theproperdepth,andtheproperspacing(seeJointSawinginchapter8,page233).

Early-Age Cracking


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Time of Occurrence

Afterconcretehassetbutbeforethepavementisopenedtotraffic.

Description

Paralleltocenterline(figure5-34).Runthroughthefulldepthoftheslab.Maybehairline(andnearlyinvisible)oropen.Mayappearwithinthefirst24hoursorafterseveralmonths.

Thecauses,effects,control,prevention,andrepairsforrandomlongitudinalcracksaresimilartothoseforrandomtransversecracks.Thefollowingaddi-tionalinformationisspecifictolongitudinalcracks.

••••

Cause

Randomlongitudinalcrackingcanoccuraftertheconcretesetsbutbeforeithasgainedenoughstrengthtoresisttensilestresses.Thestressesaregenerallycausedbyrestrained,cumulativeshrinkageoftheslabandbycurlingandwarping(seeShrinkageandTemperatureEffectsinthischapter,pages125and127,respectively).

Thestressofrestrainedthermalcontractionanddryingshrinkagemaybeincreasedbythefollowingfactors:

Nonuniformbasesupportcausedbyfrostheaving,soilsetting,orexpansivesoils.Aslabthatistoowide,i.e.,longitudinaljointsthataretoofarapart.Restraintfromanadjoiningtiedslab.Ajointcutthatistooshallow.

•

•

••

Figure 5-34. Random longitudinal crack

Control

Ifmorethanonelanewidthofunreinforcedconcreteisplacedatonce,cutorformlongitudinaljointsbetweenlaneswherecrackswillform(seeJointSawinginchapter8,page233).

Prevention

Usethesamegoodpracticesforpreventingrandomtransversecracking,aslistedonthepreviouspage.Inaddition,takecaretodothefollowing:

Placeconcreteonastable,uniformbasenotpronetofrostheaving,settling,orexpansion.Preventearlyloadingbykeepingconstruc-tionequipmentoffthepavement,particularlyalongthefreeslabedges.Donottietoomanylanestogetherwithtiebars.Tielanestogetherwhentheweatherisnottoohotortoocold(extremetemperatureswillincreasecontraction).Iftheslabis23cm(9in.)thickorless,theslabwidthshouldbenomorethan3.5m(12ft)widewithoutalongitudinaljoint.Iftheslabis25cm(10in.)thickormore,theslabshouldtypicallybenowiderthan4m(14to15ft)withoutalongitudinaljoint.

1.

2.

3.4.

5.


Random Longitudinal Cracks

Early-Age Cracking


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Time of Occurrence

Cornerbreakscanoccurafterset,beforepave-mentisopenedtothepublic.However,cornerbreakscancontinuetoformforyears,anytimethepavementlosessupportduetosoilsettling,erosion,frostheaving,expansivesoils,orexcessivecurlingandwarping.

Description

Cracksintersectingtheadjacenttransverseandlongitudinaljointsatapproximatelya45-degreeangle(figure5-35).From0.3m(1ft)longtohalftheslabwidthoneachside.Fulldepth.

•

•

•

Cause

Cornercracksoccurwhenapavementcarriesloadsthatareheavierthanitscurrentstrengthcansupportand/orwhenthereislossofbasesupport.Forinstance,earlyloadingwithheavyconstructionequipmentcancausecornercracking.

Furthermore,curledorwarpedslabslosebasesupportwheretheslabliftsawayfromthegrade.Whenloadsareapplied,thepavementwillcrackintheareasthathavereducedsupport.

Ifthebaseisnotuniform,theslabwillsimilarlylosesupportwherethebaseislessstablethaninthesurroundingareas.

Repeatedloadingsmayalsocreatevoidsunderslabcorners,andwhenloadsareappliedthepave-mentmaycrackwherethesupportisweakened.

Effect

Cornercracksusuallyrepresentstructuralfailures.

Figure 5-35. Corner break

Prevention

Topreventcornerscracking,considerthefollowingmeasures:

Makesurethebaseisuniformlystableovertime.Keepconstructiontrafficoffthenewpavementaslongaspossible.Properlycuretheconcreteaslongaspossibletohelppreventorreducecurlingandwarping.

•

•

•

Repair

Cornercracksgenerallyrequirefull-depthrepairoftheslabandperhapsstabilizationofthebase.


Corner Breaks

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AASHTOT161,ResistanceofConcretetoRapidFreezingandThawing

AASHTOT177,FlexuralStrengthofConcrete(UsingSimpleBeamwithCenter-PointLoading)

AASHTOT185,EarlyStiffeningofPortlandCement(MortarMethod)

AASHTOT186,EarlyStiffeningofHydraulicCement(PasteMethod)

AASHTOT196,AirContentofFreshlyMixedCon-cretebytheVolumetricMethod

AASHTOT197,TimeofSettingofConcreteMixturesbyPenetrationResistance

AASHTOT198,SplittingTensileStrengthofCylin-dricalConcreteSpecimen

AASHTOT231,CappingCylindricalConcreteSpecimens

AASHTOT277,ElectricalIndicationofConcrete’sAbilitytoResistChlorideIonPenetration

AASHTOT299,RapidIdentificationofASRProductsinConcrete

AASHTOT303,AcceleratedDetectionofPotentiallyDeleteriousExpansionofMortarBarsDuetoASR

AASHTOT309-99,TemperatureofFreshlyMixedPortlandCementConcrete

AASHTOTP60-00,StandardTestMethodfortheCoefficientofThermalExpansionofHydraulicCementConcrete

AASHTOTP64,StandardMethodofTestforPre-dictingChloridePenetrationofHydraulicCementConcretebytheRapidMigrationProcedure

AASHTOPP34,ProvisionalStandardtoAssessSus-ceptibilitytoCracking


ASTMC31/C31M-03a,StandardPracticeforMakingandCuringConcreteTestSpecimensintheField

ASTMC39,StandardTestMethodforCompressiveStrengthofCylindricalConcreteSpecimens

ASTMC42/C42M-04/AASHTOT24,StandardTestMethodforObtainingandTestingDrilledCoresandSawedBeamsofConcrete


AASHTOM85,StandardSpecificationforPortlandCement

AASHTOM157,StandardSpecificationforReady-MixedConcrete

AASHTOM240,BlendedHydraulicCement


AASHTOT23/AASHTOM201,StandardSpecifica-tionforMixingRooms,MoistCabinets,MoistRooms,andWaterStorageTanksUsedintheTestingofHydraulicCementsandConcretes

AASHTOT24,ObtainingandTestingDrilledCoresandSawedBeamsofConcrete

AASHTOT96,ResistancetoDegradationofSmall-SizeCoarseAggregatebyAbrasionandImpactintheLosAngelesMachine

AASHTOT97,FlexuralStrengthofConcrete(UsingSimpleBeamwithThirdPointLoading)

AASHTOT104,SoundnessofAggregatebyUseofSodiumSulfateorMagnesiumSulfate

AASHTOT119,SlumpofHydraulicCementConcrete

AASHTOT121,Density(UnitWeight),Yield,andAirContent(Gravimetric)ofConcrete

AASHTOT126,MakingandCuringConcreteTestSpecimensintheLaboratory

AASHTOT131,TimeofSettingofHydraulicCementbyVicatNeedle

AASHTOT141,SamplingFreshlyMixedConcrete

AASHTOT152,AirContentofFreshlyMixedCon-cretebythePressureMethod

AASHTOT154,TimeofSettingofHydraulicCementPastebyGillmoreNeedles

AASHTOT158,BleedingofConcrete

AASHTOT160,LengthChangeofHardenedHydrau-licCementMortarandConcrete

References


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ASTMC78,StandardTestMethodforFlexuralStrengthofConcrete(UsingSimpleBeamwithThirdPointLoading)ASTM-99a/AASHTOT104StandardTestMethodforSoundnessofAggregatesbyUseofSodiumSulfateorMagnesiumSulfate

ASTMC88-99a/AASHTOT104,StandardTestMethodforSoundnessofAggregatesbyUseofSodiumSulfateorMagnesiumSulfate

ASTMC94/C94M-04a/AASHTOM157,StandardSpecificationforReady-MixedConcrete

ASTMC131-03,StandardTestMethodforResistancetoDegradationofSmall-SizeCoarseAggregatebyAbrasionandImpactintheLosAngelesMachine

ASTMC138/C138M-01a/AASHTOT121,StandardTestMethodforDensity(UnitWeight),Yield,andAirContent(Gravimetric)ofConcrete

ASTMC143/C143M-03,StandardTestMethodforSlumpofHydraulicCementConcrete

ASTMC150/AASHTOM85,SpecificationforPort-landCement

ASTMC157M-04/AASHTOT160,StandardTestMethodforLengthChangeofHardenedHydraulicCement,Mortar,andConcrete

ASTMC172,StandardPracticeforSamplingFreshlyMixedConcrete

ASTMC173/C173M-01e1/AASHTOT196,StandardTestMethodforAirContentofFreshlyMixedCon-cretebytheVolumetricMethod

ASTMC186,TestMethodforHeatofHydration

ASTMC191/AASHTOT131,TestMethodforTimeofSettingofHydraulicCementbyVicatNeedle

ASTMC192/C192M-02/AASHTOT23andT126,StandardPracticeforMakingandCuringConcreteTestSpecimensintheLaboratory

ASTMC215-02,StandardTestMethodforFun-damentalTransverse,Longitudinal,andTorsionalFrequenciesofConcreteSpecimens

ASTMC227,StandardTestMethodforPotentialAlkaliReactivityofCement-AggregateCombinations(Mortar-BarMethod)

ASTMC231-04/AASHTOT152,StandardTestMethodforAirContentofFreshlyMixedConcretebythePressureMethod

ASTMC232,StandardTestMethodforBleedingofConcrete

ASTMC266/AASHTOT154,TestMethodforTimeofSettingofHydraulic-CementPastebyGillmoreNeedles

ASTMC289,StandardTestMethodforPotentialAlkali-SilicaReactivityofAggregates(ChemicalMethod)

ASTMC293,StandardTestMethodforFlexuralStrengthofConcrete(UsingSimpleBeamWithCenter-PointLoading

ASTMC295,C295-03,StandardGuideforPetro-graphicExaminationofAggregatesforConcrete

ASTMC359/AASHTOT185,TestMethodforEarlyStiffeningofHydraulicCement(MortarMethod)

ASTMC360-92,TestMethodforBallPenetrationinFreshlyMixedHydraulicCementConcrete(With-drawn1999)

ASTMC403M/AASHTOT197,TestMethodforTimeofSettingofConcreteMixturesbyPenetrationResistance

ASTMC418-98,StandardTestMethodforAbrasionResistanceofConcretebySandblasting

ASTMC441,StandardTestMethodforEffectivenessofPozzolansorGroundBlast-FurnaceSlaginPre-ventingExcessiveExpansionofConcreteDuetotheAlkali-SilicaReaction

ASTMC451/AASHTOT186,TestMethodforEarlyStiffeningofHydraulicCement(PasteMethod)

ASTMC452,TestMethodforPotentialExpansionofPortlandCementMortarsExposedtoSulfate

ASTMC457-98,StandardTestMethodforMicro-scopicalDeterminationofParametersoftheAir-VoidSysteminHardenedConcrete

ASTMC469,StandardTestMethodforStaticModu-lusofElasticityandPoisson’sRatioofConcreteinCompression

ASTMC496,StandardTestMethodforSplittingTen-sileStrengthofCylindricalConcreteSpecimens

ASTMC595/AASHTOM240,SpecificationforBlendedCement

ASTMC617-98/AASHTO231,StandardPracticeforCappingCylindricalConcreteSpecimens

ASTMC666-97/AASHTOT161,StandardTestMethodforResistanceofConcretetoRapidFreezingandThawing

References


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ASTMC672/C672M-03,StandardTestMethodforScalingResistanceofConcreteSurfacesExposedtoDeicingChemicals

ASTMC779/C779M-00,StandardTestMethodforAbrasionResistanceofHorizontalConcreteSurfaces

ASTMC827-01a,StandardTestMethodforChangeinHeightatEarlyAgesofCylindricalSpecimensofCementitiousMixtures

ASTMC856,StandardPracticeforPetrographicExaminationofHardenedConcrete

ASTMC873-99,StandardTestMethodforCompres-siveStrengthofConcreteCylindersCastinPlaceinCylindricalMolds

ASTMC918-97e1,StandardTestMethodforMeasur-ingEarly-AgeCompressiveStrengthandProjectingLater-AgeStrength

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ASTMC1138-97,StandardTestMethodforAbrasionResistanceofConcrete(UnderwaterMethod)

ASTMC1157,PerformanceSpecificationforHydraulicCements


ASTMC1231/C1231M-00e1,StandardPracticeforUseofUnbondedCapsinDeterminationofCompres-siveStrengthofHardenedConcreteCylinders

ASTMC1260,StandardTestMethodforPotentialAlkaliReactivityofAggregates(Mortar-BarMethod)

ASTMC1293,StandardTestMethodforDetermina-tionofLengthChangeofConcreteDuetoAlkali-SilicaReaction

ASTMC1543-02,StandardTestMethodforDeter-miningthePenetrationofChlorideIonintoConcretebyPonding

ASTMC1556-04,StandardTestMethodforDeter-miningtheApparentChlorideDiffusionCoefficientofCementitiousMixturesbyBulkDiffusion

ASTMC1567-04,StandardTestMethodforDeter-miningthePotentialAlkali-SilicaReactivityofCombinationsofCementitiousMaterialsandAggre-gate(AcceleratedMortar-BarMethod)

ASTMC1581-04,StandardTestMethodforDeter-miningAgeatCrackingandInducedTensileStressCharacteristicsofMortarandConcreteunderRestrainedShrinkage

ASTMC1585-04,StandardTestMethodforMea-surementofRateofAbsorptionofWaterbyHydraulic-CementConcretes

ASTMC1646,StandardPracticeforMakingandCuringTestSpecimensforEvaluatingFrostResistanceofCoarseAggregateinAir-EntrainedConcretebyRapidFreezingandThawing

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Liu,T.1994.AbrasionResistance.SignificanceofTestsandPropertiesofConcreteandConcreteMakingMaterials.ASTMSTP169c.Eds.PaulKliegerandJosephF.Lamond.WestConshohocken,PA:AmericanSocietyforTestingandMaterial.

Mielenz,R.C.,V.E.Wolkodoff,J.E.Backstrom,andR.W.Burrows.1958.Origin,Evolution,andEffectsoftheAirVoidSysteminConcrete:Part4–TheAirVoidSysteminJobConcrete.ACIJournal.FarmingtonHill,MI:AmericanConcreteInstitute.507–517.

Mindess,S.andJ.F.Young.1981.Concrete.Engle-woodCliffs,NJ:Prentice-HallInc.

Mindess,S.,J.F.Young,andD.Darwin.2003.Con-crete.2ndEd.UpperSaddleRiver,NJ:PearsonEducationInc.

NationalCooperativeHighwayResearchProgram(NCHRP).2004.GuideforMechanistic-EmpiricalDesignofNewandRehabilitatedPavements.NCHRPProject1-37a.Washington,D.C.:TransportationResearchBoard.

Neville,A.M.1996.PropertiesofConcrete.NewYork,NY:JohnWileyandSons.

Newlon,H.Jr.1978.ModificationofASTMC666forTestingResistanceofConcretetoFreezingandThaw-inginaSodiumChlorideSolution.VHTRC79-R16.Charlottesville,VA:VirginiaTransportationResearchCouncil.

Newlon,H.andT.M.Mitchell.1994.FreezingandThawing.SignificanceofTestsandPropertiesofConcreteandConcreteMakingMaterials.ASTMSTP169c.Eds.PaulKliegerandJosephF.Lamond.WestConshohocken,PA:AmericanSocietyforTestingandMaterial.153–163.

Poole,T.S.2005.GuideforCuringofPortlandCementConcretePavements.FHWA-RD-02-099.McLean,VA:FederalHighwayAdministration.

PCA.1982.BuildingMovementsandJoints.EB086.Skokie,IL:PortlandCementAssociation.

Powers,L.J.1999.DevelopmentsinAlkali-SilicaGelDetection.ConcreteTechnologyToday20.1:5–7.

Powers,T.C.1949.Theairrequirementsoffrostresis-tantconcrete.ProceedingsoftheHighwayResearchBoard29:184–211.

Saraf,C.L.andB.F.McCullough.1985.ControllingLongitudinalCrackinginConcretePavements.Trans-portationResearchRecord1043:8–13.

Scanlon,J.M.1994.FactorsInfluencingConcreteWorkability.SignificanceofTestsandPropertiesofConcreteandConcreteMakingMaterials.ASTMSTP169c.Eds.PaulKliegerandJosephF.Lamond.WestConshohocken,PA:AmericanSocietyforTestingandMaterial.49–64.

Springenschmid,R.1995.ThermalCrackinginCon-creteatEarlyAges.RILEMProceedings25.

Stark,D.1991.HandbookforIdentifyingAlkaliSilicaReactioninHighwayStructures.SHRPC-315.Wash-ington,D.C.:StrategicHighwayResearchProgram,NationResearchCouncil.http://gulliver.trb.org/publi-cations/shrp/SHRP-C-315.pdf.

Taylor,H.F.W.1997.CementChemistry,2ded.London:ThomasTelfordPublishing.

Tazawa,E.1999.JapanConcreteInstituteTechnicalCommitteeonAutogenousShrinkageofConcrete.CommitteeReport:AutogenousShrinkageofCon-crete.Ed.Ei-ichiTazawa.LondonandNewYork:E&FNSpon.1–62.

Tennis,P.D.,andJ.I.Bhatty.2006.Characteristicsofportlandandblendedcements:resultsofasurveyofmanufacturers.PaperpresentedattheCementIndus-tryTechnicalConference.

Tennis,P.D.andJ.I.Bhatty.2006(inpress).NorthAmericanCementCharacteristics:2004.SN2879.Skokie,IL:PortlandCementAssociation.

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References


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Thomas,M.D.A.andF.A.Innis.1998.EffectofSlagonExpansionDuetoAlkali-AggregateReactioninConcrete.ACIMaterialsJournal95.

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Vogler,R.H.andG.H.Grove.1989.Freeze-thawtest-ingofcoarseaggregateinconcrete:ProceduresusedbyMichiganDepartmentofTransportationandotheragencies.Cement,Concrete,andAggregates2.1:57–66.

Wainwright,P.J.andN.Rey.2000.InfluenceofGroundGranulatedBlastfurnaceSlag(GGBS)AdditionsandTimeDelayonthebleedingofConcrete.CementandConcreteComposites22.4:253–257.

Whiting,D.A.andM.A.Nagi.1998.ManualofCon-trolofAirContentinConcrete.EB116.Skokie,IL:PortlandCementAssociation.

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Wood,S.L.1992.EvaluationoftheLong-TermProp-ertiesofConcrete.ResearchandDevelopmentBulletinRD102.Skokie,IL:PortlandCementAssociation.

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Chapter 6

Development of Concrete Mixtures

Sequence of Development 172

Aggregate Grading Optimization 176

Calculating Mixture Proportions 179

Adjusting Properties 185

Theparametersforalmostallconcreteproper-tiesforaspecificconcretemixturearedeterminedorspecifiedduringmixdesign.Theterms“mixdesign”and“mixproportioning”areoftenincorrectlyusedinterchangeably.

Mixdesignistheprocessofdeterminingrequiredandspecifiablepropertiesofaconcretemixture,i.e.,concretepropertiesrequiredfortheintendeduse,geometry,andexposureconditions.Mixproportioningistheprocessofdeterminingthequantitiesofconcreteingredientsforagivensetofrequirements.Theobjectiveofproportioningconcretemixturesistodeterminethemosteconomicalandpracticalcombinationofreadilyavailablematerialstoproduceaconcretethatwillhavetherequiredproperties.

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Thefollowingarefactorstobeconsideredincon-cretemixdesign:

Workability.Placementconditions.Strength.Durability.Economy.

Thischaptercovershowthedesiredpropertiesofconcrete,discussedinchapter5,canbeachievedbyoptimizedselectionofconcreteingredients,discussedinchapter3.Thefirstsectionofthischapterdiscussesthesequencesofactivitieswhendesigningamix.Thenextsectionprovidesonemethodofoptimizingtheaggregatestoobtainagoodcombinedgrading.Thethirdsectionprovidesamethodofcalculatingmixproportions,followedbypossiblemodificationstoachieveselectedconcreteproperties.

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Sequence of Development

Key Points

Mixdesignspecificationsmustbeappropriatefortheconstructionsystem,loading,andenvironmentthatareexpected.

Seriesoftrialbatches,madeusingjob-specificmaterialsincludingthoseatthelikelyplacementtemperatures,areessential.

Sufficienttrialbatchesshouldbemadesothatadjustmentsoftheworkabilityandaircontentcanbemadeinthefieldwithconfidence.

Fieldtrialsusingthefull-scalebatchandhandlingplantarerecommended.

Workabilityandaircontentaretheprimaryconcretepropertiesthatcanbemanipulatedduringthebatchingprocessinthefield.

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Pre-ConstructionLongbeforeconstructionbegins,themixspecifi-

cationmustbeset,bidsevaluated,andtrialbatchesprepared.

Specifications for Mix DesignTheprocessofdesigningaconcretemixfora

pavingprojectbeginswithsettingoutthespecifica-tion.Therearetwodifferentapproachestoconcretemixturespecification:

Intheprescriptiveapproach,specifiersdefinetherequiredmaterials,proportions,andconstructionmethodsbasedonfundamentalprinciplesandpracticesthatexhibitsatisfactoryperformance.Intheperformanceapproach,specifiersidentifyfunctionalrequirements,suchasstrength,durability,andvolumechanges,andrely

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onconcreteproducersandcontractorstodevelopconcretemixturesthatmeetthoserequirements.

Allspecificationsshouldrequiretheuseofmaterialsthatmeetminimumqualityrequirementsasdescribedinchapter3.Prescriptivespecificationsshouldalsoprovidegeneralguidelinesformaterialsproportions,suchasminimumcementcontent,maximumreplace-mentratesforsupplementarycementitiousmaterials,water-cementitiousmaterialsratio(w/cm),aggregategradingrequirements,andaircontent.

Traditionally,strengthhasbeentheprimaryaccep-tancecriteriaforconcretepavements.Durability,however,isnotsimplyafunctionofstrength(seeConcreteDurabilityisAffectedbyManyConcretePropertiesinchapter5,page130).Manyotherfac-torscontributetopavementdurabilityandshouldbeconsideredinallstagesofmixdesigndevelopment.Non-strength-relatedconcretepropertiesthatinflu-encedurabilityincludebutarenotlimitedtothefollowing:

Airentrainment(spacingfactorandspecificsurfacearea).Permeability.Robustworkabilitypropertiesthatenableplace-mentatvarioustemperatureswithoutsignifi-cantchangestothew/cmratio.

Ataminimum,specificationsshouldrequirethattrialbatches(laboratorymixesorfield-batchedmixes)bepreparedwithjob-specificmaterials,andthatthetrialbatchesbetestedfortherequireddurabilityfac-torsbeforepaving.

Manylocaldepartmentsoftransportation(DOTs)mayhavealistingofmaterialsapprovedforconcretepavementconstruction.StateDOTcertificationstogetherwithotherdocumentationcanhelpfacilitatethematerialsapprovalreviewprocess.

BiddingEconomicsareintroducedasaconstraintduring

thebiddingstage.Thecontractor/concretesupplierisfacedwiththechallengeofoptimizingthemixproportionswithrespecttocost,specifications,andperformancerequirements.

Inmostinstances,prospectivebidderscanmakereasonableassumptionsbecausetheyhavepriorexpe-

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riencewiththespecificationsandmaterialsthatwillbeusedfortheproject.However,whenspecificationschange,whennewmaterialsourcesareintroduced,orwhenmaterialsourceschange,itmaybenecessarytobatchandtesttriallaboratorymixesduringthebid-dingprocesstoreducetherisksofuncertainty.Whentestingbeforethebidisdeemedappropriate,testresultsshouldbeinterpretedcarefullywithrespecttosamplesize,testprecision,andbetween-batchvariability.

Laboratory MixesAllofthedesktopapproachesthatcanbeusedto

proportionamixtureareonlythestartingpointformakingtrialbatches.Theriskofsevereproblemsissignificantifanewmixtureistakenfromthecalcula-tortothebatchplant.Itisessentialtopreparetrialbatchesinthelaboratorytoensurethatthefreshandhardenedpropertiescomplywiththerequirements,andthattherearenoincompatibilitiesbetweenthematerials,atthetemperaturesatwhichthefieldmixeswillbemade.

Itismoreconvenientandeconomicaltotestconcretemixesinthelaboratorythantobatchlargequantitiesataconcreteplant.However,projectconditionsareoftensignificantlydifferentfromthecontrolledenvironmentofalaboratory.Productionvariabilityandtestingvariabilityneedtobeconsid-eredandunderstoodwhenlaboratorytestresultsareinterpreted.Ideally,somelaboratorytestsshouldbeconductedatthesamerangeoftemperaturesexpectedinthefield.

Chooseaqualifiedlaboratoryanddesignatestingplanthatwillprovidetheinformationdesired.Laboratoriesandtechniciansshouldbeexperiencedwithconcretemixturesandaccreditedbyanindepen-dentsource.Thetestingplanshouldbespecificandincludealltestsneededtoverifythemixproperties.Asuggestedtestingplanisshownintable6-1.

Whenthepotentialforchangesinmaterialssourcesorenvironmentcanbeanticipated,itisadvisabletobatchadditionallaboratorymixeswithalternativematerialsandatdifferenttemperaturesasabackup.

Anticipating Responses to Field ConditionsLaboratorymixesshouldbebatchedsothatthe

w/cmratioandaircontentarerepresentativeofthe

mixthatwillbeusedduringpaving.Fieldadjust-mentsshouldbeanticipated.Inotherwords,differentlaboratorymixesshouldbebatchedthatwillallowtheplanttorespondtochangesinthematerialssourcesandproperties,environment,anddemandsonthecon-cretesystemwithoutdeviatingfromthespecification.

Whentherawmaterialsaredeliveredtoaconcreteplant,itistoolatetochangethecementchemistryorthephysicalpropertiesoftheaggregates.Therefore,workabilityandaircontentaretheprimaryconcretepropertiesthatcanbemanipulatedduringthebatch-ingprocessinthefield.Thiscanbeaccomplishedbyadjustingthedosageoftheappropriateadmixtureand,ifstillwithinthespecifiedw/cmratio,thewater.

Particularattentionshouldbepaidtopreparinglaboratorymixesthatarerepresentativeofthemateri-alsthatwillbeusedontheproject.Portlandcementandsupplementarycementitiousmaterialsshouldbeobtainedfromthesuppliers’normalproductionandnotbespeciallyprepared.Ifnecessary,aggregatesshouldbescreenedintoseparatesizesandrecom-binedtomatchascloselyaspossiblethegradationthatwillbeprovidedontheproject.

Thesuggestedlaboratorytestingplanshouldbefollowedforthetargetw/cmratio.Additionallabora-torymixesshouldbebatchedatdifferentw/cmratios,onehigherandonelower.

Figure6-1providesanexampleofaplotshowingthestrengthvaluesformixeswithdifferentwater-cementratios.Thisgraphprovidesabasisforsettinglimitsforfieldadjustmentsthatmayoccur.Atthepointthatconcreteproductionbegins,itisassumedthatifthe“recipe”isfollowed,anacceptablemixwillbedeliveredtothepaver.

Oncethemixproportionshavebeenproveninthelaboratory,theycanbetestedinfull-scalebatchesintheequipmentthatwillbeusedontheproject.Thismayappearexpensiveattheoutset,butthesavingsinpreventinglaterproblemswillmorethanoffsetthisinvestment.

Field TrialsJustbeforepaving,themixdesignsshouldbe

verifiedinthefield(unlessthereisexperiencewithasimilarmix).Thisprocessisnecessarytoensurethatthe



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materialsandthefinalmixaresubstantiallythesameasthosethatwereusedduringthelaboratorytrials.

Thefollowingquestionsshouldbeconsideredwhenverifyinglaboratorymixesinthefield:

Table 6-1. Suggested Laboratory Testing Plan

Arethefreshpropertiesacceptableforthetypeofequipmentandsystemsbeingused?Aretheresignsofincompatibilities?Arethestrengthsfromthefieldmixcompara-bletothosefromthelaboratorymix(±1.7MPa[250lb/in2])?

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Concrete property Test description Test method Comments

Workability Slump at 0, 5,10,15, 20, ASTM C 143 / Changes in workability may 25, & 30 minutes AASHTO T 119 indicate changes in the w/cm ratio and affects placement in the field

Concrete temperature when ASTM C 1064 Temperature differences batched in the field may affect w/cm ratio, workability, and volumetric stability

Grading and moisture content ASTM C 136 / AASHTO T 27 Use the moisture content of aggregates and in conjunction with the ASTM C 566 /AASHTO T 255 absorption of the aggregate to calculate w/cm ratio

Combined grading Coarseness / workability Project variability may affect factors “8-18” analysis workability and w/cm ratio 0.45 power chart*

Strength development Compressive or flexural strength ASTM C 39 / AASHTO T 22 Cast as many specimens and/or as possible from a single ASTM C 78 / AASHTO T 97 batch– break 3 @ 3 days, 3 @ 7 days, and 12 @ 28 days

Maturity curve ASTM C 1074 or May also be developed in Agency special provisions the field during the field trial stage

Air content Air content ASTM C 231 / AASHTO T 152 Target the middle of the specification range – laboratory batches with air contents on the low end of specification tolerances will provide higher strengths Air-void analyzer and/or ASTM C 457 Spacing factor and specific image analysis surface area are both critical for freeze/thaw durability Density Density / unit weight ASTM C 138 / AASHTO T 121 Indicates (1) volumetric quantity (yield) of concrete produced per batch, (2) air content of concrete mixture, (3) uniformity

Permeability Rapid chloride penetration ASTM C 1202 / AASHTO T 277 Results are interpreted in ranges that are broadly related to permeability

* See 0.45 power chart later in this chapter.** www.aashtotig.org/ (Click on Focus Technologies.AVA)



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________________________________________________Figure 6-1. Example w/c ratio vs. strength curve (laboratory mix target w/c = 0.40)

Table 6-2. Suggested Field Trial Batch Testing PlanFieldtrialsarealsoapreferredpracticewhenpor-tableplantsareused.Thebatchingprocess,includingmixtime,shouldbethesameasthattobeusedduringpavingoperations.

Workabilityofthefieldtrialbatchesshouldbetestedimmediatelyafterbatchingandatalatertimetosimulatethetransportationtime.Fieldtrialbatchesshouldberemadeuntilthedesiredworkabilityisachievedaftertheestimatedtimeintransportandthentested(table6-2).

When or what Action or test

Before production Test and verify scales and plant begins operations

Determine aggregate moisture contents and adjust batch proportions

The primary mix design Workability Air content

Unit weight

Three- and seven-day strength

Yield

Maturity curves

Mixer uniformity test CRD-C-55(front to rearof the batch) Slump

Air content

Unit weight

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Aggregate Grading Optimization

Key Points

Optimizedgradingofthecombinedaggregateswillenhancetheconcretemixture.

Shilstone’smethodforoptimizingaggregatestoobtainagoodcombinedgradingusesthreetools,eachofwhichprovidesadifferentkindofanalysis.

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sieve.Theseparticlesfillmajorvoidsbetweenthecoarseparticles.Fineparticlespassthe1.18-mm(#8)sieve.

Themassfractionsofthecombinedaggregatethatfallintoeachoftheabovesizegroupsareusedtocal-culatethecoarsenessfactorandtheworkabilityfactor.ThecoordinatesfromthatdataareplottedontheCFC(figure6-2).

Thecoarsenessfactoristhemassofthecoarse-sizedaggregatedividedbythesumofthemassesofthecoarseandintermediatesizes:

Thus,acoarsenessfactorof100describesagrad-ingwithnointermediateaggregate,i.e.,noparticlespassingthe9.5-mm(3/8-in.)sieveandretainedonthe2.36-mm(#8)sieve.Suchamixtureisgap-graded.Avalueof0describesamixturewithnocoarseaggregate,i.e.,noparticlesretainedonthe9.5-mm(3/8-in.)sieve.

Theworkabilityfactoristhepercentofthecom-binedaggregatethatpassesthe2.36-mm(#8)sieveplusanadjustmentfortheamountofcementitiousmaterialinamixture.Thebasecementitiousmateri-alscontentforthechartis335kg/m3(564lb/yd3).Theworkabilityfactorisincreased2.5pointsforeach56kg/m3(94lb/yd3)variationfromtheoriginalcementitiousmaterialscontent.Thecoarsenessfactorandtheworkabilityfactorestablishthecoordinatesforamixture.

Coarseness Factor Zones Thetrendbar(figure6-2)definesaregionwhere

combinedroundedorcubicalcrushedstoneandwell-gradednaturalsandareinbalance.However,suchmixtureshavelimitedapplication,astheaggregategradingmustbewell-controlled.Theymaybeusedforbucket-placedconcreteinlargefootings.Mixturesrepresentedbyplotsabovethebaridentifymixtureswithincreasingamountsoffineaggregate.Thosebelowthetrendbargenerallycontainanoverabun-danceofcoarseparticlesandarenotdesirable.

Thecoarsenessfactorzonesidentifyregionswhereplottedmixtureswillhavegenerallypredictablecharacteristics:

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Aggregatesaregenerallychemicallyanddimen-sionallystable;therefore,itisdesirabletomaximizeaggregatecontentinconcretemixturescomparedtothemorechemicallyreactivecementpaste.Well-gradedaggregatereducesthespacebetweenaggregateparticlesthathastobefilledwithcementpaste.Well-gradedaggregatealsocontributestoachievingaworkablemixwithaminimumamountofwater.

Shilstoneconsidersthatthebestmeansofspeci-fyingandselectingmixproportionsisthroughcombinedgradinganalysis(Shilstone1990)(seeCombinedGradinginchapter9,page253).Hedevel-opedthreetoolstohelpintheprocessofoptimizingthecombinedaggregategrading:

TheCoarsenessFactorChart(CFC):providesoverviewofthemixture.The0.45PowerChart:showstrends.ThePercentofAggregateRetainedonEachSieve(PARS):showsdetails.

SomemixturesthatappearexcellentbasedontheCFCmaybefound,bytheothertwoanalyses,tohaveproblemsthatneedtobeaddressed.

Coarseness Factor Chart ToanalyzedatausingtheCFC,thecombined

aggregategradingismathematicallyseparatedintothreesizegroups:

Coarseparticlesarethoseretainedonthe9.5-mm(3/8-in.)sieve.Theyprovidethepri-marybodyofthemixture.Intermediateparticlespassthe9.5-mm(3/8-in.)sieveandareretainedonthe1.18-mm(#8)

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ZoneIindicatesthatamixtureisgap-gradedandhasahighpotentialforsegregationduringplacementand/orconsolidationduetoadefi-ciencyinintermediateparticles.Thesemixturesarelikelynotcohesive;therefore,segregationmayoccur.Mixturesplottinginthiszonemayresultinlocalcracking,blistering,spalling,andscaling.ZoneIIindicatesanoptimummixtureforconcreteswithnominalmaximumaggregatesizefrom50mm(2in.)through19mm(3/4in.).Mixturesinthiszonegenerallyproducecon-sistent,high-qualityconcrete.MixturesthatplotclosetothetrendbarornearthelimitsforZonesIandIVrequireclosecontrolandadjust-mentsinproportions,assmallvariationsinconsecutivebatchescanresultintheaggregateplottingoutsideofZoneII.ZoneIIIindicatesanoptimummixtureformaximumnominalaggregatesizessmallerthan19mm(3/4in.).ZoneIVindicatesexcessivefinesandahighpotentialforsegregationduringconsolidationandfinishing.Suchmixtureswilllikelyproduce

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_______________________________________________Figure 6-2. Modified coarseness factor chart (Shilstone 1990)

_______________________________________________Figure 6-3. 0.45 power chart for 25 mm (1 in.) nominal maxi-mum aggregate (Shilstone personal communication)

variablestrength,highshrinkage,cracking,curling,spalling,andscaling.ZoneVindicatesamixturethathasanexces-siveamountofcoarseandintermediateaggre-gateandisnotplastic.

0.45 Power Chart Thechartprovidesameanstodescribeanideal

combinedaggregategrading.Sievesizes,inmicronstothe0.45power,areplottedalongtheXaxis.

Thecumulativeamountofthetotalaggregatethatpasseseachsievecanthenbeplottedandcomparedtoalineonthe0.45powerchart.Awell-gradedcom-binedaggregateinaconcretemixturewillfollowatrendfromthenominalmaximumaggregatesizetothe2.36-mm(#8)sieveandthenbenddownward,asshowninfigure6-3.Deviationsfromthislineidentifydeviationsfromtheoptimum.

Examiningplottedpointsfromrighttoleft,thefirstpointthatfallswellbelowthelineindicatesthattoomuchmaterialwasretainedonthecorrespond-ingsieve.Correspondingly,apointthatfallswellabovethelineindicatesthatnotenoughmaterialwasretainedonthecorrespondingsieve.

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The0.45powerchartshouldbeusedonlyasaguideandshouldnotbeincorporatedintospecifica-tions.Experienceshowsthatgoodgradationsplotroughlyparalleltoandwithinafewpercentofthelineonthe0.45powerchart.Trialbatchingandthebehaviorofthemixturewillindicatewhethertheselectedcombinedaggregateissatisfactory.

Percent Aggregate Retained Chart Figure6-4graphicallyillustratestheaggregate

particledistributionasaplotofthepercentofaggre-gateretainedoneachsievesize(Shilstone1990).Anoptimumcombinedaggregateparticledistributionexhibitsnogapsintheintermediateparticles,asillus-tratedbythe(a)gradingonfigure6-4.

Often,thesandavailableisfinerandthecoarseaggregateiscoarserthandesired,therebycreatingawidergapinthegrading.Duetothenatureoflocallyavailablefineaggregate,combinedaggregategrad-ingsareoftenfoundtohaveadeficiencyinparticlesretainedonthe2.36-,1.18-,and600-µmsieves(#8,#16,and#30)andanabundanceofparticlesretained

onthe300-and150-µmsieves(#50and#100).Whenthereisadeficiencyintwoadjacentsievesizes,thesizesoneithersidetendtobalancethem.However,threeadjacentdeficientsizesindicateaproblemthatshouldbecorrected,asillustratedbythe(b)gradinginfigure6-4.

Specialattentionmustbegiventocaseswherethesumofthepercentageretainedontwoadjacentsievesislessthan13percentofthetotalaggregate.

________________________________________________Figure 6-4. Percent retained chart

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Calculating Mixture Proportions

Key Points

Desktopcalculationsofmixproportionsmustbefollowedbytrialbatches.

Severalmethodsofcalculatingproportionsareavailable.One—theAbsoluteVolumeMethod—isoutlinedhere.

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Thespecifiedstrengthofaconcretemixisselectedconsideringboththestructuralanddurabilityrequire-mentsoftheconcrete.Thestrength(flexuralorcompressive)requiredtoresisttheloadsappliedtothestructureispartofthethicknessdesign(seeCon-creteStrengthinchapter2,page16).Somedurabilityissuescanbeaddressedwithalimitonthew/cmratio(seePermeabilityinchapter5,page131).

Inordertobereasonablysureofmeetingthespecifiedstrength,theaveragedesignstrengthofaconcretemix,f´cr,mustbegreaterthanthespecifiedstrength,f´c,toaccountforvariationsinmaterialsandvariationsintheproduction,curing,andtestingofcylinders.SeeACI301,section4.2.3,forastatisti-calapproachfordeterminingtherequiredaveragestrength.

Thew/cmratioissimplythemassofwaterdividedbythemassofcementitiousmaterials(portlandcement;blendedcement;flyash;ground,granulatedblast-furnace[GGBF]slag;silicafume;andnaturalpozzolans).Thew/cmratiousedinthemixdesignshouldbethelowestvaluerequiredtomeetbothstrengthanddurabilityrequirements.

ACI318(andAASHTO)hasrequirementsforvariousexposureconditions(seechapter5,table5-4,page140,forrequirementsforconcreteexposed

Step 1: Concrete Strength

Step 2: Water-Cementitious Material Ratio

Calculatedmixproportionsprovideastartingpointfortrialbatches.Aconcretemixturecanbepropor-tionedbycalculation,fromfieldexperience(historicaldata),orfromtrialmixtureswithtrialanderror.Sev-eralcalculationmethodsareavailable,oneofwhichisdescribedindetailbelow.Therequirementsforagivenmixareoftenanactofbalancingdifferent(andpossiblycontradictory)requirementsforthefreshandhardenedproperties.

Concretemixtureproportionsareusuallyexpressedonthebasisofthemassofingredientsperunitvolume.Theunitofvolumeiseitheracubicmeteroracubicfootofconcrete.Theabsolutevolumemethodofmixproportioning(ACI211),explainedbelow,involvesusingrelativedensity(specificgravity)valuesforalltheingredientstocalculatetheabsolutevolumeeachwilloccupyinaunitvolumeofconcrete(seepage184).Iftheaggregategradinghasbeencalcu-latedusingtheShilstoneapproachdescribedabove,thensomeofthestepsinACI211areunnecessary.

FurtherinformationonthisandothermethodscanbefoundinthestandardpracticesdevelopedbyACI211(2002)andKosmatkaetal.(2002).Hover(1994and1995)providesagraphicalprocessfordesigningconcretemixtures,andThomasandWilson(2002)provideaself-containedtrainingprogramoncompactdiskforselectingmixcharacteristicsinaccordancewithACI211.1.

The Absolute Volume MethodTheabsolutevolumemethodofmixturepropor-

tioningmaybesummarizedin12steps:

W/C or W/CM Ratio?

The w/cm ratio is often used synonymously with water-to-cement (w/c) ratio; however, some specifications differentiate between the two ratios. Traditionally, w/c refers to the mass ratio of water to portland cement or to blended cement, while w/cm refers to the mass ratio of water to cement plus any supplementary cementitious materials in the concrete.



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________________________________________________Figure 6-5. Approximate relationship between compressive strength and w/cm ratio for concrete using 19-mm to 25-mm (3/4 in. to 1 in.) nominal maximum size coarse aggregate. Strength is based on cylinders moist-cured 28 days per ASTM C 31 / AASHTO T 23. (Adapted from ACI 211.1, ACI 211.3, and Hover 1995)

Step 3: Aggregates

Grading(particlesizeanddistribution),shape,porosity,andsurfacetextureoftheaggregatehaveanimportantinfluenceonproportioningconcretemixturesbecausethesecharacteristicsaffectthe

________________________________________________Figure 6-6. Bulk volume of coarse aggregate per unit vol-ume of concrete. Bulk volumes are based on aggregates in a dry-rodded condition (ASTM C 29 / AASHTO T 19). For less workable concrete (slipform paving), the bulk volume may be increased by about 10 percent. (Adapted from ACI 211.1 and Hover 1995)

tosulfates).Whendurabilitydoesnotcontrol,thew/cmratioshouldbeselectedonthebasisofconcretestrength(figure6-5).Insuchcases,thew/cmratioandmixtureproportionsfortherequiredstrengthshouldbebasedonadequatefielddataortrialmix-turesmadewithactualjobmaterialstodeterminetherelationshipbetweentheratioandstrength.Aw/cmratiobelowabout0.40isnotrecommendedforslip-formpaving,althoughsomeStateshavehadsuccesswithratiosaslowas0.37.Thetypeofsupplementarycementitiousmaterialsshouldbeknownandallowedforwhenselectingthew/cmratio.

workability—andthereforethewaterdemand—oftheconcrete(seeAggregateGradationinchapter3,page44).

Themostdesirablefine-aggregategradingwilldependonthetypeofwork,thepastecontentofthemixture,andthesizeofthecoarseaggregate.Forleanermixtures,afinegrading(lowerfinenessmodu-lus)isdesirableforworkability.Forrichermixtures,acoarsegrading(higherfinenessmodulus)isusedforgreatereconomy.

Thequantity(bulkvolume)ofcoarseaggregatecanbeestimatedusingfigure6-6.Thevaluesinthetablesarebasedonaggregatesinadry-roddedcondition(ASTMC29).Theyaresuitableforproducingcon-cretewithamoderateworkabilitysuitableforgeneralconcreteconstruction.Forlessworkableconcrete(slipformpaving),thebulkvolumemaybeincreasedbyabout10percent.

Followingisalistofkeyconsiderationswhenevaluatinglocalaggregates:



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Thelargestmaximumsizeconsistentwiththerequirementsforplacingtheconcretewillproducethemosteconomicalconcretewiththeleasttendencytocrackduetothermaleffectsorautogenous,plastic,ordryingshrinkage.Themaximumsizeshouldgenerallynotexceedone-fourththethicknessofthepavementor64mm(2.5in.),whicheverisless.InareaswhereD-crackinginpavementsisknowntobeaproblem,asmallermaximumsizemayhelpmitigatetheproblem.Testingatthereducedmaximumsizeisadvisable.AggregatesshouldcontainnomorethanthespecifiedpercentagesofdeleteriousmaterialslistedinASTMC33/AASHTOM6/M43orinthecontractspecifications.

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Step 4: Air Content

Concretethatwillbeexposedtocyclesoffreez-ingandthawingshouldbeadequatelyair-entrained.Theamountofairrequired(figure6-7)isafunctionoftheseverityofexposureandthemaximumsizeofaggregateusedintheconcrete.TestingforspacingfactorsbyASTMC457oruseoftheair-voidanalyzerwillgivebetterassuranceoffreeze-thawdurability.

________________________________________________Figure 6-7. Target total air content requirements for con-cretes using different sizes of aggregate. The air content in job specifications should be specified to be delivered within –1 to +2 percentage points of the target value for moderate and severe exposures. (Adapted from ACI 211.1 and Hover 1995)

Concretemustalwaysbemadewithaworkability,consistency,andplasticitysuitableforjobplacementconditions.Theslumptestisusedtomeasurecon-creteconsistency.

However,slumpisonlyindicativeofworkabilitywhenassessingsimilarmixesandshouldnotbeusedtocomparemixesofsignificantlydifferentproportions.Also,theslumptestisnotatrueindicatorofconcreteforslipformpaving.Typicalslumprequirementforside-formconcreteis25to75mm(1to3in.)andforslipformconcreteis12.5to50mm(0.5to2in.).Workabilitymustbeassuredforthegivenmixturecharacteristics,theprojectpavingequipment,andexpectedambientconditionsattimeofpaving.

Step 5: Workability/Slump

Theamountofwaterrequiredinaconcretemixdependsonseveralfactors:

Slumprequirementsofthejob.Aggregatesize,textureandshape.Aircontent.Amountofcementitiousmaterial.Temperatureoftheconcrete.

Watercontentcanbereducedbyincorporatingwater-reducingadmixtures(seeWaterReducersinchapter3,page58).

Forbatchadjustments,theslumpcanbeincreasedbyabout10mmbyadding2kgofwaterpercubicmeterofconcrete(byabout1in.byadding10lbof

1.2.3.4.5.

Step 6: Water Content



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waterpercubicyard).Thewatercontentrequiredforair-entrainedconcretecanbedeterminedfromfigure6-8.Forsomeconcretesandaggregates,thewaterestimatescanbereducedbyapproximately10kg(20lb)forsubangularaggregate,20kg(35lb)forgravelwithsomecrushedparticles,and25kg(45lb)foraroundedgraveltoproducetheslumpsshown.Thisillustratestheneedfortrialbatchtestingoflocalmaterials,aseachaggregatesourceisdifferentandcaninfluenceconcretepropertiesdifferently.

SeeACI211.1fortherequirementfornon-air-entrainedconcrete.Air-entrainedconcretehasalowerwaterdemandthannon-air-entrainedconcrete,andthisallowsthequantityofmixwaterrequiredforagivenlevelofdurabilitytobereduced.(Ruleofthumb:Decreasewaterby3kg/m3[5lb/yd3]foreach1percentair).

(Forrequirementsregardingwaterquality,includ-ingtheuseofrecycledwater,seeWaterinchapter3,page52.)

_______________________________________________Figure 6-8. Approximate water requirement for various slumps and crushed aggregate sizes for air-entrained concrete (Adapted from ACI 211.1 and Hover 1995)

Step 7: Cementitious Materials Content

Thecementcontentiscalculatedbydividingthewatercontentbythewater-cementitiousmaterials(w/cm)ratio.

However,itisnotunusualforaminimumcementcontenttobespecifiedforaparticularjobforthepur-poseofdurability,finishability,orimprovedabrasionresistance.Forseverefreeze-thaw,deicer,andsulfateexposures,itisdesirabletospecifythefollowing:

Aminimumcementitiousmaterialscontentof335kg/m3(564lb/yd3)ofconcrete.Onlyenoughmixingwatertoachievethede-siredconsistency.

SeeACI302forminimumcementitiousmaterialcontentsforconcreteflatwork.

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Step 8: Cementitious Materials Type

Note:Mixdesignersmaydothisstepfirst,becausethetypeofcementitiousmaterialsselectedcanaffecttheprevioussevensteps.

Thecementtypeisselectedtomeetanyspecialrequirements,likesulfateresistance,alkali-silicareac-tivity,orlow-heatrequirements.Flyashreduceswaterdemand,silicafume(andtoalessereffectmetakaolin)increasesit,andGGBFslaghasaminimaleffectonwaterdemand.

Changesinthevolumeofcementitiousmaterialcomponentsduetodifferentspecificgravities(port-landcement=3.15;flyash=1.9to2.8;silicafume=2.25;GGBFslag=2.85to2.95;metakaolin=2.5)shouldbeconsidered.Supplementarycementitiousmaterials(SCMs)alsochangetherelationshipbetweenthew/cmratioandstrength.TheACI318BuildingCodeplaceslimitsonthemaximumamountofSCMallowedinconcreteexposedtodeicingsalts(table6-3)(seeSulfateResistanceinchapter5,page139).

Cementitious Materials Content = Required Water Content w/cm Ratio



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Step 9: Admixtures

Thequantitiesofadmixturesarecalculatedtoprovidetherequiredairandwater-reducingeffect.Considerationmustalsobegiventoensuringthatthechloridelimitsforreinforcedconcretearenotexceededwhenusingcalciumchlorideasanaaccel-eratororotherchloride-containingadmixtures.

Incorporatingcertainchemicaladmixtureswillresultinchangestothewaterrequirementortheaircontentofconcrete(seeSet-ModifyingAdmixturesinchapter3,page59):

Waterreducerstypicallydecreasewaterre-quirementsby5to10percentandmayin-creaseaircontentbyupto1percent.Calciumchloride-basedadmixturesreducewaterrequirementsbyabout3percentandincreaseairbyupto0.5percent.Retardingadmixturesmayincreaseaircontent.

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Table 6-3. Cementitious Materials Requirements for Concrete Exposed to Deicing Chemicals

Step 10: Fine Aggregate

Fineaggregateamountisdeterminedafterthequantitiesofcoarseaggregate,air,water,andcementi-tiousmaterialsareknown.

Intheabsolutevolumemethod,thesequantitiesareconvertedtovolumetricproportionsusingtheappro-priatespecificgravity(relativedensity)ofthematerial.Thesevolumesarethensubtractedfromaunitvolume(1m3or1ft3/1yd3)togivetherequiredvolumeofsand.

Thevolumeofsandisthenconvertedtoamassproportionusingitsspecificgravity.Ifthecombinedaggregategradinghasbeenoptimized(seepage176),thenthesandquantitycalculatedheremustbecheckedagainsttheamountindicatedbytheoptimi-zationcalculation.

Correctionsareneededtocompensateformoistureinandontheaggregates.Inpractice,aggregateswillcontainsomemeasurableamountofmoisture.Thedry-batchweightsofaggregates,therefore,havetobeincreasedtocompensateforthemoisturethatisabsorbedinandcontainedonthesurfaceofeachpar-ticleandbetweenparticles.Themixingwateraddedtothebatchmustbereducedbytheamountoffreemoisturecontributedbytheaggregates.Themagni-tudeofthewatercorrectionshouldbeequaltothecorrectionmadetotheaggregate;theoverallmassofmaterialintheunitvolumemustremainunchanged.

Considerthefollowingexampleofaggregatemoisturecorrection:Aparticularmixdesigncallsfor1,800lb/yd3offineaggregate.Themeasuredabsorp-tionoftheaggregateis1.8percentbymassofsample,andthein-storageaggregatemoisturecontentis2.8percent.Therefore,theaggregatecontains1percentofwaterabovetheamounttheaggregateabsorbed.Todeterminetheamountofwaterthatmustbewithheldfromthebatch,multiply1percentby1,800lb:18lb.

Step 11: Moisture/Absorption Correction

Cementitious materials* Maximum percent of total cementitious materials by mass**

Fly ash and natural pozzolans 25

GGBF slag 50

Silica fume 10

Total of fly ash, GGBF slag, 50***silica fume and naturalpozzolans

Total of natural pozzolans and 35***silica fume

Source: Adapted from ACI 318* Includes portion of supplementary cementitious

materials in blended cements.** Total cementitious materials include the

summation of portland cements, blended cements, fly ash, slag, silica fume, and other pozzolans.

*** Silica fume should not constitute more than 10% of total cementitious materials and fly ash or other pozzolans shall not constitute more than 25% of cementitious materials.



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Step 12: Trial Batches

Atthisstage,theestimatedbatchweightsshouldbecheckedthroughtriallaboratorybatchesandfull-sizefieldbatches.Enoughconcretemustbemixed

forappropriateairandslumptestsandforcastingthecylindersrequiredforcompressive-strengthtests,plusbeamsforflexuraltestsifnecessary.Thebatchproportionsaremultipliedbythevolumeofthebatchrequiredtoproducetheactualquantitiesrequiredformixing(seeLaboratoryMixesinthischapter,page173).

Computing Absolute Volume

The absolute volume of a granular material (such as cement and aggregates) is the volume of the solid matter in the particles; it does not include the volume of air spaces between particles. The volume (yield) of freshly mixed concrete is equal to the sum of the absolute volumes of the concrete ingredients—cementitious materials, water (exclusive of that absorbed in the aggregate), aggregates, admixtures when applicable, and air. The absolute volume is computed from a material’s mass and relative density (specific gravity) as follows:

A value of 3.15 can be used for the relative density (specific gravity) of portland cement.

Blended cements have relative densities ranging from 2.90 to 3.15.

The relative density of fly ash varies from 1.9 to 2.8, GGBF slag from 2.85 to 2.95, and silica fume from 2.20 to 2.25.

The relative density of water is 1.0, and the density of water is 1,000 kg/m3 (62.4 lb/ft3) at 4°C (39°F) (accurate enough for mix calculations at room temperature).

The relative density of normal aggregate usually ranges between 2.4 and 2.9. The relative density of aggregate as used in mix design calculations is the relative density of either saturated surface-dry (SSD) material or oven-dry material.

Relative densities of admixtures, such as water reducers, can also be considered if needed.

Absolute volume is usually expressed in cubic meters (cubic feet).

The absolute volume of air in concrete, expressed as cubic meters per cubic meter (cubic feet per cubic feet), is equal to the total air content in percent divided by 100 (for example, 7% ÷ 100) and then multiplied by the volume of the concrete batch.

The volume of concrete in a batch can be determined by either of two methods:

If the relative densities of the aggregates and ce-mentitious materials are known, these can be used to calculate concrete volume.If the relative densities are unknown or if they vary, the volume can be computed by dividing the total mass of materials in the mixer by the density of concrete. In some cases, both determinations are made, one serv-ing as a check on the other.

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Adjusting Properties

Key Points

Achievingtherequiredpropertiesintheconcretemayrequiremakingadjustmentstothematerialsselected,tomaterialsproportions,oreventootherfactorssuchastemperature.

Adjustingproportionstoachieveoneparametermayhavenegativeeffectsonotherparameters.

Refertocorrespondingsectionsofchapter5fordescriptionsofvariousconcretepropertiesandrelatedtestsandspecifications.

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Chapter5describestheproperties(andrelatedtests)thatarerequiredtoachieveaconstructableconcretemixandadurableconcretepavement.Thissectionoutlinessomeadjustmentsthatcanbemadetoamixtoachievetheseproperties.Adjustmentsmayincludechangesinthematerialsselectedorintheirproportions.

Sometimes,therequiredpropertiesmayimposemutuallyexclusivedemandsonthemixdesign.Mixproportioningisaseriesofdecisionstofindthebestcompromiseamongcompetingneeds.Inthischap-terandthroughoutthebook,therefore,suggestionstoadjustmixproportionstoobtaincertainproper-tiesmustbetakeninageneralcontext;theyarenotappropriateforeverymixture.Increasingcementcon-tent,forexample,maybegenerallydesirableinordertominimizepastecontentandthusminimizeheatandshrinkage,butitmaynegativelyaffectotherspecificperformancerequirementsforaparticularmixture.

WorkabilityFreshconcretemixturesmustpossesstheworkabil-

ity—includingmobility,compactability,stability,andfreedomfromsegregation—requiredforthejobcon-ditions(Kosmatkaetal.2002;Mindessetal.2003).Thefollowingcanbeadjustedtoachievethedesiredworkability(Scanlon1994;Mindessetal.2003):

Watercontent:Increasingthewatercontentofconcretewillgenerallyincreasetheeasewithwhichtheconcreteflows.However,increasedwatercontentwillreducethestrengthandincreasethepermeabilityofthehardenedcon-creteandmayresultinincreasedsegregation.Shrinkagealsoincreaseswithincreasedwatercontent.Proportionofaggregateandcement:Ingeneral,anincreaseintheaggregate-to-cementratiowillreduceworkability.Astheaggregategrad-ingbecomesfiner(particlesizesarereduced),morecementisrequiredtomaintainthesameconsistency.Further,mixturescontaininglittlecementareoftenharsh,whereasmixturesrichincementaregenerallymoreworkablebutmaybecomestickyanddifficulttofinishiftoomuchcementisused.Mixturesdeficientinfineaggregatewillbeharsh,pronetosegregation,anddifficulttofinish,whereasmixturesmadewithanexcessoffineaggregatearetypicallymorepermeableandlesseconomical,althoughreadilyworkable.Theuseoffinersandwillalsoreduceworkabilityunlesswatercontentisincreased,whereasconcretemadewithcoarsesandisoftendifficulttofinish.Aggregateproperties:Ingeneral,themoresphericaltheaggregate,themoreworkabletheconcrete,duetoareductioninmechani-calinterlockthatoccurswithangularparticles.Ahighdegreeofflatand/orelongatedcoarseaggregateparticleswillreduceworkability,aswilltheuseofrough-texturedaggregateversussmoothaggregate.Cementcharacteristics:Althoughlessimportantthanaggregateproperties,finercement(e.g.,TypeIII)reducesworkabilityatagivenwater-to-cementratio.Admixtures:Theuseofair-entrainingadmix-tureswillincreaseworkabilitybycreatingsmall,sphericalbubblesthatactasballbear-ingsinthefreshconcrete.Asthenameimplies,water-reducingadmixtureswillalsoincreaseworkabilityifothermixturedesignparametersarenotchanged.Pozzolansandfinelydivided

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materials,includinginert,cementitiousma-terials,generallyimproveworkabilitywhenusedtoreplacepartofthesandinsteadofthecement.Timeandtemperature:Asambienttemperatureincreases,workabilitydecreases.Yetovershortperiodsoftime,temperatureappearstohavelittleeffect.Workabilitydecreaseswithtimeashydrationproceeds.Anyimprovementsinwork-abilityachievedthroughtheuseofwater-reduc-ingadmixturesareoftenrelativelyshort-lived.

Theslumptest(ASTMC143/AASHTOT119)ismostoftenusedtomeasuretheworkabilityoffreshconcrete.Althoughthistestdoesnotmeasureallfactorscontributingtoworkability,itisconvenientasacontroltest,providinganindicationofconsis-tencyfrombatchtobatch(Scanlon1994).Ingeneral,concreteusedforpavingisrelativelystiff,withslip-formpavingmixtureshavingslumpvaluesspecifiedbetween12.5and50mm(0.5and2in.).

Stiffening and SettingTheratesofstiffeningandsettingofaconcrete

mixturearecriticallyimportantforthecontractorbecausetheywilldirectlyinfluenceitsabilitytobeplaced,finished,andsawedwithoutsurfaceblemishesandcracking.

Bothstiffeningandsettingcanbeaffectedbythefollowingintheconcretemixture:

Cementitiousmaterials:Therateofstiffeningandsettingofaconcretemixturewillbeprimar-ilycontrolledbythecementcontent,chemistry,andfineness.Generally,settingisdelayedwithincreasingdosagesofground,granulatedblast-furnaceslagandflyash,althoughthepresenceoftricalciumaluminate(C

3A)insomeflyashes

mayresultinfalseset,particularlyatelevatedtemperatures.Itisimportanttomaketrialmixesusingthematerialsavailableattheplanttocheckforincompatibility(seePotentialMateri-alsIncompatibilitiesinchapter4,page97.)Chemicaladmixtures:Chemicalretardersandacceleratorsareavailabletoassistwithcontrol-

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lingsettime,buttheygenerallydonotinflu-encetherateofslumploss.Somechemicaladmixturesmayreactwithsomecementsorsupplementarycementitiousmaterialstocauseearlystiffening.Aggregatemoisture:Ifaggregatesaredrywhenthemixtureisbatched,itislikelythatwaterabsorbedintotheaggregatewillcauseexcessiveslumploss.Theamountofwateraddeddur-ingbatchingmustbeadjustedforthemoistureconditionsoftheaggregatesinordertomeetthewaterrequirementofthemixdesignandkeepaconstantwater-cementitiousmaterialsratio(seeStep11,page183).Ideally,aggregatesshouldbeatornearanSSDstate.Temperature:Thehigherthetemperature,theshorterthesettingtimeandthehighertheriskofincompatibilityissues.Usechilledwateroricetoreducethemixtemperatureto16to21°C(60to70°F)wherepossible.Water-cementitiousmaterials(w/cm)ratio:Alowerw/cmratioreducessettime.

BleedingBleeding,asdescribedinchapter5,isthedevel-

opmentofalayerofwateronthesurfaceoffreshlyplacedconcrete.Bleedingiscausedbythesettlementofcementandaggregateparticlesinthemixtureandthesimultaneousupwardmigrationofwater(Kosmatkaetal.2002).

Anumberoftechniquescanbeusedtopreventorminimizebleedinginthemixdesignstage,includingthefollowing(Kosmatka1994):

Reducingthewatercontent,water-cementitiousmaterialsratio,andslump.Increasingtheamountofcementorsupple-mentarycementitiousmaterialsinthemix(resultinginareducedw/cmratio).Increasingthefinenessofthecementitiousmaterials.Usingproperlygradedaggregate.Usingcertainchemicaladmixturessuchasair-entrainingagentsmaybeeffectiveinreducingbleeding.

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Air-Void SystemTheair-voidsystemofconcreteisfundamentally

importanttothedurabilityofconcreteinenviron-mentssubjecttofreezingandthawing.Itincludestotalaircontent,spacingfactors,andthespecificsurface.

Theair-voidsysteminaconcretemixcanbecon-trolledwiththefollowingadjustments(WhitingandNagi1998):

Cement:Increasingalkalicontentanddecreas-ingfinenessinacementislikelytoresultinincreasedentrainedair.Supplementarycementitiousmaterials:Increas-ingthecarbon(LOI)contentofflyashwillrapidlyreducetheamountofairentrainedforagivenair-entrainingadmixture(AEA)dosage.Smallvariationsinaflyashcompositionmayresultinlargeswingsintheaircontent,makingproductionofuniformconcretedifficult.TheuseofGGBFslagorsilicafumemayrequiretheuseofanadditionalair-entrainingadmixturetoachievethedesiredaircontent.Aggregates:Increasingtheamountofmaterialretainedonthe600-to300-µmsieves(#30to#50)willresultinincreasedairentrainment.Workability:Increasingworkabilitywillresultinincreasedaircontentforagivenconcretemix.

Trialbatchingpriortothestartofthejobwillindicatetheaircontentsexpectedwiththejob-specificmaterials,batchingsequence,andmixingtimeandspeed.However,theair-voidsysteminthefieldwillbeaffectedbythefollowingfactors:

Changesinthegradingoftheaggregates:Theaircontentrequirementdecreaseswithanincreaseinlarge-sizeaggregate,andaircontentincreaseswithanincreasedfineaggregatecon-tent,especiallywithanincreaseinthe600-to300-µm(#30to#50)sizes.Water:Aircontentincreaseswithextrawater.Admixturedosage:Aircontentincreaseswithanincreaseinwater-reducingandretardingadmixturesbasedonlignin.Delays.Someairlossisexpectedduringdeliv-eryandwithtime.

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Temperature.Anincreaseintemperaturewillrequireanincreaseintheamountofair-en-trainingadmixturetomaintainthetargetaircontent(WhitingandNagi1998).

Density (Unit Weight)Conventionalconcrete,normallyusedinpave-

ments,hasadensity(unitweight)intherangeof2,200to2,400kg/m3(137to150lb/ft3).Thedensityofconcretevaries,dependingontheamountandden-sityoftheaggregate,theamountofairentrappedorpurposelyentrained,andthewaterandcementcon-tents,whichinturnareinfluencedbythemaximumsizeoftheaggregate.Densityisausefulindicatorofbatchinguniformityandconsolidation.Densityisaffectedbythefollowingfactors:

Densityofthematerialinthemixture,withthemostinfluencefromthecoarseaggregate.Moisturecontentoftheaggregates.Aircontentofthemixture.Relativeproportionsofthematerials,particu-larlywater.

Thedensity(unitweight)andyieldoffreshlymixedconcretearedeterminedinaccordancewithASTMC138(AASHTOT121)(seechapter9,page258).Theresultsshouldbesufficientlyaccuratetodeterminethevolumetricquantity(yield)ofcon-creteproducedperbatch.Thetestcanalsoindicateaircontent,providedtherelativedensitiesoftheingredientsareknown.

StrengthThepavementdesignerestablishestheconcrete

strengthrequirementthatmeetstheintentofthedesign.Strengthandrateofstrengthgainareinflu-encedbythefollowingfactors:

Water-cementitiousmaterialsratio:Reducingthew/cmratiowillincreasestrength.Cementchemistry:Cementswithhighalkaliandtricalciumsilicate,oralite(C

3S),contents

andhighfinenesswilltendtogainstrengthmorequickly,althoughlong-termstrengthsmaybeslightlyreduced.SCMs:GGBFslagandTypeFflyashmayreduceearlystrengthgain,butwillnormally

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resultinhigherlong-termstrengths.Chemicaladmixtures:Waterreducersthateffectivelydecreasethew/cmratiowillresultinincreasedstrengths.Retardersmayreduceearlystrengthsbutincreaselong-termstrengths.Aggregates:Optimizingaggregategradingwillhelpreducethewaterrequirementofthesystemwithaconsequentstrengthincrease.Usingcrushedcoarseaggregatesandincreas-ingcoarseaggregatesizewillincreaseflexuralstrengths.Temperature:Increasingtemperaturewillincreaseearlystrengthsandsuppresslaterstrengths.

Duringconstruction,changesintheenvironmentalconditionsandvariationsinmaterials,consolida-tion,andcuringaffectthestrengthataspecifiedageandaffectstrengthdevelopmentwithage.Increasedtemperatureswillincreaseearlystrengthbutmaysup-presslong-termstrengthgain.

Qualitycontrolmeasureswillindicatewhetherthestrengthgainisasexpected.Generally,thestrengthgainofthespecimenssampledandcuredinastan-dardmanneraredetermined,whichindicatesthepotentialofthemixture.However,thestrengthgainofthein-placeconcreteisalsoveryimportant,andtestproceduresareavailabletoestimatethat.

Volume StabilityConcreteexperiencesvolumechanges(i.e.,

shrinkage/contractionorexpansion)asaresultoftemperatureandmoisturevariations.

Tominimizetheriskofcracking,itisimportancetominimizethetendencytochangeinvolumebyconsideringthefollowing:

Pastecontent:Themostimportantcontrollablefactoraffectingdryingshrinkageistheamountofwaterperunitvolumeofconcrete.Shrink-agecanbeminimizedbykeepingthewatercontentofconcreteaslowaspossible.Watercontentisnormallycontrolledbycontrollingthemaximumw/cmratioThisisachievedbykeepingthetotalcoarseaggregatecontentoftheconcreteashighaspossible(minimizingpastecontent).

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Aggregates:Avoidaggregatesthathavehighdryingshrinkagepropertiesandaggregatesthatcontainexcessiveamountsofclay.Quartz,granite,feldspar,limestone,anddolomiteaggregatesgenerallyproduceconcreteswithlowdryingshrinkages(ACICommittee224).Aggregateselectionwillalsohavethegreatestinfluenceonthermalexpansion.Curing:Thelongerandmoreeffectivethecur-ingpractices,thelowertheshrinkagewillbe.Goodcuringwillalsoallowtheconcretetogainmorestrengthandthusbebetterabletoresisttheshrinkagestressesandreducetheriskofcracking.

Permeability and Frost ResistancePermeabilityisadirectmeasureofthepotential

durabilityofaconcretemixture.Lowerpermeabilitycanbeachievedbymakingthe

followingadjustments:Reducingthewater-cementitiousmaterialsratio.Usingsupplementarycementitiousmaterialsatdosagesappropriatetotheexpectedlikelihoodoffreezingweather.Usinggoodcuringpractices.Usingmaterialsresistanttotheexpectedformofchemicalattack(e.g.,GGBFslagispreferredforchloridepenetration,whileClassFflyashcanimprovesulfateresistance).UsingaggregatesthathaveaprovenhistoryofresistancetoD-cracking.Reducingmaxi-mumcoarseaggregatesizewillreducetheriskofdamageifaggregatespronetodamageareunavoidable.Ensuringthatasatisfactoryair-voidsystemisprovidedintheconcrete.

Abrasion ResistanceAbrasionresistanceisrequiredtomaintainskid

resistanceinapavement.Thiscanbeimprovedwiththefollowingadjustments:

Choosinghard,dense,siliceousaggregates.Increasingcompressivestrength.Increasingthecuringtime.

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Sulfate ResistanceSulfateattackisaproblemwhenconcreteis

exposedtosubstratesthathavehighsulfatecontents.Resistancetosuchattackscanbeimprovedbymakingthefollowingadjustments:

Reducingthew/cmratiotoreducepermeability.Usingasulfate-resistingcement(ASTMC150TypeII,orformoreresistanceTypeV;ASTMC595TypesIP[MS],IS[MS],I[PM][MS],I[SM][MS],orP[MS];orASTM1157TypesMSorHS).UsingClassFflyash(15to25percent).Using20to50percentGGBFslagformoderatesulfateresistance.Usingahigherpercentofsupplementarycementitiousmaterialsthroughternarymixes.

Alkali-Silica ReactionDeleteriousexpansionofpavementsduetoalkali-

silicareactioncanbeaseriousproblem.Reductionof

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•

thisreactioncanbeachievedbymakingthefollowingadjustments:

Usingaggregatesthathaveahistoryofsatisfac-toryperformance.Usinglow-alkalicement.(Usinglow-alkalicementsmaynotmitigateallsituations.)Usingsupplementarycementitiousmaterials,whichhasbeenshowntoreducetheexpansionofagivensystem.Perhapsusinglithiumadmixtures.(Theeffec-tivenessoflithiumhasnotbeenproven,andnoacceptedtestingprotocolsexist.)Perhapsblendingreactiveaggregatewithnon-reactiveaggregate.

ThePCA’sGuide Specification for Concrete Subject to Alkali-Silica Reactions(PCA1998)providesguid-anceonevaluatingandselectingsystemsthatcanbeusedtopreventdeleteriousexpansionwhenpoten-tiallyreactiveaggregatesareused.

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AASHTOT19,TestMethodforBulkDensityandVoidsinAggregate

AASHTOT23,TestMethodforMakingandCuringConcreteTestSpecimensintheField

AASHTOT119,TestMethodforSlumpofHydraulicCementConcrete

AASHTOT121,TestMethodforDensity,Yield,andAirContentofConcrete

AASHTOM6,SpecificationforFineAggregateforPortlandCementConcrete

AASHTOM43,SpecificationforSizesofAggregateforRoadandBridgeConstruction


ASTMC29,StandardTestMethodforBulkDensityandVoidsinAggregate

ASTMC31,StandardPracticeforMakingandCuringConcreteTestSpecimensintheField

ASTMC33,StandardSpecificationforConcreteAggregates

ASTMC138,StandardTestMethodforDensity,Yield,andAirContentofConcrete

ASTMC143,TestMethodforSlumpofHydraulicCementConcrete

ASTMC150,StandardSpecificationforPortlandCement

ASTMC457,TestMethodforMicroscopicalDeter-minationofParametersoftheAir-VoidSysteminHardenedConcrete

ASTMC595,StandardSpecificationforBlendedHydraulicCements

ASTMC1157,StandardPerformanceSpecificationforHydraulicCement

AmericanConcreteInstitute(ACI).2005.Build-ingCodeRequirementsforStructuralConcrete.ACI318-05.FarmingtonHills,MI:AmericanConcreteInstitute.

———2002.BuildingCodeRequirementsforStructuralConcreteandCommentary.ACI318.Farm-ingtonHills,MI:AmericanConcreteInstitute.

———Committee211.1991.StandardPracticeforSelectingProportionsforNormal,Heavyweightand

MassConcrete.ACI211.1-91.FarmingtonHills,MI:AmericanConcreteInstitute.

———Committee224.2001.ControlofCrackinginConcreteStructures.ACI224R-01.FarmingtonHills,MI:AmericanConcreteInstitute.

———Committee301.1999.SpecificationsforStructuralConcrete.ACI301-99.ACICommittee301Report.FarmingtonHills,MI:AmericanConcreteInstitute.

———Committee302.1996.GuideforConcreteFloorandSlaboratoryConstruction.ACI302.1R-96.FarmingtonHills,MI:AmericanConcreteInstitute.

Hover,K.1994.AirContentandUnitWeightofConcrete.ASTMSTP169C,SignificanceofTestsandPropertiesofConcreteandConcrete-MakingMateri-als.Philadelphia,PA:AmericanSocietyforTestingandMaterials.296–314.

———.1995.GraphicalApproachtoMixtureProportioningbyACI211.1-91.ACIConcreteInter-national17.9:49–53.

Kosmatka,S.H.1994.Bleeding.SignificanceofTestsandPropertiesofConcreteandConcreteMakingMaterials.SpecialTechnicalPublication169C.Philadelphia,PA:AmericanSocietyforTestingandMaterials.

Kosmatka,S.H.,B.Kerkhoff,andW.C.Panarese.2002.DesignandControlofConcreteMixtures.EB001.Skokie,IL:PortlandCementAssociation.

Mindess,S.,J.F.Young,andD.Darwin.2003.Con-crete.2nd.Ed.UpperSaddleRiver,NJ:PrenticeHall.

PortlandCementAssociation(PCA)DurabilitySub-committee.1998.GuideSpecificationforConcreteSubjecttoAlkali-SilicaReactions.IS415.Skokie,IL:PortlandCementAssociation.

Scanlon,J.M.1994.FactorsInfluencingConcreteWorkability.SignificanceofTestsandPropertiesofConcreteandConcrete-MakingMaterials.STP169C.Philadelphia,PA:AmericanSocietyofTestingandMaterials:49–64.

Shilstone,Sr.,J.M.1990.ConcreteMixtureOptimiza-tion.ACIConcreteInternational12.6:3339.

Thomas,M.andM.Wilson.2002.IntroductiontoConcreteMixDesign.CD040.Skokie,IL:PortlandCementAssociation.CD-ROM.

Whiting,D.A.andM.A.Nagi.1998.ManualofCon-trolofAirContentinConcrete.EB116.Skokie,IL:PortlandCementAssociation.

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Chapter 7

Preparation for Concrete Placement

Subgrades 192

Bases 196

Preparation for Overlays 200

Otherthanearly-agecracking,themostcommoncauseofdistressinapavementisfailureofthesub-gradeorbasebelowthepavement.Itisimportantthataconcretepavementorslabbeprovidedsuitablesup-portthatisuniform,level,andabletocarrytheloadsimposed.

Beforetheconcretepavementisplaced,theexistingsubgrademustbeproperlypreparedandcompacted.Ontopofthis,abaseisusuallyconstructed.Itisalsoessentialtopayattentiontothelevelnessorgradeof

thebase.Concreteonabasewithirregulargradeswillhavevariablethickness,potentialthinspots,andatendencytoroughness.

Thischapterprovidesabriefoutlineofsubgradeandbasepreparationissuesthatarecriticaltothelifeoftheconcretepavement.Theinformationislimitedtothatwhichisrelevanttothepavementsystem;itisnotafulldiscussionofconstructionpractice.MoredetailedinformationcanbefoundthroughtheAmericanConcretePavementAssociation,www.pavement.com.

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Subgrades

Key Points

Thesubgradeisthenaturalground,gradedandcompacted,onwhichthepavementisbuilt.

Auniformandstablesubgradeisrequiredforlong-termdurabilityofthepavement.(Ingeneral,subgradeuniformityandstabilityaremoreimportantthansubgradestrengthforpavementperformance.)

Threemajorcausesofsubgradenonuniformity—expansivesoils,frostaction,andpumping—mustbecontrolled.

Pavementsubgradesmayneedtobeimprovedtemporarily(soilmodification)orpermanently(soilstabilization)throughtheuseofadditivesorbinders.

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Thesubgradeisthenaturalground,gradedandcompacted,onwhichthepavementisbuilt.Subgradeuniformityandstabilityaffectboththelong-termperformanceofthepavementandtheconstructionprocess.Requirementsforsubgradepreparationmayvaryconsiderably,dependingonsoiltype,environ-mentalconditions,andamountofheavytraffic.Inanycase,theobjectiveistoobtainuniformsupportforthepavementthatwillprevailthroughoutitsservicelife.

Preparationofthesubgradeincludesthefollowingactivities:

Compactingsoilsatmoisturecontentsanddensitiesthatwillensureuniformandstablepavementsupport.Wheneverpossible,settinggradelineshighenoughandmakingsideditchesdeepenoughtoincreasethedistancebetweenthewatertableandthepavement.Cross-haulingandmixingsoilstoachieveuni-formconditionsinareaswherethereareabrupthorizontalchangesinsoiltypes.

1.

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Usingselectivegradingincutandfillareastoplacethebettersoilsnearertothetopofthefinalsubgradeelevation.Improvingextremelypoorsoilsbytreatingthemwithlime,cement,cementkilndust,orflyash,orbyimportingbettersoils,whicheverismoreeconomical.

Uniform SupportDuetoitsrigidnature,aconcretepavementdis-

tributesthepressurefromappliedloadsoveralargerareaofthesupportingmaterial.Asaresult,deflec-tionsaresmallandpressuresonthesubgradearelow(ACPA1995).Concretepavements,therefore,donotrequireespeciallystrongfoundationsupport.

Moreimportantthanastrongfoundationisauni-formfoundation.Thesubgradeshouldhaveauniformcondition,withnoabruptchangesinthedegreeofsupport(figure7-1).Thatis,thereshouldbenohardorsoftspots.Nonuniformsupportincreaseslocal-izeddeflectionsandcausesstressconcentrationsinthepavement.Localizeddeflectionsandconcentratedstressescanleadtoprematurefailures,fatiguecrack-ing,faulting,pumping,rutting,andothertypesofpavementdistress.

Providingreasonablyuniformsupportconditionsbeneaththeconcreteslabrequirescontrollingthreemajorcausesofsubgradenonuniformity:

Expansivesoils.Frostaction.Pumping.

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1.2.3.

________________________________________________Figure 7-1. Effects of two examples of nonuniform support on concrete slabs on the ground (Farny 2001)

a) Soft spot

b) Hard spot

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Expansive SoilsExcessivedifferentialshrinkageandswellingof

expansivesoilscancausenonuniformsubgradesup-port.Asaresult,concretepavementsmaybecomedistortedenoughtoimpairridingquality.Severalconditionscanleadtoexpansivesoilsbecomingaproblemunderconcretepavements:

Theexpansivesoilswerecompactedtoodryorallowedtodryoutbeforepaving.Theexpansivesoilshavewidelyvaryingmois-turecontents,leadingtosubsequentshrinkageandswelling.Thereareabruptchangesinsoiltypesandassociatedvolume-changecapacitiesalongtheproject.

Thekeytominimizingtheeffectsofpotentiallyexpansivesoilsistoidentifyandtreatthemearlyintheprocess.Table7-1listssimpleindexteststhatcanbeperformedtodeterminethepotentialforexpansioninsoils.

ProceduressuchasASTMD1883,ASTMD3152,ASTMD4546,ASTMD4829,andCALTRANSTest354areespeciallysuitableforevaluatingthevolumechangeofsubgradesoils.Somefactorsdeter-minedbytheseteststhatarenotindicatedbythesimpleindextestsarethefollowing:

Theeffectofcompactionmoistureanddensityonsoilswellcharacteristics.Theeffectofsurchargeloads.Expansionforthetotalsamplegradingratherthanforonlyafinergradingfractionofthesoil.

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Table 7-1. Soil Index Properties and Their Relationship to Potential for Expansion (Bureau of Reclamation 1998)

Frost ActionFrostactionincludestheeffectsofbothfrostheave

andsubgradesoftening.However,onlyfrostheaveisaconsiderationforconcretepavements.(Fieldexperi-encehasshownthatsubgradesoftening,whichoccursinthespringinmanyareasoftheUnitedStates,isnotadesignfactorbecausestrongsubgradesupportisnotrequiredunderconcretepavements.Concretepavementreducespressureonthesubgradelayersbydistributingappliedtrafficloadsoverlargeareas.Concretepavementsdesignedfortypicalsubgradeconditionswillhaveamplereservecapacityforthetwotothreeweeksofthespringsofteningofthesubgrade.)

Frostheaveoccurswhenicelensesforminthesoil,whichcontinuetoattractwaterandexpandfur-ther.Theheavingitselfisnotaproblemforconcretepavements;rather,itisthesubsequentthawinganddifferentialsettlingoftheconcreteslabsthatcanleadtoroughnessand/orcracking.

Forfrostheavetooccur,allthreeofthefollowingconditionsmustbepresent:

Afrost-susceptiblesoil.Freezingtemperaturesthatpenetratethesubgrade.Asupplyofwater.

Controllinganyoneofthethreeconditionswilldramaticallyreducethepotentialforfrostheave.

Thedegreeoffrostsusceptibilityofaparticularsoilisrelatedtoitscapillarity,orsuction,anditsperme-ability(figure7-2).

1.2.

3.

Degree of Data from index tests* Estimation of probable expansion Plasticity index, percent Shrinkage limit, percent Colloid content, expansion,** percent (ASTM D 4318) (ASTM D 427) percent minus 0.001mm total volume change (dry (ASTM D 422) to saturated condition)

Very high >35 <11 >28 >30 High 25 to 41 7 to 12 20 to 31 20 to 30 Medium 15 to 28 10 to 16 13 to 23 10 to 20 Low <18 >15 <15 <10

* All three index tests should be considered in estimating expansive properties.** Based on a vertical loading of 7 kPa (1.0 lb/in2). For higher loadings the amount of expansion is reduced, depending

on the load and the clay characteristics.

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_______________________________________________Figure 7-2. Relationship between frost action and hydraulic properties of soils (ACPA 1995)

Low-plasticity,fine-grainedsoilswithahighper-centageofsiltparticles(0.005to0.05mm[0.0002to0.002in.])areparticularlysensitivetofrostheave.Thesesoilshaveporesizessmallenoughtodevelopcapillarypotentialandlargeenoughtopermitwatertotraveltothefrozenzone.Coarsersoilsaccommodatehigherratesofflow,butlackthecapillarypotentialtoliftmoisturefromthewatertable.Morecohesivesoils,althoughtheyhavehighcapillarity,havelowperme-ability,whichpreventswaterfrommovingquicklyenoughtoformicelensesinthesoil.

PumpingPumpingistheforcefuldisplacementofamix-

tureofsoilandwater(i.e.,mud)fromunderneathaconcretepavementduringheavyappliedloads.Con-tinued,uncontrolledpumpingeventuallyleadstothedisplacementofenoughsoilsothatuniformityofthesubgradeisdestroyed,whichcanresultincracking,faulting,andsettlingoftheconcretepavement.

Threefactorsarenecessaryforpumpingtooccur:Pump-susceptiblematerialbeneaththeslab.Freewaterbetweenthepavementandsubgradeorbase.Rapidandlargedeflectionsofthepavementslabs.

Controllinganyoneofthesethreefactorsbyprovidinggoodloadtransferbetweenpanelsandwell-drainedsupportusingappropriatematerialswilldramaticallyreducethepotentialforpumping.

1.2.

3.

Grading and CompactionGradepreparationlaysthefoundationfortheentire

pavementstructure.Theuniformityandstabilityofthesubgradeaffectboththelong-termperformanceofthepavementandtherestoftheconstructionpro-cess.Theimportantelementsofsubgradepreparationincludeevaluatingsubgradestabilityanduniformity,modifyingthesubgradetoimprovestability,andevaluatingsurfacetolerances.

Pre-GradingThefirststepinpreparingthesubgradeistodeter-

mineareasforcross-haulingandmixingofsoiltypestoestablishuniformity.Siltpocketscanberemoved,andexpansivesoilscanbeplacedinlowerpartsofembankmentstoreduceswellpotential.Massgradingisaccomplishedinthisphase,whichinvolvescuttinghighpointsandfillinglowspotstoroughlyachievethedesiredalignment.

Moisture-Density ControlSubgradevolumechangesarereducedbyadequate

moistureanddensitycontrolduringcompaction.Expansiveandfrost-susceptiblesoilsshouldbecompactedatonetothreepercentaboveoptimummoistureusingASTMD698/AASHTOT99.

Improving Soil CharacteristicsPavementsubgradesmayneedtobeimprovedfora

numberofreasons:Toimprovelowstrengthsoil.Toreduceswellingpotential.Toimproveconstructionconditions.

Subgradesmaybeimprovedthroughtheuseofadditivesorbinders.Commonlyusedmaterialsforsoilimprovementincludecement,cementkilndust,lime,limekilndust,andflyash.Whichmaterialisusedandatwhatadditionratedependsonthetypeofsoil,amountofstabilitydesired,andthelengthoftimenecessary.Ifthesubgradesupportisneededonlyduringtheconstructionphasetosupportconstructionvehicles,theprocessistermed“soilmodification”or“cement-modifiedsoil.”Ifthesupportisintendedtobepermanentandfunctionasasupportivelayertothepavement,theprocessistermed“soilstabilization”or“soil-cementbase.”Certaintechniques,additives,

1.2.3.

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oradditionrateslendthemselvestotemporarysoilmodification,whileothersareoftenusedforthemorepermanentsoilstabilization.

Formoreinformationonsoilcharacteristics,refertowww.cement.org/pavements/pv_sc.asp.

TrimmingOncethesubgradehasbeencompactedtothe

desireddensityatthepropermoistureleveland/ortreatedtoreduceexpansionpotential,itistrimmedtothepropergrades,elevations,andcross-slopes.Typically,astringlineissetuptoguideanautomatictrimmeratthecorrectgrade.Forfixed-formconstruc-tion,anautomatictrimmercanrideontheformsaftertheyarefastenedintoplace(figure7-3).Forsmallerprojectsorpavementswithoutasmoothnessspecifica-tion,thegradeisnottypicallytrimmedaccordingtoastringline,butisgradedwithamotorgraderorsmallloaderandthenspot-checkedfromsurveyhubssetapproximately15m(50ft)apart(ACPA2003).

Proof-RollingProof-rollingcanlocateisolatedsoftareasthat

arenotdetectedinthegradeinspectionprocess.It

involvesdrivingaheavy,pneumatic-tiredvehicleoverthepreparedgradewhileobservingforruttingordeformation.Afully-loadedtandemaxletruckorrubber-tiredloadermaybeusedastheproof-roller.Steeldrumrollersarenotrecommendedbecausetheymaypotentiallydistributetheloadacrosssoftareaswithoutanyobservedmovement.Proof-rollingisrecommendedifanunstabilizedbaseistobeusedbetweentheconcretepavementandsubgrade.

Figure 7-3. Autograder that references the string line and trims the subgrade material (ACPA)

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Bases

Key Points

Abaseisdefinedasthelayerofmaterialthatliesimmediatelybelowtheconcretepavement.

Basesmaybeconstructedofgranularmaterials,cement-treatedmaterials,leanconcrete,oropen-graded,highlypermeablematerials,whichmaybestabilizedorunstabilized.

Aswithsubgrades,themostimportantcharacteristicsofabaseareuniformityandadequatesupport.

Balancemustbeachievedbetweenthedegreeofdrainageandthestabilityofthebaselayer.Stabilityshouldnotbesacrificedforthesakeofdrainage.

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Undercertainconditions,suchasexpectedheavytrafficorpoor-qualitysubgrades,abaselayerisneededontopofthepreparedsubgradeandimmediatelybelowtheconcretepavement.Thematerialqualityrequirementsforabaseunderconcretepavementarenotasstrictasthoseforabaseunderasphaltpavementbecausethepressuresimposedonabaseundercon-cretearemuchlowerthanthoseunderasphalt.

Forlighttrafficpavements,suchasresidentialstreets,secondaryroads,parkinglots,andlight-dutyairports,theuseofabaselayerisnotrequired,andthedesiredresultscanbeobtainedwithpropersub-gradepreparationtechniques.

Types of BasesBasesmaybeconstructedofgranularmaterials,

cement-treatedmaterials,leanconcrete,hot-mixedasphalt,oropen-graded,highly-permeablematerials,whichmaybestabilizedorunstabilized.

Whentheuseofabaseisconsideredappropri-ate,thebestresultsareobtainedbyfollowingtheseguidelines:

Selectingbasematerialsthatmeetminimumrequirementsforpreventingpumpingofsub-gradesoils.Specifyinggradingcontrolsthatwillensureareasonablyconstantbasegradingforindividualprojects.Specifyingaminimumbasedepthof100mm(4in.).Specifyingaminimumdensityforuntreatedbasesof105percentofASTMD698/AASHTOT99forheavilytraveledprojects.Specifyingacement-treatedorleanconcretebasethatprovidesastronganduniformsup-portforthepavementandjoints,providesanall-weatherworkingplatform,andcontributestosmootherpavementsbygivingfirmsupporttotheformsorpaverduringconstruction.Specifyingapermeablebaseforpavementscar-ryinghighvolumesofheavytrucksforwhichpastexperienceindicatesthepotentialforpave-mentfaultingandpumping.

Unstabilized (Granular) BasesAwidevarietyofmaterialsandgradingshasbeen

usedsuccessfullybydifferentagenciesforuntreatedbases.Thesematerialsincludecrushedstone,crushedconcrete,bank-runsand-gravels,sands,soil-stabilizedgravels,andlocalmaterialssuchascrushedwinewaste,sand-shellmixtures,andslag.

Theprincipalcriterionistolimittheamountoffinespassinga75-µm(#200)sieve.Softaggregatesshouldbeavoidedbecausefinesmaybecreatedduetotheabrasionorcrushingactionofcompac-tionequipmentandconstructiontraffic.Generally,aggregateshavinglessthan50percentlossintheLosAngelesabrasiontest(ASTMC131/AASHTOT96)aresatisfactory(seeAbrasionResistanceinchapter5,page146).

Stabilized (Treated) BasesStabilizedortreatedbasescanbeaccomplished

usinghydrauliccementorasphalt.Typesofstabilizedbasesincludecement-treated,asphalt-treated,econo-crete,leanconcrete,asphalt-treatedopen-graded,andcement-treatedopen-graded.Suchstabilizedbasesprovidethefollowingbenefits:

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Astableworkingplatformtoexpediteallconstructionoperationsandpermitlargedailyproductionofconcretepavementwithmini-mumdowntimeforinclementweather.Firmsupportforslipformpaverorsideforms.Constructionofsmoothpavementsduetostabletracklineforslipformpavers.Preventionofbaseconsolidationundertraffic.Reductioninpavementdeflectionsfromvehicleloadings.Improvedloadtransferatpavementjoints.Minimizedintrusionofhardgranularparticlesintothebottomofpavementjoints.Amoreerosion-resistantbasesurface.

Grading ControlThebaseforanindividualprojectshouldhave

areasonablyconstantgradingtoallowcompactionequipmenttoproduceuniformandstablesupport,whichisessentialforgoodpavementperformance.Abruptchangesinbasegradingcanbenearlyasharmfulasabruptchangesinsubgradesoils.

Compaction RequirementsTopreventtheconsolidationofgranularmaterials

fromtheactionofheavytrafficoncethepavementsareinservice,basesmustbecompactedtoveryhighdensities.Unstabilizedbasesunderconcretepave-mentsshouldhaveaminimumof100percentofASTMD698/AASHTOT99density.Forprojectsthatwillcarrylargevolumesofheavytraffic,thespecifieddensityshouldnotbelessthan105per-centofstandarddensityor98to100percentofASTMD1557/AASHTOT180density.

MaterialsGranularmaterialsinAASHTOSoilClassification

GroupsA-1m,A-2-4,A-2-5,andA-3areusedforcement-treatedbases.Theycontainnotmorethan35percentpassingthe75-µm(#200)sieve,haveaPIof10orless,andmaybeeitherpit-runormanufactured.Cement-treatedbaseshavebeenbuiltwithA-4andA-5soilsinsomenon-frostareasandareperformingsatisfactorily.Generally,however,suchsoils

1.

2.3.

4.5.

6.7.

8.

arenotrecommendedforbasesinfrostareasorwherelargevolumesofheavytrucktrafficareexpected.UseofA-6andA-7soilsisnotrecommended.Topermitaccurategradingofthebase,themaximumsizeofmaterialisusuallylimitedto25mm(1in.)andpreferablyto19mm(3/4in.).

Econocreteorleanconcretebasesaretypicallydesignedforaspecificapplicationandenvironment.Ingeneral,theyuseaggregatesthatdonotnecessarilymeetqualitystandardsforconventionalconcrete.Asingleaggregate,ratherthancoarseandfineaggregatesstockpiledseparately,isoftenusedinthemixture.Cementcontentislessthanthatfornormalconcreteandisselectedbasedonthetargetstrength.

Acommonsourceofaggregateforanybasetypeiscrushed,recycledconcrete.Existingconcretepave-mentscanbetakenup,crushed,andreusedinbasecourses,eitherasunstabilizedaggregatebasesorstabi-lizedbases.Thefracturedconcreteandfinematerialcontaincementthatwillbegintohydrateandtherebyprovidesomeofthebenefitofastabilizedbase.Thereisevidence,however,thatinsomeStatesleachingfromrecycledaggregatesusedasabasecancauseproblems(Mulligan2002).

ConstructionCement-treatedbasescanbeplacedusingeither

road-mixedorpre-mixedmethods.Inroadmixing,thematerialisprocessedonthegrade.Theproperamountofcementisplacedwithacementspreader,andthemixingisusuallyaccomplishedwithapulver-izerorreclaimer.Itisthencompactedwithrollers.Forpre-mixedbases,thematerialcanbemixedineitherapugmilloracentral-mixedconcreteplant.Thematerialisbatchedintodumptrucksandtypi-callyplacedusinganasphaltpavingmachine.Insomecases,themachinewillachievetheproperdensity,butadditionalcompactionmayberequired.

Econocreteorleanconcretebasesareconstructedinessentiallythesamemannerandwiththesameequipmentasnormalconcretepavements.Theyaremixedusingacentral-mixedconcreteplantandplacedusingaslipformpaver.Theonlydifferencesarethejointingpracticeandthetreatmentofthesurfaceoftheleanconcretebase.Constructionofjointsinthe

Bases


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leanconcretebaseisnotconsiderednecessaryaslongasadebondingtreatmentisappliedtothesurfaceofthebase.AfewStatesandsomeEuropeancountries,however,notchthebaseattheplannedlocationsofthejointsintheconcretepavementabove.

Becausethereisahighpotentialforbondingoftheconcretepavementtoastabilizedbase,itisimportantthatabond-breakingmediumbeappliedtothebasesurface.CurrentpracticeintheUnitedStatesincludesapplyingtwoheavycoatsofwax-basedcuringcom-poundonthebasesurface(Okamoto1994).Table7-2providessomealternativematerialsthatmaybeusedtoreducefrictionandpreventbondingofpavementconcretetobaselayers.

Table 7-2. Alternatives for Reducing Friction or Bond between Concrete Pavement and Stabilized Base Materials (ACPA 2002)

Bondingoftheconcretepavementtoahot-mixedasphaltbaseislessdetrimental,becausetheasphalthassufficientflexibilitytopreventreflectivecrackingfromoccurring.

Drainage in the Base LayerPavementdrainageisanimportantfactorinpave-

mentperformance.Inthepast,manypoorlydesignedpavementswithlittledrainagefailedbecauseofwatertrappedwithinthepavementstructure,lead-ingtosubgradepumping,reducedsubgradeandbasestrengths,andresultingpavementdistresses.

Amethodofprovidingdrainageinapavementsec-tionistospecifyadrainableorpermeablebaselayer,utilizinganopen-gradedaggregategrading.Thislayercanbedaylightedtothesideditches,oritcandirectwatertoflowintoedgedrains.

Thenumberofvoidsinthebasehasadirectrelationshiptothematerial’sstability.Dense-gradedgranularmaterialsandmaterialsstabilizedwithcementorasphaltprovidefirmsupportforcon-structionequipmentandtheconcretepavement.Unstabilizedpermeablebasesorstabilizedbuthighlypermeablebases,whichbecamepopularinthe1990s,havecausedsomeconstructionaswellasperformanceproblems,oftenrelatedtoinitialqualityormainte-nanceissues.

Animportantbalancemustbeachievedbetweenthedegreeofdrainageandthestabilityofthebaselayer.Basestabilityshouldnotbesacrificedforthesakeofdrainage.Atargetpermeabilityof60to90m/day(200to300ft/day)producesastabledrain-inglayerthatwillsupportthepavingequipmentandconstructionvehicles.Layerswithhigherpermeabilitydonothavethein-placestabilitynecessaryforcon-structionandpavementperformance.

TracklineOneofthemostsignificantdesignconsiderations

forobtainingaconsistentlysmoothconcretepave-mentisprovisionofastable,smoothtracklineorpad-line(ACPA2003).

Tracklinesarethepathsalongwhichaslipformpavingmachine’strackswillfollow.Theyareusually1m(3ft)outsideeitheredgeoftheconcreteslab

Material Comments

Curing compound Two coats of white-pigmented, wax- based compound (ASTM C 309 /

AASHTO M 148) works well.

Sand Broadcasting or dusting about 5.5 kg/m2 (12 lb/yd2) works well.

Bladed fines Recycled job site material works well as thin layer.

Asphalt emulsion Works well on smoother base surfaces. Must be an even coating.

Polyethylene Works well but difficult to usesheets* when windy. Could pose traffic hazard in

urban areas. May cause problems with moisture in slab or base.

Tar paper Works as debonding medium directly over shrinkage cracks in the base. Not recommended for application on the entire base area.

Choker stone For stabilized open-graded materials only. Chip-size material to fill near-surface voids and minimize penetration of concrete into base.

* Polyethylene (i.e., plastic) sheets are recommended for small areas only. Large sheets can be difficult to work with and have been known to contribute to (1) tearing (a form of cracking) along the edges of pavement during slipform construction, (2) slowing production to refasten dislodged sheets, and

(3) vehicle accidents if sheets are blown onto adjacent lanes with active traffic.

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(figure7-4).Agenciesthatspecifyandpayforthisextrathreefeetofbasewidthoneithersideoftheslabgetthebenefitofsmootherpavements,aswellastheadditionalsupportprovidedtotheslabedges,shoul-ders,andcurb-and-guttersections.

Trimming to GradeLikesubgrades,unstabilizedbasescanbetrimmed

tograde.Econocrete(leanconcrete)andasphaltbasesaretypicallynottrimmedafterbeingplaced,butareinsteadconstructedtotheplannedgradereferencedfromastringlineorthetrimmedsubgradesurface.Caremustbeexercised,however,whentrimmingcement-treatedbases.Trimmingthesetypesofbasespriortopavingcandisturbthebasesurface.

Aftertrimming,thebasemayberoughincertainlocations,increasingthesurfaceforbonding.Oneofthefollowingmethodswillminimizebondingintrimmedareas: Figure 7-4. Trackline of slipform paving machine (ACPA)

Reapplythecutbackasphaltcuringagentandspreadathinlayerofsandbeforepaving.Applytwocoatsofwax-basedcuringcom-poundbeforepaving.

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Preparation for Overlays

Key Points

Oneofthemostimportantaspectsofoverlaysistheinteractionwithexistingpavement.

Concreteoverlaysareusedtoextendtheservicelifeofanexistingpavementandshouldnotrequireextensiverepairstotheexistingpavement.

Thesurfaceoftheexistinglayermustbeappropriatelypreparedfortheconcreteoverlay,particularlyforbondedoverlays.

Whenthesurfacetemperatureoftheexistingasphaltpavementorasphaltseparatorlayerisatorabove48.9ºC(120ºF),surfacewateringcanbeusedtoreducethetemperatureandminimizethepotentialforshrinkagecracking.

Propercuringofbondedconcreteoverlaysisparticularlyimportantbecausetheyarethinwithlargesurfaceareascomparedwiththevolumeofconcrete.

Thetimingofjointsawingisimportant.Sawingjointstooearlycancauseexcessraveling.HIPERPAVmayhelppredicttheappropriatejointsawingwindow.

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Overlayscanbegroupedintotwomainfamilies:bondedconcreteoverlaysandunbondedconcreteoverlays(seeConcreteOverlaysinchapter2,page23).Bothfamiliescanbeplacedonexistingconcrete,asphalt,orcomposite(asphaltonconcrete)pavement.Normalconcretepavingpreparationandconstructionpracticescanbeusedtocompleteconcreteoverlayprojectsasquicklyandefficientlyasanyotherprojects.

Preparing for Bonded Concrete Overlays

Bondedconcreteoverlaysaredesignedandcon-structedsothatabondformsbetweentheexisting

pavementandthenewconcreteoverlay.Thisbondcreatesineffectonemonolithicstructurethatsupportstrafficloadings.Bondedconcreteoverlays,therefore,requirethattheexistingpavementsurfacebepreparedtomaximizebondingwiththenewlayer.

Bonded Concrete Overlays on Concrete Pavements

Pre-overlay Repairs.Thesurfaceshouldbeinspectedforisolatedpocketsofdeteriorationthatrequirerepairs.Forisolatedareasthathavewiderandomcracks,full-depthrepairsmaybenecessary.Whencracks(particularlyworkingcracks)existinthepave-menttoberesurfaced,reflectivecrackingwillalmostalwaysoccur.Crackcagesoverexistingcrackshavebeensuccessfullyusedtopreventreflectivecracking.Whenvoidsaredetectedunderexistingslabs,theslabsshouldbestabilizedthroughgroutinjectionorothermethods.Asphaltpatchesshouldberemovedandreplacedwithconcretepatches(orsimplyfilledwithconcreteatthetimeofoverlayplacement)toensurebondingoftheconcretelayers.Aconsider-ationinperformingrepairsiswhethermovementintheunderlyingpavementwillcausemovementintheoverlay.Anymovementintheoverlaythatdoesnotoccuratmatchedjointscouldcontributetodebond-ingandsubsequentdeteriorationoftheoverlay.

Surface Preparation.Surfacepreparationoftheexist-ingconcretepavementisaccomplishedtoproducearoughenedsurfacethatwillpromotebondingbetweenthetwolayers.Avarietyofsurfacepreparationproce-duresmaybeused,includingshotblasting,milling,andsandblasting.Abondinggroutorepoxyisnotrequired.Themostcommonlyusedandmosteffectivesurfacepreparationprocedureisshotblasting.Milling(usedtolowerthepavementelevation)hasthepoten-tialdrawbackofcausingsurfacemicro-crackingandfracturingtheexposedaggregate.Ifmillingisused,thesurfacemayrequireshotblastingorhighwaterpressureblastingtoremovemicro-cracks.

Surface Cleaning.Followingsurfacepreparation,theconcretesurfaceshouldbecleanedtoensureadequatebondingbetweentheexistingconcretesurfaceandthenewconcreteoverlay.Cleaningmaybeaccom-plishedbysweepingtheconcretesurface,followedbycleaninginfrontofthepaverwithcompressedair.

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Airblastingandwaterblastingshouldbeusedonlyassupplementarycleaningprocedurestoremoveloosematerialfromthesurfaceaftershotblastingorsand-blasting.Pavingshouldcommencesoonaftercleaningtominimizethechanceofcontamination.

Bonded Concrete Overlays on Asphalt Pavements

Pre-overlay Repairs.Mostsurfacedistressescanberemovedthroughmilling.Areaswithpotholes;local-ized,moderate-to-severealligatorcracking;orlossofbase/subgradesupportmayrequirepartialorfull-depthspotrepairstoachievethedesiredload-carryingcapacityandlong-termdurability.Allpatchingshouldbecompletepriortomilling.

Milling.Millingmaybeusedwheresurfacedistor-tionsare5.1cm(2in.)orgreater.Millingshouldbeminimizedbecauseitresultsinlossofstructuralsup-port.Themainobjectivesofmillingpriortoplacingabondedoverlayare(1)toremovesignificantsurfacedistortionsthatcontainsoftasphaltmaterial,whichwouldresultinaninadequatebondingsurface;(2)toroughenaportionofthesurfacetoenhancebonddevelopmentbetweenthenewconcreteoverlayandtheexistingasphalt;(3)toreducehighspotstohelpensureminimumoverlaydepthandreducethequan-tityofconcreteneededtofilllowspots;and(4)tomatchcurboradjacentstructureelevations.

Theobjectiveofremovingmaterialisnottoobtainaperfectcrosssection.Itisnotnecessarytocompletelyremoveruts.Aminimumof7.6–10.2cm(3–4in.)ofasphaltshouldbeleftaftermillingbecauseofthereli-anceontheasphaltpavementtocarryaportionoftheload.Millingshouldleaveatleasthalfoftheasphaltliftforoptimumbondingsincemillingdowntoapointjustabovethetacklinecanleadtoseparation.

Surface Cleaning.Followingrepairs,theasphaltsurfaceneedstobecleanedtoensureadequatebond-ingbetweentheexistingasphaltsurfaceandthenewconcreteoverlay.Cleaningmaybeaccomplishedbyfirstsweepingtheasphaltsurface,thencleaningwithcompressedair.Pressurewashingshouldonlybecon-sideredwhendustcontrolismandatedorwhenmudhasbeentrackedontothemilledsurface.Innocaseshouldwaterormoisturebeallowedtostandontheasphaltpavementpriortobondedoverlayplacement.

Preparing for Unbonded Concrete Overlays

Unbondedconcreteoverlaystypicallydonotrequireextensivesurfacepreparationoftheexistingpavement.

Unbonded Concrete Overlays on Concrete Pavements

Pre-overlay Repairs.Typically,onlythedistressesthatcauseamajorlossofstructuralintegrityrequirerepair.Ifsignificantlydistressedareasarenotshiftingormovingandthesubbaseisstable,costlyrepairstypicallyarenotneeded,particularlywithanadequatelydesignedoverlay.Asanalternativetonumerousrepairs,someStateDOTsincreasetheunbondedresurfacingthicknesstoprovideadditionalstrength.

Separator Layer.Useofasufficientseparatorlayercanhelpensuregoodperformanceoftheunbondedoverlay.Beforeplacementoftheseparatorlayer,theexistingpavementsurfaceshouldbesweptcleanofanyloosematerialeitherwithamechanicalsweeperoranairblower.Whenusingasphaltasaseparatorlayer,followconventionalasphaltplacementpracticesandprocedures.

Unbonded Concrete Overlays on Asphalt Pavements

Pre-overlay Repairs.Unbondedoverlaysgenerallyrequireonlyminimalpre-overlayrepairsoftheexist-ingasphalt.Ifsignificantlydistressedareasarenotshiftingormovingandthesubbaseisstable,costlyrepairstypicallyarenotneeded,particularlywithanadequatelydesignedoverlay.

Direct Placement.Directplacementwithoutmillingisrecommendedwhenruttingintheexistingasphaltpavementdoesnotexceed5.1cm(2in.).Anyrutsintheexistingpavementarefilledwithconcrete,result-inginathickeroverlayabovetheruts.Spotmillingofonlythepartsoftheprojectwithsignificantdistortionisoftenadequate.Over-millingisnotrecommendedsinceitreducesthestructuralvalueofthepavement.

Surface Cleaning.Beforeconcreteplacement,theasphaltsurfaceshouldsimplybeswept.Remainingsmallparticlesarenotconsideredaproblem.

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———.2002.Early Cracking of Concrete Pave-ment–Causes and Repairs.TB016.01P.Skokie,IL:AmericanConcretePavementAssociation.

———.1990a.Guidelines for Bonded Concrete Overlays.TB007P.Skokie,IL:AmericanConcretePavementAssociation.

———.1990b.Guidelines for Unbonded Concrete Overlays.TB005P.Skokie,IL:AmericanConcretePavementAssociation.

———.1995.Subgrades and Bases for Concrete Pave-ments.TB011.02P.Skokie,IL:AmericanConcretePavementAssociation.

BureauofReclamation.1998.Earth Manual.Denver,CO:USDepartmentoftheInterior,USGovernmentPrintingOffice.

Cable,J.K.,H.Ceylan,F.S.Fanous,T.Cackler,D.Wood,D.Frentress,T.Tabbert,S-YOh,K.Gopalakrishnan.2005.Design and Construc-tion Procedures for Concrete Overlay and Widening of Existing Pavements.IowaHighwayResearchBoardProjectTR-511,6.Ames,IA.CenterforTransporta-tionResearchandEducation,IowaStateUniversity.

CaliforniaDepartmentofTransportation.CALTRANSTest354:EvaluatingtheExpansivePotentialofSoilsBelowRigidPavements(Third-CycleExpan-sionPressureTest).HistoricalMethod.Sacramento,CA:CaliforniaDepartmentofTransportation.

Farny,J.A.2001.Concrete Floors on Ground.EB075.Skokie,IL:PortlandCementAssociation.

FHWA.2002.Portland Cement Concrete Overlays: State of the Technology Synthesis.FHWA-IF-02-045,ACPASP045P.Skokie,IL:AmericanConcretePavementAssociation.

Mulligan,S.2002.Soundness Test by Freeze/Thaw.RecycledConcreteMaterialsReport.http://www.dot.state.oh.us/testlab/In-House-Research/sndtest1.pdf.

Okamoto,P.A,P.J.Naussbaum,K.D.Smith,M.I.Darter,T.P.Wilson,C.L.Wu,andS.D.Tayabji.1994.Guidelines for Timing Contraction Joint Sawing and Earliest Loading for Concrete Pavements, Volume 1.FHWA-RD-01-079.Washington,D.C.:U.S.DepartmentofTransportation,FederalHighwayAdministration.


AASHTOT96,ResistancetoDegradingofSmall-SizeCoarseAggregatebyAbrasionandImpactintheLosAngelesMachine

AASHTOT99,Moisture-DensityRelationsofSoilsUsinga2.5-kg(5.5-lb)Rammerand305-mm(12-in.)Drop

AASHTOT180,Moisture-DensityRelationsofSoilsUsinga4.54-kg(10-lb)Rammerand457-mm(18-in.)Drop


ASTMC131-03,StandardTestMethodforResistancetoDegradingofSmall-SizeCoarseAggregatebyAbrasionandImpactintheLosAngelesMachine

ASTMD698-00a,StandardTestMethodsforLabo-ratoryCompactionCharacteristicsofSoilUsingStandardEffort(12,400ft-lb/ft3(600kN-m/m3))

ASTMD1557-02e1,StandardTestMethodsforLaboratoryCompactionCharacteristicsofSoilUsingModifiedEffort(56,000ft-lb/ft3

(2,700kN-m/m3))

ASTMD1883-99,StandardTestMethodforCBR(CaliforniaBearingRatio)ofLaboratory-CompactedSoils

ASTMD3152-72(2000),StandardTestMethodforCapillary-MoistureRelationshipsforFine-TexturedSoilsbyPressure-MembraneApparatus

ASTMD4546-03,StandardTestMethodsforOne-DimensionalSwellorSettlementPotentialofCohesiveSoils

ASTMD4829-03,StandardTestMethodforExpansionIndexofSoils

ACPA.2003.Constructing Smooth Concrete Pavements.TB006.02P.Skokie,IL:AmericanConcretePave-mentAssociation.

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Thestartingpointforconstructinggood-quality

concretepavementistodesignandusegood-quality

concreteduringconstruction.Theplasticproperties

ofconcreteaffectmanyconstructionoperations,

includingmixing,transporting,placing,paving,

curing,andjointing.Concrete’shardenedproperties

affectlong-termpavementperformance.

Theothersignificantrequirementistouse

appropriate,high-qualityequipmentandgood

practices.Failuresinconstructionareoftenduetoa

combinationofmarginalmaterialsusedinmarginalequipmentoperatedbyinsufficientlytrainedoperators.

Thischapterdiscussesvariousconcretepavementconstructionoperationsandaddresseshowdecisionsmadeabouttheconcreteoritsconstituentmaterialsmayaffectconstructabilityorpavementperformance.Theinformationisgenerallylimitedtothatwhichisrelevanttothepavementsystem;itisnotafulldiscussionofconstructionpractices.MoredetailedinformationcanbefoundthroughtheAmericanConcretePavementAssociation,www.pavement.com.

Chapter 8

Construction

Field Verification 204

Concrete Production 205

Paving 212

Dowel Bars and Tiebars 218

Finishing 220

Texturing and Smoothness 221

Curing 224

Weather Considerations 226

Crack Prediction with HIPERPAV 231

Joint Sawing 233

Joint Sealing 237

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Field Verification

Key Points

Propertiesoftheconcretemustbeverifiedbothbeforeandduringconstructiontoconfirmthatitissuitablefortheintendeduse.

Fieldverificationofmixturesshouldusetheproductionequipmentanticipatedforthejob.

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Themixturepropertiesthatarerequireddepend,inlargepart,ontheintendeduseoftheconcrete.Concretesuitableforslipformpavingisdifferentfromconcreterequiredforhandplacingflatwork.Similarly,fast-trackconcretedifferssignificantlyfromnormal-settingconcrete.Regardlessofthemixture,itspropertiesmustbeverifiedbothbeforeandduringconstructiontoconfirmthatitissuitablefortheintendeduse.Fieldverificationofmixturesshouldusetheproductionequipmentanticipatedforthejobandshouldalsoincludetheconstructionofateststrip,ifpossible.Table8-1providesavailabletestsandanaly-sistoolsforfieldanalysisandmixtureverification.Apreconstructionmeetingbetweenthecontractor,agency,constructionmanager,and/ortesting

Table 8-1. Tests/Tools for Field Verification

laboratoryisrecommendedtoestablishworkingcriteria,goals,andpracticesfortheproject.Themeet-ingprovidesanopportunityfortheprojectteamtoresolveissuesbeforetheyoccurduringconstruction.Italsogivestheteamanopportunitytoestablishasystemforrespondingtoproblemsthatmayoccurduringconstruction.Thecontractortypicallyfur-nishesaqualitycontrolplanforproductionandpaving.

Property Available tests / analysis tools

Air content ASTM C 231 (pressure meter) Unit weight AVA (air-void analyzer)

Grading See chapter 9 page 253

Shrinkage Temperature sensors to feed data for HIPERPAV analysis

Strength - early ASTM C 1074 (maturity)

Strength ASTM C 31 / AASHTO T 23 (test specimens) ASTM C 39 / AASHTO T 22 (comp. strength) ASTM C 78 (flexural strength)

Water/cement ratio Microwave oven AASHTO T 318

Loss of consistency Slump loss

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Concrete Production

Key Points

Variabilityoftheproductionprocessmustbeminimizedtoproduceconcreteofconsistentqualityanduniformity.

Materialvariationsmustberecognizedandaccountedforwithplannedandpermittedfieldadjustments.

Theplantmusthavethecapacitytomeetproductionrequirementsfortheproject.

Aqualitycontrolplanoutliningtheprocessformaterialverificationmayberequired.

Criticalaspectsofaggregatestockpilemanagementincludemaintaininguniformgradingandmoisturecontentandpreventingaggregatecontamination.

Dryingredientsinconcreteshouldbebatchedbyweight.

Theorderinwhichmaterialsareintroducedintothemixermustbeconsistent.

Sufficientmixingtimemustbeallowedtoensureahomogeneousmixtureandtoentraintherequiredair-voidsystem.

Concretedeliverymustbeconsistentandontime.

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Concreteproductionisamanufacturingprocess.However,concreteismanufacturedfromraworcommoditymaterialssuchasaggregates,flyash,andcement,whichhaveinherentvariability.Becauseofvariabilityintheconstituentmaterialsandinmeasur-ingthem,aconcretemixture’spropertieswillvary.Toproduceconcreteofconsistentqualityanduniformity,thevariabilityoftheproductionprocessmustbe

minimizedwhilerecognizingandaccountingformaterialvariations.

Itisimportanttoconsiderthemixdesign,aswellasarealisticproductionschedule,intheplantselec-tionprocess.Productioncapacityandlimitsonhaultimerequireforethought.Projectsincongestedareas,whichdonotallowforon-siteproduction,mayrequireamixdesignthatallowsforextendedhaulingandplacementtimes.

Setting Up the PlantConcreteplantsareeitherpermanent,stationary

facilities,ortheymayconsistofportableequipmentthatiserectedadjacenttothepavingsite(figure8-1).Plantlocationandsetupdependprimarilyonsitefactorslikezoning,accesstoutilities,availabilityofmaterials,andpublictraffic(urbanorrural).Theplantmusthavethecapacitynecessarytomeettheproductionrequirementsfortheproject.

Optimizingtrafficflowattheplantisimportant.Itemstoconsiderincludethefollowing:

Deliveryofrawmaterials.Deliveryofconcrete.Qualitycontrol-relatedtrafficoperationsandtestingpersonnelsafety.Operationofequipmentformanagingtheaggregatestockpiles.Plantsafety.Environmentalimpact.

Theconcreteplantneedstobeingoodcondition,operatereliably,andproduceacceptableconcrete

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Figure 8-1. Portable concrete plant (ACPA)

Concrete Production


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uniformlyfrombatchtobatch.Plantsshouldbeinspectedpriortothestart(orrestart)ofeachpavingprojectandwhenuniformityorstrengthproblemsareencounteredduringproduction.Table8-2providesachecklistforinspection.

Handling MaterialsMaterialmanagementrequirementsaresimilarfor

eithertransitmixorcentralmixoperations.Thecon-tractorand/ormaterialsuppliersshouldhandleallofthematerialsinsuchawayastomaintainqualityanduniformity.Thecontractorshoulduseonlycertifiedmaterialsontheprojectandfollowthemanufacturer’srecommendations,asappropriate.

Thecontractorandtheconcreteproducershouldensurethatallmaterialsmeettheprojectspecifica-

Table 8-2. Concrete Plant Checklist

tions.Manyprojectsrequireaqualitycontrol(QC)planoutliningtheprocessformaterialverification.TheagencytypicallyreviewstheQCplanandassumesanintegralroleintheprocess.

Attheplant,differentcementitiousmaterials(cement,flyash,orground,granulatedblast-fur-naceslag)mustbekeptinseparatesilosorstorageunits.Systemsmustbeimplementedtoensurethatthecementitiousmaterialsareloadedintothecor-rectsiloswhentheyaredelivered.Unloadingofthecementitiousmaterialstakestime.Therefore,itisvitaltomaintainanadequateareaforcement,flyash,andothersupplementarymaterialdeliveries.

Stockpile ManagementStockpilemanagementisthecoordinationofthe

aggregatedelivery,storage,andloadingintothemixingplant,whichisavitalaspectofconsistent,qualityconcreteproduction(ACPA2004a).Locatingthestockpilesisanimportantfirstconsideration.Arelativelyflatareaispreferredtofacilitateunloadingandstockpilingtheaggregates.Also,placeapadoraggregateseparationlayerinthestockpilearea.Thiswillminimizecontaminationoftheaggregatefromthesoilbelowaswellaspreventmaterialloss.

Thegoalwithaggregatestockpilesistomaintainuniformgradationandmoisturecontentandpreventaggregatecontaminationthroughouttheproject.Con-sistentaggregatewillcontributetoconsistentconcrete.Afewbasicstockpilingconceptsincludethefollowing:

Pilethematerialinlifts.Completeeachliftbeforebeginningthenext.Donotdumpmaterialovertheedgesofastockpile.Minimizefree-fallheightsofaggregatestoavoidsegregation.Onlystockpileasmuchmaterialaspractical.Minimizecrushingoftheaggregatebytheloader.Managethestockpilecarefullytoobtainclosetosaturatedsurfacedry(SSD)condition.Forexample,thoroughlywettheaggregate,thenletitstandanhourbeforebatching.Monitorthemoisturecontentoftheaggregateusingprobesinthestockpile.

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No. Inspection item

1 Check foundations of stockpiles for proper separation and adequate drainage.

2 Check bins for adequate partitions to prevent intermingling of aggregates.

3 Check scales with test weights throughout range to be used.

4 Check scales for seals by approved agency.5 Check water meter for accuracy.6 Check for leakage of lines.

7 Check capacity of boilers and chillers if their use is anticipated.

8 Check admixture dispensers for accuracy.9 Check mixers for hardened concrete around

blades.

10 Inspect concrete hauling units for cleanliness.11 Check to ensure that all concrete-making materials

have been certified and approved for use.12 Observe stockpiling operations. Verify that

segregation and contamination will not occur.

13 Observe charging of the bins. Verify that segregation and contamination will not occur.

14 Review aggregate moisture tests.15 Observe batching operations at start and

periodically during production.

16 Check scales for zeroing.17 Check to ensure proper batch weights are set on

the scales.


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Insomecases,theaggregatesmaybecontaminatedwithclayorsoilbeforearrivingontheplantsite.Dirtyaggregatesrequirewashingorcleaningorshouldberejected.Inadditiontocausingclayballproblemsintheconcrete,dirtyaggregatescanleadtoproblemssuchaslowstrength.Theloaderoperatorhasakeyroleinpreventingclayormudfrombeingdepositedintotheplant’sfeedhoppers(figure8-2).Theopera-tormustcontroltheelevationoftheloaderbladetopreventpickingupcontaminationfrombelowtheaggregatestockpile.

Portablecentral-mixplantsareusuallymoresus-ceptibletoproducingconcretecontaminatedwithclayballs,simplybecausetheyaretemporarilyplacedneartheprojectsiteandmayhaveclayorloosesoilunderneaththestockpiles.Thebatchplantorcon-creteforemanmustkeepacloseeyeonstockpilemanagementatportableplantsites.Stationaryready-mixplantsoftenhaveapavedsurfaceorbunkersonwhichthestockpilesareplacedorstoredandwheretheloaderoperates.Thisreducesthelikelihoodofclaybeingintroducedintotheready-mixedconcrete.

Theaggregateloaderoperatorisanimportantpersonintheproductionofconsistentqualitycon-crete.Theprimaryfunctionsoftheloaderoperatorincludethefollowing:

Workingthestockpiletoprovideuniformwatercontentandgradation,whileavoidingsegregation.Minimizingcontamination.

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Figure 8-2. Loader operation is key to stockpile manage-ment. (ACPA)

Observingandreportingmoisturevariations.Addingmaterialtothefeedhopper(s)appropriately.Notifyingtheplantforemanofanticipatedaggregateshortages.

BatchingBatchingistheprocessofmeasuringthemixture

ingredientsandintroducingthemintothemixer(forinformationaboutbatchingasitaffectsconcreteuniformity,seeBatchingOperationsinchapter5,page106).

Dryingredientsinconcreteshouldbebatchedbyweight,whilewaterandchemicaladmixturesmaybebatchedbyvolume.Thepotentialerrorinvolumebatchingcementandaggregatesislargebecauseofbulkingofthematerialswithhandlingandincreasingmoisturecontent.

Theorderinwhichmaterialsareintroducedintothemixerisimportanttoensureuniformmixingandmaximizeconcreteconsistency.Figure8-3showsthetypicalsequenceofaddingcomponentsintoastation-arymixplant.Materialsareblendedinapproximateproportionsastheyenterthemixer.Onebatchofconcreteismixedwhileanotherisbeingbatched.Theseoperationsoccursimultaneously.

Specificsequencesmayvarydependinguponthematerials.Theplantoperatormayadjustthesequencetoaccommodateproductionrequirementsorif

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________________________________________________Figure 8-3. Typical sequence of adding material in a stationary mix plant (Ayers et al. 2000)

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incompatibilityisreportedtooccur.Therequirementsforspecialtymaterials,suchasfibersandotheraddi-tives,mayalsoalterthesequence.Thesequencingofingredientswillalsohaveanimportanteffectontheuniformityoftruck-mixedconcrete.Elevatingtherearportionofthetruckmixerduringchargingoperationsminimizesthetimerequiredtogetalltheingredientsintothedrum.Figure8-4showsthetypicalsequenceofaddingcomponentsintoatruckmixer.Onceagreedupon,thesequencingprocessshouldremainthesame.Asequencingdiagramisausefultoolinclearlyshowingtheparticularchargingprocessfortheconcretemixandplant.

Inadrybatchprocess,thefirstingredientsintothedrumareusuallyaportionofthewaterandapor-tionofthecoarseaggregate.Thewaterisshutoffandaggregatesandcementitiousmaterialsarecombineduntilallofthecementitiousmaterialisinthedrum.Thefinalportionofwatergoesinwiththelastoftheaggregatestocleanandwashanycementitiousmaterialclingingtothemixer’shoppers,rearfins,andchutes(Ayersetal.2000).

Ribbonloadingisamethodofbatchingconcreteinwhichthesolidingredients,andsometimesthewater,enterthemixersimultaneously.Thisprocesswilloftenproduceaveryconsistentproductiftheplantisconfiguredappropriately.

Aggregatemoisturecontent(monitoredusingprobesinthestockpile)andthepotentialforsegrega-tiongreatlyaffectconcretequalityanduniformity.

_______________________________________________Figure 8-4. Typical sequence of adding material in a truck mixer (Ayers et al. 2000)

Iftheaggregatemoisturecontentislowerthanthesaturatedsurfacedry(SSD)moisture,additionalwaterwillberequiredinthemixtopreventstiffeningcausedbywaterbeingabsorbedintotheaggregate(seeStep11,Moisture/AbsorptionCorrection,inchapter6,page183).

ThemaximumadditionalamountofwaterrequiredinthemixiscalculatedasthedifferencebetweentheSSDmoistureandthecurrentmoisturecontent,multipliedbythemassoftherespectiveaggregate.However,iftheaggregateiswetterthantheSSDvalue,theamountofwatermustbereducedatthebatchplant.Otherwise,thewater-cementitiousmateri-alsratioofthemixwillbeexceeded,leadingtolowstrengthandpoordurability.

Aggregatemoisturecontentandthepotentialforsegregationgreatlyaffectconcretequalityandunifor-mity.ASTMC94providesfourkeyitemstocontroluniformity:

Foreachsizeofcoarseaggregate,useseparateaggregatebinsthatcanshutoffmaterialwithprecision.Usecontrolstomonitoraggregatequantitiesduringhoppercharging.Usescalesaccurateto+0.2percenttestedwithineachquarterofthetotalscalecapacity.Adequatestandardtestweightsforcheckingscaleaccuracyshouldbeavailable.Addwatertoanaccuracyofonepercentoftherequiredtotalmixingwater.

Thefollowingconsiderationsareimportantforcon-trollingbatch-to-batchconsistency(ACPA2004a):

Theplantforepersonmustensureaconsistentandcleanoperationofthefront-endloadersandotherheavyequipmentthatmoverawmaterialsatthebatchplant.Somecoarseaggregatesforuseinconcretearehighlyabsorptive,requiringasignificantquantityofwatertocreatethesaturatedsurfacedryconditionassumedinthemixturedesign,sothatadditionalwaterisnotabsorbedduringmixing.Wateringthestockpilesmaybeneces-saryforthesecoarseaggregates.Periodicmoisturetestsonthefineandcoarseaggregatearenecessary(atleasttwiceaday).

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Concrete Production

Truckoperatorsmustbesuretoremovefreewaterorexcessreleasecompoundsfromdrumsordumpbedsafterwashingorrainfall.

Mixing ConcreteConcreteisnormallymixedanddeliveredusing

oneoracombinationofthefollowingoperations:Centralmixing.Inacentralmixconcreteplant,theplantoperatoraddsthebatched(weighedormetered)ingredientsintoastationarymixer.Themixerthencompletelymixesthecompo-nentsbeforedischargingtheconcreteintoadeliveryvehiclefortransportingtothepointofdischarge.Thedeliveryvehiclecanbeatruckmixeroperatingatlowmixingspeed,atruckagitator,oranonagitating(e.g.,flatbed)truck.Shrinkmixing.Shrink-mixedconcreteispartiallymixedinastationarymixerandthentransportedwhilemixingiscompletedinatruckmixer.Truckmixing.Truck-mixedconcreteismixedentirelyinthetruckmixer.

Changestothemixture’swaterandadmixturesarepossibleatthejobsiteifatruckmixerisusedtotransportthemixture(Ayersetal.2000).

Therequiredmixtimevariesdependingontheequipment,proportions,andmaterialsused,grada-tionsofaggregates,amountandtypesofadmixtures,temperature,etc.However,ifthereareanyprob-lemsregardingaconcretemix,suchassegregation,bleeding,finishing,etc.,thefirstandeasiestaspecttochangeorconsideristhemixtime.Forapor-tablecentralmixbatchplantthatisusedtoproduceaconcretepavingmixture,mixtimeswilltypicallyrangefrom60to90seconds.Shortmixingperiodswillreducetheamountofentrainedairandwilllikelyleadtoanonuniformmixture(seeTypeofPlantandProductionProceduresinchapter5,page134).

Delivering ConcreteConsistentdeliveryofconcretetothepaving

projectsiteisanimportantelementinmaintainingauniform,trouble-freepavingoperation(ACPA2003b).Thisisusuallylesschallenginginruralareasthaninurbanareasbecausehaulroadsarewiderandhaul

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trucksmaytravelfreely.However,denselypopulatedurbanareasrequirecarefulevaluationtodeterminewhethertrafficdelayswillhamperconcretedelivery.Considerationoftheconcretemixture’sstiffeningpropertiesisalsonecessary,withnormal-settingmixturesallowinglongertraveltimesthanfast-settingmixturesallow.

Feedingconcreteintothepavingmachineconsis-tentlyrequiresanadequatenumberofbatchdeliverytrucks.Thenumberoftruckswilloftendictatetheslipformorplacementspeed.Theentirecycleofmixing,discharging,traveling,anddepositingconcretemustbecoordinatedforthemixingplantcapacity,haulingdistance,andspreaderandpavingmachinecapabilities.Extratrucksmaybeneededasthehaultimeincreases.

________________________________________________Figure 8-5. Depositing concrete in front of the paving machine (ACPA)

a) Too much

b) Appropriate amount

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Themannerinwhichthecrewdepositsconcreteinfrontofthepavingoperationisanimportantfactorinthiscycleandincreatingasmoothpavementsurface.Forslipformpaving(seethesectionlaterinthischap-teronPaving),theamountofconcretebeingcarriedinfrontofthepavingmachine(thehead)mustbecontrolledtoensurethatitdoesnotgettoohighortoolow(figure8-5).

Iftheheadgetstoohigh,itcreatesapressuresurgeunderthepavingmachine.Thesurgecancausetheconcretebehindthemachine’sfinishpantoswell,creatinganuncorrectablesurfacebump.Theslipformmachinemayevenlosetractionandsteering.Thisismoreofaproblemwithsmalltwo-trackpavers.Largerfour-trackpaversmaybeheavyenoughtohandleconcretebuildupinfrontofthepaver.Ifthereisnotenoughmaterialinfrontofthepavingmachine,

Figure 8-6. A belt placer/spreader ensures a consistent amount of concrete in front of the paver. (ACPA)

thentheconcreteheadmayrunoutorthegroutboxmayrunempty,creatingalowspotandvoidsorpocketsinthepavementsurface.Avoidsuchproblemsbyusingaplacer/spreadermachine(figure8-6)orbycarefullydepositingconcretefromthehaultrucks.

Field AdjustmentsAsdiscussedabove,thematerialsusedtomake

concreteareinherentlyvariable,asaretheconditionsinwhichtheyaremixedandplaced.Itisthereforeimportantthatthepropertiesofthefreshconcretearecloselymonitoredandproportionsadjusted(withintheconstraintsofthespecification)toimprovetheuniformityofthefinalproductfrombatchtobatchanddaytoday.Thefollowingsectionsdiscusssomeofthevariablesthathavetobeconsideredaspavingproceeds.

Ambient TemperaturesDailyandseasonaltemperaturevariationsimpact

thefreshandhardenedconcrete.Concretestiffens,sets,andgainsstrengthfasterasthetemperaturerises.Hotweatheristhereforeasourceofproblemsforthepavingprocess.

Sprinklingaggregatestockpilesandchillingthemixwaterorusingicearenormallythefirstresponsestohotweather;however,thetotalamountofwaterandiceinthemixturemustnotexceedthemixingwaterinthemixdesign.Ifagreatertemperaturereductionisrequired,theinjectionofliquidnitrogenintothemixermaybethebestalternativemethod.

ASTMC94/AASHTOM157allowwatertobeaddedtoremixtheconcretewhenthetruckarrivesonthejobsiteandtheslumpislessthanspecified,providingthefollowingconditionsaremet:

Tips for Managing a Head of Concrete

Allow old concrete being carried in the head in front of the paver to be placed. This material can contain a large amount of lighter aggregate particles that, after the first winter, can cause scaling of the pavement surface. The old concrete can also create honeycombed areas or seams that can lead to cracking.

Do not allow the head to run out. When the head is gone, there may not be enough concrete to completely build the slab. Paving operations must then be stopped and concrete brought to the paver to fill the required head. This is disruptive and costly.


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Maximumallowablewater-cementratioisnotexceeded.Maximumallowableslumpisnotexceeded.Maximumallowablemixingandagitatingtime(ordrumrevolutions)arenotexceeded.Concreteisremixedforaminimumof30revo-lutionsatmixingspeedoruntiltheuniformityoftheconcreteiswithinthelimitsdescribedinASTMC94/AASHTOM157.

Watershouldnotbeaddedtoapartialload.Adjustmentsforcoolorfallingtemperaturesareless

criticalwithrespecttothewater-cementitiousmateri-als(w/cm)ratioandstrength.However,consistentworkabilitymuststillbemaintained.Heatedwaterisacommonmixadjustmentwhencoolertemperaturesoccur.Consistentcooltemperaturesmayalsorequiretheadditionofanacceleratingadmixturetominimizetheriskofrandomcrackingduetolatesawing.

Materials VariabilityThreeofthefourlargestconstituentsinaconcrete

mixbyvolume—coarseaggregate,fineaggregate,andcementitiousmaterials—areprocessed/manufacturedfromrawmaterialsthatareextractedfromnaturallyoccurringdeposits.Theverynatureofthesemateri-alsdictatesthattheyhaveaninherentvariability.Thequarryingandmanufacturingprocessesusedtopro-duceconcretematerialsreducebutdonoteliminateallofthisvariability.

Moisturecontentintheaggregatestockpilesisthemostcommonparameterthatrequiresanadjustmentintheplantprocess.Stockpilesshouldbesampledandtestedatleastdaily,or,dependingonweathercondi-tions,morefrequenttestingmayberequired.Theplantshouldbecapableofcompensatingforthisfree

1.

2.3.

4.

waterfromtheaggregates.Mostmodernbatchplantshavemoisturesensorsintheirfineaggregatebins.

Aggregategradingshouldbemonitoredatleastdaily.Frequentdensity(unitweight)testscanbehelpfulindetectingchangesinaggregategradation.Modestvariationsingradationarenormalandwillnotgenerallyhaveanadverseimpactonperformance.Severelysegregatedstockpilesshouldberejectedorrebuilt.Batchproportionsoftheaggregatesshouldbeadjustedifthecombinedgradationofthemixdeviatessignificantlyfromthemixdesignandtheworkabilitypropertiesarecausingplacementdifficulties.

Variabilityincementitiousmaterialsalsoimpactsworkability.Therearecurrentlynostandardtestmethodsthatprovidetimelyfeedbackonthisvariable;thereforeadjustmentsmustbemadebasedonfeedbackfromthepaver.Changesincementandsupplementarycementitiousmaterialsfineness(higherBlainevalue)mayrequireadditionalwaterandair-entrainingadmix-turetomaintainworkabilityandaircontent.

Material Supply ChangesWhenaningredientsourceisreplacedorthe

amountrequiredchangesbymorethanfivepercent(excludingadmixturesusedwithinrecommendeddosages),trialbatchesareneeded.Timeisusuallyacriticalfactorinhowmaterialchangesarehandled.Thebestandsafestalternativeistobatchanticipatedmixdesignsinthelaboratorywellbeforeconstruc-tionstarts.Otherwise,workability,settingtime,airentrainmentproperties,andearly-agestrength(matu-rityand/orthree-daystrength)shouldbecloselycomparedwiththeinitialmixdesign;proceedwithcautionandincreasetestingfrequencyduringtheinitialdayswiththenewmaterials.

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Paving

Key Points

Slipformorfixed-formpavingmethodscanbeused.

Concretemixturesandpracticesvaryaccordingtoplacementmethod.

Slipformpavingisgenerallyforplacementsrequiringhigherproductionrates.

Fixed-formpavingisadaptabletonearlyanyplacementcircumstance.

Thepavingtrainisatermreferringtothecombinationofindividualmachinesthatplaceandfinishconcretepavement.

Allequipmentmustbesuitablefortheapplication,well-maintained,andoperatedbysuitablytrainedpersonnel.

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____________________________________________________________________________________________________Figure 8-7. Components of a typical slipform paving machine (ACPA)

Contractorsuseeitherslipformorfixed-formpavingmethods,dependinguponthenatureoftheplacement.Theconcretemixturesrequiredbyeitherplacementmethodvarysignificantly.Slipformpavingoperationsrequirealow-slumpmixturethatwillnotsloughafterextrusionbythepavingmachine,whileafixed-formpavingoperationreliesonahigherslumpmixturethatwillfloweasilytofilltheforms.

Slipformpavingisgenerallyforplacementsrequir-inghigherproductionrates,suchasmainlinepaving(figure8-7).Fixed-formpavingisadaptabletonearlyanyplacementcircumstance,butbecauseitrequiressettingupsideformstoholdtheconcrete,itisgener-allynotasefficient.Table8-3listscommonelementsofpavingmachines.

Thepavingtrainisatermreferringtothecombi-nationofindividualmachinesthatplaceandfinishconcretepavement.Forhighwayapplications,atypicalpavingtrainincludesthefollowing:

Spreaderwithbeltplacer.Slipformpaver.Texturingmachine.Curingcart(usuallytogetherwiththetexturingunit).

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Table 8-3. Common Elements of Paving MachinesEquipment Setup

The following critical elements should be in place before production paving starts (IPRF 2003):

Check all the equipment in the paving train to make sure it is in operational condition.

Verify that an acceptable length of grade is avail-able for concrete paving.

Check that approved test reports are available for all materials in storage at the job site and the plant site.

Verify that backup testing equipment is available; develop extra equipment backup plans.

Verify that all necessary concrete placement tools are available, such as hand tools, straight edges, hand floats, edgers, and hand vibrators.

Verify that radio/telephone communication with the plant is operational.

Verify that equipment is available to water the grade, if necessary.

Monitor the string line regularly and re-tension as necessary (slipform only).

Check the forms for proper bracing (fixed-form only).

Verify that the day’s work header is in place (or just saw off the excess).

Develop an extreme-weather management plan.

Check the weather forecast for each day of paving.

Make sure a sufficient length of plastic covering is available in case of sudden and unexpected rain.

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Placement (Fixed Form)Thereareavarietyoffixed-formpavingmachines;

thelesscomplexequipmentincludeshand-operatedandself-propelledvibratoryscreeds,single-tubefin-ishers(figure8-8),andrevolvingtripletubes.Larger,form-ridingmachinescanplaceandconsolidatetheconcretebetweenformsinonepass.Thesemachinesrideontheformsoronpipeslaidoutsidetheforms.

Tomakepavingeasier,itisimportanttoevenlydepositconcreteontothegradeinfrontofthefixed-formplacementmachine.Pilingtoomuchconcretein

frontofthemachineleadstodifficultywithstrikingoff.Theconcreteshouldnotoverlyexceedtheheightoftheforms.However,pilingtoolittleconcreteinfrontofthemachinemayproducelowspotsinthepavementsurface.Althoughitisidealtodistributetheconcreteevenlywiththechutefromtheready-mixorotherconcretehaulingtruck,somedistributionoftheconcretewithhandtoolsisusuallynecessary.Concreteshouldneverbemovedwithavibrator,whichisforconsolidationonly.Usingvibratorstomoveconcretecansegregatethemix,sendingthelargeaggregate

Slipform Fixed form

Self-propelled with Ride on the forms or on self-either two or four propelled wheels.tracks.

Steering and elevation Steering and elevationcontrolled from controlled by fixed forms.reference stringlines.

Paving width: up to 15 m Paving width: various.(50 ft), depending upon model and available attachments. (Most are 7.3 to 8.5 m (24 to 28 ft) width.)

Weight: about 3,000 kg Weight: about 1,500 kg/mor more per meter (1,000 lb/ft) of paving width.(2,000 lb/ft) of pavinglane width.

Continuous auger or Suspended screw auger to hydraulic plow-pans to spread concrete in front of distribute concrete in screed or roller.front of the screed (may carry head of concretein front of paver).

Contain variable speed Have one or two vibrators that hydraulically controlled move transversely in front of the internal vibrators, with screed. May also use fixed optional vibration vibrators near the form edges. monitoring capabilities.

Provide the consolidation Provide the consolidation energy required for energy required for pavements highway pavements as as thick as 250 mm (10 in.).thick as 375 mm (15 in.).

Allow for various finishing attachments.

Source: IPRF (2003)

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tothebottomandpastetothetop.Thiscanalsocompromisetheairentrainmentandultimatelytheconcretedurability.

Foreaseofremovalandcleaning,formsrequireathinapplicationofoilbeforepaving.Withoutoil,concretecanadheretothesteelorwoodensurfacesoftheforms.Thepavingcrewshouldinspectformsjustbeforepavingandcarefullyreapplyoiltoanydryareas.Ifsteelreinforcementisinplace,careisneces-sarytoavoidgettingoilontothebars.

Consolidation (Fixed Form)Theexternal(surface)vibrationthatavibratory

orrollerscreedproducesisadequatetoconsolidateprimarilyjustthesurfaceofmostpavementslabs.Supplementaryinternalvibrationwithhand-operatedspudvibratorsisusuallynecessaryforadequatecon-solidationofunreinforcedconcreteslabsthickerthan75mm(3in.).Acombinationofinternalandsurfacevibrationispreferableforreinforcedslabsatanythick-ness(Kosmatka,Kerkhoff,andPanarese2002).Becausesurfacevibrationofconcreteslabsisleasteffectivenearthefixedforms,itisalsobeneficialtoconsolidatecon-cretealongtheformswithaspudvibrator.

Supplementalvibrationwithhand-heldspudvibratorsshouldprecedetheplacementscreed.Standardpracticeforthickerslabscallsforverticalplungesofthevibratorhead.Forthinslabs,itis

Figure 8-8. A roller screed (i.e., single-tube finisher) is one type of equipment that can be used to strike off the con-crete in fixed-form placements. Others include vibratory screeds, form riders, and bridge deck finishers. (ACPA)

preferabletoinsertthevibratorheadatanangleorhorizontallytokeepitcompletelyimmersedintheconcrete.Operatorsshouldneitherdragspudvibratorsthroughtheconcretenorattempttomovetheconcretelaterally,aseitherwilltendtocausesegregation.Leavingthevibratorheadinsertedforabout5to15secondswillusuallyprovideadequateconsolidation.Ingeneral,properconsolidationofair-entrainedconcretetakeslesstimethannon-air-entrainedcon-crete,evenwhenbothmixturesarepreparedwiththesameconsistency(slump).

Placement (Slipform)Slipformpavingmachinesoperatebyextrudingthe

concreteintotheshapeoftheslab.Aseriesoftoolsmountedonthefrontofaslipformpaverfilltheformsandcreateauniformshape.Thesetoolsaretheaugerspreader(orspreaderplow),strike-off,vibrators,tamperbar,profilepan,oranycombinationoftheseitems(seefigure8-7).

Themoldingcomponentsarethebottomoftheprofilepanorformingplateandthesideforms.Alloftheseelementsconfinetheconcreteandformitsshapeinthesamemannerthatacaulkinggunnozzleconfinesthecaulkanddefinestheshapeofthebead.

Inpaving,themoldisforcedthroughoroveravolumeofconcretethatremainsstaticonthegrade.Vibratorsmountedtotheslipformmachinefluidizetheconcreteandmakeiteasiertomold.Theslipformpaverthuspassesoverthefluidizedconcretewhileitsmasskeepsthepanandsideformssteadytoconfineandshapetheconcrete.Thefollowingfactorsinflu-encetherequiredextrusionpressure:

Weightofthemachine.Taperofsideformsrelativetothedesiredpave-mentedgeplanes.Angleoftheprofilepanrelativetothedesiredpavementsurfaceplane.Vibratorpowerandfrequency.Paverspeed.Concreteworkability.

Theprimaryadjustmentsaslipformpavingmachineoperatorcanmakearethemachine’sspeedandthefrequencyofitsinternalvibrators.Iftheconcrete’splasticpropertiesvarywidely,requiring

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frequentadjustmentsoftheplacingspeedorvibrationfrequency,theresultwillbeanonuniformsurface.Aslipformpavingmachinemustspreadandconsoli-datetheconcreteasitmovesforward,anditcannotproduceasmoothridingsurfaceifitmuststopoftenorpushalargepileofconcreteaheadofit.

Consolidation (Slipform)Vibrationisnecessaryforconsolidatingthecon-

crete.Onslipformpavers,aseriesofvibratorsfluidizetheconcreteandremovelargeairvoids(figure8-9).

Thevibratorsaretypicallysetataconstantfrequency,whichcanbemonitoredandadjustedbythepaveroperator.Someadjustmenttovibratorfrequencymaybehelpful,butrunningthevibratoratahigherfrequencyshouldnotbeusedtoovercomepoorequip-mentsetup,pooralignment,orpoormixtures(seeVibrationMonitoringinchapter9,page248).

Vibratorsmaycauseundesirableeffects,suchaslossofairentrainmentorvibratortrails,whenoper-atingataveryhighfrequency.Formostmixtures,afrequencyfrom5,000to8,000vibrationsperminute,atpaverspeedsgreaterthan0.9m(3ft)perminute,canadequatelyfluidizeandconsolidatetheconcretewithoutlossofentrainedairorsegregationofparticles(TymkowiczandSteffes1997).Vibratorfrequencymightneedtobelowerifthepavingspeedfallsbelow0.9m(3ft)perminute.

Vibratorsensingsystemsareavailabletoprovideareal-timereadoutofvibrationfrequencyforallofthe

vibratorsonaslipformpavingmachine.Theseunitspermitalarmsettingsthatalerttheoperatorofhighorlowfrequencies,forallorindividualvibrators,ortotallossofvibration.

Studieshaveshownthatheavyvibrationisnotdet-rimentaltothefrost-resistanceofwell-proportionedmixtures,providedthattheproperair-voidsystemisinitiallyestablishedintheconcrete(Tynes1975;Simonetal.1992).Mixturesthataregap-graded(oversanded)willtendtosegregatemoreeasilythanwell-gradedmixturesatagivenvibrationfrequency.Therefore,theneedforreducedvibrationfrequenciesiscriticalforgap-gradedmixtures.

Iftheoperatorslowsthepavertomatchthedeliv-eryofconcrete,reductionofthevibrationfrequencyisalsolikelytobenecessarytomaintainconsistentextrusionpressure.Theadjustmentisrelativetothenormalvibrationfrequencyandpavingspeedoftheoperation.

String LinesTheprofilepan,thepartofaslipformpaverthat

controlsthepavementsurface,referencesitspositionfromsensorsthatfollowstringlinesplacedalongthegrade,usuallyonbothsidesofthepaver.Thestringlineistheprimaryguidancesystemforaslipformpaver(ACPA2003b).Thepaver’selevationsensingwandridesbeneaththestring,andthealignmentsensingwandridesagainsttheinsideofthestring.Neitherofthesewandsshoulddeflectthelineanymeasurableamount.Atypicalsetupincludesastringlineoneachsideofthemachine(figure8-10).Thestringlineitselfmaybewire,cable,wovennylon,polyethylenerope,oranothersimilarmaterial.

Thestringlinesarenotnecessarilyparalleltothegrade,butaresettoformthesurfaceregardlessofthegradeelevation.Awell-positionedstringlinecanhelpovercomeminorsurfacedeviationsinabaseortrackline,butitisnotasubstituteforasmooth,stabletracklinebuilttoatighttolerance.Thehydraulicsystemsthatcontrolaslipformpavingmachine’sprofilepancannotadjustquicklyenoughtosignificantvariationsinthemachine’sverticalpositioncausedbyanunstablebaseortrackline.Anunstabletracklinecausestheprofilepantocontinuallyattempttoadjustitspositionrelativeto

Figure 8-9. An array of vibrators under a slipform paver (ACPA)

Paving


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themachine’sframe.Thesetypesofmechanicaladjust-mentscausebumpsordipsintheresultingpavementsurfaceiftooabruptortoofrequent(ACPA2003b).

Achievingasmoothsurfacewithslipformpavingrequirescloseattentiontothesetupandmaintenanceofthestringline.Thestringlinematerial,stakes,stakinginterval,splices,andrepositioningfrequencyallmayimpacttheresultingpavementsurface.Stakesthatsecurethestringlineshouldbelongenoughtobefirmwhendrivenintothesubgrade.Theremustbeanadequatestakelengthexposedabovethegradetoallowadjustmentofthestringlinetothedesiredheightabovethesubgradesurveyhub,typically450to750mm(1.5to2.5ft).Amaximumspacingbetweenstakesofnomorethan7.5m(25ft)ontan-gentsectionswillproducethebestresults.Decreasingthisintervalinhorizontalandverticalcurvesisalsonecessary.

Reducinghowoftenastringlinemustbesetupduringtheprojectcanleadtobettersmoothnesscon-trol.Wherepossible,itisadvantageoustosetuponestringlineoneachsideofthepavingareatoservealloperations,includingsubgradepreparation,subgradestabilization,baseconstruction,andpavementplace-

____________________________________________________________________________________________________Figure 8-10. Typical string line setup (ACPA 2003b)

ment.Formultioperationalusage,thestakesandstringsmustbeoffsetfartherfromthepavementareatokeepthemclearoftheequipmentandoperations.

Inmanyinstances,thehaulroadisnexttothestringline.Thisarrangementnecessitatesregularinspectionofthestringlinebyeyetodeterminewhetheranyheavingorsettlingofthegradedisturbedthehubsand/orlinestakes.Ittakesconsiderableexperiencetoproperly“eyeball”correctionstoastringlineduetoadeviationinthegrade.Whennoticed,thesurveyorstringlinecrewshouldreposi-tionmisalignedstakesimmediately.Itissometimesadvantageoustocheckastringlineatnightusinglightshonefromvehicleheadlights.Thisnightsmoothingtechniquereducesvisibilityofbackgroundobjectsandenhancestheabilitytofocussolelyontheilluminatedstringline.

Alternatives to String LinesRecentimprovementsintechnologyhaveresulted

inconcretepaverguidancesystemsthatrequiremuchlesssurveying,setup,andmaintenancethanstringlines.Thesetechnologiesincludelaser-guidedsystemsandglobalpositioningsystems(GPS).Anotheralter-

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nativetousingstringlinesistheuseofaski,whichridesoffofapreviouslypavedsurface.Thistechniqueisalsoreferredtoasbeing“lockedtograde.”Theuseofthesealternativesisdictatedbytheirneedinspecialsituationsandthedemonstrationofsimilartolerancesandaccuraciestostringline-guidedpavers.

Edge SlumpEdgeslumpoccurswhenthetopedgeofafreshly-

placed,slipformedconcretepavementsagsdownaftertheslabisextrudedfrombehindthepaver.Asmallamountofedgeslumpisusuallyacceptablewhenthelongitudinaljointwillhavesometrafficmovingacrossit.Whentheslabedgeformstheabsoluteextent(freeedge)ofthepavement,alargeramountofedgeslumpisacceptable.

Edgeslumpisgenerallymorecommoninthickerpavements,whichhavetostandhigherandarethere-foremoresusceptible.Themostcommonformofedgeslumpiswhenthetopedgeslumpsdown(figure8-11).Thebottomedgeslumpingoutusuallyindi-catesamoreseriousproblemwiththemixdesignandisoftenassociatedwithhigherslumpmixturesthatarenotintendedforslipformpaving.Whenthistypeofedgeslumpoccurs,pavingshouldbesuspendeduntiltheconcretemixturehasbeenmodifiedtoworkwithslipformpaving.

Thefollowingfactorsaffectedgeslump:Concreteconsistency.Concretemixturecompatibilitywithplacementtechniques.Paveradjustmentsandoperation.Excessivefinishing.Segregationonthebeltplacer.

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Mostspecificationslimittheareaconsideringedgeslumptowithin150to300mm(6to12in.)awayfromtheslabedge.Ingeneral,mostagenciesspecifyanedgeslumpof6mm(1/4in.)asthetriggerforcorrec-tiveaction.

Correctiveactionusuallyinvolvesthefinishersbehindthepaverreworkingtheedgetoremovetheirregularity.However,continualcorrectionofexcessiveedgeslumpinfreshconcretecanleadtounacceptablelevelsofjointspallinginthefinishedconcrete.Ifsuchaproblemdevelops,pavingshouldbestoppedandmeasurestakentocorrectexcessiveedgeslump.

Inmostcases,edgeslumpcanbecorrectedimme-diatelybehindthepaver,andthemixturecanbecalibratedtopreventreoccurrence.

Iftheedgeslumpisnotdetectedintime,itmayrequirepatchingand/ordiamondgrindingtocorrecttheirregularity.

________________________________________________Figure 8-11. Two types of edge slump

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Dowel Bars and Tiebars

Key Points

Dowelbarsandtiebarsareusedtoprovidesupportbetweenadjacentpanelsinapavement.

Dowelassembliesmustbeplacedintherequiredlocationsandfixedtopreventmovementwhenconcreteisplacedoverthem.

Usingspecializedequipment,dowelscanbeinsertedafterconcreteplacement.

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Dowelbarsandtiebarsareusedtoprovidesupportbetweenadjacentpanelsinapavement.

Theyareeitherplacedinbaskets(dowels)orchairs(tiebars)orinsertedintothefreshconcrete(seeDowelBars,Tiebars,andReinforcementinchapter3,page61;andDowelBarTolerancesinchapter9,page248).

Pre-Placed BarsDowelassembliesarefastenedtothesubbaseusing

steelstakingpinsforgranularmaterialsornailingclipsforstabilizedmaterials(figure8-12).

Ifnotproperlyfastened,thebarsmaytipormoveunderthepressureofpaving.Itisthereforeimportantthatthecagesbepinnedsecurelysothattheydonotmovewhenthepaverpassesoverthem.

Careinpositioningthebasketsisalsonecessarysothatjointscanbesaweddirectlyoverthebasketandperpendiculartothecenterline.Apermanentmarkindicatingthelocationofthedowelbaskets,oranindentationintheedgeoftheslabintheareaofthebasket,shouldbemadeforlaterreferencewhensawingthecontractionjoints.

Dowelbasketsandtiebarsmaydisrupttheconsoli-dationpressureduringthepassageofaslipformpaver.Theresultisabumpand/ordipinthesurfaceofthe

pavementoranindentationintheedgeoftheslabintheareaofthebasket.Somecontractorsfindthatplacingconcreteoverdowelassembliesbeforepas-sageofthepavingmachineeliminatesdowelassemblypressureeffects.Othersfindtheuseofv-floatsorproperlyadjustedoscillatingbeamfloatstoremovespring-backandripplingeffects.

Insomecasesforlongitudinaljoints,contractorselecttoplacetiebarsintopositionaheadofpaving.Straightdeformedbarsonsupportingchairsfastentothesubbaseorsubgradeinamannersimilartodowelbaskets.Infixed-formconstruction,standardtiebarsortwo-piecebarswithathreadedcouplinginsertthroughholesinsideformsforlongitudinalconstruc-tionjoints.Careinconsolidatingtheconcretearoundthesebarsisnecessary.

a) Staking

b) Pinning

________________________________________________Figure 8-12. (a) Staking or (b) pinning dowel cages (ACPA)

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Inserted BarsThealternativetoplacingdowelbarsinbasket

assembliesistouseautomaticdowelinsertionequip-

ment(figure8-13).

Thekeytocontrollingthelocationandpositioning

ofautomaticallyinserteddowelbarsistheconcrete

mixture.Mixtureswithwell-gradedaggregateand

appropriateworkabilityproduceexcellentresultswith

dowelinsertion.Mixtureswithgap-gradedaggregate,

ontheotherhand,tendtoallowthedowelstomigrate

withintheconcretemass.Figure 8-13. Dowel bar insertion equipment (ACPA)

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Finishing

Key Points

Itisbesttolimittheamountofhandandmechanicalfinishing.

Handfinishingofthepavementsurfaceusingbullfloatsisnecessaryonlywherethesurfaceleftfromthepavingequipmentcontainsvoidsorimperfections.

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Handfinishingofthepavementsurfaceusingbullfloatsisnecessaryonlywherethesurfaceleftfromthepavingequipmentcontainsvoidsorimperfections.Mechanicallongitudinalfloatsareoftenoverusedbehindthescreedorslipformequipment.Ingeneral,itisbesttolimithandandmechanicalfinishing.Iflongitudinalfloatingistheonlymethodtoproduceanacceptablyclosedsurface,adjustmentsareneededintheconcretemixtureand/ortothepavingequipment.Theagencyandcontractorshouldreviewandadjusttheirdesignandoperationstoimprovetheresultsachievedbythepavingmachinealone.

Checkingthesurfacebehindthepavingequip-mentwitha3-to6-m(10-to20-ft)hand-operatedstraightedgeisarecommendedprocedure.Successivestraightedgechecksshouldoverlapbyone-halfthe

lengthofthestraightedgetohelpensurethatthetooldetectshighandlowspotsinthesurface.Experiencedfinisherscanusethestraightedgetoremovenoticeablebumpsbyemployingascrapingmotion.Otherwise,theyusealong-handledfloattosmoothbumpsanddisturbedplacesinthesurface.

Headers(transverseconstructionjoints)areoneofthemostconsistentcontributorstotheroughnessofconcretepavement.Thisisbecauseheadersoccurattheendofadayofworkorataninterruptionforabridge,intersection,orleave-out.Thepavingmachinemuststopattheselocations,andthemostcommonpracticeistoplaceawoodenformtocreatethejoint.Theformingofaheaderinthismannerincreasesthechanceofabumpinthesurfaceduetothehandworknecessarytoblendthemechanizedpavingsurfacewiththehand-placedarea.

Handformingheaderscanbeavoidedbyusingacutbackmethodtocreatethejoint.Thepaveropera-torcontinuespavinguntilalloftheconcreteisused.Thefollowingmorning,atransversesawcutismadeabout1.6m(5ft)fromtheendofthehardenedcon-creteslab.Theendmaterialisremovedanddowelsaregroutedintoholesdrilledintothesmoothsawface.Thismethodofheaderconstructionislesslaborintensiveandproducesasmootherridingconstruc-tionjointthanisgenerallyachievedusingtheformandhandfinishingtechnique.

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Texturing and Smoothness

Key Points

Thetextureofaconcretesurfaceaffectsskidresistanceandnoise.

Textureisusuallyappliedtothepavementsurfacewhileitisplastic.

Itisimportanttoapplytextureuniformly.

Pavementconditionisoftenjudgedbyitssmoothness.Followgoodconstructionpractices.

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Concretepavementsurfacesaregenerallytexturedtoprovideadequatefrictionandskidresistance.Thetexturealsoaffectstire-pavementnoise.

Itisgenerallypreferabletoconducttexturingsoonerratherthanlater,sothatcuringcanbeappliedearly.

Texturing OptionsMosttexturingtechniquesinvolvedraggingatex-

turingtoolacrossplasticconcrete(table8-4).Workisongoingtofindtheoptimummeansofachievingsatisfactoryskidresistancewhilereducingnoiseeffects(Rasmussenetal.2004)(seeFunctionalPerformanceinchapter2,page13).

Table 8-4. Description of Various Concrete Pavement Texture Options

Texture for fresh concrete Description

Burlap drag Produced by dragging moistened coarse burlap from a device that allows control of the time and rate of texturing – usually a construction bridge that spans the pavement. Produces 1.5 to 3 mm (1/16 to 1/8 in.) deep striations.

Artificial turf drag Produced by dragging an inverted section of artificial turf from a device that allows control of the time and rate of texturing – usually a construction bridge that spans the pavement. Produces 1.5 to 3 mm (1/16 to 1/8 in.) deep striations when using turf with 77,500 blades/m2 (7,200 blades/ft2).

Transverse broom Obtained using either a hand broom or mechanical broom device that lightly drags the stiff bristles across the surface. Produces 1.5 to 3 mm (1/16 to 1/8 in.) deep striations.

Longitudinal broom Achieved in similar manner as transverse broom, except that broom is pulled in a line parallel to the pavement centerline.

Random transverse Achieved by a mechanical device equipped with a tining head (metal rake) thattining* moves across the width of the paving surface laterally or on a skew. (A hand tool is(perpendicular or skewed) sufficient on smaller areas.)

Longitudinal tining* Achieved in similar manner as transverse tining, except the tines are pulled in a line parallel to the pavement centerline.

Exposed aggregate Occasional European practice of applying a set retarder to the new concrete pavement, covering with plastic sheeting, and then washing or brushing away mortar at a later age to expose durable aggregates. Other techniques involve the uniform application of aggregates to the fresh concrete, and mechanically abrading the mortar at a later age.

Diamond grinding Longitudinal, corduroy-like texture made by equipment using diamond saw blades gang-mounted on a cutting head. The cutting head produces 164 to167 grooves/m (50 to 60 grooves/ft) and can remove 3 to 20 mm (1/8 to 3/4 in.) from the pavement surface.

* For best results, most agencies precede with a burlap or artificial drag texture.



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Drag TexturesDraggingartificialturformoistened,coarseburlap

acrossthesurfaceofplasticconcretecreatesashallowsurfacetexture.

Longitudinal TiningLongitudinallytinedtexturesarecreatedbymoving

atiningdevice(commonlyametalrakecontrolledbyhandorattachedtoamechanicaldevice)acrosstheplasticconcretesurfaceparalleltothecenterline.

Transverse TiningTransverselytinedtexturesarecreatedbymoving

atiningdeviceacrossthewidthoftheplasticpave-mentsurface.Thetinescanbeuniformlyorrandomlyspaced,orskewedatanangle.

Iftransversetiningisused,itisrecommendedthataspacingof12.5mm(0.5in.)beused.

Diamond GrindingDiamondgrindingisaprocessofremovingathin

layerofhardenedconcretepavementusingcloselyspaceddiamondsawblades.

Innovative TechniquesExposedaggregatepavementsandperviouspave-

mentsarebeinginvestigatedfortheirfrictionandnoiselevels.

Texture VariabilityWhichevertechniqueisused,itisimportantto

applythetextureasuniformlyaspossibletoproduceuniformfrictionandnoiselevels.Thefollowingfac-torsinfluencevariabilityintextures:

Consistencyofconcreteproperties(workability).Timeoftexturingrelatedtoconcreteplacement.Presenceofbleedwateronthefreshconcretesurface.Totalpressureandvariabilityofpressureonthetexturingtools.

•••

• Figure 8-14. Artificial turf drag texturing rig, weighted with sand (ACPA)

Evennessofthetoolsonthesurface.Angleoftinestothesurface.Cleanlinessofburlap,turf,ortines.

Thetexturedepthprimarilydependsonboththetimeatwhichthetexturingtoolisapplied(relativetowhentheconcretewasplacedandfinished)andtheamountofpressureappliedtothetexturingtool.Dragtextures,suchasburlapdragandartificialturfdrag,canbeweighteddowntoimprovetexturedepthbypilingsandorrockonthematerial(figure8-14).

Itisimportanttodeterminetheoptimumtimetobegintexturingandtheamountofpressurerequiredtoachievethedesireddepth,andthentoconsistentlyapplythetextureatthattimeandpressure.

Pavement SmoothnessPavementsmoothnessistheusers’primarymeasure

ofapavement’scondition,anditisthereforeaveryimportantaspectintermsofquality.Severalimpor-tantspecificationandconstructionfactorsinfluencethesmoothnessofapavement.Tables8-5and8-6describethesefactors(ACPA2003b).

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Table 8-6. Construction Factors that Influence Pavement Smoothness

Table 8-5. Specification Factors that Influence Pavement Smoothness

Design / specification factor Comment

Base/subbase and trackline Needs to be stable and built to a tolerance; additional subbase width should be in pay item.

Horizontal alignment, cross-slope, Curves and cross-slopes can add to roughness by design.and super-elevated curves

Grade and staking calculations Accuracy of surveying and staking is essential.

Embedded reinforcement and Manholes, valve access covers, rebar, etc., have potential to affect smoothness.fixtures

Concrete mixture Proper proportioning and gradation is key to having a mix that will slipform easily.

Access to businesses and local Block-outs or leave-outs will add to roughness; minimize or eliminate if at all residences possible.

Construction factor Comment

Preparing the grade Pay attention to the smoothness of each layer of material under the pavement.

Producing consistent concrete Batch-to-batch consistency is key to producing a smooth pavement.

Delivering concrete Keeping the paver moving is essential to minimize bumps; hauling is often the critical step in consistent paving.

Setting up fixed forms Top of form determines surface of the pavement.

Setting and maintaining the stringline Check often; use night-smoothing techniques; ensure that it is taut.

Operating the paving machine Keep proper amount of concrete (head) consistently in front of paver; minimize stops; monitor vibrators.

Paving on vertical grades and curves Adjust the paver’s profile pan; adjust staking interval.

Handling dowel bars and reinforcement Place concrete on dowel assemblies before paver; turn down vibrators.

Finishing the surface and headers Check surface (cut bumps) behind paver with longest straightedge possible; use cut-back header if formed headers cause bumps.

Educating and motivating the crew Training is essential; explain factors involved and consequences of actions; implement an employee bonus system for smoothness incentives.

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Curing

Key Points

Curingistheactiontakentomaintainmoistureandtemperatureconditionsinafreshlymixedconcretetoallowhydrationandpozzolanicreactionstoproceed.

Curingprimarilyaffectsthequalityofthesurfaceofapavement,thezonethatisimpactedmostbytheenvironmentandloadingconditions.

Curingcompoundsprovidethemostefficientmeansofprovidingcuringforpavementconcrete.

Curingactivitiesincludecontrollingthetemperatureoftheconcreteduringextremeweather.

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Curingistheactiontakentomaintainmoistureandtemperatureconditionsinafreshlymixedcon-cretetoallowhydrationandpozzolanicreactionstoproceed.Internaltemperatureandmoisturedirectlyinfluencebothearlyandultimateconcreteproper-ties(Kosmatka,Kerkhoff,andPanarese2002).Propercuringmeasurespreventrapidwaterlossfromthemixtureandallowmorethoroughcementhydration.Itisessentialtoapplycuringmeasuresasearlyaspos-sibleafterplacingconcreteandtocontinuethemuntilenoughhydrationhastakenplacethattherequiredhardenedpropertieshavebeenachieved.

Avarietyofcuringmethodsandmaterialsareavail-ableforconcretepavement,includingthefollowing:watersprayorfog,wetburlapsheets,plasticsheets,insulatingblankets,andliquidmembrane-formingcompounds.Themostcommonmethodofcuringistheapplicationofacuringcompound.

Curing CompoundCuringcompoundsareorganicmaterialsthatform

askinoverthesurfaceoftheconcreteandreducethe

rateofmoisturelossfromtheconcrete(seeCuringCompoundsinchapter3,page64).

Note:Ondry,windydays,orduringperiodswhenadverseweatherconditionscouldresultinplasticshrinkagecracking,anapplicationofevaporationretardersimmediatelyafterfinalfinishingandbeforeallfreewateronthesurfacehasevaporatedwillhelppreventtheformationofcracks.Evaporationretardersshouldnotbeconfusedwithcuringcompounds.

Themostcommoncuringmethodforconcretepavementsistheapplicationofaliquidmembrane-formingcompoundtotheconcretesurface.Thismateriallimitswaterevaporationtoabout20percentofunprotectedconcretewhenproperlyapplied.Aliquidmembrane-formingcompoundthatmeetsASTMC309/AASHTOM148materialrequirementsisadequateformostsituationswhenappliedatthefollowingrates(ACPA1994):

5.0m²/L(200ft²/gal)fornormalpavingapplications.3.75m²/L(150ft²/gal)forfast-trackconcrete.2.5m²/L(100ft²/gal)forthinoverlays.

Ifthecuringregimenisinadequateorappliedtoolate,theconcretewillbesusceptibletoplasticshrink-agecracking,excessivecurling,andscaling.

Therefore,forcuringcompoundtobeofbenefititshouldbeappliedassoonaspossibleafterthewatersheenhasleftthesurfaceandtexturingiscomplete.Theconcretesurfaceshouldbedampwhenthecom-poundisapplied.

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Figure 8-15. A curing machine coats both the top surface and sides of a slipform paving slab. (ACPA)

Curing


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Theinitialapplicationofcuringcompoundshould

coatboththetopandedgesofslipformedconcrete

(figure8-15).Forfixed-formpaving,thecuring

compoundshouldinitiallycoattheexposedconcrete

surface.Ifremovingformsearly,asecondcoatis

necessarytoanyexposedverticaledgesoftheslabin

ordertoprovideacompleteseal.Timelyapplicationis

important;curingcompoundshouldbeappliedasthe

watersheenisdisappearingonthesurface.

Curingcompoundsshouldbeappliedbyhand-

operatedorpower-drivensprayequipmentimme-

diatelyafterfinalfinishingoftheconcrete.

Hand-operatedequipmentshouldbeusedonlyfor

smallareas.

Power-drivensprayequipmentisrecommended

foruniformapplicationofcuringcompoundsonlarge

pavingprojects.Spraynozzlesandwindshieldson

suchequipmentshouldbearrangedtopreventwind-

blownlossofcuringcompound.

Completecoverageofthesurfacemustbeattained

becauseevensmallpinholesinthemembranewill

increasetheevaporationofmoisturefromtheconcrete.

Normally,onlyonesmooth,evencoatisapplied.If

twocoatsarenecessarytoensurecompletecoverage,

thesecondcoatshouldbeappliedatrightanglesto

thefirstassoonasthefirstcoatbecomestacky.

Curingcompoundsmightpreventbonding

betweenhardenedconcreteandafreshlyplaced

concreteoverlay.Consequently,theyshouldeither

betestedforcompatibilityornotusedwhena

bondedoverlayisused.

Whitepigmentationinthecompoundispreferable

toaclearcompoundbecausetheamountofcoverage

iseasytosee.Thepigmentalsoreflectssolarradiation

thatmayotherwiseheattheconcretesurfaceexcessively.

Recommendationsforcuringcompoundapplica-

tionincludethefollowing(IPRF2003):

Applyliquidcuringcompoundsusingspray

equipmentmountedonaself-propelledframe

thatspansthepavinglane.

Limithand-heldsprayersforcuringapplication

onsmallareas.

Eventhoughavisualcheckisfeasiblewith

white-pigmentedcuringcompound,measure

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thevolumeonagivenareaandcompareittothespecifiedorrecommendedapplicationrate.Applycuringcompoundtoallexposedfacesoftheconcreteafterslipformingorafterformsareremoved.Whenmoistcuring,maintainthemoistcondi-tionovertheentireconcretesurfacefortheentirecuringperiod(typicallysevendays)oruntilacuringcompoundisapplied.

Acuringcompoundisnotthesameasanevapo-rationretarder,whichisawater-based,spray-onliquidthatformsamono-molecularfilmovertheplasticconcretesurface.Itisintendedtobeappliedimmediatelyafterstrike-offand/orbetweenfinish-ingoperationstoreducetheriskofplasticshrinkagecracking.Evaporationretarderswillnotretardthesettingcharacteristicsoftheconcrete,buttheywillminimizetheamountofwaterlossintheconcreteduetoevaporation.Theyareusefulinverydryorwindyconditions,whenevaporationratesarehigh.Acuringcompound(orothercuringmethod)muststillbeappliedsubsequenttofinalfinishingortexturing.

Evaporationretardersshouldnotbeusedasfinish-ingaidsbecause,astheyareworkedintothesurfaceoftheconcrete,theywillelevatethewater-cementratioatthesurface.Thismayleadtolowerstrengthsatthesurface,pooraircontentorair-voidstructure,andnondurablesurfacemortar.

Other Curing MethodsPlasticsheetingcanbeusedforcuring,usuallyas

asupplementtocuringcompound,tofacilitatecoldweatherplacements,ortoprotectthefreshlyplacedslabfromrain(seeWeatherConsiderationsonthenextpage).Plasticsheetingisalsousefultosustaintheheatofhydrationandincreasethestrengthgainbecauseitmaintainsalayerofairabovethesurface,whichactsasaninsulator.

Curingblanketscanalsobeusedforcuring.Con-sidercarefullybeforeremovingcuringsheetingorblanketsfromarecentlyplacedconcretepavement.Iftheconcreteisstillwarmwhentheblanketsareremoved,andtheambienttemperatureislow,thermalshockcanoccur,whichmaycausecracking(Huangetal.2001).

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Weather Considerations

Key Points

Preparationsforhot-orcold-weatherpaving,aswellasforarainevent,shouldbemadewellinadvance.

Duringhot-weatherpavingconditions,itiscriticaltoreducetheevaporationratefromconcretetominimizeplasticshrinkagecracking.

Duringcold-weatherpavingconditions,primaryconcernsaretokeeptheconcretetemperatureabovefreezingsothathydrationcontinuesandtocontrolcrackingthroughjointplacement.

Duringarainevent,itisimportanttocoverandprotectthenewconcretesurfaceaswellastoprepareforapossiblecoldfrontfollowingtherain.Asudden,significantdropintemperaturecanincreasetheriskofuncontrolledcracking.

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Hot-Weather ConcretingTheAmericanConcreteInstitutecategorizeshot

weatherasaperiodwhen,formorethanthreeconsec-utivedays,thefollowingconditionsexist(ACI1999):

Theaveragedailyairtemperatureisgreaterthan25°C(77°F).Theaveragedailytemperatureisthemeanofthehighestandthelowesttempera-turesoccurringduringtheperiodfrommidnighttomidnight.Theairtemperatureformorethanone-halfofany24-hourperiodisnotlessthan30°C(86°F).

Preparationforhot-weatherpavingshouldtakeplacelongbeforepavingbegins.

Whenevertheconstructionteamanticipatesbuildingaprojectinthesummer,theyshouldverifytheconcretemixturefortheseconditions.Verifica-tiontestingisconductedinthelaboratoryduringthemixdesignphase.Thetestinglaboratorymixes

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trialbatchesandcastsspecimensattemperaturesrepresentativeofthesiteconditionstoflagwhethercompatibilityproblemsmayarise.

Duringhotweather,problemsthatmayoccurincludethefollowing:

Rapidslumploss.Reducedaircontents.Prematurestiffening.Plasticshrinkagecracking.Thermalcracking.

Duringhotweather,theconstructionteamshouldtakestepstoreducetheevaporationratefromtheconcrete.Thelikelihoodofplasticshrinkagecrackingincreaseswhentheevaporationrateincreases.Plasticshrinkagecrackingresultsfromthelossofmoisturefromtheconcretebeforeinitialset.Theevaporationrateisafunctionofthefollowing:

Airtemperature.Concretetemperature.Relativehumidity.Windspeed.

Iftheevaporationrateexceeds1.0kg/m2/hr(0.2lb/ft2/hr),itisadvisabletoprovideamoreeffectivecuringapplication,suchasfogspraying,ortoapplyanapprovedevaporationreducer.

Oneormoreofthefollowingprecautionscanminimizetheoccurrenceofplasticshrinkagecracking(Menzel1954,aspublishedinKosmatka,Kerkhoff,andPanarese2002).Theyshouldbeconsideredwhileplanningforhot-weatherconcreteconstructionorwhiledealingwiththeproblemafterconstructionhasstarted.Theprecautionsarelistedintheorderinwhichtheyshouldbedoneduringconstruction:

Moistendry,absorptiveaggregates.Keeptheconcretetemperaturelowbycoolingaggregatesandmixingwater.Dampenthesubgradeandfogformsbeforeplacingtheconcrete.Erecttemporarywindbreakstoreducewindvelocityovertheconcretesurface.Erecttemporarysunshadestoreduceconcretesurfacetemperatures.

Ifconditionsoftemperature,relativehumidity,andwindaretoosevere(figure8-16)topreventplasticshrinkagecracking,orifcorrectivemeasuresarenot

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effective,pavingoperationsshouldbestoppeduntilweatherconditionsimprove(IPRF2003).

Thefollowingaregeneralrecommendations/options/considerationsforhot-weatherconcreting(IPRF2003):

Donotexceedthemaximumallowablewater/cementitiousmaterialsratioorthemanufac-turer’smaximumrecommendeddosageofanyadmixture.Considerretardingadmixturesiftheirperfor-mancehasbeenverifiedduringtrialbatches.Substituteground,granulatedblast-furnaceslagorclassFflyashforpartoftheportlandce-ment.Thesematerialshydratemoreslowlyandgeneratelowerheatsofhydrationthancement,

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_____________________________________________________________________________________________________Figure 8-16. A monograph to estimate the rate of evaporation (PCA)

reducingtendenciestowardslumploss,prema-turestiffening,andthermalcracking.CertainclassCflyashes,withhighcalciumandalumi-numcontents,maycauseprematurestiffening.Lowaircontentscanbecorrectedbyincreasingthedosageofair-entrainingadmixture.Bet-terorlongermixingmayallowmaintenanceofaconstantair-voidspacingfactorwithoutagreateraircontent.Usingadditionalwaterreducermayalsobehelpful.Riskofearly-agethermalcrackingisreducedbyensuringthatthetemperatureoftheplasticconcreteisaslowaspractical.

Sprinklingwithwatermaycoolaggregates;besuretocorrectfortheaggregatemoisture.

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Aggregatesneedtobebatchedinasaturatedsurface-dryconditiontoavoidabsorbingmixturewater.Chillingthemixingwateroraddingchippediceinsubstitutionforsomeofthewaterlowersthemixtemperature.Besurethatalloftheicemeltsduringmixing.

Considerpaintingthemixingandtransport-ingequipmentwhiteoranotherlightcolortominimizetheheatabsorbedfromthesun.Inextremeconditions,considerschedulingcon-creteplacementsforduringtheeveningornight.Moistenthebasebeforetheconcreteisplacedtokeepthetemperaturedownandtokeepitfromabsorbingwaterfromtheconcrete.Placeandfinishtheconcreteasrapidlyaspossibletoapplythecuringcompoundattheearliestpossibletime.Theuseofawhitecuringcompoundwillreflectthesun’sheat.Ifthereisanydelayinapplyingthecuringcompound,useafogsprayorevaporationretardanttokeepthesurfacefromdryingout.RefertoACI305,HotWeatherConcreting,foradditionalinformation.

Cold-Weather PlacementColdweatherisdefinedbyACIasaperiodwhen,

formorethanthreeconsecutivedays,thefollowingconditionsexist(ACI1988):

Theaveragedailyairtemperatureislessthan4°C(40°F).Theaveragedailytemperatureisthemeanofthehighestandlowesttempera-turesoccurringduringtheperiodfrommid-nighttomidnight.Theairtemperatureisnotgreaterthan10°C(50°F)formorethanone-halfofany24-hourperiod.

Cold-weatherpavingrequiresspecialconsider-ations.Thecontractorandmaterialsuppliershouldaddresstheseconsiderationswellbeforetemperatureforecastspredicttemperaturestodropclosetoorbelowfreezing.Theprimaryconcernistokeepthetemperatureoftheconcreteabovefreezingsothatthehydrationreactioncontinuesandtocontrolcrackingthroughjointplacement.

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Trialbatchesareneededtoverifythattheproposedmixtureswillachievethedesiredstrengthatthepotentialtemperatures.Mixtureswithacceleratingadmixturesmustbetreatedcarefullytoensurethattheyacceleratethesettingand/orearlystrengthgainofconcretebutdonotleadtoworkabilityorconstruc-tabilitychallenges.

Thefollowingarerecommendations/options/con-siderationsforcold-weatherconcreting(IPRF2003):

Considerusingahigherportlandcementcon-tentinconcretemixturedesignsforplacementatcoolertemperatures.Reduceoreliminateground,granulatedblast-furnaceslag,flyash,andnaturalpozzolansfromthemixtureunlesstheyarerequiredfordurability.Thenecessaryair-entrainingadmixturedos-agewilllikelybelowerforcold-weathercon-cretethanforconcretedesignedfornormaltemperatures.AnacceleratingadmixtureconformingtoASTMC494TypeCorEmaybeused,provideditsperformancehasbeenpreviouslyverifiedbytrialbatches.Donotuseadmixturescontainingaddedchlo-rides.Also,donotusecalciumchloride.Aggregatesmustbefreeofice,snow,andfrozenlumpsbeforebeingplacedinthemixer.Becausetheconcretewilltakelongertoset,thereismoreriskforplasticshrinkagecrack-ing,especiallyiftheconcreteismuchwarmerthantheambientairorthewindspeedissignificant.Considerheatingthemixwater(ifpracticalforthesizeofthepour).Thetemperatureofthemixedconcreteshouldnotbelessthan10°C(50°F).

Themixturewaterand/oraggregatesmaybeheatedto66°C(150°F).Thematerialmustbeheatedevenly.

Insulatingblanketsalsoarenecessaryforcuringconcretepavementincoolweather.Theblan-ketsreduceheatlossandlessentheinfluenceofbothairtemperatureandsolarradiationonthepavementtemperature.Theblanketsare

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notasubstituteforcuringcompound,whichisstillneededtocontainmoistureforcompletehydration.Theconcretetemperatureshouldbemain-tainedat10°C(50°F)oraboveforatleast72hoursafterplacementandatatemperatureabovefreezingfortheremainderofthecuringtime(whentheconcreteattainsacompressivestrengthof20MPa[3,000lb/in2]).Cornersandedgesarethemostvulnerabletofreezing.Concreteshouldnotbeplacedwhenthetem-peraturesoftheairatthesiteorthesurfacesonwhichtheconcreteistobeplacedarelessthan4°C(40°F).Concreteplacedincoldweathergainsstrengthslowly.Concretecontainingsupplementaryce-mentitiousmaterialsgainsstrengthveryslowly.

Sawingofjointsandopeningtotrafficmaybedelayed.Verifythein-placestrengthbyamaturitymethod,temperature-matchedcuring,nondestructivetesting,ortestsofcoresfromthepavementbeforeopeningthepavementtotraffic.

Allowtheslabstocoolbeforecompletelyremov-inginsulatingblanketstoavoidathermalshocktothepavementthatmightinducecontractioncracking.Insulatingblanketsmaybetemporarilyrolledasidetosawcontractionjoints.RefertoACI306,ColdWeatherConcreting,foradditionalinformation.

Protection From RainPlasticsheeting(figure8-17)andsteelsideformsor

woodenboardsmustbeavailableatalltimestoprotectthesurfaceandedgesofthenewlyplacedconcretepavementwhenitrains.Ifrainisexpectedonnewlyplacedconcretepavementthathasnothardened,coverthesurfacewiththeplasticsheeting.

Thesheetsmustbeweighteddowntopreventthemfromblowinginthewind.Whenitstartsraining,a“ruleofthumb”todeterminehowmuchofthepave-menttocoveristogobacktothepointwheretherainisnotindentingthepavementsurface.Thecoveringdoesnotneedtobeextendedtoareaswheretherain

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isonlywashingawaythecuringmembrane(ACPA2003a).

Climaticconditionsduringaraineventcanactu-allybeconducivetogoodconcretecuring.Duringrain,thehumidityisatornear100percentandthereislittlechancefortheevaporationofmixwater.Temperaturesaregenerallymoderateduringrain,whichisalsobeneficial.Inthesesituations,therainessentiallyprovidesabeneficialmoistcuringenviron-ment,whichassistswithstrengthdevelopmentanddecreasesthechanceforuncontrolledcracking.Thisprovidesanaturalcure.

Sometimes,particularlyiftheprevailingweatherishotandhumid,rainprecedesthepassageofacoldfront,whichmaydroptheairtemperaturemorethan11°C(20°F).Wherethisoccurs,andwhenthepave-mentisunderconstruction,theriskofuncontrolledcrackingwillincrease(ACPA2002).Thedropinthedewpointthatusuallyoccurswithacoldfrontmayalsoleadtoalowerrelativehumidityabovethewarmconcreteandthusagreatersusceptibilityforplasticshrinkagecracking.

Somemarringoftheconcretesurfacemayoccurfromtheplasticsheetingusedtoprotecttheslabsfromrain.Exceptforanundesirableappearance,thereisnothingwrongwithsurfacesaffectedbyplasticsheeting.Asimilarappearancecanoccurwhenusingplasticsheetingtocureconcrete.

Figure 8-17. Plastic sheeting ready for placement to protect the fresh surface from rain (ACPA)



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Additionalwateronthepavementsurfacewillele-vatethesurfacewater-cementitiousmaterials(w/cm)ratio,potentiallyreducingdurability.Donotfinishrainwaterintotheconcretesurface.Thiselevatesthew/cmratio,creatinganondurabletopsurface,suscep-

Figure 8-18. Typical scaling of concrete pavement due to rain on nondurable paste surface (ACPA 2003a)

Figure 8-19. Edge erosion of freshly placed slab due to rain (ACPA 2003a)

tibletocrazing,scaling,anddusting(figure8-18).Forslipformpavingoperations,itisadvantageous

toinstallsideformswheresevereerosionofthefreshconcreteedgemayoccur(figure8-19)becauseofheavyrain.

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Crack Prediction with HIPERPAV

Key Points

Anumberofvariablesinfluencetheriskofcrackinginconcrete:

Temperature.

Rateofdrying.

Shrinkage.

Restraint.

Strength.

Modulusofelasticity.

Coefficientofthermalexpansion(CTE).

Acomputerprogram,HIPERPAV,isavailabletomodeltheriskofcrackingforagivenmixunderagivenenvironment.

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Manyvariablescaninfluencethepropensityofaspecificconcretetocrack(seeEarly-AgeCrack-inginchapter5,page148).However,itispossibletonumericallymodelthedominantmechanismsoffailuretoassesstheriskofcrackingwithinagivensetofconditions.

SuchamodelhasbeendevelopedforpavementsbytheFederalHighwayAdministration(FHWA1999).Theresultingsoftwarepackage,knownasHIPERPAV,isbrieflydescribedbelow.Formoreinformation,seewww.fhwa.dot.gov/pavement/pccp/hipemain.cfm.

HIPERPAVisacommerciallyavailablecomputersoftwarepackage.Itisintendedtobeusedbyperson-nelinvolvedinconstructingconcretepavementsandbondedoverlays.Itspurposeistomodelandpredictwhethertheconcreteislikelytocrackatanearlyageduetoconditionsunrelatedtostructuralslabloading.

Theprogramisalsousedtoestimatethewindowofopportunityforsawcutting.

Theprogramconsidersthefollowingfactors:Concretepavementtemperature.•

Concretecoefficientofthermalexpansion(CTE).Dryingshrinkage.Slab-sub-baserestraint.Modulusofelasticity.Strength.

Alloftheseperformanceparametersarecalculatedbasedoncomplexnumericalmodelsthatusethefol-lowingfactorsasinputparameters:

Pavementdesigninputs:Subbasetype.Slab-basefriction(bond).Transversejointspacing.Tensilestrength.Modulusofelasticity.Slabthickness.Mixdesigninputs.Concretestrengthdevelopment.Portlandcementtype.Cementchemistry.Cementcontent.Silicafumecontent.Flyash(typeFandC)content.GGBFslagcontent.Watercontent.Coarseaggregatetypeandcontent.Fineaggregatecontent.Waterreducercontent.Superwaterreducer.Retardercontent.Acceleratorcontent.

Environmentalinputs:Airtemperature.Relativehumiditydistribution.Cloudconditions.Moistureconditions.Windspeed.

Constructioninputs:Curingmethod.Timeofcuringapplication.Timeofcuringremoval.Timeofconstruction.Timeofsawcutting.Initialportlandcementconcrete.Mixtemperature.Initialsubbasetemperature.

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www.fhwa.dot.gov/pavement/pccp/hipemain.cfm

Crack Prediction with HIPERPAV


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TheoutputoftheHIPERPAVprogramisaplotofallowablestress(developedstrengthanticipated)intheconcreteandthepredictedstressoveraperiodof72hours(figure8-20).Ifthepredictedstressexceedstheallowablestress,crackingisindicatedaslikelyandchangesneedtobemadetotheconcretemixorconstructionpractices,and/orprotectionmustbeprovidedfromtheenvironment.

TheHIPERPAVpackagehasbeenvalidatedforarangeofdesigns,materials,andclimaticconditionsatseveralsitesintheUnitedStates.Pavementsunderconstructionwereinstrumentedandthedatawerecomparedwiththepredictionsmadebytheprogram.Thesystemwasfoundtopredictcrackformationwithanaccuracyofapproximatelyfivehours.

________________________________________________Figure 8-20. An example plot reported by HIPERPAV show-ing a high risk of cracking at about six hours after paving

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Joint Sawing

Key Points

Jointsarerequiredtocontrolcrackinginaconcretepavement.

Jointscanbesawcutaftertheconcretehasbeenplaced.

Itiscriticaltocutthejointsattherighttime:tooearlywillresultinraveling,toolatewillbeafterrandomcrackingoccurs.

Thetimingofsaw-cuttingisdependantonconcretepropertiesandtheenvironment.

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Likeallmaterials,concreteexpandsandcontractswithvariationsintemperatureandmoisturecontent,whichcancausecrackingifitisrestrained.Cuttingthepavementintosmallerelementshelpsrelievetherestraintandgenerallyensuresthatthecracksformwheredesiredratherthanatrandom(seeEarly-AgeCrackinginchapter5,page148).

Concreteslabscrackwhentensilestresseswithintheconcreteovercomethetensilestrength.Atearlyages,thetensilestressesdevelopfromrestraintoftheconcrete’svolumechangeorrestraintofslabbendingfromtemperatureandmoisturegradientsthroughtheconcrete(Okamotoetal.1994).Earlyvolumechangesareassociatedwiththeconcrete’sdryingshrinkageandtemperaturecontraction.Eachtransverseandlon-gitudinalsawcutinducesapointofweaknesswhereacrackwillinitiateandthenpropagatetothebottomoftheslab.

Inmostcases,cracksfirstappearatlargeintervals,10to45m(30to150ft),andthenformatcloserintervalsovertime.Intermediatesawedjoints,nor-mallyrequiredtocontrolcrackingfromdifferentials,sometimesdonotcrackforseveralweekstomonthsafteropeningthepavementtotraffic.However,thismaynotbetrueoneverypavement,anditmaybeverydifficulttodeterminewhetherrestrainttovolumechangesorrestrainttotemperatureormoisturegradi-entscausethefirstcracks.

Afundamentalofjointedconcretepavementdesignistointroduceajointingsystemtocontroltheloca-tionofthisexpectedcracking.Ofthethreejointtypes,contraction,construction,andisolation,contractionjointsarespecificallyforcrackcontrol.Forunrein-forcedconcretepavement,jointspacingorslablengthdependsuponslabthickness,concreteaggregate,subbase,andclimate(ACPA1991).Inmostareas,thetypicalmaximumtransversejointspacingforunrein-forced(plain)pavementisabout4.5m(15ft)(ACPA2004b).Longitudinaljointsaretypicallyabout3.0to4.2m(10to13ft)apartandservethedualpurposeofcrackcontrolandlanedelineation.

Theclimateandconcretecoarseaggregatecommontosomegeographicregionsmayallowtransversejointstobefurtherapartorrequirethemtobeclosertogetherthantheaverage.Forexample,concretemadefromgraniteandlimestonecoarseaggregateismuchlesssensitivetotemperaturechangethanconcretemadefromsiliceousgravel,chert,orslagaggregate.Alesstemperature-sensitiveconcretedoesnotexpandorcontractmuchwithtemperaturechange,whichallowsalongerspacingbetweenpave-mentcontractionjointswithoutagreaterchanceofrandomcracking.

Variouskindsofsawscanbeusedforcuttingjoints(figure8-21).

Saw TimingThereisanoptimumtimetosawcontractionjoints

innewconcretepavements.Thattimeoccurswithinthesawingwindow(figure8-22)(Okamotoetal.1994).Thewindowisashortperiodafterplacementwhentheconcretepavementcanbecuttosuc-cessfullycontrolcrackformation(seetheStagesofHydrationchartsandConcreteStrengthGain,TensileStress,andtheSawingWindowinchapter4,pages76and93,respectively).Thewindowbeginswhenconcretestrengthissufficientforsawingwithoutexcessiveravelingalongthecut.Thesawingwindowendswhenrandomcrackingstartstooccur.

Sawingtooearlycausesthesawbladetobreakorpullaggregateparticlesfreefromthepavementsur-facesalongthecut.Theresultingjagged,roughedgesaretermedraveling.Someravelingisacceptablewhere

Joint Sawing


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________________________________________________Figure 8-22. Sawing window (ACPA)

asecondsawcutwouldbemadeforajointsealant.Iftheravelingistoosevere,itwillaffecttheappearanceand/ortheabilitytomaintainthejoint.Figure8-23showsdifferentdegreesofraveling.

_______________________________________________Figure 8-21. Common sawing equipment. (a) For most proj-ects, transverse or longitudinal contraction joints are cut with single-blade, walk-behind saws. (b) For wider paving, contractors may elect to use span-saws that are able to saw transverse joints across the full pavement width in one pass. (c) A newer class of saw, the early-entry dry saw, is a walk-behind saw that allows sawing sooner than with conventional saws. (ACPA)

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________________________________________________Figure 8-23. Close-up of different degrees of raveling caused by joint sawing (ACPA)

a) No raveling—sawed later in the window

b) Moderate raveling—sawed early in the window

c) Unacceptable raveling—sawed too early

When Can You Begin Sawing Joints?

To determine when the concrete is ready to cut, use the Scratch Test (Saraf 1985). Scratch the concrete surface with a nail or knife blade. As the surface hardens, the scratch depth decreases. In general, if the scratch removes the surface texture, it is probably too early to saw.

Joint Sawing


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Onestudy(Okamotoetal.1994)foundthatravel-ingwaswithinacceptablelimitswhentheconcretecompressivestrengthswerefrom2.1to7.0MPa(300to1,015lb/in2),dependinguponthetypeofaggregateusedinthemixture.Refinementofaspecificstrengththresholdnumbertobeusedonaprojectwouldrequiretrialanderrortestingofthejobmateri-alsusingtheconcretematurityprinciple.

Thelengthofthesawingwindowdependsuponmanyfactorsandislikelytobedifferentforeachprojectandeachdayofconstruction.Certaindesignfeatures,materials,orweatherconditionscanconsid-erablyshortenthewindow(table8-7).Undermostweatherconditionsandfortypicalpavementdesigns,thewindowwillbelongenoughtocompletesawingwithexcellentresults.Inextremeconditions,thewindowcanbesoshortastobeimpracticableforcrackcontrol.Concretematuritytestinghelpsindicatetheinfluenceofambientconditionsontheconcretestrengthprofileandconsequentlyhelpsdefinethesawingwindow.OthertoolsincludetheFHWA’s

HIPERPAVsoftware,whichcanhelpdeterminesawtimingandpredictthepossibilityofuncontrolledcracking(McCulloughandRasmussen1999).

Early-entrysawsarenowfrequentlyusedonpavingprojects.Smallerandlighterthanconventionalsaws,theyhavetheadvantageofallowingsawingtobeginwithinanhourortwoofpaving.Thesawingwindowforearly-entrysawsbeginsearlierandendssoonerthanforconventionalsaws;seefigure4-13.

First-generationearly-entrysawsweredesignedtocutshallow(25mm[1in.)])joints.Atfinalset(thebeginningofthesawingwindowforearly-entrysaws),ashallowjointisenoughtocreateaplaneofweaknesswhereacrackwillformtorelievestressesasthepave-mentbuildsstrength.Early,shallowcutsworkwellfortransversejoints,regardlessofthethicknessofthepavement.

Longitudinaljoints,ontheotherhand,aretypicallycutlater(oftenwithinthefirstday).Bythistime,theconcretehasdevelopedmorestrength,andadeepercutisrequiredtoeffectivelycreateaplaneofweak-ness.Therefore,first-generationearly-entrysawswerenotnormallyusedforlongitudinaljoints.Today,bothearly-entryandconventionalsawscancutlongitudi-naljointstotherequiredone-thirddepthoftheslab.

Noteaboutjointwidth:Becausetransversejointsmove,theyaretypicallysealedtopreventtheinfil-trationofwaterandforeignmatter.Thesawcuts(whetherearly-entryorconventional)mustthereforebeatleast6mm(1/4-in.)widetoformadequateres-ervoirsforthesealant.Becauselongitudinaljointsaretiedandthusdesignednottomove,manystatesdonotsealthem.Thesawcutscanthereforebenarrow(3mm[1/8in.]).Withgranularbasesandsubdrains,anywaterenteringlongitudinaljointswilleffectivelydrainaway.

Mixture EffectsRegardlessofotherrelatedfactors,theconcrete

mixtureitselfisaprimaryfactorindefiningthepoten-tialtocontrolcrackingwithajointingsystem.

Theprimaryinfluencesofthemixturearethefollowing:

Pastecontent.Water-cementitiousmaterialsratio.

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Table 8-7. Factors that Shorten the Sawing Window

Category Factor

Concrete mixture High water demand Rapid early strength Retarded set Fine aggregate (fineness and

grading) Coarse aggregate (maximum size

and/or percentage)

Weather Sudden temperature drop or rain shower

Sudden temperature rise High winds and low humidity Cool temperatures and cloudy Hot temperatures and sunny

Subbase High friction between the subbase and concrete slab

Bond between the underlying subbase and concrete slab

Dry surface Porous aggregate subbase

materials

Miscellaneous Paving against or between existing lanes

Saw blade selection Delay in curing protection

Source: ACPA (2002)

Joint Sawing


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Typeofcementitiousmaterials.Aggregatetype.

Thevolumeofshrinkageisdirectlycontrolledbythetotalwatercontent,whichisafunctionofcementitiouscontentandw/cmratio.Increasedwatercontentincreasesshrinkage.Ontheotherhand,thegreaterthestrength,thebetterabletheconcreteistoresistshrinkageinducedstresses.

Mixturescontainingflyashesorground,granulatedblast-furnace(GGBF)slagorblendedcementmayexperienceadelayinearly-agestrengthdevelopment,especiallyincoolerweather.Thiscoulddelaythe

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concretesettimeandtheabilitytosawwithoutexcessiveraveling.Aftersetting,thetimeavailableforsawingbeforecrackingbeginsmaybeshorterthannormal.Thisdecreaseinavailablesawingtimeincreasestheriskofuncontrolledcrackingincoolerweather.Manyagenciesspecifyausageperiodforsuchmixtures,prohibitingtheiruseinearlyspringorlatefall.

Thecoarseaggregateinfluencesthetemperaturesensitivityoftheconcrete.Concretethatismoretem-peraturesensitivewillexpandorcontractmorewithtemperaturechange,increasingcrackingpotential.

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Joint Sealing

Key Points

Jointsarefilledwithsealanttopreventingressofdeleteriousmaterials.

Manyalternativesealantsystemsareavailable.

Sealantmaterialselectionconsiderationsincludetheenvironment,cost,performance,jointtype,andjointspacing.

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Thepurposeofsealingjointsistominimizeinfiltra-tionofsurfacewaterandincompressiblematerialintothepavementlayers(ACPA1993).Excesswatercancontributetosubgradeorbasesoftening,erosion,andpumpingofsubgradeorbasefinesovertime.Thisdegradationresultsinalossofstructuralsupport,pavementsettlement,and/orfaulting.

Therearemanyacceptablematerialsavailableforsealingjointsinconcretepavements.Sealantsareeitherplacedinaliquidformorarepreformedandinsertedintothejointreservoir.Sealantsinstalledinaliquidformdependonlong-termadhesiontothejointfaceforsuccessfulsealing.Preformedcompres-sionsealsdependonlateralreboundforlong-termsuccess.Formorespecificinformationonjointsealingmaterialsandrequiredshapefactorsandsizes,consultreferenceACPA1993.

Sealingpreventsincompressibleobjectsfromenter-ingjointreservoirs.Incompressiblescontributetospallingand,inextremecases,mayinduceblow-ups.Ineithercase,excessivepressurealongthejointfacesresultsasincompressiblesobstructpavementexpan-sioninhotweather.Yearsago,thetermjointfillersdescribedliquidasphaltmaterialsplacedinjoints.Jointfillersaidmoreinkeepingoutincompressiblesthanminimizingwaterinfiltration.

Sealantmaterialselectionconsiderationsincludethefollowing:

Environment.Cost.

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Performance.Jointtypeandspacing.

Thefollowinglistincludescommonfactorsforwhichtheconcretematerialorinstallationtechniqueaffectsthejointsealperformance:

Siliconesealantsareknowntohavepooradhe-siontoconcretecontainingdolomiticlime-stone.Aprimerapplicationtothesealantreser-voirwallswillensurethatthesiliconeadheres.Concretecontaininghardercoarseaggregates,suchasgravelorgranite,willexpandandcontractmorewithagiventemperaturechangethanaconcretecontaininglimestone.Theseal-antwillbestretchedfartherforagivenjointspacing.Tokeepthisundercontrol,useanap-propriateshapefactorforsealant(figure8-24).Hot-pouredasphalt-basedsealantstypicallyneedareservoirshapefactor(width/depthratio)ofone.Siliconeandtwo-componentcold-pouredsealantstypicallyneedareservoirshapefactorof0.5.Compressionsealantsareselectedsuchthatthemaximumcompressionofthesealis50percentandtheminimumis20percentthroughtheanticipatedambienttemperaturecyclesinthearea.

Chemicalsolventsusedtocleanthejointres-ervoirmaybedetrimental.Solventscancarrycontaminantsintoporesandsurfacevoidsonthereservoirfacesthatwillinhibitbondingofthenewsealant.Forcleaningjoints,theairstreammustbefreeofoil.Manymoderncompressorsautomaticallyinsertoilintotheairhosestolubricateair-pow-eredtools.Newhosesoranoilandmoisturetrappreventscontaminationofthejointfacefromoilinthecompressororairhoses.

Aboveall,themostcriticalaspectinsealantper-formanceisreservoirpreparation.Aconsiderableinvestmentinjointpreparationandcleaningactivi-tiesisnecessaryforalmostallsealanttypes.Thereislittledoubtthatpoorlyconstructedjointswillperformpoorly.Propercleaningrequiresmechanicalactionandpurewaterflushingtoremovecontaminants.The

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Joint Sealing


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followingoutlinestherecommendedprocedures(ACPA1993):

Immediatelyaftersawing,awaterwashremovestheslurryfromthesawingoperation.Contractorsperformthisoperationinonedirectiontominimizecontaminationofsurroundingareas.Afterthejointhassufficientlydried,asand-blastingoperationremovesanyremainingresidue.Donotallowsandblastingstraightintothejoint.Holdingthesandblastnozzleclosetothesurfaceatanangletocleanthetop25mm(1in.)ofthejointfaceprovidescleaningwhereneeded.Onepassalongeachreservoirfaceprovidesexcellentresults.Thisnotonlycleans

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thejointfaces,butprovidestexturetoenhancesealantadhesion.Anairblowingoperationremovessand,dirt,anddustfromthejointandpavementsurface.Conductingthisoperationjustpriortosealantpumpingensuresthatthematerialwillenteranextremelycleanreservoir.Thecontractormustprovideassurancethattheaircompres-sorfiltersmoistureandoilfromtheair.Thecompressorshoulddeliverairataminimum3.4m3/min(120ft3/min)anddevelopatleast0.63MPa(90lb/in2)nozzlepressure.Somecontractorsalsouseavacuumsweeperandhandbroomstokeepthesurroundingpave-mentclean.

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____________________________________________________________________________________________________Figure 8-24. Different forms of joint sealant (adapted from ACPA)

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AASHTOM148,LiquidMembrane-FormingCom-poundsforCuringConcrete

AASHTOM157,Ready-MixedConcrete


ASTMC94/C94M-04a,StandardSpecificationforReady-MixedConcrete

ASTMC309-03,StandardSpecificationforLiquidMembrane-FormingCompoundsforCuringConcrete

ASTMC494/C494M-04,StandardSpecificationforChemicalAdmixturesforConcrete

ACI.1999.HotWeatherConcreting.ACI305R-99.FarmingtonHills,MI:AmericanConcreteInstitute.

———.1988.ColdWeatherConcreting.ACI306-88.FarmingtonHills,MI:AmericanConcreteInstitute.

ACPA.1991.DesignandConstructionofJointsforConcreteHighways.TB010P.ArlingtonHeights,IL:AmericanConcretePavementAssociation.

———.1993.JointandCrackSealingandRepairforConcretePavements.TB012P.ArlingtonHeights,IL:AmericanConcretePavementAssociation.

———.1994.FastTrackConcretePavements.TB004P.Skokie,IL:AmericanConcretePavementAssociation.

———.2000.ConcretePavementSurfaceTextures.SR902P.Skokie,IL:AmericanConcretePavementAssociation.

———.2002.EarlyCrackingofConcretePavements,CausesandRepairs.TB016P.ArlingtonHeights,IL:AmericanConcretePavementAssociation.

———.2003a.HowtoHandleRained-onConcretePavement.Research&TechnologyUpdate4.04.Skokie,IL:AmericanConcretePavementAssociation.

———.2003b.ConstructingSmoothConcretePave-ments.TB006.02P.Skokie,IL:AmericanConcretePavementAssociation.

———.2004a.ClayBallPreventionandRepair:StockpileManagementisKey.Research&TechnologyUpdate5.04.Skokie,IL:AmericanConcretePave-mentAssociation.

———.2004b.DatabaseofStateDOTConcretePavementPractices.Skokie,IL:AmericanConcretePavementAssociation.http://www.pavement.com/PavTech/Tech/StPract/Query.asp.

Ayers,M.,J.McDaniel,andS.Rao.2000.Con-structionofPortlandCementConcretePavementsParticipant’sWorkbook.FHWAHI-02-018.Wash-ington,D.C.:NationalHighwayInstitute,FederalHighwayAdministration.

Huang,H.,C.A.Beckemeyer,W.S.Kennedy,andL.Khazanovich.2001.FiniteElementModelingofCrackinginFast-TrackFullDepthPCCRepairs.Papersubmittedfor80thAnnualMeetingoftheTransporta-tionResearchBoard,Washington,D.C.

IPRF.2003.Evaluation,DesignandConstructionTechniquesfortheUseofAirfieldConcretePave-mentasRecycledMaterialforSubbase.Project03-5.Skokie,IL:InnovativePavementResearchFoundation.

Okamoto,P.,R.Rollings,R.Detwiler,R.Perera,E.Baren-berg,J.Anderson,M.Torres,H.Barzegar,M.Thompson,J.Naughton.2003.BestPracticesforAirportPortlandCementConcretePavementConstruction(RigidAirportPavement).IPRF-01-G-002-1.Washington,D.C.:Inno-vativePavementResearchFoundation.

Kosmatka,S.,B.Kerkhoff,andW.C.Panarese.2002.DesignandControlofConcreteMixtures.EB001.Skokie,IL:PortlandCementAssociation.

McCullough,B.F.andR.O.Rasmussen.1999.Fast-TrackPaving:ConcreteTemperatureControlandTrafficOpeningCriteriaforBondedConcreteOverlaysVolumeII:HIPERPAVUser’sManual.FHWA-RD-98-168.Washington,D.C.:FederalHighwayAdministration.

Okamoto,P.A,P.J.Naussbaum,K.D.Smith,M.I.Darter,T.P.Wilson,C.L.Wu,andS.D.Tayabji.1994.Guide-linesforTimingContractionJointSawingandEarliestLoadingforConcretePavements,Volume1.FHWA-RD-01-079.Washington,D.C.:U.S.DepartmentofTransportation,FederalHighwayAdministration.

Rasmussen,R.O.,Y.A.Resendez,G.K.Chang,andT.R.Ferragut.2004.ConcretePavementSolutionsForReducingTire-PavementNoise.FHWA/ISUCoop-erativeAgreementDTFH61-01-X-00042.Ames,IA:CenterforTransportationResearchandEducation,IowaStateUniversity.

Tymkowicz,S.andB.Steffes.1997.VibrationStudyforConsolidationofPortlandCementConcrete.InterimReportforMLR-95-4.Ames,IA:IowaDepart-mentofTransportation.

Wiegand,P.T.Ferragut,D.S.Harrington,R.O.Rasmussen,S.Karamihas,andT.Cackler.2006.EvaluationofU.S.andEuropeanConcretePavementNoiseReductionMethods.FHWA/ISUCooperativeAgreementDTFH61-01-X-00042(Project15).Ames,IA:CenterforTransportationResearchandEducation,IowaStateUniversity.

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Chapter 9Quality and TestingQuality Assurance/Quality Control Records 242

Batching 246

Construction Monitoring 247

Test Methods 250

Thischapterdiscussesqualityassuranceandqualitycontrolsystemsastheyinfluenceconcretepavement,withspecialemphasisonmaterials.Thechapteralsodescribesactionsthatareneededtomonitorandadjustforcriticalparameters,suchasthewater-cementitiousmaterials(w/cm)ratio,air-voidsystem,andtheriskofcracking.Finally,testscommonlyusedtomonitorthematerialsandconcretequality

aredescribed.Thesedescriptionsdonotrepresentallqualitycontroltestsdiscussedthroughoutthismanual.Rather,theyhavebeenidentifiedascurrentqualitycontrolbestpracticesinthe“MaterialsandConstructionOptimizationforPreventionofPrema-turePavementDistressinPortlandCementConcretePavements”projectatIowaStateUniversity(TPF-5[066]).

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Quality Assurance/Quality Control Records

Key Points

Qualityassurance(QA)isthesetofactivitiesconductedbytheownertobesurethattheproductdeliveredcomplieswiththespecification.

Qualitycontrol(QC)isthesetofactivitiesconductedbythecontractortomonitortheprocessofbatching,placing,andfinishingtheconcretetobesurethatthepavementwillmeetorexceedQAtestcriteria.

TestlaboratoriesmustbeappropriatelycertifiedbyAASHTO(orequivalent)andmeettherequirementsofASTMC1077.

QAandQCpersonnelshouldreceivepropertraining.Mostagenciesrequiretechnicianstobecertified(AmericanConcreteInstitute,NationalInstituteforCertificationinEngineeringTechnologies,etc.).

Manytestsperformedonconcretehavevaryinglevelsofprecision.OpencommunicationandcooperationbetweenQCandQAorganizationsisimportanttoavoidandresolveconflicts.

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Thissectiondefinesanddistinguishesbetweenqualityassuranceandqualitycontrol.

Quality AssuranceQualityassurance(QA)isthesetofactivities

conductedbytheowner/agencyoritsrepresentativessothattheycanconfirmthattheproductdeliveredcomplieswiththespecifications.Amoreformaldefinitionforqualityassuranceis“Allthoseplannedandsystematicactionsnecessarytoprovideadequateconfidencethataproductorservicewillsatisfygivenrequirementsforquality”(AASHTO1996).

FormostcontractsQAisassociatedwithaccep-

tancetestingbytheowner.Emphasisisnormallyplacedonstrength,thickness,smoothness,andaircontent,becauseitiseasytocomparethesetestresultstospecifiedvaluesanddeterminewhetherthecriteriahavebeensatisfied.Otherqualityfactorsthatareoftenoverlookedbecausetheyarenotaseasilymeasuredincludesteelplacementforjoints,air-voidstructure,permeability,andcuring.

QAtestingmayalsobeperformedbycontractorpersonnel,butthenindependenttestingisneededforverification.Thisisespeciallyimportantwhenstatis-ticalacceptanceprocedures,suchaspercentwithinlimits(PWL)specifications,areused.

Mostofthesespecificationsarebasedontheassumptionthatthepropertybeingtestedisnormallydistributed.Theaverageandstandarddeviation(vari-ability)oftestresultsarethenusedtoestimatethepercentofmaterialthatiswithinthespecifiedlimits.Sinceaportionoftheoverallstandarddeviationisduetotestingvariability,thecontractorshouldhavetherighttomaintaincontrolovertheacceptancetesting.

Acceptancemethodsthatincludecriteriathatareatleastpartiallybaseduponstandarddeviationshouldnotpenalizecontractorsiftheyarenotallowedtheopportunitytobeincontrolofallfactorsthatcon-tributetotheoverallvariability.Whetherexplicitlystatedornot,PWLspecificationsrequirecontractorstoreducevariability.Thiscanbeaccomplishedbyproducingmoreconsistentconcreteandattemptingtoreducetestingvariability.Reducingtestingvariabilitymaybeaseasyassimplyhavingthesametechnicianperformthesametestforallofthesamplesincludedinagivenlotofpavement.

Quality ControlIncontrasttoqualityassurance,qualitycontrol

(QC)isalwaysthecontractor’sresponsibility.Thisworkfocusesontheprocessofbatching,placing,andfinish-ingthepavementtobesurethatthepavementwillmeetorexceedQAtestcriteria.AASHTOhasdefinedqualitycontrolas“Thesumtotalofactivitiesperformedbythesellertomakesurethataproductmeetscontractspecificationrequirements”(AASHTO1996).

QualitycontrolisnotanexerciseinmirroringQAtests.Materialsandprocessesaremonitoredatall



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stagesofthebatchingandpavingprocesstopreemptproblemsorvariationsratherthansimplyreacttothem.AcomprehensiveQCprogramencompassesallaspectsoftheconcretepavingprocess:

Trainingofallcontractorpersonnel:Everypersonontheprojectcontributestoquality.Preliminarymaterialtesting:Testmaterialbeforeitisbatched.Equipmentandprocessmonitoring:Developchecklists,procedures,andresponsestopreventqualitydeficiencies.Testingofconcreteandindividualmaterialsduringtrialbatchingandproduction:Concen-trateontimelyandearlyresults.AdditionalsamplesmaybeusedasabackupforQAtests.AnalysisofQCtestresultsandprocessmoni-toring:Organizedataandidentifychangesintheprocess.AdjustprocessesinresponsetoQCanalysis:Proactivelyreacttoprocesschangesandinputsinatimelymanner.

AqualitycontrolplanisrequiredtoensurethatQCactivitiesarecarefullyplanned,sufficient,andconsis-tent.Theplanshouldprovideadetaileddescriptionofthetypeandfrequencyofinspection,sampling,andtestingtomeasurethevariouspropertiesdescribedinthespecification(AASHTO1996).

Preliminaryinformationfromthe“MaterialsandConstructionOptimizationforPreventionofPrematurePavementDistressinPortlandCementConcretePavements”projectatIowaStateUniversity(TPF-5[066])providesastartingpointforalterna-tiveandnewtestmethodsthatcanbeusedforQCpurposes.Thisresearchprojecthasidentifiedasuiteofteststhatcanbeconsideredtobethecurrentbest

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New QC Skills and Tools

Shifting the focus from conventional concrete testing to process control requires a different set of skills and tools. Statistical control charts can be utilized, along with new and/or alternative test methods to improve on traditional QC methods.

practices.Summarysheetsdescribingthewhyandhowofeachtestareprovidedlaterinthischapter.Thesesummariescanbeusedforguidanceindeter-miningwhichtestsmaybeusefulinthedesignofaQCprogramforconstructingdurableconcretepavements.

Quality of TestingAllpersonnelinvolvedwithQAandQCshould

receivepropertraining.LaboratoriesshouldbecertifiedbyAASHTO(orequivalent)andmeettherequirementsofASTMC1077.MostagenciesrequirecertificationforQCandQAtechnicians(AmericanConcreteInstitute,NationalInstituteforCertificationinEngineeringTechnologies,etc.).

Athoroughunderstandingoftestproceduresisnecessary.Justasimportant,however,istheknowl-edgethatmanytestsperformedonconcretehavevaryinglevelsofprecision.

Disputesanddisagreementsoftenstemfromdiffer-ingtestresultsbetweenanowner’slaboratoryandacontractor’slaboratory.Whentwotechnicianssplitasampleofconcreteandtheresultingflexuralstrengthresultsdifferby90lb/in2,whichtestiscorrect?Assum-ingthatbothtestswereperformedaccordingtotheprescribedtestmethod(ASTMC78-02),bothanswersarecorrect—neitherismorerightthantheother.

Thenatureofthematerialsandthetestsissuchthattestresultsareestimatesofactualpavementprop-erties.Fromtheowner’sperspective,itisimperativetounderstandthateachtesthasaninherentlevelofprecisionandvariability;asingletestresultmaynotrepresentthetrueconditionofthepavement.Fromacontractor’sperspective,thisimpliesthatthemixshouldbedesignedtoaccommodatetestingvariabilityinordertomeetspecifiedtolerances.

Ataminimum,marginaltestresultsshouldbeevaluatedorexaminedwiththeprecisionandbiasassociatedwiththattestmethod(table9-1).

Onewaytointerpreteachoftheexampletestresultsintable9-1istosaythatyoucanbe95percentconfidentthatthetruevalueofthematerialbeingtestedisbetweenthelowerandupperlimitsshown.Inotherwords,thereisarangeofvaluesassociated



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Table 9-1. Example of Testing Precision

witheachtestresult.Therangedependsonthepreci-sionofeachtestmethod.

OpencommunicationandcooperationbetweenQCandQAorganizationsisimportanttoavoidandresolveconflicts.

Record KeepingProperdocumentationofQCandQAtestsisakey

factorforinterpretingdata,makinginformeddeci-sions,andtroubleshootingproblemsthatmayariseinthefuture.Testdatacanbecomeoverwhelmingonmoderatetolargeconcretepavingprojects.Withoutclearandaccuratedata,processcontroladjustmentscannotbemadewithconfidence.

Someprimaryelementsofagoodrecord-keepingsystemincludethefollowing:

Consistentandclearlabeling/identificationofsamples.Accuratesamplelocation(centerlinesurveysta-tion)and/ortimeofsampling.Legiblehandwritingontestingworksheets.Organizedfilingsystem.

Statisticalcontrolchartsareusefultoolsforana-lyzingchangesinmaterialsandthepavingprocess.Practicallyanyvariablethatismeasuredinconcretepavingcanbeplottedonacontrolchart.Anexampleoftestdataforconcreteunitweightisshownintable9-2.Thecorrespondingcontrolchartforthatunitweightdataisshowninfigure9-1.

Figure9-1isaplotofexampleconcreteunitweight

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________________________________________________Figure 9-1. Example control chart: concrete unit weight

testresults.Eachpointisatestresult(SampleID).Thegreenhorizontallinerepresentsthemeanofthetestresults.Thedashedhorizontallinesaredrawnatintervalsofoneandtwotimesthestandarddeviationonbothsidesofthemean.Thesolidredhorizontallinesaretheupperandlowercontrollimitsandareplacedthreestandarddeviationsawayoneachsideofthemean.

Standardcriteriacanbeusedtodetermineifanout-of-controlconditionexists.Thesecriteriaareasfollows(Seward2004):

Anyonepointisoutsideofthecontrollimits(morethanthreestandarddeviationsfromtheaverage).Ninepointsinarowareonthesamesideofthemean.Sixpointsinarowareallincreasingoralldecreasing.Fourteenpointsinarowarealternatingupand

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Test Sample result 95% lower limit 95% upper limit

Grading (% passing 1/2”) 28% 24% 32%

Slump 21/2in. 2 in. 3 in.

Air content 5.5% 4.9% 6.1%

Rodded unit weight for aggregate 120 lb/ft3 114.5 lb/ft3 125.5 lb/ft3

Compressive strength 3,600 lb/in2 3,390 lb/in2 3,810 lb/in2

Flexural strength 700 lb/in2 602 lb/in2 798 lb/in2

Smoothness (0.2” blanking band) 4.00 in./mi 2.58 in./mi 5.42 in./mi(data from DOT equipment certification)



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Table 9-2. Example Concrete Unit Weight Test Resultsdown.Twooutofthreepointsaremorethantwostandarddeviationsfromthemean(andonthesamesideofthemean).Fouroutoffivepointsaremorethanonestan-darddeviationfromthemean(andonthesamesideofthemean).Fifteenpointsinarowareallwithinonestan-darddeviationofthemean.Eightpointsinarowareallmorethanonestandarddeviationfromthemean(oneithersideofthemean).

Onceanout-of-controlconditionisidentifiedusingthesecriteria,theprocessmustbeanalyzedtodeterminethecauseofthevariability,andappropriateactionmustbetakentocorrecttheprocess.

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Sample ID Unit weight (lb/ft3)

1-1 150.3 1-2 151.0 1-3 148.0 1-4 150.7 1-5 151.7 1-6 148.6

2-1 147.4 2-2 149.2 2-3 147.1 2-4 149.8 2-5 147.2 2-6 147.9

3-1 147.3 3-2 148.0 3-3 150.7

Average 149.0 Standard deviation 1.6

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Batching

Key Points

Thebatchplantmustbecontinuouslymonitoredandregularlycalibrated.

Criticalissuesduringbatchingareaggregatemoisture,thewatercontentandwater-cementitiousmaterialsratio,and

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BatchingisacriticalpartofconcreteQC.Toensureauniformmixfrombatchtobatch,thematerialsmustbeuniformandthehandlingsystemsconsistent.Thebatchplantmustbecontinuouslymonitoredandregularlycalibrated(seeStockpileManagementandBatchinginchapter8,pages206and207).

Aggregate MoistureAggregates’moisturecontentcaninfluenceconcrete’s

workability,strength,anddurability.Allnormal-weightaggregatespossesssomeporosityaccessibletowater.Iftheaccessibleporesarefilled,filmsofwatermayexistontheaggregatesurface.Dependingonitsmoisturecondition,aggregatecaneithertakeuporaddtothemixwater.Itisthereforeimportanttoknowtheaggregate’smoistureconditionduringbatchingandtoadjustthemixwateraccordingly.

Mostdifficultieswithsurfacemoisturestemfromfineaggregate.Coarseaggregatewillcommonlyhaveabsorptionlevelsof0.2to4percentandfreewatercontentfrom0.5to2percent.Fineaggregatewillhaveabsorptionlevelsof0.2to2percentandfreewatercontentfrom2to6percent(Kosmatkaetal.2003).

CoarseaggregatemoisturecanbeassessedusingASTMC127-04/AASHTOT85;fineaggregate,usingASTMC70-94orASTMC128-04/AASHTOT84.Totalmoisturecontent,fineorcoarse,canbedeter-minedusingASTMC566-97/AASHTOT255.Itiscommontoconductmoisturetestsonrepresentativeexamplesofstockpiledaggregatesonceortwiceaday(Kosmatka,Kerkhoff,andPanarese2003).

Goodstockpilemanagement,allowingfineaggre-gatestockpilestodrain,willhelpmaintainuniformmoisturecontent(Landgren1994).Becauseaggre-gatesurfacemoisture,especiallyofthefines,isso

importanttopropermixcontrol,surfacemoistureisoftenmonitoredwithmoisturemeters(electricormicrowaveabsorption)onthebatchingequipment(Landgren1994).Properlyplacedandmaintainedmeterscanprovideaccurate,continuousmeasurementofaggregatesurfacemoisture,aslongasotherplantoperationsareconductedproperly.

Water-Cementitious Materials Ratio Thew/cmratioofamixtureisperhapsthesingle

mostimportantvariableandmustbecarefullycon-trolledtomaintainconcretequality.Thew/cmratioisdifficulttomeasuredirectly,soitisimportanttocare-fullymonitorandcontrolthebatchingprocess.

Themicrowavetestmethod(AASHTOT318)hasbeenusedtodeterminethewatercontentoffreshconcrete.Bycouplingthewatercontent(lesswaterabsorbedintheaggregates)withbatchweightsofthedryingredients,thew/cmratioiscomputed.

Batching TolerancesAggregatesandcementitiousmaterialsshouldbe

batchedbyweight.(Ifbatchingisdonebyvolume,widevariationscanoccurinbatchquantitiesduetobulkingofmoistaggregate,resultinginsignificanterrors.)Liquidconstituents,suchasmixwaterandliquidadmixtures,canbebatchedbyvolume.

ASTMC94,Specifications for Ready-Mixed Concrete,stipulatestherecommendedtolerancesforbatchingconcretetolerances(table9-3).

Table 9-3. Recommended Batch Tolerances for Ready-Mixed Concrete* (ASTM C 94)

Constituent Individual**, % Cumulative***,%

Cementitious +1 +1materialsWater +1 +3Aggregates +2 +1Admixtures +3 N.R.****

* Batch weights should be greater than 30 percent of scale capacity.

** Individual refers to separate weighing of each constituent.

*** Cumulative refers to cumulative weighing of cement and pozzolan, of fine and coarse aggregate, or water from all sources (including wash water).

**** Not recommended.

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Construction Monitoring

Key Points

Duringconstruction,theconcretetemperature,air-voidsystem,amountofvibration,anddowelbarlocationsmustbemonitoredandadjustedasnecessary.

Anoptimaltemperatureforfreshlyplacedconcreteisfrom10to15°C(50to60°F).

Routinelyachievingthetargetaircontentandair-voidsystemisoneofthemostchallengingaspectsofcontrollingconcretemixturesbecausesomanyvariablesaffecttheairsystem.Testsshouldbeconductedatthebatchplantand,ifpractical,behindthepaver.

Vibratormonitorshelpequipmentoperatorsensurethattheproperamountofvibrationisappliedtoproducehomogeneous,denseconcretewithoutadverselyaffectingtheentrainedair.

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TemperatureThetemperatureofconcreteasplacedandshortly

thereaftercanhavealargeimpactonboththefreshandhardenedproperties(seeTemperatureEffectsinchapter5,page127).

Problemsassociatedwithhighconcretetempera-turesincludethefollowing:

Increasedwaterdemandtomaintainworkability.Decreasedsettingtime.Increaseddangerofplasticandearly-ageshrinkagecracking.Reductioninair-voidsystemeffectiveness.Lowerultimatestrength.

Problemsassociatedwithlowconcretetemperaturesincludethefollowing:

Reducedrateofhydration,thusincreasingtheriskofplasticshrinkagecrackingandchangingthesaw-cuttingwindow.Reducedearlystrength.

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Freezingoftheconcretebeforeitsets(withextremelycoldtemperatures).

Anoptimaltemperatureforfreshlyplacedcon-creteisintherangeof10to15°C(50to60°F),anditshouldnotexceed30to33°C(85to90°F)(Mindess,Young,andDarwin2003).

Foragivenconcretemixture,thetemperatureoffreshconcretecanbecontrolledbyadjustingthetem-peratureoftheconstituentmaterialsoralteringthelocalenvironment,orthroughacombinationofthetwo(seeFieldAdjustments,chapter8,page210).

Timeofplacementwillaffectthetemperatureoftheconcreteatearlyages.Heatingduetosolarenergymaycoincidewithhydrationheatingtoraisethetem-peratureoftheconcretemorethanifthehydrationpeakoccursovernight.

Inhotweather,theconcretemixturecanbecooledbyaddingliquidnitrogenorbycoolingoneormoreofthecomponents.Duetotheirhighvolume,cool-ingtheaggregatescanbeveryeffectiveinreducingtheconcretetemperature.Eventhoughwaterisarelativelysmallcomponent,ithasahighspecificheatcapacityandismorepracticaltocoolthanaggregates(Kosmatka,Kerkhoff,andPanarese2003).

Incoldweather,theconcretecanbesufficientlywarmedthroughtheuseofhotwater.Waterneartheboilingpointcanbeusedifitismixedwiththeaggregatepriortotheadditionofcement(Kosmatka,Kerkhoff,andPanarese2003).Whenairtemperaturesarewellbelowfreezing,itmaybenecessarytoheatthefineaggregateaswell.Inallcases,frozenlumpsmustbeavoidedintheaggregate.

ASTMC1064/AASHTOT309providethestandardfordeterminingthetemperatureoffresh

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Cooling Concrete in Hot Weather

During hot weather, the concrete temperature can often be lowered to an acceptable level simply by doing the following: Cool the aggregates by shading the stockpiles and sprinkling them with water, and chill the mix water or use ice. (Be sure the total of all water and ice does not exceed the mix design.)



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concrete.Thetemperaturemeasuringdeviceusedmustbeaccurateto±0.5°C(±1.0°F)andmustremainintheconcreteforatleasttwominutesoruntilthetemperaturestabilizes.

Air ContentRoutinelyachievingthetargetaircontentandair-

voidsystemisoneofthemostchallengingaspectsofcontrollingconcretemixtures.Projectspecificationsoftenallowtheaircontentoftheconcretetobewithin-1to+2percentagepointsofthetargetvalue.

Thefactorsthataffecttheair-voidsystemincon-creteincludethefollowing:

Ingredients.Anychangeinsourceoramountofanyofthemixingredientsmaychangetheair-voidsystem.Temperature.Increasingconcretetemperaturetendstoreduceaircontentforagivendosageofair-entrainingadmixture.Mixingtime.Aircontenttendstoincreasewithcontinuingmixingtimeuptoalimit,atwhichpointitwilldecrease.Batchingsequence.Changingthebatchingsequencemaychangetheaircontentofaconcrete.Slump.Moreairwillbeentrainedinahigh-slumpmixturethaninasimilarlow-slumpmixture.However,watershouldnotbeaddedtothetruckinordertoraisetheaircontentofagivenbatch.Admixtureinteractions.Somewater-reducingadmixtureswillaffectair.Haultime.Increasinghaultimemayreduceaircontent.Vibration.Excessivevibrationmayremoveairfromtheconcrete.

Testsshouldbeconductedatthebatchplantformonitoringpurposesand,ifpractical,behindthepaverforpaymentpurposes.Testsshouldbeconductedeveryhourorevery100lane-meters(300lane-feet)ofpaving,every50m3(70yd3)ofcon-crete,orwhensamplesforstrengtharemade.

Testsaremosteasilyconductedusingthepressureairmeter(ASTMC231-04/AASHTOT152)(seeAirContent[Pressure]inthischapter,page266)orthe

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air-voidanalyzer(seeAir-VoidAnalyzerinthischap-ter,page265).

Othermethodsmaybeconsideredifnecessary:volumetricairmeter(ASTMC173/AASHTOT196),orgravimetric(ASTMC138/AASHTOT121)(seeAir-VoidSysteminchapter5,page133).

Vibration MonitoringVibrationandconsolidationcanhaveasignificant

impactonthedurabilityofconcretepavement.Propervibrationproducesapavementthatishomogeneousanddensewithoutadverselyimpactingtheentrainedairinthemix.

Priorto1996,vibratormonitorswerenotavailablecommercially(SteffesandTymkowicz2002).Today’svibratormonitorsgivethepaveroperatoraconvenientwaytopreprogram,operate,andmonitorvibratoroperation.Vibratormonitorsalsoenabletheinspec-tionpersonneltoviewinrealtimethefrequencyofallvibratorsduringthepavingoperation.Datacanalsobedownloadedtoacomputerforlateranalysis.

Inwell-proportionedmixtures,vibrationsinexcessof8,000vibrationsperminute(vpm)canbeused;however,inoversandedmixtures,reductioninvibrationsisneededtopreventthedisappearanceofbeneficialairvoids.Insuchcases,5,000to8,000vpmaregenerallyused(SteffesandTymcowics1997).

Dowel Bar TolerancesThemostcriticalfactorsindowelperformanceare

diameter,alignment,andembedmentlength.Dowelstransferappliedloadacrossjointsandmustallowfreedomofmovementbetweenadjacentslabs.Dowelplacementtolerancesusuallyreferencethethreewaysthatdowelbarscanbeoutofalignment:

Longitudinaltranslation.(Isthejointcenteredonthedowel/isthereadequateembedmentlength?)Depth.(Isthedowelnearthemidpointoftheslabthickness?)Horizontalandverticalskew.(Isthedowelparalleltoboththecenterlineandtopsurfaceofthepavement?)

Numerousstudieshavebeenconductedonthetopicofdowelbaralignment,manyofthembrought

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aboutbytheadventofdowelbarinsertionequipment.Inmorerecentyears,themajorityofstudieshaveconcludedthatstrictertolerancesarenotnecessary.Summarizedbelowisthecurrentstateofthepractice,aswellasrecommendedguidelinesfordowelalignment.

AlignmentAlignmentofeachdowelmustbewithincertain

tolerancestoallowadequatefreedomofmovementbetweenslabs.Alignmentrequirementsaretypicallyinboththehorizontalandverticalplanes,aswellasacombinationthereof.AsurveybytheAmericanCon-cretePavementAssociationin1999ofStatehighwayagencyconcretepavementpracticesshowedthattheaveragetolerancefordowelskewinboththeverticalandhorizontaldimensionscurrentlyallowedbyStateagenciesis6mm(0.25in.)per300mm(12in.)ofdowellength,orthreepercent.

Embedment LengthToprovideaconstructiontolerance,thetotal

lengthofdowelbarsisalwayssomewhatlongerthanrequiredforembedmentdepth.Thecurrentstateofthepracticeistoprovideaminimumdowelembed-mentlengthof150mm(6in.)forajointtobeeffectiveundermostloadingconditions.Inhighwaywork,450-mmlong(18-in.long)dowelsarespeci-

fied,providing150mm(6in.)oftolerance(±75mm[3in.]).Foranygivendowelsize,themaximumbearingstressesoccurnearthejointface.Anylossofloadtransferthatmightoccurfrominadequatedowelembedmentrelatestothemagnitudeofthebearingstressesalongthedowel.

Dowel DepthThereisusuallynoproblemwithplacingdowels

slightlyaboveorbelowmid-depthinaconcreteslab.Theverticallocationisnotascriticaltodowelorjointperformanceasaremanyotherfactors.Increas-ingtheslabthicknessbyevenseveralcentimetersorinchesonaconcretepavement,basketassemblieswillpositionthedowelsbelowmid-depth.Thisisnotasignificantvariationonthedesign,andtherehavebeennocasesofdecreasedperformanceofthedowelsorthepavement.Manymilesofconcretepavementhavebeenplacedwiththedowelsabovenominalmid-depth(ACPA1994).

Thecurrentindustry-recommendedtolerancefordowelskewis9.5mm(0.375in.)per300mm(12in.)ofdowellength,orthreepercent,inthehori-zontalandverticalplanes.Dowelsmustbelubricatedsufficientlytopreventbondingandallowmovement.

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Test Methods

Key Points

AsuiteoftestsrepresentsQCbestpracticesbutdoesnotincludealltestsdiscussedinthismanual.

Readersshouldrefertotherelevantfullmethodstatementbeforeattemptingtoconductanytest.

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Asuiteoftestsforuseduringconstructionofcon-cretepavementshasbeenidentifiedintheTPF-5(066)project(table9-4).Themethodsshouldbeusedduringthethreephasesofamix’sevolutionduringaproject:

Materialselectionandmixdesign.Preconstructionverification.Qualitycontrol.

Thetestshavebeencategorizedaccordingtofiveconcreteproperties:

Workability.Strengthdevelopment.Aircontent.Permeability.Thermalmovement.

1.2.3.

1.2.3.4.5.

TestdescriptionsonthefollowingpagesarenotintendedtosubstituteforformalmethodsdescribedinvariousspecificationsandmethodspublishedbyorganizationslikeAASHTOandASTM.Sufficientinformationisgivenheretohelpreadersunderstandhowandwhyatestshouldbeconducted.Readersshouldrefertotherelevantfullmethodstatementbeforeattemptingtoconductanytest.

Table 9-4. Suite of QC Tests from TPF-5(066)

Current Best Practices

The tests listed in table 9-4 and described in the following pages do not represent all QC tests discussed throughout this manual. Rather, they have been identified as current best practices for QC purposes in the “Materials and Construction Optimization for Prevention of Premature Pavement Distress in Portland Cement Concrete Pavements” project at Iowa State University (TPF-5[066]). These tests provide a starting point for alternative and new test methods that may yet be identified as best practices through the TPF-5(066) project.

Concrete property Test name (standard test method) Laboratory

Workability Differential scanning calorimetry (DSC) Central laboratory Blaine fineness (ASTM C 204 / AASHTO T 153) Central laboratory Combined grading Mobile laboratory Penetration resistance (false set) (ASTM C 359 / AASHTO T 185) Mobile laboratory Cementitious materials temperature profile - “coffee cup test” Mobile laboratory Water/cementitious materials ratio (microwave) (AASHTO T 318) Mobile laboratory Unit weight (ASTM C 138 / AASHTO T 121M / AASHTO T 121) Mobile laboratory Heat signature Mobile laboratory Concrete temperature, subgrade temperature, project environmental conditions (weather data) Mobile laboratory

Strength development Concrete maturity (ASTM C 1074 / AASHTO T 325) Mobile laboratory Flexural strength and compressive strength (ASTM C 78 / ASTM C 39 / ASTM C 39M / AASHTO T 97 / AASHTO T 22) Mobile laboratory

Air content Air-void analyzer Mobile laboratory Air content (pressure) (ASTM C 231 / AASHTO T 152) Mobile laboratory Air content (hardened concrete) (ASTM C 457) Central laboratory

Permeability Chloride ion penetration (ASTM C 1202 / AASHTO T 277) Central laboratory

Thermal movement Coefficient of thermal expansion (ASTM C 531 / AASHTO TP 60) Central laboratory

Test Methods


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Purpose – Why Do This Test?

The form and content of calcium sulfate in portland cement are important factors affecting the workability and durability of concrete mixes. Commonly observed setting problems, such as false set and flash set, are caused by inadequate content and proportions of gypsum and plaster in the cement. Monitoring the level of sulfate forms in cement can help prevent problems during construction and improve the quality of the

Differential Scanning Calorimetry (DSC)

Principle – What is the Theory?

Gypsum and plaster are two forms of calcium sulfate that greatly affect setting properties, strength development, and volume stability of cement and concrete. Gypsum is added to clinker to control the setting of cement. The addition of gypsum is optimized to produce the highest concrete strength. Gypsum dehydrates to plaster in a cement mill under high temperatures and low humidity conditions.

The amount of gypsum and plaster present in cement can be determined by subjecting the sample to heating and monitor-ing the change in energy of the sample. The intensity of the endothermic peaks associated with the dehydration process of gypsum and plaster is proportional to the amount of these constituents present in the sample.

Test Procedure – How is the Test Run?

The differential scanning calorimetry (DSC) method consists of determining the areas of endothermic peaks associ-ated with the dehydration process of gypsum and plaster. To obtain these endothermic peaks, a cement sample is subjected to heating under specific conditions, such as atmo-sphere and heating rate.

Test Method

Place a 100-µL aluminum crucible containing a sample and a reference crucible on a sensor plate of the DSC measuring cell.

Heat the cell at a constant rate of 10˚C/min (18˚F/min) from 50 to 400˚C (122 to 205˚F). The experiment is performed under regular atmospheric conditions.

The cell obtains measurements and continuously sends data to the control unit. An endothermic profile of the tested sample is demonstrated by an obtained experimental curve.

The area under the peaks, proportional to the heat of the endothermic reaction, is integrated and expressed in joules/gram (J/g). This heat corresponds to specific gypsum and/or plaster content of the submitted sample. The actual amount of gypsum and plaster is determined from a calibration curve (weight percent vs. J/g) estab-lished for the DSC used in testing.

The total amount of sulfate added to the clinker is the calculated sum of sulfate present in gypsum and plaster.

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Test Apparatus

Sample crucible: 100-µL aluminum crucible sealed with a pierced lid needed to create the pressure leak necessary to resolve peaks of the two-stage gypsum dehydration.

Reference crucible: 100-µL aluminum crucible sealed with a lid.

Measuring cell: Measures the difference between the heat that flows to a sample and a reference crucible. The unit has a high signal resolution and detects changes in the heat flow. The cell obtains measurements, and con-tinuously sends data to the control unit.

Software: Helps evaluate experimental curves within selected integration limits.

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Output – How Do I Interpret the Results?

The results of the DSC testing are expressed as weight percent of gypsum and plaster present in the tested samples. The test can also be used to determine other components of the cementitious system.

The effect of various sulfate forms content on the concrete performance greatly depends on the physical and chemical properties of cement, the use of alternate materials such as fly ash and ground, granulated blast-furnace slag in con-crete, the type of admixture, and weather conditions.

Data from the test can be used as a monitoring tool, to assess variability of the cement, or to assess the risk of incompatibility.

Construction Issues – What Should I Look For?

Changes in workability that may be due to portland cement chemistry variability include the following:

False set, an initial stiffening that disappears when remixing or vibration occurs.

Increased slump loss.

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Concrete property: workability

Test Methods


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The particle size (fineness) of portland cement affects concrete’s rate and heat of hydration, workability, water requirement, rate of strength gain, and permeability. Fine cements can increase early strength gain, but ultimately sacrifice long-term strength gain, which can result in higher concrete permeability. Daily monitoring of the fineness of the cement used on a project will provide a means of tracking variability in the cement.

The smaller a particle is, the greater the relative surface area. Therefore, the finer a given sample of cement is, the greater the surface area per unit mass.

Blaine Fineness


The test is based on the theory that the permeability of a powder is related to the fineness of the powder. The test is run under controlled conditions and calibrated against the fineness of a known control sample.


ASTM C 204, Standard Test Method for Fineness of Hydraulic Cement by Air-Permeability Apparatus (Blaine Fineness), consists of determining the time required to draw a standard volume of air through a standard volume of portland cement and comparing that to the time required for the same proce-dure performed on a reference material (calibration).

Test Method

Place the perforated disk inside the permeability cell, and place a paper filter on top of the perforated disk.

Weigh a cement sample equal to the mass of the calibration sample and place the cement sample in the permeability cell on top of the paper filter.

Place another paper filter inside the permeability cell on top of the cement sample. (The cement sample is sand-wiched between two paper filters on top of the perforated disk.)

Insert the plunger into the permeability cell to compact the cement sample, then withdraw the plunger.

Attach the permeability cell to the manometer tube, and draw a given volume of air through the cement sample.

Record the time required to draw a given volume of air through the cement sample.

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Test Apparatus

Manometer, half filled with mineral oil.

Permeability cell: A steel cylinder (1/2-in. diameter) that holds the compacted cement sample.

Plunger: A steel rod with a 1/8-in. flat slot that fits tightly inside the permeability cell.

Perforated disk: A steel disk with 1/32-in. holes that fits in the bottom of the permeability cell.

Paper filters: Circular paper disks cut to fit the cell.

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Results of the test are expressed in m2/kg (preferred) or cm2/g. Typical values for Blaine fineness of portland cement range from 300 to 450 m2/kg (3,000 to 4,500 cm2/g). Higher values of Blaine fineness indicate a finer grind of cement.


The risk of incompatibility increases with increasingly finer cement. Changes in fineness that affect the rate of strength gain may have an impact on sawing operations, either delaying or accelerating the timing of initial saw-cutting of contraction joints to avoid random cracking.

Increased water demand due to cement fineness may be compensated for by the use of a water-reducing admixture.

An increase in slump loss after initial mixing may indicate variability in the fineness of the cement, and the use of chilled water may offset this impact during warm weather conditions.


Test Methods


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Aggregate grading may influence the water requirement, workability, and paste contents. These in turn may impact the risk of segregation, bleeding, and increased shrinkage of concrete paving mixes.

It is desirable to blend different aggregate sizes to obtain a smooth grading curve for the combined aggregates system.

Combined Grading


The sieve analysis (amount of material retained or passing a series of sieves with different-sized openings) is compared to optimized systems using a number of numerical and graphi-cal models. The closer the batch grading is to the optimum, the lower the risk of grading-related problems in the mixture.


Sieve analyses are conducted in accordance with ASTM C 136 for the coarse and fine fractions, and the data are applied to the following models.

The coarseness/workability chart plots a single point on a graph, with the coarseness factor on the horizontal axis and the workability factor on the vertical axis,

where

Coarseness factor = [(percent retained on 3/8-in. sieve) / (percent retained on #8 sieve)] • 100

Workability factor = percent passing #8 sieve

The 0.45 power chart plots the combined grading on a chart with sieve size on the horizontal axis (scale = sieve size 0.45 µm) and percent passing on the vertical axis.

The 8-18 chart plots the material retained on each sieve with sieve size on the horizontal axis and percent passing on the vertical axis.


Points on the coarseness/workability chart (figure 9-2) rep-resent the coarseness factor and the workability factor for a mix based on the grading test results of each individual aggre-gate. For an optimized grading mix, the points should plot above the control line (28<workability factor<44) and inside the zone labeled well-graded (45<coarseness factor<75).

When the sample combined grading plot on the 0.45 power chart (figure 9-3) crosses back and forth across the refer-ence line, it indicates gap grading.

A general rule of thumb for optimized grading is to have between 8 and 18 percent retained on each individual sieve on the 8-18 chart (figure 9-4).

Each of the charts (figures 9-2 through 9-4) provides a dif-ferent perspective of gradation. When used together, the information in these three charts can provide the contractor and the agency with a basis for evaluating the combined grading of a concrete mix.


Modest variations in grading are to be expected from batch to batch and generally do not have a significant impact on performance. Extreme variations in grading and workability should be addressed as they occur.

Workability concerns attributable to aggregate grading can be identified by observing the following conditions:

Stockpile segregation and/or inconsistent stockpiling methods.

Inconsistent slump (mix water is static while grading changes).

Excessive bleeding.

Variation in vibrator frequencies.

Edge slump.

Poor consolidation observed in cores.

Segregation observed in cores.

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Combined Grading, continued

________________________________________________Figure 9-2. Sample coarseness/workability chart

________________________________________________Figure 9-3. Sample combined aggregate gradation 0.45 power curve

________________________________________________Figure 9-4. Percent retained chart

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Some portland cements and combinations of cement and pozzolans may be prone to false set. False set reduces the workability of the concrete mix. Workability can be restored by remixing without the addition of water.

Since remixing is not normally possible when a central mix plant and dump trucks are used for delivery, the false set condition is usually offset by adding more mixing water, which increases the water-cementitious materials (w/cm) ratio. This is poor practice.

Performing the penetration resistance test on the cementitious materials will indicate whether the mix is prone to false set.

Penetration Resistance (False Set)


As concrete mortar stiffens (sets), the resistance required for a 10-mm diameter rod to penetrate into the mortar will increase. The depth of penetration of the 10-mm rod into a mortar sample is measured and recorded over time. If, after remixing, the 10-mm rod penetrates the mortar sample to a depth greater than was measured before remixing, then a false set condition is occurring.


ASTM C 359, the Standard Test Method for Early Stiffen-ing of Portland Cement (Mortar Method) (false set), tests a laboratory-mixed mortar. It may be advisable to test a mortar sample obtained from the actual project-mixed concrete. The test method uses a Vicat apparatus to measure the depth of penetration of a 10-mm diameter plunger 10 seconds after it is released into the mortar at fixed time intervals.

Test Method

Obtain a mortar sample from the project-mixed concrete.

Place the mortar sample in the mold, consolidate it, and strike it off.

Using the Vicat, hold the 10-mm rod in contact with the top surface of the mortar by a set screw.

Release the rod from the set screw and allow it to pen-etrate into the mortar. Record the depth of penetration 10 seconds after the rod is released.

Take penetration readings 3 minutes, 5 minutes, 8 minutes, and 11 minutes after batching.

After the 11-minute reading, remix the mortar sample for 1 minute.

Replace the mortar sample in the mold, consolidate it, and strike it off.

Measure the penetration 45 seconds after remixing.

Record the depths of penetration for each of the five repetitions.

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Test Apparatus

Vicat: A frame holding the 10-mm rod and an indicator to measure the depth of penetration in mm.

Mortar Mold: A box 51-mm wide x 51-mm high x 152-mm long (2- x 2- x 6-in.) used for containing the mortar sample.

Mixer: A laboratory mixer used for remixing the mortar sample.

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The depths of penetration are reported in tabular format, as in this example:

Initial penetration 50 mm5-minute penetration 40 mm8-minute penetration 25 mm11-minute penetration 10 mmRemix penetration 25 mm

If, as shown in the example, the penetration after remixing is greater than the 11-minute penetration, false set is likely, affecting the workability of the project mix.


Situations that may indicate the occurrence of false set include the following:

Excessive vibration that essentially remixes the concrete.

Loss of workability during moderate temperatures.

Workability changes that occur when pozzolans and/or admixtures are removed or added.

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Concrete property: workability (see Testing and Prevention for Incompatibilities in chapter 4, page 100).

Test Methods


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Chemical reactions that occur during hydration of portland cement generate heat. The coffee cup test is a quick guide to temperature viariations that can indicate possible uniformity, workability, and/or compatibility problems. Variability of the cementitious materials and interactions with chemical admixtures may also be tracked by daily monitoring of the coffee cup test results.

Cementitious Materials Temperature Profile (“Coffee Cup Test”)


Measuring the temperature change of a cement paste mixture during hydration may indicate compatibility and workability issues.


Portland cement, supplementary cementitious materials (SCMs), chemical admixtures, and water are mixed and stored in an insulated container. The temperature of the paste is recorded at intervals for up to 24 hours and compared to a sample of sand kept under the same conditions. The first few minutes are the most critical.

Test Method

There is no standard method for this test at present. The fol-lowing is one approach:

Acquire representative material samples from the plant.

Record the temperature of the materials at the time of sampling.

Prepare a paste representative of the field concrete.

Place the paste in an insulated container and a sample of sand in a similar container.

Record the temperature of the sample and the sand during the first minute and periodically for up to 24 hours.

Calculate the difference in temperature between the sample and the sand and plot the temperatures, with time on the x-axis and temperature on the y-axis.

Repeat this procedure with alternative mixtures, chang-ing one parameter (e.g., SCM dosage, admixture dosage, w/cm ratio) at a time.

Observe the effects of changing the system on the shape of the temperature plot.

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Test Apparatus

Mixing containers: Insulated containers.

Scale for measuring the mass of portland cement, SCMs, and water.

Digital thermometer or data logger with thermocouples.

Laboratory mixer.

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Changes in the temperature profile of cementitious materials will indicate potential changes in the following properties:

Workability.

Slump loss.

Setting time.

Strength gain.

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The strength of concrete used for pavements is mainly a function of the water-cementitious materials (w/cm) ratio of the concrete mix being used. Acceptance strength tests on hardened concrete are normally performed at least seven days after placement of the concrete. The microwave method can be used to obtain w/cm ratio results within hours, instead of waiting days for strength results. Monitoring the test results may provide an early flag of potentially low-strength concrete, allowing the contractor to adjust operations sooner than conventional strength testing might indicate.

Concrete strength varies inversely with the amount of water in the mix. In simplest terms, less water implies higher strength. Other factors, such as consolidation, curing, aggregate quality, air content, and aggregate shape, affect strength as well. For a given mix with a constant amount of cement, the w/cm ratio has the greatest impact on strength.

Water-Cementitious Materials Ratio (Microwave)


The total water in a concrete mix comes from the following sources:

Moisture absorbed in the aggregate.

Free water on the aggregate.

Water added in the batching process.

The mass of water removed from a fresh mixture by drying in a microwave can be used to calculate the w/cm ratio of the mixture.

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The test is described in AASHTO T 318. A sample of fresh concrete from the project is weighed and then dried in a microwave. It is then reweighed to determine the mass of water that was contained in the mix. The water absorbed in the aggregate is subtracted from this total, and the remain-der is used to calculate the w/cm ratio using the batched cementitious materials content.

Test Method

Weigh the glass pan and fiberglass cloth (tare).

Place the concrete sample in the glass pan on top of the fiberglass cloth.

Weigh the glass pan, fiberglass cloth, and concrete sample.

Heat the concrete sample in the microwave oven for five minutes.

Remove the sample from the microwave, weigh, and break up the sample using the pestle.

Reheat the sample for five minutes.

Repeat the weighing, breaking, and heating cycle at two-minute intervals until the sample loses less than 1 g of mass between reheating cycles.

Record the mass of the wet concrete sample and the mass of the dry concrete sample and the difference between the two masses (mass of total water content).

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Test Apparatus

Microwave oven for drying the concrete sample.

Glass pan and fiberglass cloth (a container for the concrete sample).

Scale to obtain the mass of the sample.

Porcelain pestle for grinding the sample as it is dried.

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The total water content of the concrete sample can be expressed as a percentage:

Total water content % (Wt) = (wet sample mass – dry sample mass) / wet sample mass

This Wt can be monitored and used as a relative indicator of potential variability in pavement strength.

It should be noted that the value of Wt will not provide the true w/cm ratio because the microwave test drives out all of the water in the concrete, including the water that is absorbed in the aggregate. As such, the value of Wt will be greater than the true w/cm ratio of the mix. By compensating for the measured absorption of the aggregate, the result from this test can be used to monitor variability in the concrete from batch to batch.


When variations in Wt are noted, plant operations should be reviewed to ensure that materials are being batched in the proper proportions.


Test Methods


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The unit weight of fresh concrete is a general indicator that the concrete has been batched in the correct proportions. It is an excellent indicator of uniformity of a mixture from batch to batch.

Unit Weight


A concrete mix design is composed of several ingredients: portland cement, supplementary cement materials (SCMs), fine aggregate, coarse aggregate, admixtures, air, and water. All of these materials have different specific gravities. A variation in the unit weight of a mixture will indicate a change in proportioning of the mixture, often in the water or air content.


A sample of mixed concrete is consolidated in a container of known volume and weighed to determine the unit weight of the mixed concrete (ASTM C 138).

Test Method

Determine the level-full volume of the air pot.

Weigh the empty air pot.

Consolidate a sample of fresh concrete in the air pot using the tamping rod or vibrator and mallet.

Strike off the concrete so that it is level-full with the top rim of the air pot.

Clean off all excess concrete from the exterior of the air pot.

Weigh the air pot full of concrete.

Record the empty mass, full mass, and volume of the air pot.

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Test Apparatus

Measure: cylindrical container, usually a standard pres-sure air pot.

Scale for weighing the sample.

Tamping rod, vibrator, mallet, and strike-off plate for con-solidating the sample in the air pot.

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The unit weight of the concrete mix is reported in pounds per cubic foot (lb/ft3):

Unit weight = (full mass – empty mass) / volume

The unit weight of the mix should be compared with the unit weight of the mix design to identify potential problems in the batching process or changes in raw materials.

A variability of more than 5 lb/ft3 may be considered significant.


When variations in unit weight measurements are observed, the following potential causes should be reviewed:

Sample consolidation. (Was the sample fully consolidated in the air pot?)

Air content of the concrete.

Batch proportions of each material.

Changes in raw material densities (specific gravities).

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Test Methods


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Heat signature is a representation of the heat of hydration generated by a specific concrete mix over time. Variations in the chemistry and dosage of portland cement and supplementary cement materials (SCMs), along with interactions between them and chemical admixtures, may be flagged by the heat signature.

Heat Signature (Adiabatic Calorimetry Test)


Chemical reactions that occur as concrete hardens emit heat (heat of hydration). By insulating a standard cylinder of concrete from the influence of outside temperatures and using sensors to record the heat generated by the concrete, it is possible to measure the adiabatic heat signature of a concrete mix. A chart that plots time on the x-axis and tem-perature on the y-axis is produced from this data.


A concrete cylinder is placed inside an insulated drum that is equipped with temperature sensors that record the temperature inside the drum at 15-minute intervals. The temperature and time data are transmitted by computer to a centralized database, where data are stored and analyzed. For this research, the analysis period will range from 2 days up to 10 days.

Test Method

Cast a standard concrete cylinder (6 in. x 12 in.) from concrete sampled at the project site.

Place the cylinder inside the adiabatic calorimeter and seal it.

Temperature sensors record the temperature inside the drum at 15-minute intervals.

Data from the sensors are uploaded to a central database.

Centralized database software is utilized for analysis and downloading reports.

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Test Apparatus

Adiabatic calorimeter: insulated container equipped with temperature sensors.

Standard concrete cylinder.

Personal computer to transmit data to a central database.

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The basic analysis of the heat signature consists of a graph of time vs. temperature. Plotting multiple samples on the same chart may reveal differences in the mix’s chemistry (figure 9-5).


Changes in heat signature may impact the following concrete properties:

Workability and consolidation.

Rate of strength gain.

Ultimate concrete strength.

Initial window for saw-cutting contraction joints.

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________________________________________________Figure 9-5. Heat signature sample plots


Test Methods


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Interactions between concrete pavements and the environment during construction have the potential to adversely impact the constructability and durability of concrete pavements. Collecting, monitoring, recording, and responding to the temperature of the concrete and the underlying base course, along with weather data from the construction site, will allow the construction team to minimize the risk of problems.

Concrete Temperature, Subgrade Temperature, and Project Environmental Conditions


Environmental factors, such as temperature, relative humid-ity, wind velocity, solar radiation, and temperature changes, influence the rate of drying, setting time, and strength gain of a slab, thus changing the saw-cutting window and the risk of cracking or curling.


Concrete (ASTM C 1064) and ambient temperatures are taken with a thermometer when the concrete is sampled.

Subgrade temperature is taken with a pyrometer in front of the paver when the concrete is sampled.

Project environmental conditions are recorded using a por-table weather station.

Test Method

Measure and record concrete temperature when the sample is taken.

Measure and record subgrade temperature in front of the paver, using a handheld infrared pyrometer aimed at the base layer directly beneath the concrete pavement.

Measure and record environmental conditions using a portable weather station located at the test trailer near the project.

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Test Apparatus

Thermometer to measure concrete temperature.

Pyrometer to measure subgrade temperature.

Weather station to measure project environmental conditions.

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Temperatures and environmental conditions should be reported in a tabular format. Changes in temperature, wind speed, and humidity should be monitored and may require changes in mix proportions or construction practice.


Concrete temperatures should be monitored. Temperatures above 35˚C (95˚F) may affect workability.

Thermal gradients in the slab at an early age may cause pre-mature cracking. This condition may be aggravated by high subgrade temperatures at the time of placement.

Many combinations of weather conditions can adversely affect a concrete pavement. The following are the primary conditions to observe:

High temperatures may increase the risk of incompatibility between the reactive components of a mix.

Hot, dry, windy conditions contribute to plastic shrinkage cracking.

Cold conditions will increase the time of setting.

Sudden cold fronts can cause premature cracking.

Rain can damage the surface of fresh concrete unless the surface is protected.

There is an inherent risk in the construction of concrete pavements. Weather conditions are beyond the contractor’s control, and forecasts are not entirely reliable. Even with experience and the best judgment, conditions may arise that will result in damage to a concrete pavement.

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Test Methods


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Measuring the maturity of concrete pavements is a nondestructive test method for estimating in-place concrete strength. It is quickly becoming standard practice. Maturity may be used as a criterion for opening a pavement to traffic and for quality control purposes.

Concrete Maturity


The degree of hydration (leading to strength) of a given mix design is a function of time and temperature. Correlation curves can be developed for a mix design that estimate con-crete strength based on its maturity. The in-place strength of a pavement can be estimated by monitoring the temperature of the slab over time and using the correlation curve that was developed for that mixture.

A maturity curve (strength estimate based on maturity) is only applicable to a specific mix design.


The maturity curve is developed by casting, curing, and testing standard strength specimens while measuring and recording the temperature of those specimens over time (ASTM C 1074).

Maturity testing is performed by inserting a temperature sensor in the slab and then downloading the temperature data to a computer that compares the slab temperature data to the maturity curve.

Test Method

Maturity Curve:

Cast 13 strength specimens from materials that are mixed at the project site.

Completely embed a temperature sensor in one of the specimens. This specimen is used only for recording the temperature over time and will not be broken.

Cure all the strength specimens in the same location (constant temperature).

Test the strength of the specimens at one, three, five, and seven days. Break and average three specimens at each age.

Download and record the time/temperature factor (TTF) for each set of strength specimens when they are broken.

Plot the strength and TTF data for the strength speci-mens on a graph, with TTF on the x-axis and concrete strength on the y-axis.

Fit a smooth curve through the plotted points.

In-Place Maturity (estimated strength):

Completely embed a temperature sensor in the pavement.

Download the TTF from the sensor at any time.

Estimate the strength of the concrete pavement using computer software and the appropriate maturity curve.

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Test Apparatus

Beams, cylinders, and hydraulic loading frame for strength testing to develop the maturity curve.

Sensors to measure the temperature of the test specimens and of the pavement.

Computer software to analyze strength, temperature, and time data for developing the maturity curve and estimating the pavement strength.

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Commercially available maturity systems normally include software that provides the estimated concrete strength based on the maturity of the concrete (TTF). A sample maturity curve is shown in figure 9-6.

Concrete property: strength development (see Maturity Testing, chapter 5, page 121).

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Concrete Maturity, continued


Maturity testing is a way of nondestructively estimating the strength of a concrete pavement.

It cannot be overemphasized that the maturity vs. strength relationship is mix-specific. Maturity estimates of in-place strength are valid only if the pavement being tested is con-structed using the same mix design that was used to develop the maturity curve.

Changes in the water-cementitious materials ratio, air con-tent, grading, aggregate proportions, admixtures, etc., may cause a maturity curve to overestimate the strength of the pavement. ________________________________________________

Figure 9-6. Sample maturity curve

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Purpose – Why Do this Test?

Concrete strength is critical because it reflects concrete’s ability to carry intended loads. Flexural and compressive strength testing are currently the standard methods of evaluating and assessing pay factors for pavement concrete. The tests are required for calibrating maturity-based monitoring systems.

Flexural Strength and Compressive Strength (Seven Day)


A measured force is applied to concrete samples of consistent cross-sectional area (beams and cylinders) until the samples fail. The force required to break the sample is used to calculate the strength based on the cross-sectional area of the sample.


Samples of fresh concrete from the project are cast in cylinder and/or beam molds. These test specimens are cured in laboratory conditions until they are broken in accordance with ASTM C 39 (compression) or ASTM C 78 (flexure). A consistent and continuously increasing force is applied to the test specimens by a hydraulic testing machine. The maximum force required to break the sample and the actual dimensions of each sample are recorded.

Test Method

Sample and cast cylinder and beam specimens in accor-dance with standard procedures.

Cover and protect specimens from evaporation and damage for 24 hours.

Remove specimens from the molds and transfer to the curing tanks.

Cure the specimens in a controlled environment until they are broken.

Remove the specimens from the curing tanks. Trim cylin-ders to length, taking care to avoid letting the specimens dry out before they are tested.

Place the specimens in the hydraulic testing frame and apply a force until the specimen breaks.

Record the maximum force applied and the dimensions of the specimen.

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Test Apparatus

Cylinder and beam molds for casting strength specimens (6-in. diameter x 12-in. height for cylinders and 6-in. width x 6-in. height x 24-in. length for beams).

Curing tanks to provide consistent curing conditions for the specimens.

Hydraulic testing frame for applying the force.

Cutoff saw, neoprene cap, and miscellaneous tools for preparing the specimens.

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Strength results are reported in a tabular format in units of pounds per square inch (lb/in2). Other data in the report should include specimen identification, specimen dimensions, span length (beams), and maximum force applied.

Formulas for concrete strength calculations:Flexural strength = ([force x span] / [width x depth2]) Compressive strength = force / (π x radius2)

Concrete property: strength development (see Strength Testing, chapter 5, page 119).

Test Methods


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Flexural Strength and Compressive Strength (Seven Day), continued


Laboratory-cured strength tests are a representation of the concrete mix’s strength properties. The strength of the pave-ment will differ from laboratory-molded and laboratory-cured specimens due to differences in consolidation and differ-ences in the curing environment. Core specimens taken from the slab can be used to verify pavement strengths.

Conditions that may prevent the strength tests from being representative of the actual concrete strength include the following:

The load rate does not conform to standard procedures; faster load leads to higher strength test results.

Beam specimens are allowed to dry before testing, result-ing in lower strength test results.

Specimen dimensions are not uniform, resulting in lower strength test results.

Specimens are not adequately consolidated, resulting in lower strength test results.

The strength of the concrete pavement structure is influ-enced by the following factors:

Water-cementitious materials ratio.

Air content.

Consolidation.

Curing conditions.

Aggregate grading, quality, and shape.

Concrete strength has long been an acceptance criterion for concrete pavements. From a long-term performance standpoint, characteristics other than strength have a significant impact on pavement durability. Adequate strength is a good indicator of concrete quality, but it does not guar-antee performance. Focusing on strength alone may ignore important properties, such as air entrainment, permeability, and workability.

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Freeze-thaw resistance of concrete is primarily controlled by the spacing factor of the air-void system. The air-void analyzer provides a method of measuring the spacing factor in fresh concrete, rather than waiting for microscopical analysis of hardened concrete. A sample of mortar is taken from the concrete after it has been through the paver and tested immediately, with a result obtained in about 30 minutes.

Air-Void Analyzer (AVA)


Gently stirring a sample of fresh concrete mortar releases the air bubbles through a viscous fluid and then through a column of water. The air bubbles are captured under a submerged bowl that is connected to a scale. As the air bubbles collect, the buoyancy (mass) of the bowl is recorded over time. The distribution of different-sized bubbles is a function of Stoke’s Law (larger bubbles rise faster than smaller bubbles).


A sample of fresh concrete mortar is taken from the slab behind the paver using a vibrating cage attached to a hand drill. A 20-cc portion of the mortar sample is injected into the instrument, which then gently stirs it to release the air bubbles into the fluid.

The measurement continues for 25 minutes or until the weight of the bowl remains unchanged for 3 minutes.

Software then processes the scale readings that were taken over time and, using an algorithm, calculates the air bubble spacing factor and bubble size.

Test Method

Obtain a sample of fresh mortar behind the paver.

Using a syringe, extract 20 cc of mortar from the sample.

Eject the 20-cc sample from the syringe and gently agitate it for 30 seconds.

The bubbles are released from the mortar sample and, over time, rise through the separation liquid and through a column of water.

As the bubbles rise, they are collected underneath a submerged bowl.

The buoyancy (mass) of the submerged bowl is measured over time as the bubbles are collected.

The test is concluded when the mass of the submerged bowl remains constant for 3 minutes or at the end of 25 minutes, whichever occurs first.

The computer and software collect and analyze the data from the scale, which is part of the AVA.

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Test Apparatus

Portable drill with vibrating cage for obtaining mortar sample.

Air-void analyzer (AVA) with all supplies, cables, etc.

Personal computer with AVA software.

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Software provided with the AVA produces tabular and graphical reports. Values are reported for the following:

Spacing factor: Values less than 0.01 in. are desirable.

Specific surface (bubble size): Values greater than 0.04 mm are desirable.

Air-void content of paste.

Air-void content of concrete.

The results may not reflect actual spacing factors, but the output can be calibrated from trial batches in which the data have been collected using both the AVA and linear traverse methods.

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Construction issues – What Should I Look For?

The AVA permits monitoring of the spacing factor and specific surface (bubble size) in fresh concrete. Changes in the results will indicate changes in the concrete mixture, which should then be investigated.

The instrument may also be used to assess changes in the air-void system due to transporting and handling through the paver.

Concrete property: air content (see Testing / Air-Void System, chapter 5, page 136).

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Entrained air is essential to the long-term durability of concrete pavements that are subject to freezing and thawing. Air content is a commonly specified parameter in paving specifications. It is usually measured at the truck using a pressure meter (normally a type B meter).

Air Content (Plastic Concrete, Pressure Method)


The fresh concrete is fully consolidated in an airtight container. Pressure from a fixed-volume cell is applied to the sample in the container. Air in the sample is compressed, and the reduction in pressure in the cell is directly related to the volume of air in the sample. The air content of the sample is thus read directly from the gauge of a properly calibrated meter.


The test is described in ASTM C 231. A sample of fresh concrete is fully consolidated in the air meter and struck off level-full. A known volume of air at a known pressure is applied to the sample in an airtight container. The air content of the concrete is read from the gauge on the pressure meter apparatus.

Test Method

Consolidate the concrete in the measuring bowl using a tamping rod or vibrator and mallet.

Strike off the concrete in the measuring bowl so that it is level-full with the top rim.

Clean the edge and rim of the measuring bowl and clamp the cover on to form an airtight seal.

Pump air into the air chamber until the gauge needle is stabilized on the initial pressure line.

Open the valve between the air chamber and the measuring bowl.

Tap the measuring bowl with the mallet to ensure that pressure is equalized.

Tap the gauge lightly if necessary to stabilize the needle indicator.

Record the percentage of air content indicated on the gauge.

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Test Apparatus

Measuring bowl and airtight cover (type B meter) for hold-ing the sample and measuring the air content.

Tamping rod/vibrator and mallet for consolidating the sample.

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Air content of the fresh concrete mix is read directly from the gauge of a calibrated type B pressure meter.

This is a measure of the percentage of total air content in a con-crete mix. Both entrained air and entrapped air are measured.

The results are compared to the specified limits.


Air content should be monitored regularly during paving (minimum one test every two hours).

Generally, air contents greater than 4.5 percent (depending on exposure and aggregate size) provide adequate protec-tion from freeze/thaw conditions. However, the use of an AVA is recommended to be sure that proper bubble spacing and bubble size are present.

High air contents are less worrisome than low air contents, unless the strength is reduced to critical levels due to the high air content.

Air content can be affected by many factors, ranging from cement and admixture chemistry to mixing time and aggre-gate grading.


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Another method of determining the quality of an air-void system in concrete is microscopical analysis of hardened concrete. This method provides information on the total air content, as well as the spacing factor and other parameters.

Air Content (Hardened Concrete)


The air-void structure of concrete can be measured and characterized by examining a section of a core with a microscope. The linear traverse method consists of measur-ing the air voids as a polished concrete sample travels under a microscope in regularly spaced lines. The length of travel across air voids is compared to the length of travel across paste and aggregate, and the data are used to calculate the air content, spacing factor, and specific surface of the air voids in the concrete sample.


The method is described in ASTM C 457. A core from the slab is sectioned and polished. The apparatus is used to move a core sample under a microscope (or vice versa) in straight lines. The total length traversed and the length traversed across air voids are recorded.

Test Method

Obtain a core from the pavement.

Cut a section of the core.

Grind, lap, and polish the core section until it is smooth and flat.

Cover the polished face of the core section with black ink from a stamp pad.

Heat the core to 54°C (130°F) and coat the ink-covered core section with a zinc paste.

Allow the core section to cool, and scrape the zinc paste off the surface. The melted zinc paste will remain in the air voids of the surface, providing a white contrast to the black ink surface of the core section.

Mount the prepared core section in the image analysis apparatus.

Start the image analysis apparatus.

The image analysis hardware and software automatically traverse the section and record the data.

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Test Apparatus

Saw for cutting a section of a core.

Polishing tools for grinding, lapping, and preparing the core section.

Hardware and software for measuring air voids in the core section.

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The software produces a tabular report showing air content, spacing factor, and specific surface area of the air voids. A digital image of the core section can also be viewed or printed.

The air content is expressed as a percent of volume of the concrete sample.

Spacing factor is the average distance from any point to the nearest air void, or the maximum length measured from the cement paste to the edge of an air void.

Specific surface area is the surface area of the air voids divided by the air voids’ volume.


Spacing factors should be less than 0.2 mm (0.008 in.).

Air-void spacing can be impacted by many factors, ranging from cement and admixture chemistry to mixing time to aggregate grading.


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The ability of concrete to resist the transportation of chlorides is an important factor in its potential durability. If chlorides can be prevented from reaching any steel in the concrete, then the risk of corrosion is reduced.

The test method also provides an indirect measure of the permeability of the concrete, a critical parameter in all durability-related distress mechanisms. The lower the permeability, the longer the concrete will survive chemical attack.

Chloride Ion Penetration


The permeability of concrete can be indirectly assessed by measuring the electrical conductance of a sample of concrete.


The test is described in ASTM C 1202. A 2-in. thick section is obtained from a 4-in. diameter pavement core. The core section is completely saturated with water in a vacuum apparatus. Electrical current is passed from one side of the core section to the other side while it is contained within a cell that has a sodium chloride solution on one side of the core and a sodium hydroxide solution on the other side. The electric current is applied and measured for six hours.

Test Method

Completely saturate the core section with water.

Place the saturated core section in the sealed cell con-taining the two different sodium solutions on either side of the core section.

Connect the power supply and voltmeter.

Apply a 60-volt DC current across the cell for six hours.

Convert the ampere-seconds curve recorded from the test to coulombs.

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Test Apparatus

Vacuum saturation apparatus: Completely saturates the sample.

Sodium chloride solution.

Sodium hydroxide solution.

Sealed cell: Holds the core specimen with each liquid solution on opposite sides of the core section and has electrical leads for connecting a DC electrical source.

DC power supply: Provides constant DC power to the test specimen.

Voltmeter: Measures and records volts and amps on both sides of the core specimen.

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The electrical current is conducted through the concrete by chloride ions that migrate from one side of the core section to the other side. Higher permeability will result in a higher cur-rent being carried through the core section.

The test results are expressed in coulombs, and the perme-ability of the concrete is classified according to table 9-5. Note that differences within the range of 1,300 to 1,800 coulombs are not significant.


Mix design issues that can influence permeability include the following:

Lower water-cementitious materials ratio will lead to lower conductivity.

Use of fly ash; ground, granulated blast-furnace slag; and silica fume will generally reduce conductivity.

Paving process inputs that influence permeability include the following:

Improved consolidation will reduce conductivity.

Premature final finishing when excessive bleed water is present will increase surface permeability.

Proper curing will reduce conductivity.

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Table 9-5. Relationship Between Coulombs and Permeability

Concrete property: permeability (see Permeability / Testing, chapter 5, page 131).

Coulombs Permeability

> 4,000 high2,000 to 4,000 moderate1,000 to 2,000 low100 to 1,000 very low

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The expansion and contraction of concrete due to temperature changes can impact the durability of joints and the risk of cracking in concrete pavements.

Coefficient of Thermal Expansion


Concrete expands and contracts as its temperature changes. When a saturated cylinder of concrete is exposed to chang-ing temperature conditions, its change in length can be measured by a linear variable differential transformer (LVDT).


A saturated concrete cylinder or core is subjected to tem-perature changes from 10 to 50°C (50 to 120°F). The change in length of the cylinder is measured and recorded at different temperatures (AASHTO TP 60).

Test Method

Soak a 4-in. diameter core in water for a minimum of 48 hours.

Measure the length of the saturated core using calipers.

Place the core in the support frame that is submerged in the water tank.

Adjust the temperature of the water tank to 10°C (50°F).

Maintain the temperature until three consecutive LVDT readings taken every 10 minutes change by less than 0.00025 mm (0.00001 in.). Record the initial LVDT and temperature values.

Set the temperature of the water tank to 50°C (120°F).

Maintain the temperature until three consecutive LVDT readings taken every 10 minutes change by less than 0.00025 mm (0.00001 in.). Record the second LVDT and temperature values.

Adjust the temperature of the water tank to 10°C (50°F).

Maintain the temperature until three consecutive LVDT readings taken every 10 minutes change by less than 0.00025 mm (0.00001 in.). Record the final LVDT and tem-perature values.

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Test Apparatus

Caliper to measure the initial length of the core specimen.

Water tank: Maintains saturation of the sample and varies the temperature of the water from 10 to 50°C (50 to 120°F).

Support frame: Holds the core specimen and the LVDT.

Thermometer: Measures the water temperature.

LVDT: Measures the length change of the specimen (resolution = 0.00025 mm [0.00001 in.]).

•

•

•

•

•


The coefficient of thermal expansion (CTE) is a function of length change due to a change in temperature.

CTE = (measured length change / specimen length) / measured temperature change

The CTE reported is the average of both test values.

The CTE is reported in microstrain/°F. Typical values for con-crete can range from 4(10-6) to 7(10-6)°F. CTE is most affected by aggregate type. Concrete produced with siliceous aggre-gates has a higher CTE than concrete produced with limestone aggregates.


Thermal expansion/contraction is a factor that should be considered in the design phase. During construction, the following items should be monitored for conformity with the plans to avoid the possibly adverse effects of thermal expan-sion and contraction:

Joint layout and spacing.

Joint width.

•

•

Concrete property: thermal movement (see Thermal Expansion/Contraction, chapter 5, page 129).

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AASHTOT84,SpecificGravityandAbsorptionofFineAggregate

AASHTOT85,SpecificGravityandAbsorption

ofCoarseAggregate

AASHTOT97,FlexuralStrengthofConcrete(UsingSimpleBeamwithThird-PointLoading)

AASHTOT121,Density(UnitWeight),Yield,andAirContent(Gravimetric)ofConcrete

AASHTOT152,AirContentofFreshlyMixedCon-cretebythePressureMethod

AASHTOT153,FinenessofHydraulicCementbyAirPermeabilityApparatus

AASHTOT185,EarlyStiffeningofPortlandCement(MortarMethod)

AASHTOT196,StandardMethodofTestforAirContentofFreshlyMixedConcretebytheVolumetricMethod

AASHTOT255,StandardMethodofTestforTotalEvaporableMoistureContentofAggregatebyDrying

AASHTOT277,StandardMethodofTestforElectri-calIndicationofConcrete’sAbilitytoResistChlorideIonPenetration

AASHTOT309,StandardTestMethodforTempera-tureofFreshlyMixedHydraulic-CementConcrete

AASHTOT318,WaterContentofFreshlyMixedConcreteUsingMicrowaveOvenDrying

AASHTOT325,StandardMethodofTestforEsti-matingtheStrengthofConcreteinTransportationConstructionbyMaturityTests

AASHTOTP60,StandardTestMethodfortheCoef-ficientofThermalExpansionofHydraulicCementConcrete

ASTM standards may be found in Annual Book of ASTM Standards 2004, ASTM International, 2004. www.astm.org.

ASTMC39,StandardTestMethodforCompressiveStrengthofCylindricalConcreteSpecimens

ASTMC70-94(2001),StandardTestMethodforSur-faceMoistureinFineAggregate

ASTMC78-02,StandardTestMethodforFlexuralStrengthofConcrete(UsingSimpleBeamwithThird-PointLoading)

ASTMC94/C94M-04a,StandardSpecificationforReady-MixedConcrete

ASTMC127-04/AASHTOT85,StandardTestMethodforDensity,RelativeDensity(SpecificGravity),andAbsorptionofCoarseAggregate

ASTMC128-04/AASHTOT84,StandardTestMethodforDensity,RelativeDensity(SpecificGrav-ity),andAbsorptionofFineAggregate

ASTMC136,StandardTestMethodforSieveAnalysisofFineandCoarseAggregates

ASTMC138/C138M-01a/AASHTO121,StandardTestMethodforDensity(UnitWeight),Yield,andAirContent(Gravimetric)ofConcrete

ASTMC173/C173M-01e1/AASHTOT196,StandardTestMethodforAirContentofFreshlyMixedCon-cretebytheVolumetricMethod

ASTMC204,StandardTestMethodforFinenessofHydraulicCementbyAirPermeabilityApparatus

ASTMC231-04/AASHTOT152,StandardTestMethodforAirContentofFreshlyMixedConcretebythePressureMethod

ASTMC359,StandardTestMethodforEarlyStiffen-ingofHydraulicCement(MortarMethod)

ASTMC457-98,StandardTestMethodforMicro-scopicalDeterminationofParametersoftheAir-VoidSysteminHardenedConcrete

ASTMC531,StandardTestMethodforLinearShrinkageandCoefficientofThermalExpansionofChemical-ResistantMortars,Grouts,MonolithicSur-facings,andPolymerConcretes

ASTMC566-97(2004)/AASHTOT255,StandardTestMethodforTotalEvaporableMoistureContentofAggregatebyDrying

References


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ASTMC1064/C1064M-04/AASHTOT309,Stan-dardTestMethodforTemperatureofFreshlyMixedHydraulic-CementConcrete

ASTMC1074,StandardPracticeforEstimatingStrengthbytheMaturityMethod

ASTMC1077,StandardPracticeforLaboratoriesTestingConcreteandConcreteAggregatesforUseinConstructionandCriteriaforLaboratoryEvaluation


AASHTO.1996.Quality Assurance Guide Specification.ReportoftheAASHTOHighwaySubcommitteeonConstruction.Washington,D.C.:AmericanAssocia-tionofStateHighwayandTransportationOfficials.

ACPA.1999.Database of State DOT Concrete Pavement Practices.Skokie,IL:AmericanConcretePavementAssociation.www.pavement.com.

———.1994.VerticalDowelLocation:TechTip.Concrete Pavement Progress34.3.

Kosmatka,S.,B.Kerkhoff,andW.C.Panarese.2003.Design and Control of Concrete Mixtures.EB001/CD100.Skokie,IL:PortlandCementAssociation.

Landgren,R.1994.UnitWeight,SpecificGravity,Absorption,andSurfaceMoisture.Significance of Tests and Properties of Concrete and Concrete Making Materials.STP169c.Eds.PaulKliegerandJosephF.Lamond.WestConshohocken,PA:AmericanSocietyforTestingandMaterial.421–428.

Mindess,S.,J.F.Young,andD.Darwin.2003.Concrete.2ndEd.UpperSaddleRiver,NJ:PrenticeHall.

Seward,K.,etal.[pleaselistallauthorshere].2004.OklahomaDepartmentofTransportationDraftSpecialProvision.Quality Control and Acceptance Procedures for Optimized Portland Cement Concrete Pavements.414-10QA(a-x)99.OklahomaCity,OK:OklahomaDepartmentofTransportation.

Steffes,R.andS.Tymkowicz.1997.VibratorTrailsinSlipformedPavements.Concrete Construction.April.361–368.

Steffes,R.andS.Tymkowicz.2002.ResearchPaysOff:VibratorMonitorsConcretePavingTechnology,GeneratesBuzz.Transportation Research News.November.

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Chapter 10Troubleshooting and PreventionTable 10-1. Problems Observed Before the Concrete Has Set 275

Table 10-2. Problems Observed in the First Days After Placing 282

Table 10-3. Preventing Problems That Are Observed at Some Time After Construction 286

Table 10-4. Assessing the Extent of Damage in Hardened Concrete 290

Mostmaterials-relatedproblemsthatoccurduringpavingoperationsareduetoactionstakenorcon-ditionsmetduringmaterialsselectionorconcretemixingandplacing.Topreventandfixproblems,allmembersoftheconstructionteamneedtounderstandthematerialstheyareworkingwithandbepreparedtoaddresspotentialscenarios.

Inappropriaterepairsmayfailtocorrecttheprob-lemandmaynotlast.Itisessential,therefore,thatbeforechangesaremadeorremedialactiontaken,thecauseofaproblembeunderstood.Thismayinvolveinvestigativeworktouncoverthemechanismsandsourcesofthedistress.Whendoingthistypeofinves-tigation,itisimportanttomakeaccurateobservationsanddocumentthem.

Beginanyinvestigationbylookingforpatternsthatmayconnectcauseandeffect.Inparticular,lookforchangesthatmayhaveledtotheproblem.

Possibilitiesmayincludeweatherchanges,achangeinmaterialsourceorquality,andstaffingchanges.Problemsareoftenacombinationoffactors,includingdesignanddetailing,materialsselectionandpropor-tioning,andconstructionpractices.Aprojectmaybeproceedingsatisfactorily,butasmallchangeinanyoneofthesefactorsmaytipthebalanceandresultinaproblem.

Notallproblemsareobservedintheearlystages.Somebecomeapparentonlylater.Thischaptercon-tainstablesthatrecommendactionstobetakenwhenproblemsareobserved—beforetheconcretehasset(seeplasticshrinkagecracksinfigure10-1),duringthefirstfewdaysafterplacement(seeotherearly-agecracksinfigure10-1),orsometimeafterthecon-cretehashardened.Thetablesrefertosectionsinthemanualwheredetailedinformationcanbefoundaboutthefactorsthatmayhavecontributedtotheproblem.

____________________________________________________________________________________________________Figure 10-1. Early-age cracking

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Thetablesonthispageareoutlinesonlyofthoseonpages275–290.

Before the Concrete Has SetSomepavingproblemsaregenerallyobservedupto

thetimeoffinalset(table10-1).Someofthese,likeinsufficientairintheconcrete,canbefixedimmedi-atelyiftheyareobservedearlyenough.Insuchcases,admixturedosagesmightbeadjustedifallowedbythespecificationandgoodpractice.Others,suchasrapidstiffening,maybeobservedonlyaftertheconcretehasbeenplaced.Insuchcases,thematerialmayhavetoberemoved,oritmaybeacceptedaslongasfuturedeliveriesarecorrected.

Table 10-1. Problems Observed Before the Concrete Has SetMixture and Placement issues 1.Slumpisoutofspecification. 2.Lossofworkability/slumploss/earlystiffening. 3.Mixtureissticky. 4.Mixturesegregates. 5.Excessivefreshconcretetemperature. 6.Aircontentistoolowortoohigh. 7.Variableaircontent/spacingfactor. 8.Mixsetsearly. 9.Delayedset. 10.Supplierbreakdown,demandchange,raw

materialchanges.

Edge and Surface Issues 11.Fiberballsappearinmix. 12.Concretesurfacedoesnotclosebehindpaving

machine. 13.Concretetearsthroughpavingmachine. 14.Pavingleavesvibratortrails. 15.Slabedgeslump. 16.Honeycombedslabsurfaceoredges. 17.Plasticshrinkagecracks.

Someproblems,suchasearly-agecracking,areobservedsometimebetweenfinalsetanduptoafewdaysafterplacing(table10-2).Someremedialworkmaybepossible(andrequired),andstepsshouldbetakentopreventtheirrecurrence.Finally,someprob-lemsareobservedonlyafterthepavementhasbeen

inplaceforsometime(table10-3).Inthesecases,somerepairmaybepossibleorreplacementmayberequired.Table10-3providesguidelinesonhowto

preventsuchproblemsinfutureconstruction.

Table 10-2. Problems Observed in the First Days after Placing Strength 18.Strengthgainisslow. 19.Strengthistoolow.

Cracking 20.Early-agecracking.

Joint Issues 21.Ravelingalongjoints. 22.Spallingalongjoints. 23.Dowelsareoutofalignment.

Table 10-3. Preventing Problems That Are Observed Some Time after Construction

Edge and Surface Issues 24.Clayballsappearatpavementsurface. 25.Popouts. 26.Scaledsurface. 27.Dustingalongsurface. 28.Concreteblisters. 29.Surfacebumpsandroughridingpavement. 30.Surfaceismarredormortariswornaway. 31.Cracking.

Table10-4isalistingofsomeofthenondestructivetesting(NDT)techniquesavailabletoassesstheextentofdamageinhardenedconcrete.

Table 10-4. Assessing the Extent of Damage in Hardened Concrete 1.Dowelbaralignment. 2.Pavementthickness. 3.Pavementsubgradesupportandvoiding. 4.Concretequalityinpavement. 5.Overlaydebondingandseparation. 6.Concretestrength.

Key to Troubleshooting Tables

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Change in water content or aggregate grading Check aggregate moisture contents and absorptions. 44, 47, 183, 206,

Check for segregation in the stockpile. 207, 211

Make sure the batch water is adjusted for aggregate moisture content.

Conduct batch plant uniformity tests.

Check whether water was added at the site.

Potential Cause(s) Actions to Consider/Avoid See Page

�. Slump is Out of Specification

Dry coarse aggregates Make sure the aggregate stockpile is kept consistently at 206 saturated surface-dry (SSD) (use soaker hoses if necessary).


�. Loss of Workability/Slump Loss/Early Stiffening

Mix proportions Check batch equipment for calibration. 207

Admixture dosage Check delivery ticket for correct admixture dosage. 207

Concrete temperature too high or too low Adjust the concrete placement temperature. 127

Haul time Check the batch time on the concrete delivery ticket. 209 Haul times should not be excessive.

Ambient temperature increases Do not add water. 179, 182, 183, 206,

Chill the mix water or add ice. 210, 226

Sprinkle the aggregate stockpiles.

Use a water reducer or retarder.

Do not increase the water/cement ratio to a value greater than the maximum approved mix design.

Use a mix design that includes slag or fly ash.

Transport time too long Reject the load if greater than specified. 183, 209

Use retarder in the mixture.

Use an agitator rather than dump trucks.

Mix proportions have changed Check/monitor the moisture contents of the aggregate 206, 207, 246 stockpiles.

Check the batch weigh scales.

Verify that aggregate gradations are correct.

False setting (temporary) Check for changes in cementitious materials. 58, 209, 211

Reduce Class C fly ash replacement.

Change the type of water reducer.

Try restoring plasticity with additional mixing.

Contact the cement supplier.

Incompatibility Check for changes in the cementitious materials. 97, 246, 247

Reduce Class C fly ash replacement.

Change chemical admixtures.

Change the batching sequence.

Cool the mixture.

Variation in air content Check the air content/air entrainer dosage. 56

Table 10-1. Problems Observed Before the Concrete Has Set

Mixture and Placement Issues

Problems Observed Before the Concrete Has Set


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Inconsistent concrete material—batching, Check aggregate gradation; poorly graded mixtures may mixing, placing tend to segregate.

Verify batching/mixing procedures so that the mixture is adequately mixed.

Check aggregate stockpile, storage, and loading procedures to prevent aggregate segregation.

Place concrete as close to final position as possible to minimize secondary handling.

Perform uniformity testing on batch plant, if necessary, use agitator trucks for transport.

Reduce the vibration energy if consolidation efforts cause segregation. (Vibration at 5,000–8,000 vpm is sufficient for most well-graded mixtures.)


�. Mixture Segregates

Hot ingredients Do not add water. 128, 226, 247

Follow hot-weather concreting practice as appropriate.

Chill the mix water or use ice.

Shade and sprinkle the aggregate stockpiles.


�. Excessive Fresh Concrete Temperature

Long haul times Adjust the hauling operation to minimize haul times. 209

Adjust paving time to off-peak traffic time if hauling through public traffic.

Hot weather Follow hot-weather concreting practice as appropriate. 129, 226, 247

Chill the mix water; sprinkle the aggregate stockpiles.

Pave at night or start paving in afternoon.

Mixture and Placement Issues, continued

Sand too fine Change the sand grading. 44, 109

Mix too sandy Check the sand and combined aggregate grading. 180

Cementitious materials Check the cementitious materials contents. 31, 109, 179, 214 (Mixtures containing GGBF slag and fly ash appear sticky but finish well and respond well to vibration energy.)

Lower the vibration energy to avoid segregation.

Adjust the mix proportioning.

Using wood float on air-entrained concrete Use magnesium or aluminum floats.


�. Mixture is Sticky

176, 206, 207, 208,

213, 215, 246



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Temperature changes The air-entraining admixture dosage may need to be 186 adjusted during hot/cold weather.


�. Air Content is Too Low or Too High

Materials have changed Check for uniformity of materials.

Mix proportions have changed Altering other admixture dosages may impact the 176, 180, 246 effectiveness of the air-entraining admixture.

Check slump; it is easier to entrain air with increasing concrete workability.

Check/monitor the moisture contents of the aggregate stockpiles.



Verify sand quantity.

Short or inadequate mixing Check the charging sequence. 209 Increase mixing time. Check if the blades of the mixer are missing or dirty.

Incorrect or incompatible admixture types Change types or brands of admixtures. 186, 248

Try to work within one manufacturer’s family of admixtures if air-entraining agent is being combined with other admixtures.

Admixture dosage Check the batching equipment for calibration and settings. 207, 248

Change the sequence of batching.

Mix proportions have varied or changed Check/monitor the moisture contents of the aggregate 176, 183, 206, 248 stockpiles.



Cementitious materials Check for changes in cementitious materials, particularly the 34 loss-on-ignition (LOI) content of fly ash.

Poor plant configuration Introduce aggregates together on the plant’s belt feed 207 (requires a multiple weigh hopper).

Poor aggregate grading Use a more well-graded coarse and fine aggregate mixture. 176

Check variation in the amount of materials retained on the #30 through #100 sieves.

Temperature changes Air-entraining admixture dosage may need to be adjusted 56, 133, 247 during hot/cold weather.

Altering other admixture dosages may impact the effectiveness of the air-entraining admixture; air-entraining admixtures work more efficiently with increasing workability.

Variable mixing Ensure that each batch is handled consistently in the plant. 207


�. Variable Air Content/Spacing Factor




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Excessive retarder dosage Verify the proper batch proportions. 56, 183, 207, 246

Check the batching equipment.

Reduce the dosage of the retarder.

Excessive water reducer dosage Verify the proper batch proportions. 56, 183, 207

Reduce the dosage of the water reducer.

Retarder not dispersed well Improve mixing to disperse the retarder. 209

Supplementary cementitious materials Reduce GGBF slag content; GGBF slag in excess of 31interference 25 percent can cause a dramatic increase in set time.

Eliminate/reduce fly ash content in the mix.

Cold placement temperature Follow cold-weather concreting practices if appropriate. 228

Organic contamination Verify the proper batch proportions. 39, 52, 207

Check for contamination of water and aggregates.


�. Delayed Set


Cementitious materials Check for changes in the cementitious materials; differing 99 sources or changes in properties of a given material may result in incompatibility; changes in proportions may also affect setting times.

Admixture dosage Check the dosage of chemical admixtures, particularly 56, 99, 183, 207, 246 accelerators.

Check the batching equipment.

Hot weather Adjust the mix proportions. 31, 99, 185, 186,

Use mix designs that include GGBF slag or fly ash. 226, 246

Use a retarder.

Reduce haul time if possible.

Reduce the placement temperature of the concrete.

In hot weather, use a hot weather mix design.

Cool the concrete ingredients.


�. Mix Sets Early



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Cement Refer to backup lab mixes if conditions were anticipated. 28, 171, 211

Switch sources, batch new mix designs, and develop new laboratory strength gain and maturity information. (This action may require a project delay. To avoid unacceptable delays, a contractual agreement should be arranged prior to paving, which allows for unforeseen material supply changes, burden of delay costs, and risk of paving during batch revision testing. If paving activity is continued during testing, compare early-age strengths (1- and 3-day) and maturity data to confirm that the new mix will perform adequately.)

Supplementary cementitious materials See cement supply change. 31

Switch sources and compare early-age strengths (1- and 3-day) and maturity data to confirm that the mix will perform adequately.

Aggregates See cement supply change. 39

Switch sources and compare early-age strengths (1- and 3-day) and maturity data to confirm that the mix will perform adequately.

Chemical admixtures See cement supply change. 55

Switch admixture sources and compare early-age strengths (1- and 3-day) and maturity data to confirm that the mix will perform adequately.


�0. Supplier Breakdown, Demand Change, Raw Material Changes


Fibers not thoroughly dispersed in mix If added in bags, check the timing of addition and subsequent 62 mixing.

Some mixes do not break down bags as easily as others (i.e., smaller sized rounded coarse aggregate mixes); check compatibility.

Use a blower for synthetic fibers or a belt placer for steel fibers instead of bags.


��. Fiber Balls Appear in Mixture

Edge and Surface Issues



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Non-workable concrete mix Review concrete workability field test data. 109

See no. 2: Loss of workability/slump loss/early stiffening.

Over-sanded mixes Increase the coarse aggregate. 176, 180

Poor combined aggregate grading Check the combined aggregate grading. 176

Insufficient volume contained in the grout box Place more material in front of the paver; consider using 209 a spreader.

The concrete is stiffening in the grout box Check for premature concrete stiffening (admixture 97 compatibility). (See no. 2: Loss of workability/slump loss/early stiffening.)

The fine/coarse aggregate volume or paste Check mixture proportions, particularly aggregate gradations. 171, 176volume is too low Check the uniformity of aggregate materials/supplies.

The finishing pan angle needs adjustment Adjust the pan angle.

The paver speed is too high or vibrators Slow the paver. 215, 248need to be adjusted Lower the vibrator frequencies or use vibrators with greater

force. Adjust the location of the vibrators; raise them closer to the

surface. Place more material in front of the paver; consider using a

spreader. Change the vibrator angle.


��. Concrete Surface Does Not Close Behind Paving Machine

Excessive concrete slump loss Check for slump loss and mixture or weather changes. 109


Insufficient concrete slump Check the mixture proportions. 171

Angular fine aggregate (manufactured sand) Replace a portion of the manufactured sand with natural sand. 39

Paver speed too high Slow the paver.

Coarse aggregate is segregated Check the stockpile. 109, 206

Coarse aggregate is gap-graded Check the combined aggregate grading. 109, 176

Blend the aggregate with intermediate aggregates to achieve a uniform combined grading.


��. Concrete Tears Through Paving Machine

Vibrator frequency too low Check if the seals on the vibrators are leaking. 248

Vibrator frequency too high Lower the vibrator frequency. 248

Paver speed too slow Increase the paver speed. 212


��. Paving Leaves Vibrator Trails

Edge and Surface Issues, continued



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Hot weather may induce premature stiffening Follow hot-weather concreting practices if appropriate. 226


Inadequate vibration Check that all vibrators are working properly, at the right 248 frequency and amplitude; the paver speed should not be too high.

Add an additional vibrator near the slipformed edge.

Poor workability Check for changes in the aggregate grading. 176


��. Honeycombed Slab Surface or Edges

Poor and/or nonuniform Verify the mix design and batching procedures. 176, 246concrete—gap-graded aggregate, Check the aggregate grading—use a well-graded combined high water/cement ratio, etc. aggregate gradation.

Inadequate operation of equipment Check the construction procedures. 212

Adjust the outside vibrator frequency.

Adjust the side form batter.

Improper equipment setup Adjust the overbuild. 212

Check the track speed (same on both sides).

Check the pan profile.


��. Slab Edge Slump


High evaporation rate (excessive loss of Apply the curing compound as soon as possible to protect 158, 176, 191, 206, moisture from surface of fresh concrete; the concrete from loss of moisture. 224, 226i.e., evaporation rate > bleed rate) Use additional curing measures: fogging, evaporation retarder,

windbreaks, shading, plastic sheets, or wet coverings. Make sure the absorptive aggregates are kept moist; a dry

concrete mixture from concrete aggregates that are not saturated tends to surface dry at mixing. This is problematic if not accounted for.

Use a well-graded combined aggregate (gap gradation requires more paste and causes more shrinkage).

Refer to hot-weather concreting practices if appropriate.

Pave at night.

Chill the mixing water.

Dampen the subgrade.

Avoid paving on hot, windy days.

Consider adding fibers to the mix.

Delayed setting time Check the time of set. 114


17. Plastic Shrinkage Cracks (figures 5-30, 5-31, 10-1)

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Cold temperature during/after placement Heat the mix water. 31, 59, 121, 182,

Use burlap/insulating blankets for protection from freezing. 224, 228, 233

Use an accelerating admixture.

Eliminate/reduce GGBF slag and fly ash content in the mix.

Increase the cement content.

Use a Type III cement.

Utilize early-entry sawing to reduce the potential for random cracking.

Monitor the slab temperature with maturity sensors.

Mix proportions or materials have changed Check/monitor the moisture contents of the aggregate 176, 206, 207, 211 stockpiles.

Check for uniformity of the cementitious materials.



Verify that batch weights are consistent with the mix design.


��. Strength Gain is Slow

Cementitious materials Check for changes in the cementitious materials. 211

Check that the correct materials have been loaded into the cement/fly ash/slag silos.

Water Check the water content. 181, 206, 207

Verify the aggregate moisture contents and batch weights.

Change in sand grading Check the sand stockpile to see whether the grading has 176 changed.

Contamination with organics Contamination of one of the ingredients with organics can also effect a sudden change in the required dosage of air-entraining admixture; try to isolate the source.

Inadequate or variable mixing Examine the mixer and mixing procedures. 207, 208

Check for worn mixer blades.

Check for mixer overloading.

Batch smaller loads.

Check the sequencing of batching.

Check for mixing time consistency.

Conduct batch plant uniformity testing.

Plant operations Verify the acceptability of the batching and mixing process. 207, 208

Check for adequate mixing times.

Check if water was added to the truck.

Testing procedures Verify proper making, curing, handling, and testing of strength 263 specimens. (Flexural strength specimens are particularly vulnerable to poor handling and testing procedures.)

Verify the machine acceptability testing.

Test the cores sampled from the pavement to verify acceptance.

Air-void clustering Use a vinsol resin-based air-entraining admixture. 100, 209

Avoid retempering.

Increase the mixing time.


��. Strength is Too Low

Table 10-2. Problems Observed in the First Days After Placing

Strength

Problems Observed in the First Days After Placing


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Concrete mixture Check the combined aggregate grading. 31, 39, 55, 88, 97, 148,

Examine the fine aggregates; fine aggregates may be too fine 176, 228, 235 and angularity may cause harsh finishing (i.e., manufactured sands).

Reduce the paste content (minimize shrinkage potential).

Materials incompatibility may lead to delayed set and/or higher concrete shrinkage; consider mixture component adjustments.

Eliminate or reduce the content of fly ash or GGBF slag in cool-weather conditions.

Consider using an accelerator in cold weather.

Sawing Saw as early as possible but avoid excessive raveling. 231, 233

Saw in the direction of the wind.

Check that the diamond saw blade is appropriate for concrete aggregate hardness, fines, etc.

Use early-entry dry sawing.

Use HIPERPAV to model stress versus strength gain for conditions to determine the optimum sawing time.

Curing Improve/extend curing. 224

Apply the curing compound at a higher rate.

Apply the curing compound sooner.

Use blankets between placing and saw-cutting.

Insufficient joint depth Check the saws for depth setting. 231, 233

Check the saw blade for wear (carbide blades).

Check that saw operators are not pushing saws too fast, causing them to ride up.

Look for base bonding or mortar penetration into the open-graded base-altered effective section; increase the saw depth to create an effective weakened plane.

Check the slab thickness.

Excessive joint spacing Reduce spacing between the joints.

Slabs are too wide in relation to thickness and length; add intermediate joints.

Maintain a reasonable length-width ratio.

Warping (slab curvature due to moisture Check the moisture state of base. 150, 224gradient; the term “curling,” however, Improve or extend curing. is commonly used in the industry to cover Minimize the shrinkage potential of the concrete mixture. both moisture- and temperature-related slab distortion) Cover the slab, particularly when night/day temperatures

vary widely.


20. Early-Age Cracking (figures 5-32, 5-33, 5-34, 5-35, and 10-1)

Cracking




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High temperature Cool the raw materials before mixing the concrete: shade, 64, 156, 224, 226 spray, ice, liquid nitrogen

Cool the equipment.

Work at night.

Watch for shaded areas where drying and strength gain may vary within a single day’s work.

Delay paving if conditions are too hot (>38°C [100°F]).

Apply an evaporative retardant prior to texturing.

Apply the curing compound at an additional dosage rate and consider a non-water-based compound with better membrane-forming solids.

Too many lanes tied together Do not exceed 15 m (50 ft) of pavement tied together. (generally only a consideration Add an untied construction or isolation joint. for longitudinal direction) To prevent additional cracking, consider sawing through a

longitudinal joint to sever bars.

Edge restraint (paving against an Cracks occur due to restraint to movement (sometimes existing or previously placed lane) referred to as sympathy cracks).

Tool the joint or use an early-entry dry saw to form the joints as early as possible.

Match the joint location and type.

Eliminate tiebars in a longitudinal construction joint that is within 24 inches on either side of transverse joint locations. Match all locations of the joints in the existing pavement (cracks, too).

Slab/base bonding or high frictional restraint Moisten the base course prior to paving (reduce the base 191 temperature by evaporative cooling).

Use a bond-breaking medium (see reflective cracks).

If the base is open graded, use a choker stone to prevent the penetration of concrete into the base’s surface voids.

Misaligned dowel bars Investigate whether the joints surrounding the crack have 218 activated and are functioning; misaligned or bonded dowels may prevent joint functioning, causing cracks.

Cold front with or without rain shower Use early-entry sawing to create a weakened plane prior to 231, 233 temperature contraction.

Skip-saw (saw every other joint or every third joint) until normal sawing can be resumed.

Use HIPERPAV to model stress versus strength-gain conditions that may warrant a suspension or change of paving activities.


20. Early-Age Cracking, continued

Cracking, continued



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Excessive hand finishing Check for mixture problems that would necessitate 220 overfinishing.

Improve construction practice.

Trying to fix edge slump of low spots Check for mixture problems that would cause edge slump. 217by hand manipulating concrete Improve construction practice.

Mortar penetration into transverse joints Mortar penetration occurs when paving against an existing (after hardening mortar prevents joint closure) previously placed lane; apply duct tape or other means to

block the penetration of mortar into the transverse joints of the existing lane.

Collateral damage from equipment, Protect the edges of the slab from damage using gravel or slipform paver tracks, screeds, etc. dirt ramps.

Delay placement of the next phase of construction until the concrete gains sufficient strength.


��. Spalling Along Joints

Movement in dowel basket assemblies Cover the dowel baskets with concrete ahead of the paver. 218, 248

Use stakes to secure the baskets to the granular base.

Increase the length and number of stakes.

Use nailing clips on both sides of basket to secure the basket to the stabilized base.

Dumping directly on dowel baskets Deposit the concrete a few feet from the dowel basket to 212 allow the concrete to flow around the dowel bars.

Poor aggregate gradation Dowel insertion into mixtures with gap-graded aggregates 176 does not work well; improve the aggregate grading.


��. Dowels Are Out of Alignment

Sawing too soon Wait longer to saw. 233

Use formed joints.

Blank out transverse tining at transverse contraction joints.

Saw equipment problem Blade selection for the concrete (coarse aggregate type) 233 may be inadequate.

A bent arbor on the saw causes the blade to wobble.

The second saw cut can go back and forth; consider a single-cut design.

Sawing too fast Slow down. 233


��. Raveling Along Joints

Joint Issues

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Aggregate stockpile contamination Educate the loader operator on proper stockpile 206generally caused by the following: management techniques.

• Haul trucks tracking clay and Keep end-loader buckets a minimum of 2 ft off the ground. mud to stockpiles Do not stockpile aggregates on soft foundations. • Loader operator digging into dirt Stabilize the haul road at the plant site to avoid tracking • Dirt coming from the quarry contaminants.

Use belt placers at stockpiles rather than end loaders.

Check the aggregate producer’s stockpiles.

Check for contamination in the hauling equipment.

Do not drive over a bridge to unload the aggregate.

Mud being thrown into concrete trucks Cover the trucks. from muddy haul roads


��. Clay Balls Appear at Pavement Surface

Unsound aggregates Use only aggregates that have been tested for chert, shale, 45, 48 and/or other undesirable fine particles.

Reduce vibration to minimize the flotation of particles.

Alkali-silica reactions Use non-alkali silica reactive aggregates. 31, 48, 141

Use blended cements or SCMs proven to control ASR.


��. Popouts

Table 10-3. Preventing Problems That Are Observed at Some Time After Construction

Edge and Surface Issues

Premature finishing Improve the finishing technique. 220

Improper finishing Do not add water to the surface during finishing. 220

Over-finishing Improve the finishing technique. 220

Frost related Protect the concrete from freezing until a sufficient strength 186, 228 is achieved.

Concrete damaged by freezing must be removed and replaced.

Check the air content and spacing factor in the hardened concrete.

Premature salting; salts should not be applied to immature concrete.

Check the de-icing salts being used.


��. Scaled Surface

Adding water during finishing or Prevent the addition of water during finishing. 220finishing in bleed water Delay finishing until after the dissipation of the bleed water.


��. Dusting Along Surface

Preventing Problems That Are Observed at Some Time After Construction


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Premature closing of surface Check for bleed water trapping. 220, 221

Consider using a double burlap drag to open the surface.

Extremely high/variable air content Check for the consistency of the air content. 186

Vibrators too low Check the vibrator depth. 215

Vibrator frequency too high Reduce the vibrator frequency. 215

Over-sanded mixes Increase the coarse aggregate. 176

Poor combined aggregate grading Check the combined aggregate grading. 176(gap grading)


��. Concrete Blisters

Placement operations Construct and maintain a smooth and stable paver 212, 215 track line.

Check the string line tension and profile.

Maintain a consistent quantity of concrete in front of the paver.

Maintain a consistent forward motion; avoid a stop-and-go operation.

Check the paver tracks.

Check that the machine is level.

Check the sensors on the paver.

Verify that the paver electronics/hydraulics are functioning properly.

Nonuniform concrete Check the batching, mixing, and transport procedures for 176, 206, 207 consistency.

Check the aggregate grading and moisture contents for variations that might lead to wet and dry batches.

Damming or rebound from dowel baskets Lack of consolidation to achieve a uniform concrete density 215, 218, 248 within the dowel basket area may create a rough surface because the concrete may settle or slough over the dowels.

Check that the dowel baskets are secured.

The basket assembly deflects and rebounds after the slipform paver profile pan passes overhead and the extrusion pressure is released. The result is a slight hump in the concrete surface just ahead of the basket. Spring-back is more apt to occur on steeper grades and when there is too much draft in the pan; do not cut the basket spacer wires to prevent the basket from springing under the paver’s extrusion pressure.

Do not overvibrate the concrete at the baskets in an effort to prevent basket movement.


��. Surface Bumps and Rough Riding Pavement





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Rained-on surface Cover the slab to protect from rain. 229

Remove the damaged surface by grinding.

Restore the surface texture (if required) by grinding.

Improper curing type or application Place a curing blanket or plastic sheets after the bleed water 224 sheen disappears.

Consider using a membrane-forming curing compound instead of sheets/blankets.

Use of higher dosages (>25%) of GGBF slag Do not add water to the mixture. 248

Reduce the vibration energy to avoid bringing too much moisture to the surface; vibration at 5,000–8,000 vpm is sufficient for most well-graded mixtures.

Over-sanded mixes Increase the coarse aggregate. 176

Abrasion Use a hard, wear-resistant aggregate. 50, 116

Use a concrete mix with sufficient strength.


�0. Surface is Marred or Mortar is Worn Away

Reinforcement ripple Address reinforcement ripple issues with well-graded 106, 176 aggregates and uniform concrete; consolidation is achieved at lower vibration energy and extrusion pressure.

Reinforcement ripple occurs when plastic concrete is restrained by the reinforcing bars, resulting in a ripple in the surface, with the surface slightly lower near each bar than in the area between the bars.

Longitudinal depressions are caused when longitudinal bars limit the restitution of the surface level behind the profile pan by restraining the rebound of the concrete beneath the bars.

Transverse ripple is caused by the transverse bars in the same way as longitudinal depressions, except that transverse ripple is found to be less noticeable than the prominent ridge caused by the damming effect of the transverse bars upon the upsurge flow of concrete behind the profile pan.

The prominence of surface rippling depends on the finishing techniques and depth of cover to the reinforcement, with less cover producing more prominent rippling.

Vertical grades (exceeding 3 percent) Lower the slump of the concrete; the need to make an 212, 215 adjustment depends upon whether it is difficult to maintain a uniform head of concrete in front of the paver.

Adjust the profile pan attitude, draft, or angle of attack. (When paving up a steeper grade, the pan elevation may be adjusted to about 25 mm [1.0 in.] below the surface grade. When paving down a steeper slope, the pan may be adjusted to about 25 mm [1.0 in.] above the surface grade. This adjustment must be made carefully to avoid reinforcement ripple, particularly a spring-back of the embedded dowel baskets.)

Adjust the staking interval; closely follow the grade and staking calculations for these circumstances to reduce the semi-chord effect enough to produce a smooth surface.


��. Surface Bumps and Rough Riding Pavement, continued




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Applied loads Keep construction traffic away from the slab edges; early 152, 157 loading by traffic or equipment causes higher edge stresses.

Keep public traffic away from the slab edges.

Loss of support Ensure that the subgrade and base have been properly 151, 157, 192, 237 prepared.

Ensure that the joints are properly filled and sealed where appropriate.

Reflective cracks from stabilized bases Isolate the slab from cracks in the base course by using bond 198 breakers. (Acceptable bond breakers include two coats of wax-based curing compound, dusting of sand, bladed fines, asphalt emulsion, polyethylene sheets, and tar paper. Sheet goods are difficult to handle in windy or other harsh conditions.)

Joint the base course to match the joints in the pavement.

Slab/base bonding or high frictional restraint Moisten the base course prior to paving (reduce the base 198 temperature by evaporative cooling).

Use a bond-breaking medium (see “Reflective cracks from stabilized bases,” immediately above)

If the base is open-graded, use a choker stone to prevent the penetration of concrete into the base’s surface voids.

Mortar penetration into transverse joints Mortar penetration occurs when paving against an existing (after hardening mortar prevents joint closure) previously placed lane; apply duct tape or other means to

block the penetration of mortar into the transverse joints of the existing lane.

Differential support condition created by frost Check base compaction, particularly above utility, culvert, 192, 196heaving, soil settling, or expansive soils and other trenches.

Proof roll the base.

Stabilize the subgrade soil.

Use selective grading techniques; cross-haul the soils to create smooth transitions between cut and fill sections and soil transitions.

Misaligned dowel bars Investigate whether the joints surrounding the crack have 218 cracked and are functioning; misaligned or bonded dowels may prevent joint functioning, causing cracks.

Designate personnel to ensure dowel alignment.

Alkali-silica reactions Avoid using reactive aggregates if possible. 31, 48, 141, 157, 179

Use appropriate amounts of SCMs.

Use blended cements or SCMs proven to control ASR.

Use a low w/cm ratio.

Chemical attack Use a low w/cm ratio, maximum 0.45. 28, 157, 179

Use an appropriate cementitious system for the environment.

Frost related Ensure that the air-void system of the in-place concrete is 49, 135, 136, 157, 179 adequate.

Use a low w/cm ratio.

Use frost-resistant aggregates.

Reduce the maximum particle size.


��. Cracking

Cracking

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1. Dowel bar alignment Twin antenna radar

Pulse induction (MIT-scan)

2. Pavement thickness Impact-echo, radar

3. Pavement subgrade support and voiding Impulse response

Benkelman beam

Falling weight deflectometer (FWD)

4. Concrete quality in pavement: Impulse radar

a) Inclusions in pavement (clay balls) Impulse response

b) Honeycombing and poor concrete consolidation

5. Overlay debonding and separation: Hammer sounding

a) Traditional Chain drag

b) Advanced techniques Impulse response

Impact-echo, radar

6. Concrete strength Rebound hammer (for comparative tests)

Windsor probe

Problem Nondestructive Testing Method(s)

Table 10-4. Assessing the Extent of Damage in Hardened Concrete

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ACI.1998.Nondestructive Test Methods for Evaluation of Concrete in Structures.ACI228.2R-98.FarmingtonHills,MI:AmericanConcreteInstitute.

ACPA.2002.Early Cracking of Concrete Pavement – Causes and Repairs.TB016P.Skokie,IL:AmericanConcretePavementAssociation.

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Davis,A.G.,B.H.Hertlein,M.K.Lim,andK.A.Michols.1996.Impact-EchoandImpulseResponseStress-WaveMethods:AdvantagesandLimitationsfortheEvaluationofHighwayPavementConcreteOverlays.Proceedings of the International Society for Optical Engineering, Nondestructive Evaluation of Bridges and Highways.Scottsdale,AZ:InternationalSocietyforOpticalEngineering.

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Hertlein,B.H.andA.G.Davis.1996.Performance-basedandCondition-basedNDTforPredictingMaintenanceNeedsofConcreteHighwaysandAirportPavements.Conference on Nondestructive Evaluation of Aging Aircraft, Airports and Aerospace Hardware.ProcSPIE2945273-281.Eds.R.D.RemptandA.L.Broz.Scottsdale,AZ:InternationalSocietyforOpticalEngineering.

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Kosmatka,S.H.,B.Kerkhoff,andW.C.Panarese.2002.DesignandControlofConcreteMixtures.EB001.14.Skokie,IL:PortlandCementAssociation.

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Lerch,W.1946.TheInfluenceofGypsumontheHydrationandPropertiesofPortlandCementPastes.RX012.Skokie,IL:PortlandCementAssociation.

Ramachandran,V.S.1995.ConcreteAdmixturesHandbook:PropertiesScience,andTechnology.ParkRidge,NJ:NoyesPublishers.

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Chapter �


ASTMC33-03,StandardSpecificationforConcreteAggregates

ASTMC294-04,StandardDescriptiveNomenclatureforConstituentsofConcreteAggregates

ASTMC535-03/AASHTOT96,StandardTestMethodforResistancetoDegradationofLarge-SizeCoarseAggregatebyAbrasionandImpactintheLosAngelesMachine

ASTMC563,TestMethodforOptimumSO3inHydraulicCementUsing24-hCompressiveStrength

ASTMC618-03/AASHTOM295,StandardSpecifica-tionforCoalFlyAshandRaworCalcinedNaturalPozzolanforUseasaMineralAdmixtureinConcrete

ASTMC684-99,StandardTestMethodforMaking,AcceleratedCuring,andTestingConcreteCompres-sionTestSpecimens

ASTMC989-04/AASHTOM302,StandardSpecifica-tionforGroundGranulatedBlast-FurnaceSlagforUseinConcreteandMortars

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Simon,M.J.,R.B.Jenkins,andK.C.Hover.1992.TheInfluenceofImmersionVibrationontheVoidSystemofAir-EntrainedConcrete.DurabilityofConcrete,G.M.IdornSymposium,ACISP-131.FarmingtonHills,Michigan:AmericanConcreteInstitute.99–126.

Stark,D.1976.CharacteristicandUtilizationofCoarseAggregatesAssociatedwithD-Cracking.RD047.Skokie,IL:PortlandCementAssociation.

StarkD.,S.H.Kosmatka,J.A.Farny,andP.D.Tennis.2002.ConcreteSpecimensinthePCAOutdoorTestFacility.ResearchandDevelopmentBulletinRD124.Skokie,IL:PortlandCementAssociation.

Stark,D.1982.LongtimeStudyofConcreteDura-bilityinSulfateSoils.RD086.Skokie,IL:PortlandCementAssociation.

Stark,D.1989.DurabilityofConcreteinSulfate-RichSoils.RD097.Skokie,IL:PortlandCementAssociation.

Stark,D.2002.PerformanceofConcreteinSulfateEnvironments.RD129.Skokie,IL:PortlandCementAssociation.

Steffes,R.andS.Tymkowicz.1997.VibratorTrailsinSlipformedPavements.ConcreteConstruction(April):361–368.

Taylor,H.F.W.CementChemistry.2ndEd.London:ThomasTelford.

Tynes,W.O.1975.InvestigationofHigh-StrengthFrost-ResistantConcrete.MiscellaneousPaperC-75-6.Vicksburg,MS:U.S.ArmyCorpsofEngineersWater-waysExperimentStation.

Verbeck,G.J.andC.W.Foster.1949.LongTimeStudyofCementPerformanceinConcretewithSpecialReferencetoHeatsofHydration.RX032.Skokie,IL:PortlandCementAssociation.

Chapter �

AASHTO standards may be found in Standard Specifications for Transportation Materials and Methods of Sampling and Testing, 24th Edition, and AASHTO Provisional Standards, 2004 Edition, HM-24-M. https://www.transportation.org/publications/bookstore.nsf.

AASHTOPP34,ProvisionalStandardtoAssessSus-ceptibilitytoCracking


AASHTOT27,TestMethodforSieveAnalysisofFineandCoarseAggregates

AASHTOT97,FlexuralStrengthofConcrete(UsingSimpleBeamwithThirdPointLoading)

AASHTOT152,TestMethodforAirContentofFreshlyMixedConcretebythePressureMethod

AASHTOT177,FlexuralStrengthofConcrete(UsingSimpleBeamwithCenter-PointLoading)

AASHTOT198,SplittingTensileStrengthofCylindri-calConcreteSpecimen

AASHTOT255,TestMethodforTotalEvaporableMoistureContentofAggregatebyDrying

AASHTOT277,TestMethodforElectricalIndicationofConcrete’sAbilitytoResistChlorideIonPenetration

AASHTOT303,AcceleratedDetectionofPotentiallyDeleteriousExpansionofMortarBarsDuetoASR

AASHTOT309,TemperatureofFreshlyMixedPort-landCementConcrete

AASHTOTP60,StandardTestMethodfortheCoef-ficientofThermalExpansionofHydraulicCementConcrete


ASTMC39,TestMethodforCompressiveStrengthofCylindricalConcreteSpecimens

ASTMC78,TestMethodforFlexuralStrengthofConcrete(UsingSimpleBeamwithThirdPointLoading)

Bibliography


ASTMC136,TestMethodforSieveAnalysisofFineandCoarseAggregates

ASTMC157,StandardTestMethodforLengthChangeofHardenedHydraulicCement,Mortar,andConcrete

ASTMC215,TestMethodforFundamentalTransverse,Longitudinal,andTorsionalResonantFrequenciesofConcreteSpecimens

ASTMC231,TestMethodforAirContentofFreshlyMixedConcretebythePressureMethod

ASTMC293,TestMethodforFlexuralStrengthofConcrete(UsingSimpleBeamWithCenter-PointLoading

ASTMC418,TestMethodforAbrasionResistanceofConcretebySandblasting

ASTMC469,TestMethodforStaticModulusofElas-ticityandPoisson’sRatioofConcreteinCompression

ASTMC496,StandardTestMethodforSplittingTen-sileStrengthofCylindricalConcreteSpecimens

ASTMC566,TestMethodforTotalEvaporableMois-tureContentofAggregatebyDrying

ASTMC617,PracticeforCappingCylindricalCon-creteSpecimens

ASTMC666,TestMethodforResistanceofConcretetoRapidFreezingandThawing

ASTMC672,TestMethodforScalingResistanceofConcreteSurfacesExposedtoDeicingChemicals

ASTMC779,TestMethodforAbrasionResistanceofHorizontalConcreteSurfaces

ASTMC827,StandardTestMethodforChangeinHeightatEarlyAgesofCylindricalSpecimensofCementitiousMixtures

ASTMC944,TestMethodforAbrasionResistanceofConcreteorMortarSurfacesbytheRotating-CutterMethod

ASTMC1064,StandardTestMethodforTemperatureofFreshlyMixedHydraulic-CementConcrete

ASTMC1074,PracticeforEstimatingConcreteStrengthbytheMaturityMethod

ASTMC1138,TestMethodforAbrasionResistanceofConcrete(UnderwaterMethod)

ASTMC1202,TestMethodforElectricalIndicationofConcrete’sAbilitytoResistChlorideIonPenetration

ASTMC1231,PracticeforUseofUnbondedCapsinDeterminationofCompressiveStrengthofHardenedConcreteCylinders

ASTMC1260,TestMethodforPotentialAlkaliReac-tivityofAggregates(Mortar-BarMethod)

ASTMC1293,TestMethodforDeterminationofLengthChangeofConcreteDuetoAlkali-SilicaReaction

ASTMC1556,StandardTestMethodforDetermin-ingtheApparentChlorideDiffusionCoefficientofCementitiousMixturesbyBulkDiffusion

ASTMC1567,StandardTestMethodforDeterminingthePotentialAlkali-SilicaReactivityofCombinationsofCementitiousMaterialsandAggregate(AcceleratedMortar-BarMethod)

ASTMC1581,StandardTestMethodforDeterminingAgeatCrackingandInducedTensileStressChar-acteristicsofMortarandConcreteunderRestrainedShrinkage

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———.1998.NondestructiveTestMethodsforEvaluationofConcreteinStructures.ACI228.2R-98.FarmingtonHills,MI:AmericanConcreteInstitute.

Baalbaki,W.,B.Benmokrane,O.Chaallal,andP.C.Aítcin.1991.InfluenceofCoarseAggregateonElasticPropertiesofHigh-PerformanceConcrete.ACIMateri-alsJournal88.5:499.

Bérubé,M.A.,J.Duchesne,andD.Chouinard.1995.WhytheacceleratedmortarbarmethodASTMC1260isreliableforevaluatingtheeffectivenessofsupplementarycementingmaterialsinsuppressingexpansionduetoalkali-silicareactivity.Cement,Con-crete,andAggregates17.1:26–34.

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NewlonJr.,H.1978.ModificationofASTMC666forTestingResistanceofConcretetoFreezingandThaw-inginaSodiumChlorideSolution.VHTRC79-R16.Charlottesville,VA:VirginiaTransportationResearchCouncil.

Powers,T.C.1968.ThePropertiesofFreshConcrete.NewYork,NY:Wiley&SonsInc.

Roper,H.1960.VolumeChangesofConcreteAffectedbyAggregateType.ResearchDepartmentBulletinRX123.Skokie,IL:PortlandCementAssociation.http://www.portcement.org/pdf_files/RX123.pdf.

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Tazawa,Ei-ichi.1999.AutogenousShrinkageofCon-crete.NewYork,NY:E&FNSponandRoutledge.

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Touma,W.E.,D.F.Fowler,R.L.Carrasquillo.2001.Alkali-SilicaReactioninPortlandCementCon-crete:TestingMethodsandMitigationAlternatives.ICAR-301-IF.Austin,TX:InternationalCenterforAggregatesResearch.

UnitedStatesArmyCorpsMethodCRD-C38.1973.MethodofTestforTemperatureRiseinConcrete.http://www.wes.army.mil/SL/MTC/handbook/crd_c38.pdf.

UnitedStatesArmyCorpsMethodCRD-C55.1992.TestMethodforWithin-BatchUniformityofFreshlyMixedConcrete.http://www.wes.army.mil/SL/MTC/handbook/crd_c55.pdf.

Weymouth,C.A.G.1933.EffectsofParticleInterfer-enceinMortarsandConcrete.RockProducts36.2:26–30.

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Chapter �

ASTMC309-03/AASTOM148,StandardSpecifica-tionforLiquidMembrane-FormingCompoundsforCuringConcrete

Okamoto,P.,R.Rollings,R.Detwiller,R.Perera,E.Barenberg,J.Anderson,M.Torres,H.Barzegar,M.Thompson,J.Naughton.2003.BestPracticesforAirportPortlandCementConcretePavementCon-struction(RigidAirportPavement).IPRF-01-G-002-1.Washington,D.C.:InnovativePavementResearchFoundation.

Chapter �

ACPA.1997.ConcreteIntersections–AGuideforDesignandConstruction.TB019P.Skokie,IL:Ameri-canConcretePavementAssociation.

ASTMC33-03,StandardSpecificationforConcreteAggregates

Ledbetter,W.andA.Meyer.1976.WearandSkidResistanceofFull-ScaleExperimentalConcreteHigh-wayFinishes.TransportationResearchRecord602:62–68.

Zollinger,D.,T.Tang,D.Xin.1994.SawcutDepthConsiderationsforJointedConcretePavementBasedonFractureMechanicsAnalysis.TransportationResearchRecord1449:91–100.

Chapter �

ACI.2004.InPlaceMethodtoEstimateConcreteStrength.ACI228.1R-95.FarmingtonHills,MI:AmericanConcreteInstitute.

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ACPA.1992.MaturityTestingofConcretePavements:ApplicationsandBenefits.IS275.Skokie,IL:Ameri-canConcretePavementAssociation.www.cement.org/pdf_files/IS275.pdf.

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———.2003.ConstructingSmoothConcretePave-ments.TB006.02P.Skokie,IL:AmericanConcretePavementAssociation.

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———.1999.FastTrackPaving:ConcreteTempera-tureControlandTrafficOpeningCriteriaforBondedConcreteOverlays,VolumeII,HIPERPAVUser’sManual.FHWA-RD-98-168.McLean,VA:FederalHighwayAdministration.

———.17May1989.MemorandumfromDirector,OfficeofHighwayOperationstoRegionalFederalHighwayAdministrators.

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Morey,R.M.1998.GroundPenetratingRadarforEvaluatingSubsurfaceConditionsforTransportationFacilities.NCHRPSynthesisofHighwayPractice255.Washington,D.C.:TransportationResearchBoard.

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Chapter �0


ASTMC1383-04a.StandardTestMethodforMeasur-ingtheP-WaveSpeedandtheThicknessofConcretePlatesUsingtheImpact-EchoMethod.(2)

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———.2003.HowtoHandleRained-onConcretePavement.Research&TechnologyUpdate4.04.Skokie,IL:AmericanConcretePavementAssociation.

———.2003.WhattoDoWhenFacedWithEarly-AgeCracking.Research&TechnologyUpdate4.07.Skokie,IL:AmericanConcretePavementAssociation.

———.2003.ConstructingSmoothConcretePave-ments.TB006.02P.Skokie,IL:AmericanConcretePavementAssociation.

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AAASHTO AmericanAssociationofStateHighwayandTransportationOfficials

Absolute Specific Gravity Theratiooftheweightreferredtoinavacuumofagivenvolumeofmaterialatastatedtemperaturetotheweightreferredtoavacuumofanequalvolumeofgas-freedistilledwateratthesametemperature.

Absolute Volume (of ingredients of concrete or mortar) Thedisplacementvolumeofaningredientofconcreteormortar;inthecaseofsolids,thevolumeoftheparticlesthemselves,includingtheirpermeableorimpermeablevoidsbutexcludingspacebetweenparticles;inthecaseoffluids,thevolumewhichtheyoccupy.

Absorbed Moisture Themoistureheldinamaterialandhavingphysicalpropertiesnotsubstantiallydifferentfromthoseofordinarywateratthesametem-peratureandpressure.

Absorbed Water Water held on surfaces of a material by physical and chemicalforces,andhavingphysicalpropertiessubstantiallydifferentfromthoseofabsorbedwaterorchemicallycombinedwateratthesametemperatureandpressure.

Absorption Theamountofwaterabsorbedunderspecificconditions,usuallyexpressed as a percentage of the dry weight of the material; theprocessbywhichthewaterisabsorbed.

Acceleration Increaseinrateofhardeningorstrengthdevelopmentofconcrete.

Accelerator An admixture which, when added to concrete, mortar, or grout,increasestherateofhydrationofhydrauliccement,shortensthetimeofset,orincreasestherateofhardeningorstrengthdevelopment.

ACIAmericanConcreteInstitute

ACPA AmericanConcretePavementAssociation

ACR Alkali-carbonatereaction

Admixture A material other than water, aggregates, and portland cement(including air-entraining portland cement, and portland blastfurnaceslagcement)thatisusedasaningredientofconcreteandis added to the batch before and during the mixing operation.

Aggregate Granular material, such as sand, gravel, crushed stone, crushedhydraulic-cementconcrete,orironblastfurnaceslag,usedwithahydrauliccementingmediumtoproduceeitherconcreteormortar.

Aggregate Blending Theprocessof intermixing twoormore aggregates toproduce adifferentsetofproperties,generally,butnotexclusively,toimprovegrading.

Aggregate Gradation The distribution of particles of granular material among varioussizes,usuallyexpressedintermsofcumulativepercentageslargeror smaller than each of a series of sizes (sieve openings) or thepercentagesbetweencertain rangesof sizes (sieveopenings). SeealsoGrading.

Aggregate Interlock The projection of aggregate particles or portion of aggregateparticlesfromonesideofajointorcrackinconcreteintorecessesintheothersideofthejointorcracksoastoeffectloadtransferincompressionandshearandmaintainmutualalignment.

Aggregate, Angular Aggregateparticles thatpossesswell-definededges formedat theintersectionofroughlyplanarfaces.

Aggregate, Coarse SeeCoarseAggregate

Aggregate, Dense-graded Aggregates graded to produce low void content and maximumweightwhencompacted.

Aggregate, Fine SeeFineAggregate

Aggregate, Gap- graded Aggregatesogradedthatcertainintermediatesizesaresubstantiallyabsent.

Aggregate, Maximum Size SeeMaximumSizeofAggregate

Aggregate, Nominal Maximum Size In specifications for and descriptions of aggregate, the smallestsieve opening through which the entire amount of the aggregateispermittedtopass;sometimesreferredtoas“maximumsize(ofaggregate).”

Aggregate, Open-graded Concreteaggregateinwhichthevoidsarerelativelylargewhentheaggregateiscompacted.

Agitating Truck Avehicleinwhichfreshlymixedconcretecanbeconveyedfromthepointofmixingtothatofplacing;whilebeingagitated,thetruckbodycaneitherbestationaryandcontainanagitatororitcanbeadrumrotatedcontinuouslysoastoagitatethecontents.

Agitation The process of providing gentle motion in mixed concrete justsufficienttopreventsegregationorlossofplasticity.

Glossary (based on ACPA definitions)



Air Content Theamountofairinmortarorconcrete,exclusiveofporespaceintheaggregateparticles,usuallyexpressedasapercentageof totalvolumeofmortarorconcrete.

Air Void A space in cement paste, mortar, or concrete filled with air; anentrappedairvoidischaracteristically1mm(0.04in.)ormoreinsizeandirregularinshape;anentrainedairvoidistypicallybetween10µmand1mm(34ftand.04in.)indiameterandspherical(ornearlyso).

Air-Entraining Thecapabilitiesofamaterialorprocesstodevelopasystemofminutebubblesofairincementpaste,mortar,orconcreteduringmixing.

Air-Entraining Agent Anadditionforhydrauliccementoranadmixtureforconcreteormortarwhichcausesair,usuallyinsmallquantity,tobeincorporatedin the form of minute bubbles in the concrete or mortar duringmixing,usuallytoincreaseitsworkabilityandfrostresistance.

Air-Entraining Cement Acementthathasanair-entrainingagendaaddedduringthegrindingphaseofmanufacturing.

Air-Entrainment Theinclusionofairintheformofminutebubblesduringthemixingofconcreteormortar.

Alkali-Aggregate Reaction Chemicalreactioninmortarorconcretebetweenalkalis(sodiumandpotassium)releasedfromportlandcementorfromothersources,and certain compounds present in the aggregates; under certainconditions,harmful expansionof theconcreteormortarmaybeproduced.

Alkali-Carbonate Reaction Thereactionbetweenthealkalies(sodiumandpotassium)inportlandcement binder and certain carbonate rocks, particularly calcitedolomiteanddolomiticlimestones,presentinsomeaggregates;theproductsofthereactionmaycauseabnormalexpansionandcrackingofconcreteinservice.

Alkali-Silica Reaction Thereactionbetweenthealkalies(sodiumandpotassium)inportlandcement binder and certain siliceous rocks or minerals, such asopalinechert,strainedquartz,andacisicvolcanicglass,presentinsomeaggregates;theproductsofthereactionmaycauseabnormalexpansionandcrackingofconcreteinservice.

Artificial Turf Surfacetextureachievedbyinvertingasectionofartificialturfthatisattachedtoadevicethatallowscontrolofthetimeandrateoftexturing.

Asphalt A brown to black bituminous substance that is chiefly obtainedas a residue of petroleum refining and that consists mostly ofhydrocarbons.

ASR See,Alkali-SilicaReaction

ASTM AmericanSocietyforTestingandMaterials

BBacker Rod Foamcordthatinsertsintoajointsealantreservoirandisusedtoshapealiquidjointsealantandpreventsealantfromadheringtoorflowingoutofthebottomofthereservoir.

Bag (of cement) Aquantityofcement;42.6kgintheUnitedStates,39.7kginCanada;portlandorair-entrainingportlandcement,orasindicatedonthebagforotherkindsofcement.

Ball Test Atesttodeterminetheconsistencyoffreshconcretebymeasuringthedepthofpenetrationof a steelball.The apparatus isusuallycalledaKellyball.

Bar Amemberusedtoreinforceconcrete,usuallymadeofsteel.

Barrel (of cement) Aunitofweightforcement:(170.6kg)net,equivalentto4USbagsforportlandorair-entrainingportlandcements,orasindicatedbythemanufacturerforotherkindsofcement.(InCanada,158.8kg.netperbarrel.)

Base Theunderlyingstratumonwhichaconcreteslab,suchasapavement,isplaced.

Base Course Alayerofspecifiedselectmaterialofplannedthicknessconstructedonthesubgradeorsubbasebelowapavementtoserveoneormorefunctionssuchasdistributingloads,providingdrainage,minimizingfrostaction,orfacilitatingpavementconstruction.

Batch Quantityofconcreteormortarmixedatonetime.

Batch Plant Equipmentusedforbatchingconcretematerials.

Batch Weights Theweightsofthevariousmaterials(cement,water,theseveralsizesofaggregate,andadmixtures)thatcomposeabatchofconcrete.

Batched Water Themixingwateraddedtoaconcreteormortarmixturebeforeorduringtheinitialstagesofmixing.

Batching Weighingorvolumetricallymeasuringandintroducingintothemixertheingredientsforabatchofconcreteormortar.



Beam Test Amethodofmeasuringtheflexuralstrength(modulusofrupture)ofconcretebytestingastandardunreinforcedbeam.

Binder SeeCementPaste

Blast-Furnace Slag Thenon-metallicbyproduct,consistingessentiallyofsilicatesandaluminosilicates of lime and other bases, which is produced ina molten condition simultaneously with iron in a blast furnace.

Bleeding Theself-generatedflowofmixingwaterwithin,or itsemergencefrom,freshlyplacedconcreteormortar.

Bleeding Rate Therateatwhichwaterisreleasedfromapasteormortarbybleeding,usuallyexpressedascubiccentimetersofwaterreleasedeachsecondfromeachsquarecentimeterofsurface.

Blended Cement SeeCement,Blended

Blended Hydraulic Cement SeeCement,Blended

Blistering Theirregularrisingofathinlayerofplacedmortarorconcreteatthesurfaceduringorsoonaftercompletionofthefinishedoperation.

Bond Theadhesionofconcreteormortartoreinforcementorothersurfacesagainstwhichitisplaced;theadhesionofcementpastetoaggregate.

Bond Area The interface area between two elements across which adhesiondevelopsormaydevelop,asbetweenconcreteandreinforcingsteel.

Bond Breaker Amaterialusedtopreventadhesionofnewlyplacedconcretefromothermaterial,suchasasubstrate.

Bond Hardness Thesupport (bondstrength) that themetalmatrix inadiamondsaw blade segment provides to each diamond that is embeddedwithinthematrix.

Bond Strength Resistancetoseparationofmortarandconcretefromreinforcingsteelandothermaterialswithwhichitisincontact;acollectiveexpressionfor all forces such as adhesion, friction due to shrinkage, andlongitudinalshearintheconcreteengagedbythebardeformationsthatresistseparation.

Bond Stress Theforceofadhesionperunitareaofcontactbetweentwosurfacessuchasconcreteandreinforcingsteeloranyothermaterialsuchasfoundationrock.

Bonded Concrete Overlay Thinlayerofnewconcrete5to10cm(2to4in.)placedontoslightly

deterioratedexistingconcretepavementwithstepstakentoprepareoldsurfacetopromoteadherenceofnewconcrete.

Bonding Agent Asubstanceappliedtoanexistingsurfacetocreateabondbetweenitandasucceedinglayer,asbetweenabondedoverlayandexistingconcretepavement.

Broom Thesurfacetextureobtainedbystrokingabroomoverfreshlyplacedconcrete.Asandytextureobtainedbybrushingthesurfaceoffreshlyplacedorslightlyhardenedconcretewithastiffbroom.

Bulk Cement Cementthatistransportedanddeliveredinbulk(usuallyinspeciallyconstructedvehicles)insteadofinbags.

Bulk Density Themassofamaterial(includingsolidparticlesandanycontainedwater)perunitvolume,includingvoids.

Bulk Specific Gravity The ratio of the weight in air of a given volume of a permeablematerial (including both permeable and impermeable voidsnormal to the material) at a stated temperature to the weight inairofanequalvolumeofdistilledwateratthesametemperature.

Bulking Factor Ratioofthevolumeofmoistsandtothevolumeofthesandwhendry.

Bull Float Atoolcomprisingalarge,flat,rectangularpieceofwood,aluminum,ormagnesiumusually20cm(8in.)wideand100to150cm(39to60in.)long,andahandle1to5m(1.1to5.5yd)inlengthusedtosmoothunformedsurfacesoffreshlyplacedconcrete.

Burlap Acoarsefabricofjute,hemp,orlesscommonlyflax,foruseasawater-retainingcoverforcuringconcretesurfaces;alsocalledHessian.

Burlap Drag Surfacetextureachievedbytrailingmoistenedcoarseburlapfromadevicethatallowscontrolofthetimeandrateoftexturing.

CCalcareous Containing calcium carbonate, or less generally, containing theelementcalcium.

Calcium Chloride A crystalline solid, CaC1

2; in various technical grades, used as a

dryingagent,asanacceleratorofconcrete,adeicingchemical,andforotherpurposes.

Calcium Lignosulfonate An admixture, refined from papermaking wastes, employed inconcretetoretardthesetofcement,reducewaterrequirementandincreasestrength.



Caliche Gravel,sand,ordesertdebriscementbyporouscalciumcarbonateorothersalts.

California Bearing Ratio Theratiooftheforceperunitarearequiredtopenetrateasoilmasswitha19.4cm2circularpistonattherateof1.27mm(.05in.)permintotheforcerequiredforcorrespondingpenetrationofastandardcrushed-rockbasematerial;theratioisusuallydeterminedat2.5mm(.10in.)penetration.

California Profilograph Rolling straight edge tool used for evaluating pavement profile(smoothness)consistingofa25-ftframewithasensingwheellocatedatthecenteroftheframethatsensesandrecordsbumpsanddipsongraphpaperorinacomputer.

Capillary Incementpaste,anyspacenotoccupiedbyunhydratedcementorcementgel(airbubbles,whetherentrainedorentrapped,arenotconsideredtobepartofthecementpaste).

Capillary Absorption Theactionofsurfacetensionforceswhichdrawswaterintocapillaries(i.e.,inconcrete)withoutappreciableexternalpressures.

Capillary Flow Flowofmoisturethroughacapillaryporesystem,suchasconcrete.

Capillary Space

Incementpaste,anyspacenotoccupiedbyanhydrouscementorcementgel.(Airbubbles,whetherentrainedorentrapped,arenotconsideredtobepartofthecementpaste.)

Carbonation Reaction between carbon dioxide and the products of portlandcementhydrationtoproducecalciumcarbonate.

Cast-In-Place Concreteplacedandfinishedinitsfinallocation.

Cement SeePortlandCement

Cement Content Quantityofcementcontainedinaunitvolumeofconcreteormortar,ordinarily expressed as pounds, barrels, or bags per cubic yard.

Cement Factor SeeCementContent

Cement Paste Constituentofconcreteconsistingofcementandwater.

Cement, Blended Ahydrauliccementconsistingessentiallyofanintimateanduniformblend of granulated blast-furnace slag and hydrated lime; or anintimate and uniform blend of portland cement and granulatedblast-furnaceslagcementandpozzolan,producedbyintergrindingPortland cement clinkerwith the othermaterials or byblendingPortland cement with the other materials, or a combination ofintergrindingandblending.

Cement, Expansive Aspecialcementwhich,whenmixedwithwater,formsapastethattendstoincreaseinvolumeatanearlyage;usedtocompensateforvolumedecreaseduetodryingshrinkage.

Cement, High Early-Strength Cement characterizedbyproducing earlier strength inmortar orconcretethanregularcement,referredtointheUnitedStatesasTypeIII.

Cement, Hydraulic Cementthatiscapableofsettingandhardeningunderwater,suchasnormalportlandcement.

Cement, Normal Generalpurposeportlandcement,referredtointheUnitedStatesasTypeI.

Cement, Portland Pozzolan Ahydrauliccementconsistingessentiallyofanintimateanduniformblendofportlandcementorportlandblast-furnaceslagcementandfinepozzolanproducedbyintergrindingportlandcementclinkerand pozzolan, by blending portland cement or portland blast-furnaceslagcementandfinelydividedpozzolan,oracombinationofintergrindingandblending,inwhichthepozzolanconstituentiswithinspecifiedlimits

Cement-Aggregate Ratio Theratio,byweightorvolume,ofcementtoaggregate.

Cementitious Materials Substances that alone have hydraulic cementing properties (setandhardeninthepresenceofwater);includesground,granulatedblast-furnace slag, natural cement, hydraulic hydrated lime, andcombinationsoftheseandothermaterials.

Central Mixer A stationary concrete mixer from which the fresh concrete istransportedtothework.

Central-Mixed Concretethatiscompletelymixedinastationarymixerfromwhichitistransportedtothedeliverypoint.

Chalking Aphenomenonofcoatings,suchascementpaint,manifestedbytheformationofaloosepowderbydeteriorationofthepaintatorjustbeneaththesurface.

Coarse Aggregate SeeAggregate,Coarse

Coefficient of Thermal Expansion Changeinlineardimensionperunitlengthorchangeinvolumeperunitvolumeperdegreeoftemperaturechange.

Cohesion Loss Thelossofinternalbondwithinajointsealantmaterial;notedbyanoticeabletearalongthesurfaceandthroughthedepthofthesealant.

Cohesiveness Thepropertyofaconcretemixwhichenablestheaggregateparticlesand cement paste matrix therein to remain in contact with each



otherduringmixing,handling,andplacingoperations;the“stick-togetherness”oftheconcreteatagivenslump.

Combined Aggregate Grading Particlesizedistributionofamixtureoffineandcoarseaggregate.

Compacting Factor The ratio obtained by dividing the observed weight of concretewhichfillsacontainerofstandardsizeandshapewhenallowedtofallintoitunderstandardconditionsoftest,bytheweightoffullycompactedconcretewhichfillsthesamecontainer.

Compaction The process whereby the volume of freshly placed mortar orconcrete is reduced to the minimum practical space, usually byvibration,centrifugation,tamping,orsomecombinationofthese;tomolditwithinformsormoldsandaroundembeddedpartsandreinforcement,andtoeliminatevoidsotherthanentrainedair.SeealsoConsolidation.

Compression Test Testmadeonaspecimenofmortarorconcrete todeterminethecompressivestrength;intheUnitedStates,unlessotherwisespecified,compressiontestsofmortarsaremadeon50-mm(2-in.)cubes,andcompressiontestsofconcretearemadeoncylinders152mm(6in.)indiameterand305mm(12in.)high.

Compressive Strength Themeasuredresistanceofaconcreteormortarspecimentoaxialloading;expressedaspoundspersquareinch(psi)ofcross-sectionalarea.

Concrete Acompositematerialthatconsistsessentiallyofabindingmediuminwhichisembeddedparticlesorfragmentsofrelativelyinertmaterialfiller.Inportlandcementconcrete,thebinderisamixtureofportlandcementandwater;thefillermaybeanyofawidevarietyofnaturalorartificialaggregates.

Concrete Spreader Amachinedesignedtospreadconcretefromheapsalreadydumpedinfrontofit,ortoreceiveandspreadconcreteinauniformlayer.

Concrete, Normal-Weight Concretehavingaunitweightofapproximately2,400kg/m3madewithaggregatesofnormalweight.

Concrete, Reinforced Concreteconstructionthatcontainsmeshorsteelbarsembeddedinit.

Consistency Therelativemobilityorabilityoffreshconcreteormortartoflow.Theusualmeasuresofconsistencyareslumporballpenetrationforconcreteandflowformortar.

Consolidate Compaction usually accomplished by vibration of newly placedconcretetominimumpracticalvolume,tomolditwithinformshapesoraroundembeddedpartsandreinforcement,andtoreducevoidcontenttoapracticalminimum.

Consolidation Theprocessofinducingacloserarrangementofthesolidparticlesin

freshlymixedconcreteormortarduringplacementbythereductionof voids, usually by vibration, centrifugation, tamping, or somecombinationoftheseactions;alsoapplicabletosimilarmanipulationofother cementitiousmixtures, soils, aggregates, or the like. SeealsoCompaction.

Construction Joint Thejunctionoftwosuccessiveplacementsofconcrete,typicallywithakeywayorreinforcementacrossthejoint.

Continuously Reinforced Pavement Apavementwithcontinuouslongitudinalsteelreinforcementandnointermediatetransverseexpansionorcontractionjoints.

Contract Decrease in length or volume. See also Expansion, Shrinkage,Swelling.

Contraction Joint A plane, usually vertical, separating concrete in a structure ofpavement,atadesignatedlocationsuchastopreventformationofobjectionableshrinkagecrackselsewhereintheconcrete.Reinforcingsteelisdiscontinuous.

Control Joint SeeContractionJoint

Core Acylindricalspecimenofstandarddiameterdrilledfromastructureor rock foundation to be bested in compression or examinedpetrographically.

Corner Break Aportionoftheslabseparatedbyacrackthatintersectstheadjacenttransverse or longitudinal joints at about a 45º angle with thedirectionoftraffic.Thelengthofthesidesisusuallyfrom0.3meterstoone-halfoftheslabwidthoneachsideofthecrack.

Course Inconcreteconstruction,ahorizontallayerofconcrete,usuallyoneofseveralmakingupalift;inmasonryconstruction,ahorizontallayerofblockorbrick.SeealsoLift.

CPCD Concretepavementcontractiondesign;termusedinTexasforjointedplainconcretepavement(seeJPCP).

Crack Saw Smallthree-wheeledspecialtysawusefulfortracingthewanderingnatureofatransverseorlongitudinalcrack;usuallycontainsapivotwheelandrequiresasmalldiametercracksawingblade.

Cracking Theprocessofcontractionorthereflectionofstressinthepavement.

Crazing Minutesurfacepatterncracksinmortarorconcreteduetounequalshrinkageorcontractionondryingorcooling.

CRC Pavement (CRCP) Continuously reinforced concrete pavement; see ContinuouslyReinforcedPavement.



Cross Section Thesectionofabodyperpendiculartoagivenaxisofthebody;adrawingshowingsuchasection.

Crushed Gravel Theproductresultingfromtheartificialcrushingofgravelwithaspecifiedminimumpercentageoffragmentshavingoneormorefacesresultingfromfracture.SeealsoCoarseAggregate.

Crushed Stone Theproductresultingfromtheartificialcrushingofrocks,boulders,orlargecobblestones,substantiallyallfacesofwhichpossesswell-defined edges and have resulted from the crushing operation.

Crusher-Run Aggregate Aggregatethathasbeenbrokeninamechanicalcrusherandhasnotbeensubjectedtoanysubsequentscreeningprocess.

Cubic Yard Normalcommercialunitsofmeasureofconcretevolume,equalto27ft3.

Cure Maintenanceoftemperatureandhumidityforfreshlyplacedconcreteduring some definite period following placing and finishing toensure proper hydration of the cement and proper hardening oftheconcrete.

Curing Themaintenanceofasatisfactorymoisturecontentandtemperatureinconcreteduringitsearlystagessothatdesiredpropertiesmaydevelop.

Curing Blanket Abuilt-upcoveringof sacks,matting,Hessian, straw,waterproofpaper,orothersuitablematerialplacedoverfreshlyfinishedconcrete.SeealsoBurlap.

Curing Compound Aliquid thatcanbeappliedasacoating to thesurfaceofnewlyplacedconcretetoretardthelossofwateror,inthecaseofpigmentedcompounds,alsotoreflectheatsoastoprovideanopportunityfortheconcretetodevelopitspropertiesinafavorabletemperatureandmoistureenvironment.SeealsoCuring.

DDamp Eithermoderateabsorptionormoderatecoveringofmoisture;implieslesswetnessthanthatconnotedby“wet,”andslightlywetterthanthatconnotedby“moist.”SeealsoMoistandWet.

Deformed Bar Areinforcingbarwithamanufacturedpatternofsurfaceridgesthatprovidealockinganchoragewithsurroundingconcrete.

Deformed Reinforcement Metalbars,wire,orfabricwithamanufacturedpatternofsurfaceridgesthatprovidealockinganchoragewithsurroundingconcrete.

Density Massperunitvolume;bycommonusage inrelationtoconcrete,weightperunitvolume,alsoreferredtoasunitweight.

Density (dry) Themassperunitvolumeofadrysubstanceatastatedtemperature.SeealsoSpecificGravity.

Density Control Controlofdensityofconcreteinfieldconstructiontoensurethatspecified values as determined by standard tests are obtained.

Design Strength Load capacity of a member computed on the basis of allowablestressesassumedindesign.

Deterioration 1) Physical manifestation of failure (e.g., cracking delamination,flaking,pitting,scaling,spalling,staining)causedbyenvironmentalorinternalautogenousinfluencesonrockandhardenedconcreteaswellasothermaterials.

2)Decompositionofmaterialduringeithertestingorexposuretoservice.SeealsoDisintegrationandWeathering.

Diamond Grinding Theprocessusedtoremovetheuppersurfaceofaconcretepavementtoremovebumpsandrestorepavementrideability;also,equipmentusingmanydiamond-impregnatedsawbladesonashaftorarbortoshavethesurfaceofconcreteslabs.

Dispersing Agent Admixturescapableof increasingthefluidityofpastes,mortarorconcretesbyreductionofinter-particleattraction.

Distress Physicalmanifestationofdeteriorationanddistortioninaconcretestructureastheresultofstress,chemicalaction,and/orphysicalaction.

Dolomite Amineralhavingaspecificcrystalstructureandconsistingofcalciumcarbonateandmagnesiumcarbonateinequivalentchemicalamounts(54.27and45.73percentbyweight,respectively);arockcontainingdolomiteastheprincipalconstituent.

Dowel 1)Aloadtransferdevice,commonlyaplainroundsteelbar,whichextendsintotwoadjoiningportionsofaconcreteconstruction,asatajointinapavementslab,soastotransfershearloads.

2) A deformed reinforcing bar intended to transmit tension,compression,orshearthroughaconstructionjoint.

Dowel Bar (Dowelbar) SeeDowel

Dowel Bar Retrofit (DBR) SeeRetrofitDowelBars

Dowel Basket SeeLoad-TransferAssembly

Drainage Theinterceptionandremovalofwaterfrom,on,orunderanareaorroadway;theprocessofremovingsurplusgroundorsurfacewaterartificially; a general term for gravityflowof liquids in conduits.



Dry Process Inthemanufactureofcement,theprocessinwhichtherawmaterialsareground,conveyed,blended,andstoredinadrycondition.SeealsoWetProcess.

Dry Mix Concrete, mortar, or plaster mixture, commonly sold in bags,containing all components except water; also a concrete of nearzeroslump.

Dry Mixing Blendingofthesolidmaterialsformortarorconcretepriortoaddingthemixingwater.

Drying Shrinkage Contractioncausedbydrying.

Durability The ability of concrete to remain unchanged while in service;resistance to weathering action, chemical attack, and abrasion.

Dynamic Load Avariableload;i.e.,notstatic,suchasamovingliveload,earthquake,orwind.

Dynamic Loading Loadingfromunits(particularlymachinery)which,byvirtueoftheirmovementorvibration,imposestressesinexcessofthoseimposedbytheirdeadload.

EEarly Strength Strengthofconcretedevelopedsoonafterplacement,usuallyduringthefirst72hours.

Early-Entry Dry Saw Lightweightsawequippedwithabladethatdoesnotrequirewaterfor cooling and that allows sawing concrete sooner than withconventionalwet-diamondsawingequipment.

Econocrete Portlandcementconcretedesigned fora specificapplicationandenvironment and, in general, making use of local commerciallyproduced aggregates. These aggregates do not necessarily meetconventionalquality standards foraggregatesused inpavements.

Edge Form Formworkusedtolimitthehorizontalspreadoffreshconcreteonflatsurfaces,suchaspavementsorfloors.

Edger Afinishing toolusedontheedgesof freshconcrete toprovidearoundededge.

Efflorescence Depositofcalciumcarbonate(orothersalts),usuallywhiteincolor,appearinguponthesurfaceorfoundwithinthenear-surfaceporesof concrete. The salts deposit on concrete upon evaporation ofwaterthatcarriesthedissolvedsaltsthroughtheconcretetowardexposedsurfaces.

Entrained Air Round, uniformly distributed, microscopic, non-coalescing airbubblesentrainedbytheuseofair-entrainingagents;usuallylessthan1mm(.04in.)insize.

Entrapped Air Air in concrete that is not purposely entrained. Entrapped air isgenerallyconsideredtobelargevoids(largerthan1mm[.04in.]).

Equivalent Single Axle Loads (ESALs) Summation of equivalent 18,000-pound single axle loads usedto combine mixed traffic to design traffic for the design period.

Evaporable Water Watersetincementpastepresentincapillariesorheldbysurfaceforces; measured as that removable by drying under specifiedconditions.

Expansion Increaseinlengthorvolume.SeealsoAutogenousVolumeChange,Contraction,MoistureMovement,Shrinkage,andVolumeChange.

Expansion Joint SeeIsolationJoint

Exposed Aggregate Surfacetexturewherecementpasteiswashedawayfromconcreteslabsurfacetoexposedurablechip-sizeaggregatesfortheridingsurface.

Exposed Concrete Concretesurfacesformedsoastoyieldanacceptabletextureandfinishforpermanentexposuretoview.SeealsoArchitecturalConcrete.

External Vibrator SeeVibration

FFalse Set The rapid development of rigidity in a freshly mixed portlandcementpaste,mortar,orconcretewithouttheevolutionofmuchheat, which rigidity can be dispelled and plasticity regained byfurther mixing without addition of water; premature stiffening,hesitationset,earlystiffening,andrubbersetaretermsreferringtothesamephenomenon,but falseset is thepreferreddesignation.

Fast-Track Seriesoftechniquestoaccelerateconcretepavementconstruction.

Faulting Differentialverticaldisplacementofaslaborothermemberadjacenttoajointorcrack.

FHWA FederalHighwayAdministration

Fibrous Concrete Concretecontainingdispersed,randomlyorientedfibers.



Field-Cured Cylinders Testcylinderscuredasnearlyaspracticableinthesamemannerastheconcreteinthestructuretoindicatewhensupportingformsmayberemoved,additionalconstructionloadsmaybeimposed,orthestructuremaybeplacedinservice.

Final Set Adegreeofstiffeningofamixtureofcementandwatergreaterthaninitialset,generallystatedasanempiricalvalueindicatingthetimeinhoursandminutesrequiredforacementpastetostiffensufficientlytoresisttoanestablisheddegree,thepenetrationofaweightedtestneedle;alsoapplicabletoconcreteandmortarmixtureswithuseofsuitabletestprocedures.SeealsoInitialSet.

Final Setting Time The time required for a freshly mixed cement paste, mortar, orconcretetoachievefinalset.SeealsoInitialSettingTime.

Fine Aggregate Aggregate passing the 9.5-mm (3/8-in.) sieve and almost entirelypassingthe4.75-mm(#4)sieveandpredominantlyretainedonthe75-mm(#200)sieve.

Finish Thetextureofasurfaceaftercompactingandfinishingoperationshasbeenperformed.

Finishing Leveling,smoothing,compacting,andotherwisetreatingsurfacesoffreshorrecentlyplacedconcreteormortartoproducedesiredappearanceandservice.SeealsoFloatandTrowel.

Finishing Machine Apower-operatedmachineusedtogivethedesiredsurfacetexturetoaconcreteslab.

Fixed Form Paving A type of concrete paving process that involves the use of fixedformstouniformlycontroltheedgeandalignmentofthepavement.

Flash Set Therapiddevelopmentofrigidityinafreshlymixedportlandcementpaste,mortar,orconcrete,usuallywiththeevolutionofconsiderableheat,whichrigiditycannotbedispellednorcantheplasticityberegainedbyfurthermixingwithoutadditionofwater;alsoreferredtoasquicksetorgrabset.

Flexible Pavement A pavement structure that maintains intimate contact with anddistributesloadstothesubgradeanddependsonaggregateinterlock,particlefriction,andcohesionforstability;cementingagents,whereused,aregenerallybituminous(asphaltic)materialsascontrastedtoportlandcementinthecaseofrigidpavement.SeealsoRigidPavement.

Flexural Strength A property of a material or structural member that indicates itsability to resist failure inbending. See alsoModulusofRupture.

Float Atool (notadarby)usuallyofwood,aluminum,ormagnesium,usedinfinishingoperationstoimpartarelativelyevenbutstillopentexturetoanunformedfreshconcretesurface.

Float Finish Aratherroughconcretesurfacetextureobtainedbyfinishingwithafloat.

Floating Processofusingatool,usuallywood,aluminum,ormagnesium,infinishingoperationstoimpartarelativelyevenbutstillopentexturetoanunformedfreshconcretesurface.

Flow 1) Time dependent irrecoverable deformation. See Rheology.

2)Ameasureoftheconsistencyoffreshlymixedconcrete,mortar,orcementpasteintermsoftheincreaseindiameterofamoldedtruncatedconespecimenafterjiggingaspecifiednumberoftimes.

Flow Cone Test Testthatmeasuresthetimenecessaryforaknownquantityofgrouttocompletelyflowoutofandemptyastandardsizedcone;usuallyusedinslabstabilizationtodeterminethewaterquantitynecessaryforstabilizationgrout.

Fly Ash Thefinelydividedresidueresultingfromthecombustionofgroundorpowderedcoalandwhichistransportedfromthefireboxthroughtheboilerbyflugasses;usedasmineraladmixtureinconcretemixtures.

Form Atemporarystructureormold for the supportofconcretewhileit is setting and gaining sufficient strength to be self-supporting.

Free Moisture Moisturehaving essentially theproperties of purewater inbulk;moisture not absorbed by aggregate. See also Surface Moisture.

Free Water SeeFreeMoistureandSurfaceMoisture

Full-Depth Patching Removingandreplacingatleastaportionofaconcreteslabtothebottomof theconcrete, inorder torestoreareasofdeterioration.

Full-Depth Repair SeeFull-DepthPatching

GGap-Graded Concrete Concretecontainingagap-gradedaggregate.

Gradation SeeGrading

Grading The distribution of particles of granular material among varioussizes,usuallyexpressedintermsofcumulativepercentageslargeror smaller than each of a series of sizes (sieve openings) or thepercentages between certain ranges of sizes (sieve openings).

Gravel Granular material predominantly retained on the 4.75 mm (#4)



sieveandresultingfromnaturaldisintegrationandabrasionofrockorprocessingofweaklyboundconglomerate.

Green Concrete Concretethathassetbutnotappreciablyhardened.

Green Sawing The process of controlling random cracking by sawing uniformjointspacinginearlyageconcrete,withouttearingordislocatingtheaggregateinthemix.

Grooving Theprocessusedtocutslotsintoaconcretepavementsurfacetoprovidechannelsforwatertoescapebeneathtiresandtopromoteskidresistance.

Gross Vehicle Load Theweightofavehicleplustheweightofanyloadthereon.

Gross Volume (of concrete mixers) Inthecaseofarevolving-drummixer,thetotalinteriorvolumeoftherevolvingportionofthemixerdrum;inthecaseofanopen-topmixer,thetotalvolumeofthetroughorpancalculatedonthebasisthatnoverticaldimensionofthecontainerexceedstwicetheradius of the circular sectionbelow the axis of the central shaft.

HHairline Cracking Barelyvisiblecracksinrandompatterninanexposedconcretesurfacewhichdonotextendtothefulldepthorthicknessoftheconcrete,andwhicharedueprimarilytodryingshrinkage.

Hardening Whenportlandcementismixedwithenoughwatertoformapaste,thecompoundsofthecementreactwithwatertoformcementitiousproducts that adhere to each other and to the intermixed sandandstoneparticlesandbecomeveryhard.Aslongasmoistureispresent,thereactionmaycontinueforyears,addingcontinuallytothestrengthofthemixture.

Harsh Mixture Aconcretemixturethatlacksdesiredworkabilityandconsistencyduetoadeficiencyofmortar.

Harshness Deficientworkabilityandcohesivenesscausedbyinsufficientsandorcement,orbyimproperlygradedaggregate.

Header A transverse construction joint installed at the end of a pavingoperation or other placement interruptions. To a contractor, aheaderisthelocationatwhichpavingwillresumeonthenextday.

Heat of Hydration Heatevolvedbychemicalreactionsofasubstancewithwater,suchasthatevolvedduringthesettingandhardeningofportlandcement.

Heavy-Weight Aggregate Anaggregateofveryhighunitweight,suchasbarium,boron,orironore,steelshotorpunchings,whichformsahighdensitymortarof concrete when bound together with hardened cement paste.

Heavy-Weight Concrete Concreteinwhichheavyaggregateisusedtoincreasethedensityoftheconcrete;unitweightsintherangeof165to330lb/ft3areattained.

High Range Water-Reducing Admixture SeeWater-ReducingAdmixture(highrange)

High Early-Strength Cement SeeCement,HighEarly-Strength

High Early-Strength Concrete Concrete that, through the use of high-early-strength cement oradmixtures, iscapableofattainingspecifiedstrengthatanearlieragethannormalconcrete.

Honeycomb Concrete that, due to lack of the proper amount of fines orvibration,containsabundantinterconnectedlargevoidsorcavities;concrete thatcontainshoneycombswas improperlyconsolidated.

Horizontal-Axis Mixer Concretemixersoftherevolvingdrumtypeinwhichthedrumrotatesaboutahorizontalaxis.

Hot-Pour Sealant Jointsealingmaterialsthatrequireheatingforinstallation,usuallyconsistingofabaseofasphaltorcoaltar.

Hydrated Lime Adrypowderobtainedbytreatingquicklimewithsufficientwatertoconvertittocalciumhydroxide.

Hydration The chemical reaction between cement and water which causesconcretetoharden.

Hydraulic Cement Acementthatiscapableofsettingandhardeningunderwaterduetothechemicalinteractionofthewaterandtheconstituentsofthecement.

Hydroplaning Togooutofsteeringcontrolbyskimmingthesurfaceofawetroad.

IIncentive Barely visible cracks in random pattern in an exposed concretesurface which do not extend to the full depth or thickness ofthe concrete, and which are due primarily to drying shrinkage.

Inclined-Axis Mixer Atruckwitharevolvingdrumthatrotatesaboutanaxisinclinedtothebedofthetruckchassis.

Incompressibles Smallconcretefragments,stones,sandorotherhardmaterialsthatenter a joint sealant, joint reservoir, or other concrete pavementdiscontinuity.

Initial Set Adegreeofstiffeningofamixtureofcementandwater lessthan



finalset,generallystatedasanempiricalvalueindicatingthetimeinhoursandminutesrequiredforcementpastetostiffensufficientlytoresisttoanestablisheddegreethepenetrationofaweightedtestneedle;alsoapplicabletoconcreteormortarwithuseofsuitabletestprocedures.SeealsoFinalSet.

Initial Setting Time Thetimerequiredforafreshlymixedcementpastetoacquireanarbitrarydegreeofstiffnessasdeterminedbyspecifictest.

Inlay A form of reconstruction where new concrete is placed into anarea of removed pavement; The removal may be an individuallane,alllanesbetweentheshouldersoronlypartlythroughaslab.

Isolation Joint Apavementjointthatallowsrelativemovementinthreedirectionsandavoidsformationofcrackselsewhereintheconcreteandthroughwhichallorpartofthebondedreinforcementisinterrupted.Largeclosuremovementtopreventdevelopmentoflateralcompressionbetweenadjacentconcreteslabs;usuallyusedto isolateabridge.

JJoint A plane of weakness to control contraction cracking in concretepavements. A joint can be initiated in plastic concrete or greenconcreteandshapedwithlaterprocess.

Joint Depth Themeasurementofasawcutfromthetopoftheslabtothebottomofthecut.

Joint Deterioration SeeSpalling,Compression

Joint Filler Compressiblematerialusedtofillajointtopreventtheinfiltrationofdebrisandtoprovidesupportforsealant.

Joint Sealant Compressible material used to minimize water and solid debrisinfiltrationintothesealantreservoirandjoint.

Joint Shape Factor Ratiooftheverticaltohorizontaldimensionofthejointsealantreservoir.

Joint, Construction SeeConstructionJoint

Joint, Contraction SeeContractionJoint

Joint, Expansion SeeExpansionJoint

Jointed Plain Concrete Pavement (JPCP) Pavement containing enough joints to control all natural cracksexpected in the concrete; steel tiebars are generally used atlongitudinal joints toprevent jointopening,anddowelbarsmaybe used to enhance load transfer at transverse contraction jointsdependingupontheexpectedtraffic.

Jointed Reinforced Concrete Pavement (JRCP) Pavement containing some joints and embedded steel meshreinforcement(sometimescalleddistributedsteel)tocontrolexpectedcracks; steel mesh is discontinued at transverse joint locations.

KKeyway Arecessorgrooveinoneliftorplacementofconcrete,whichisfilledwithconcreteofthenextlift,givingshearstrengthtothejoint.SeealsoTongueandGroove.

LLaitance Alayerofweakmaterialcontainingcementandfinesfromaggregates,brought to the top of overwet concrete, the amount of which isgenerallyincreasedbyoverworkingandover-manipulatingconcreteatthesurfacebyimproperfinishing.

Layer SeeCourse

Lean Concrete Concreteoflowcementcontent.

Life-Cycle Cost Analysis Theprocessusedtocompareprojectsbasedontheirinitialcost,futurecostandsalvagevalue,whichaccountsforthetimevalueofmoney.

Lift Theconcreteplacedbetweentwoconsecutivehorizontalconstructionjoints,usuallyconsistingofseverallayersorcourses.

Liquid Sealant Sealantmaterialsthatinstallinliquidformandcoolorcuretotheirfinalproperties;relyonlong-termadhesiontothejointreservoirfaces.

Load Transfer Device SeeDowel

Load Transfer Efficiency Theabilityofajointorcracktotransferaportionofaloadappliedonthesideofthejointorcracktotheothersideofthejointorcrack.

Load Transfer Restoration (LTR) SeeRetrofitDowelBars

Load-Transfer Assembly Mostcommonly,thebasketorcarriagedesignedtosupportorlinkdowelbarsduringconcretingoperationssoastoholdtheminplace,inthedesiredalignment.

Longitudinal Broom Surfacetextureachievedinsimilarmannerastransversebroom,exceptthatbroomispulledinalineparalleltothepavementcenterline.

Longitudinal Joint Ajointparalleltothelongdimensionofastructureorpavement.



Longitudinal Reinforcement Reinforcement essentially parallel to the long axis of a concretememberorpavement.

Longitudinal Tine Surface texture achieved by a hand held or mechanical deviceequippedwitharake-liketiningheadthatmovesinalineparalleltothepavementcenterline.

Lot Adefinedquantity.

MM-E PDGGuide for Mechanistic-Empirical Design of New and Rehabilitated Pavements(NCHRP2004).

Map Cracking 1) Intersecting cracks that extendbelow the surfaceofhardenedconcrete;causedbyshrinkageofthedryingsurfaceconcretewhichisrestrainedbyconcreteatgreaterdepthswhereeitherlittleornoshrinkageoccurs;varyinwidthfromfineandbarelyvisibletoopenandwell-defined.

2) The chief symptom of chemical reaction between alkalis incement and mineral constituents in aggregate within hardenedconcrete; due to differential rate of volume change in differentportions of the concrete; cracking is usually random and ona fairly large scale, and in severe instances the cracks mayreach a width of ½-in. See also Crazing and Pattern Cracking.

Maximum Size Aggregate Thelargestsizeaggregateparticlespresentinsufficientquantitytoaffectpropertiesofaconcretemixture.

Membrane Curing A process that involves either liquid sealing compound (e.g.,bituminousandparaffinicemulsions,coaltarcut-backs,pigmentedandnon-pigmentedresinsuspensions,orsuspensionsofwaxanddrying oil) or non-liquid protective coating (e.g., sheet plasticsor “waterproof”paper), bothofwhich types function asfilms torestrictevaporationofmixingwaterfromthefreshconcretesurface.

Mesh Thenumberofopenings(includingfractionsthereof)perunitoflengthineitherascreenorsieveinwhichtheopeningsare6mm(¼-in.)orless.

Mesh Reinforcement SeeWelded-WireFabricReinforcement

Method and Material Specification Specificationthatdirectsthecontractortousespecifiedmaterialsindefiniteproportionsandspecifictypesofequipmentandmethodstoplacethematerial.

Mix The act or process of mixing; also mixture of materials, such asmortarorconcrete.

Mix Design SeeProportioning

Mixer Amachineused forblending theconstituentsofconcrete,grout,mortar,cementpaste,orothermixture.

Mixer, Batch SeeBatchMixer

Mixer, Transit SeeTruckMixer

Mixing Cycle Thetimetakenforacompletecycleinabatchmixer;i.e.,thetimeelapsingbetweensuccessiverepetitionsofthesameoperation(e.g.,successivedischargesofthemixer).

Mixing Plant SeeBatchPlant

Mixing Speed Rotationrateofamixerdrumorofthepaddlesinanopen-top,pan,ortroughmixer,whenmixingabatch;expressedinrevolutionsperminute (rpm),or inperipheral feetperminuteofapointon thecircumferenceatmaximumdiameter.

Mixing Time Theperiodduringwhichthemixeriscombiningtheingredientsforabatchofconcrete.Forstationarymixers,thetimeismeasuredfromthecompletionofbatchingcementandaggregateuntilthebeginningofdischarge.Fortruckmixers,mixingisgivenintermofthenumberofrevolutionsofthedrumatmixingspeed.

Mixing Water Thewaterinfreshlymixedsand-cementgrout,mortar,orconcrete,exclusiveofanypreviouslyabsorbedbytheaggregate(e.g.,waterconsideredinthecomputationofthenetwater-cementratio).SeealsoBatchedWaterandSurfaceMoisture.

Mixture Theassembled,blended,commingledingredientsofmortar,concrete,orthelike,ortheproportionsfortheirassembly.

Modulus of Rupture A measure of the ultimate load-carrying capacity of a beam,sometimesreferredtoas“rupturemodulus”or“rupturestrength.”It is calculated for apparent tensile stress in the extremefiber ofa transverse test specimenunder the load thatproduces rupture.SeealsoFlexuralStrength.

Moist Slightlydampbutnotquitedrytothetouch;theterm“wet”impliesvisiblefreewater,“damp”implieslesswetnessthan“wet,”and“moist”impliesnotquitedry.SeealsoDampandWet.

Moisture Barrier Avaporbarrier.

Moisture Content of Aggregate Theratio,expressedasapercentage,oftheweightofwaterinagivengranularmasstothedryweightofthemass.

Moisture-Free Theconditionofamaterialthathasbeendriedinairuntilthereisnofurthersignificantchangeinitsmass.SeealsoOvendry.



Mortar Concretewithessentiallynoaggregatelargerthanabout3/16inch.

Mud Balls Ballsofclayorsilt(“mud”).

NNatural Sand Sandresultingfromnaturaldisintegrationandabrasionofrock.SeealsoSandandAggregate,Fine.

NCHRP NationalCooperativeHighwayResearchProgram

NHI NationalHighwayInstitute

Nominal Maximum Size (of aggregate) In specifications for and descriptions of aggregate, the smallestsieve opening throughwhich the entire amount of the aggregateispermittedtopass;sometimesreferredtoas“maximumsize(ofaggregate).”

Non-Agitating Unit Atruck-mountedcontainerfortransportingcentral-mixedconcretethat is not equipped to provide agitation (slow mixing) duringdelivery;adumptruck.

Non-Air-Entrained Concrete Concreteinwhichneitheranair-entrainingadmixturenoranair-entrainingcementhasbeenused.

No-Slump Concrete Concretewithaslumpof6mm(¼-in.)orless.SeealsoZero-SlumpConcrete.

NRMCA NationalReadyMixedConcreteAssociation

OOpen-Graded Subbase Unstabilizedlayerconsistingofcrushedaggregateswithareducedamountoffinestopromotedrainage.

Ovendry Theconditionresultingfromhavingbeendriedtoessentiallyconstantweight, inanoven,ata temperature thathasbeenfixed,usuallybetween105and115ºC(221and239ºF).

Overlay The addition of a new material layer onto an existing pavementsurface.SeealsoResurfacing

Overlay, Bonded SeeBondedConcreteOverlay

Overlay, Unbonded SeeUnbondedConcreteOverlay

Overlay, UTW SeeUltra-ThinWhitetopping

Overlay, Whitetopping SeeWhitetopping

Over-Sanded Containing more sand than would be required for adequateworkabilityandsatisfactoryfinishingcharacteristics.

Over-Vibrated Concretevibratedmorethanisnecessaryforgoodconsolidationandeliminationofentrappedair.

Over-Wet Theconsistencyofconcretewhenitcontainsmoremixingwaterandhenceisofgreaterslumpthanisnecessaryforreadyconsolidation.

PParticle-Size Distribution Thedivisionofparticlesofagradedmaterialamongvarioussizes;for concrete materials, usually expressed in terms of cumulativepercentageslargerorsmallerthaneachofaseriesofdiametersorthepercentageswithincertainrangesofdiameter,asdeterminedbysieving.

Paste Constituentofconcreteconsistingofcementandwater.

Pattern Cracking Fine openings on concrete surfaces in the form of a pattern;resultingfromadecreaseinvolumeofthematerialnearthesurface,an increase involumeof thematerialbelowthesurface,orboth.

Pavement (concrete) Alayerofconcreteoversuchareasasroads,sidewalks,canals,airfields,and those used for storage or parking. See also Rigid Pavement.

Pavement Structure Thecombinationofsurfacecoursesandbase/subbasecoursesplacedonapreparedsubgradetosupportthetrafficload.

Paving Train Anassemblageofequipmentdesignedtoplaceandfinishaconcretepavement.

PCA PortlandCementAssociation

PCC Portlandcementconcrete

Pea Gravel Screenedgraveltheparticlesizesofwhichrangebetween3/16and3/8inchindiameter.

Percent Fines Amount, expressed as a percentage, of material in aggregatefiner than a given sieve, usually the 75-mm (#200) sieve; also,the amount of fine aggregate in a concrete mixture expressed asa percent by absolute volume of the total amount of aggregate.



Performance-Based Specification Specification that describes the desired levels of fundamentalengineering properties (for example, resilient modulus and/orfatigueproperties)thatarepredictorsofperformanceandappearinprimarypredictionrelationships(i.e.,modelsthatcanbeusedtopredictpavementstress,distress,orperformancefromcombinationsof predictors that represent traffic, environmental, roadbed, andstructuralconditions).

Performance-Related Specification Specificationthatdescribesthedesiredlevelsofkeymaterialsandconstructionqualitycharacteristicsthathavebeenfoundtocorrelatewithfundamentalengineeringpropertiesthatpredictperformance.These characteristics (for example, strength of concrete cores)are amenable to acceptance testing at the time of construction.

Permeable Subbase Layerconsistingofcrushedaggregateswithareducedamountoffinestopromotedrainageandstabilizedwithportlandcementorbituminouscement.

Phasing Thesequencesusedbyacontractortobuildelementsofaproject.

Pitting Alocalizeddisintegrationtakingtheformofcavitiesatthesurfaceofconcrete.

Placement The process of placing and consolidating concrete; a quantity ofconcreteplacedandfinishedduringacontinuousoperation;alsoinappropriatelyreferredtoas“pouring.”

Placing The deposition, distribution, and consolidation of freshly mixedconcrete in the place where it is to harden; also inappropriatelyreferredtoas“pouring.”

Plain Bar A reinforcing bar without surface deformations, or one havingdeformationsthatdonotconformtotheapplicablerequirements.

Plain Concrete Concretewithoutreinforcement.

Plain Pavement Concretepavementwithrelativelyshortjointspacingandwithoutdowelsorreinforcement.

Plane of Weakness Theplanealongwhichabodyunderstresswill tend to fracture;mayexistbydesign,byaccident,orbecauseof thenatureof thestructureanditsloading.

Plastic Conditionoffreshlymixedcementpaste,mortar,orconcretesuchthatdeformationwill be sustained continuously in anydirectionwithoutrupture;incommonusage,concretewithslumpof80to100mm(3to4in.).

Plastic Consistency A condition of freshly mixed concrete such that it is readilyremoldableandworkable,cohesive,andhasanamplecontentofcementandfines,butisnotover-wet.

Plastic Cracking Crackingthatoccursinthesurfaceoffreshconcretesoonafteritisplacedandwhileitisstillplastic.

Plastic Deformation Deformation thatdoesnotdisappearwhen the force causing thedeformationisremoved.

Plastic Shrinkage Cracking Cracks,usuallyparallelandonlyafewinchesdeepandseveralfeetlong,inthesurfaceofconcretepavementthataretheresultofrapidmoisturelossthroughevaporation.

Plasticity That property of fresh concrete or mortar which determines itsresistancetodeformationoritseaseofmolding.

Plasticizer A material that increases the plasticity of a fresh cement paste,mortar,orconcrete.

Pneumatic Movedorworkedbyairpressure.

Popout Pitorcraterinthesurfaceofconcreteresultingfromcrackingofthemortarduetoexpansiveforcesassociatedwithaparticleofunsoundaggregate or a contaminating material, such as wood or glass.

Porosity Theratio,usuallyexpressedasapercentage,ofthevolumeofvoidsinamaterialtothetotalvolumeofthematerial, includingvoids.

Portland Cement A commercial product which when mixed with water alone orin combination with sand, stone, or similar materials, has thepropertyofcombiningwithwater,slowly,toformahardsolidmass.Physically,portlandcementisafinelypulverizedclinkerproducedbyburningmixturescontaining lime, iron,alumina,andsilicaathightemperatureandindefiniteproportions,andthenintergrindinggypsumtogivethepropertiesdesired.

Portland Cement Concrete Acompositematerialthatconsistsessentiallyofabindingmedium(portlandcementandwater)withinwhichareembeddedparticlesorfragmentsofaggregate,usuallyacombinationoffineaggregatecourseaggregate.

Portland-Pozzolan Cement SeeCement,PortlandPozzolan

Pozzolan A siliceous or siliceous and aluminous material, which in itselfpossesseslittleornocementitiousvaluebutwill,infinelydividedformandinthepresenceofmoisture,chemicallyreactwithcalciumhydroxideatordinarytemperaturestoformcompoundspossessingcementitiousproperties.

Pozzolan-Cement Grout Commonslabstabilizationgroutconsistingofwater,portlandcementandpozzolan;usuallyflyash.

Preformed Compression Seal Jointsealantthatismanufacturedreadyforinstallationandisheld



inajointbylateralpressureexertedagainstthereservoirbythesealafterbeingcompressedduringinstallation.

Preservation Theprocessofmaintainingastructureinitspresentconditionandarrestingfurtherdeterioration.SeealsoRehabilitation,Repair,andRestoration.

Pressure-Relief Cutmadeinaconcretepavementtorelievecompressiveforcesofthermalexpansionduringhotweather.

Process Control Those quality assurance actions and considerations necessary toassessproductionandconstructionprocessessoastocontrolthelevelofqualitybeingproducedintheendproduct.Thisincludessamplingandtestingtomonitortheprocessbutusuallydoesnotincludeacceptancesamplingandtesting.

Profile Index Smoothnessqualifyingfactordeterminedfromprofilographtrace.Calculated by dividing the sum of the total counts above theblankingbandforeachsegmentbythesumofthesegmentlength.

Profile Line Onaprofiletrace,linedrawnbyhandonthefieldtracetoaverageoutspikesandminordeviationscausedbyrocks,texturing,dirtortransversegrooving.

Project Scoping Anearlyplanningstepinthedevelopmentofaprojectwhereallprojectrequirementsaredefinedandaplanisdevelopedtoaddressthem.

Proportioning Selectionofproportionsofingredientsformortarorconcretetomakethemosteconomicaluseofavailablematerialstoproducemortarorconcreteoftherequiredproperties.

PSI 1)Poundspersquareinch;ameasureofthecompressive,tensileorflexuralstrengthofconcreteasdeterminedbyappropriatetest.

2)Inpavements,thePerformanceServiceabilityIndex.

Pumping Theforcefuldisplacementofamixtureofsoilandwaterthatoccursunderslabjoints,cracksandpavementedgeswhicharedepressedandreleasedquicklybyhigh-speedheavyvehicleloads;occurswhenconcretepavementsareplaceddirectlyonfine-grained,plasticsoilsorerodiblesubbasematerials.

Punchout In continuously reinforced concretepavement, the area enclosedbytwocloselyspacedtransversecracks,ashortlongitudinalcrack,andtheedgeofthepavementorlongitudinaljoint,whenexhibitingspalling,shattering,orfaulting.Also,areabetweenYcracksexhibitingthissamedeterioration.

QQA/QC See,QualityAssuranceandQualityControl

Quality Assurance Plannedandsystematicactionsbyanownerorhisrepresentativeto provide confidence that a product or facility meet applicablestandardsofgoodpractice.This involvescontinuedevaluationofdesign,planandspecificationdevelopment,contractadvertisementandaward,construction,andmaintenance,andtheinteractionsoftheseactivities.

Quality Assurance/Quality Control Specification Statisticallybasedspecificationthatisacombinationofendresultandmaterialandmethodspecifications.Thecontractorisresponsiblefor quality control (process control), and the highway agency isresponsibleforacceptanceoftheproduct.

Quality Control Actionstakenbyaproducerorcontractortoprovidecontroloverwhatisbeingdoneandwhatisbeingprovidedsothattheapplicablestandardsofgoodpracticefortheworkarefollowed.

RRadius of Relative StiffnessAcharacterorpropertyofaconcreteslabwhichmeasuresthestiffnessoftheslabinrelationtothatofthesubgrade;itisexpressedbytheequation:

Random Crack SeeUncontrolledCrack

Raveling Displacementofaggregateorpasteneartheslabsurfacefromsawing;normally indicates that concrete strength is too low for sawing.

Reactive Aggregate Aggregatecontainingcertainsilicaorcarbonatecompoundsthatarecapableofreactingwithalkalisinportlandcement,insomecasesproducingdamagingexpansionofconcrete.

Ready-Mixed Concrete Concretemanufacturedfordeliverytoapurchaserinaplasticandunhardenedstate.

Rebar Abbreviationfor“reinforcingbar.”SeeReinforcement.

Rebound Hammer An apparatus that provides a rapid indication of the mechanicalpropertiesofconcretebasedonthedistanceofreboundofaspring-drivenmissile.

Reconstruction Theprocessofremovinganexistingpavementfromitsgradeandreplacingitwithacompletelynewpavement.

Recycled Concrete Concrete that has been processed for use, usually as aggregate.

Recycling Theactofprocessingexistingpavementmaterialintousablematerialforalayerwithinanewpavementstructure.



Rehabilitation The process of repairing or modifying a structure to a desireduseful condition. See also Preservation, Repair, and Restoration.

Reinforced Concrete Concrete containing adequate reinforcement (prestressed or notprestressed)anddesignedontheassumptionthatthetwomaterialsacttogetherinresistingforces.

Reinforcement Bars, wires, strands, and other slender members embedded inconcreteinsuchamannerthatthereinforcementandtheconcreteacttogetherinresistingforces.

Reinforcement, Transverse Reinforcementatrightanglestothelongitudinalreinforcement;maybemainorsecondaryreinforcement.

Relative Humidity The ratio of the quantity of water vapor actually present to theamountpresentinasaturatedatmosphereatagiventemperature;expressedasapercentage.

Release Agent Materialusedtopreventbondingofconcretetoasurface.SeealsoBondBreaker.

Remoldability The readiness with which freshly mixed concrete responds to aremoldingeffort,suchasjiggingorvibration,causingittoreshapeitsmassaroundreinforcementandtoconformtotheshapeoftheform.SeealsoFlow.

Repair To replace or correct deteriorated, damaged, or faulty materials,components, or elements of a structure. See also Preservation,Rehabilitation,andRestoration.

Reservoir Thepartofaconcretejointthatnormallyholdsasealantmaterial.Usuallyawideningsawcutabovetheinitialsawcut.

Restoration Theprocessofreestablishingthematerials,form,andappearanceofastructuretothoseofaparticulareraofthestructure.SeealsoPreservation,Rehabilitation,andRepair.

Resurfacing The addition of a new material layer onto an existing pavementsurfaceforthepurposesofcorrectingafunctional factor,suchassmoothnessortexture.

Retardation Reductionintherateofhardeningorstrengthdevelopmentoffreshconcrete,mortar,orgrout;i.e.,anincreaseinthetimerequiredtoreachinitialandfinalset.

Retarder Anadmixturethatdelaysthesettingofcementandhenceofmixturessuchasmortarorconcretecontainingcement.

Retempering Additionofwaterandremixingofconcreteormortarthathaslostenoughworkabilitytobecomeunplaceableorunusable.SeealsoTempering.

Retrofit Dowel Bars Dowels that install into slots cut into the surface of an existingconcretepavement.

Revibration A second vibration applied to fresh concrete, preferably as longafter thefirstvibrationas theconcretewillstill respondproperly.

Rheology Thescienceofdealingwithflowofmaterials,includingstudiesofdeformationofhardenedconcrete,thehandlingandplacingoffreshlymixedconcrete,andthebehaviorofslurries,pastes,andthelike.

Ribbon Loading Methodof batching concrete inwhich the solid ingredients, andsometimesthewater,enterthemixersimultaneously.

Rich Mixture Aconcretemixturecontainingalargeamountofcement.

Rigid Pavement Pavementthatwillprovidehighbendingresistanceanddistributeloadstothefoundationoveracomparativelylargearea.

Rock Pocket Aportionofhardenedconcreteconsistingofaconcentrationofcoarseaggregate that isdeficient inmortar;causedbyseparationduringplacementorinsufficientconsolidation,orboth;seeHoneycomb.

Rod Aspecifiedlengthofmetal,circularincrosssectionwithoneendrounded; used to compact concrete or mortar test specimens.

Rod, Tamping Astraightsteelrodofcircularcrosssectionhavingoneorbothendsroundedtoahemisphericaltip.

Rodability The susceptibility of fresh concrete or mortar to compaction bymeansofatampingrod.

Rodding Compactionofconcretebymeansofatampingrod.SeealsoRod,Tamping,andRodability.

SSack SeeBag

Sample Agroupofunits,orportionofmaterial,takenfromalargercollectionofunitsorquantityofmaterial,whichservestoprovideinformationthatcanbeusedasabasisforactiononthelargerquantityorontheproductionprocess;thetermisalsousedinthesenseofasampleofobservations.

Sampling, Continuous Samplingwithout interruptions throughoutanoperationor for apredeterminedtime.



Sampling, Intermittent Sampling successively for limited periods of time throughout anoperationorforapredeterminedperiodoftime.Thedurationofsample periods and of the intervals between are not necessarilyregularandarenotspecified.

Sand Thefinegranularmaterial(usuallylessthan3/16inchindiameter)resultingfromthenaturaldisintegrationofrock,orfromthecrushingoffriablesandstone.

Sand Grout Grout mixture containing water, portland cement, and sand.

Sand Streak Astreakofexposedfineaggregateinthesurfaceofformedconcretecausedbybleeding.

Saturated Surface-Dry Conditionofanaggregateparticleorotherporoussolidwhenthepermeablevoidsarefilledwithwaterbutthereisnowaterontheexposedsurface.

Saturated Surface-Dry (SSD) Particle Density The mass of the saturated surface-dry aggregate divided by itsdisplacedvolumeinwaterorinconcrete.(AlsocalledBulkSpecificGravity).

Saturation 1)Ingeneral,theconditionofthecoexistenceinstableequilibriumofeitheravaporandaliquidoravaporandsolidphaseofthesamesubstanceatthesametemperature.

2)Asappliedtoaggregateorconcrete,theconditionsuchthatnomoreliquidcanbeheldorplacedwithinit.

Saw Blade, Abrasive Concretesawingmediumthatusesnon-diamondabrasionelements.Thesebladesdonotneedwatertocool,butwaterissometimesused.

Saw Blade, Diamond Concrete sawing medium that uses industrial diamonds as theprimaryabrasionelement.Bladesarecooledwithwatertoprotectthehostmetalfrommeltingandprematurelydislodgingthediamonds.

Saw Cut Acut inhardenedconcreteutilizingdiamondorsilicone-carbidebladesordiscs.

Sawed Joint Ajointcutinhardenedconcrete,generallynottothefulldepthofthemember,bymeansofspecialequipment.

Sawing Cuttingofjointsinhardenedconcretebymeansofspecialequipmentutilizingdiamondorsiliconcarbidebladesordiscs;cutgoesonlypartwaythroughtheslab.

Scaling Flaking or peeling away of the near-surface portion of hydrauliccementconcreteormortar.

Schmidt Hammer (trade name), Swiss Hammer, or Rebound Hammer A device used to estimate the compressive strength of hardenedconcretebymeasuringsurfacehardness.

Scoping SeeProjectScoping

Screed 1) To strike off concrete lying above the desired plane or shape.

2)Atool forstrikingoff theconcretesurface,sometimesreferredtoasaStrikeoff.

Screed Guide Firmlyestablishedgradestripsorsideformsforunformedconcretethatwillguidethestrikeoffinproducingthedesiredplaneorshape.

Screeding Theoperationofformingasurfacebytheuseofscreedguidesandastrikeoff.SeealsoStrikeoff.

Sealant SeeJointSealantandMembraneCuring

Sealant Reservoir Thesawkerforformedslotinwhichajointsealantisplaced.Manytimesthisreferstoacutmadetowidentheoriginalsawcutmadeforacontractionjoint.

Sealing The process of filling the sawed joint with material to minimizeintrusion into the joint of water and incompressible materials.

Sealing Compound SeeJointSealantandMembraneCuring

Secondary Sawing Thesawingthat takesplacetoestablishshape inthe joint.Manytimesthisshapeisthereservoirofthejoint.

Segregation The tendency, as concrete is caused to flow laterally, for coarseaggregateanddriermaterialtoremainbehindandformortarandwettermaterialtoflowahead.Thisalsooccursinaverticaldirectionwhenwetconcreteisover-vibrated,themortarandwettermaterialrisingtothetop.Intheverticaldirection,segregationmayalsobecalledStratification.

Semiautomatic Batcher Abatcherequippedwithgatesorvalvesthatareseparatelyopenedmanuallytoallowthematerialtobeweighedbutwhichareclosedautomaticallywhenthedesignatedweightofeachmaterialhasbeenreached.

Separation Thetendency,asconcreteiscausedtopassfromtheunconfinedendsofchutesorconveyorbelts, forcoarseaggregatetoseparatefromtheconcreteandaccumulateatoneside;thetendency,asprocessedaggregateleavestheendsofconveyorbelts,chutes,orsimilardeviceswithconfiningsides,forthelargeraggregatetoseparatefromthemassandaccumulateatoneside;thetendencyforsolidstoseparatefromthewaterbygravitationalsettlement.SeealsoBleedingandSegregation.



Set Theconditionreachedbyacementpaste,mortar,orconcretewhenit has lost plasticity to an arbitrary degree, usually measured intermsofresistancetopenetrationordeformation.Initialsetreferstofirststiffening.Finalsetreferstoattainmentofsignificantrigidity.

Set-Accelerating Admixture SeeAccelerator

Set-Retarding Admixture SeeRetarder

Setting of Cement Developmentofrigidityofcementpaste,mortar,orconcreteasaresultofhydrationofthecement.Thepasteformedwhencementismixedwithwaterremainsplasticforashorttime.Duringthisstageitisstillpossibletodisturbthematerialandremixwithoutinjury,butas thereactionbetweenthecementandwatercontinues, themasslosesitsplasticity.Thisearlyperiodinthehardeningiscalledthe“settingperiod,”althoughthereisnotawell-definedbreakinthehardeningprocess.

Setting Time The time required for a specimenof concrete,mortar or cementpaste,preparedandtestedunderstandardizedconditions,toattainaspecifieddegreeofrigidity.

Settlement Sinkingofsolidparticlesingrout,mortar,orfreshconcrete,afterplacementandbeforeinitialset.SeealsoBleeding.

Settlement Shrinkage Areductioninvolumeofconcretepriortothefinalsetofcementitiousmixtures;causedbysettlingofthesolidsanddecreasesinvolumeduetothechemicalcombinationofwaterwithcement.SeePlasticShrinkage.

Shrinkage Decreaseinlengthorvolume.

Shrinkage Crack Crack from restraint of volume reduction due to shrinkage ortemperaturecontraction;usuallyoccurringwithinthefirstfewdaysafterplacement.

Shrinkage Cracking Cracking of a slab due to failure in tension caused by externalor internal restraints as reduction in moisture content develops.

Shrink-Mixed Concrete Ready-mixedconcretemixedpartiallyinastationarymixerandthenmixedinatruckmixer.

Sieve A metallic plate or sheet, a woven-wire cloth, or other similardevice,withregularlyspacedaperturesofuniformsize,mountedinasuitableframeorholderforuseinseparatinggranularmaterialaccordingtosize.

Sieve Analysis Theclassificationofparticles,particularlyofaggregates,accordingtosizesasdeterminedwithaseriesofsievesofdifferentopenings.

Silicone Aresin,characterizedbywater-repellentproperties, inwhichthemainpolymerchainconsistsofalternatingsiliconandoxygenatoms,withcarbon-containingsidegroups;siliconesmaybeusedinjointsealingcompounds,caulkingorcoatingcompounds,oradmixturesforconcrete.

Silicone Sealant Liquidjointsealantconsistingofsilicone-basedmaterial.

Skid Resistance Ameasureofthefrictionalcharacteristicsofasurface.

Slipform Paving A type of concrete paving process that involves extruding theconcrete through a machine to provide a uniform dimension ofconcretepaving.

Slipform Aformthatispulledorraisedasconcreteisplaced;maymoveinagenerallyhorizontaldirectiontolayconcreteevenlyforhighwaypavingoronslopesandinvertsofcanals,tunnels,andsiphons;orverticallytoformwalls,bins,orsilos.

Slump A measure of consistency of freshly mixed concrete, equal tothe subsidence measured to the nearest 6mm (¼-in.) of themolded specimen immediately after removal of the slump cone.

Slump Cone Amoldintheformofthelateralsurfaceofthefrustumofaconewithabasediameterof203mm(8in.),topdiameter102mm(4in.),andheight305mm(12in.),usedtofabricateaspecimenoffreshlymixedconcretefortheslumptest.

Slump Loss Theamountbywhichtheslumpoffreshlymixedconcretechangesduringaperiodoftimeafteraninitialslumptestwasmadeonasampleorsamplesthereof.

Slump Test Theprocedureformeasuringslump.

Slurry Mixture of water and concrete particles resulting from concretesawingorgrinding.

Solid Volume SeeAbsoluteVolume

Sounding Processoftappingconcreteslabsurfacewithmetalobject,listeningfor tone from the impact, to determine areas of delamination.

Soundness Inthecaseofacement,freedomfromlargeexpansionaftersetting.Inthecaseofaggregate,theabilitytowithstandaggressiveconditionstowhichconcretecontainingitmightbeexposed,particularlythoseduetoweather.

Spalling, Compression Cracking,breaking,chipping,or frayingofslabedgeswithin0.6meterofatransversejoint.



Spalling, Sliver Chipping of concrete edge along a joint sealant; usually within12mmofthejointedge.

Spalling, Surface Cracking, breaking, chipping, or fraying of slab surface; usuallywithinaconfinedarealessthan0.5squaremeters.

Specific Gravity Theratiooftheweightinairofagivenvolumeofmaterialatastatedtemperaturetotheweightinairofanequalvolumeofdistilledwateratthesametemperature.

Specific Gravity Factor The ratioof theweightof aggregates (includingallmoisture), asintroducedintothemixer,totheeffectivevolumedisplacedbytheaggregates.

Split Batch Charging Methodofchargingamixerinwhichthesolidingredientsdonotallenterthemixertogether;cement,andsometimesdifferentsizesofaggregate,maybeaddedseparately.

Spud Vibrator A vibrator used for consolidating concrete, having a vibratingcasingorheadthatisusedbyinsertionintofreshlyplacedconcrete.

Standard Deviation Therootmeansquaredeviationofindividualvaluesfromtheiraverage.

Static Load Theweightofasinglestationarybodyorthecombinedweightsofallstationarybodiesinastructure(suchastheloadofastationaryvehicleonaroadway);duringconstruction,thecombinedweightofforms,stringers,joists,reinforcingbars,andtheactualconcretetobeplaced.

Stationary Hopper Acontainerusedtoreceiveandtemporarilystorefreshlymixedconcrete.

Storage Hopper SeeStationaryHopper

Straight-Edging Processofusingarigid,straightpieceofeitherwoodormetaltostrikeofforscreedaconcretesurfacetopropergradeortochecktheplanenessofafinishedsurface.

Stratification Theseparationofover-wetorover-vibratedconcreteintohorizontallayers with increasingly lighter material toward the top; water,laitance,mortar,andcoarseaggregatewilltendtooccupysuccessivelylowerpositions(inthatorder).

Strength Agenerictermfortheabilityofamaterialtoresiststrainorruptureinducedbyexternalforces.SeealsoCompressiveStrength,FlexuralStrength,andTensileStrength.

Stress Intensityofinternalforce(i.e.,forceperunitarea)exertedbyeitheroftwoadjacentpartsofabodyontheotheracrossanimaginedplaneofseparation;whentheforcesareparalleltotheplane,thestressis

calledshearstress;whentheforcesarenormaltotheplanethestressiscallednormalstress;whenthenormalstressisdirectedtowardthepartonwhichitactsitiscalledcompressivestress;whenitisdirectedawayfromthepartonwhichitactsitiscalledtensilestress.

Strikeoff Toremoveconcreteinexcessofthatrequiredtofilltheformevenlyor bring the surface to grade; performed with a straightedgedpieceofwoodormetalbymeansofaforwardsawingmovementor by a power operated tool appropriate for this purpose; alsothe name applied to the tool. See also Screed and Screeding.

Structural Capacity Expression of the ability of a pavement to carry traffic loads;Expressedasnumberofequivalent singleaxle loads inAASHTOdesignmethodology.

Subbase Alayerinapavementsystembetweenthesubgradeandbasecourseorbetweenthesubgradeandaportlandcementconcretepavement.

Subgrade The soil prepared and compacted to support a structure or apavementsystem.Alsosometimescalledgrade.

Sulfate Attack Chemicalorphysicalreactionbetweencertainconstituentsincementandsulfatesinthesoilorgroundwater;sufficientattackmaydisruptconcretethatissusceptibletoit.

Sulfate Resistance The ability of aggregate, cement paste, or mixtures thereof towithstandchemicalattackbysulfateioninsolution.

Superplasticizer SeeWater-ReducingAdmixture(highrange)

Supplementary Cementitious Material Mineraladmixturesconsistingofpowderedorpulverizedmaterials,whichareaddedtoconcretebeforeorduringmixingto improveorchangesomeof theplasticorhardenedpropertiesofPortlandcementconcrete.Materialsaregenerallynaturalorby-productsofothermanufacturingprocesses.

Surface Moisture Water retained on surfaces of aggregates capable of mixing withportlandcementinconcrete;distinguishedfromabsorbedmoisture,whichiscontainedinsidetheaggregateparticles.

Surface Retarder A retarder used by application to a form or to the surface ofnewlyplacedconcrete todelay settingof thecement to facilitateconstruction jointcleanupor to facilitateproductionofexposed,aggregatefinish.

Surface Tension Thatproperty, due tomolecular forces, that exists in the surfacefilmofallliquidsandtendstopreventtheliquidfromspreading.

Surface Texture Degree of roughness or irregularity of the exterior surfaces ofaggregateparticlesorhardenedconcrete.



Surface Vibrator Avibratorusedforconsolidatingconcretebyapplicationtothetopsurfaceofamassoffreshlymixedconcrete;fourprincipaltypesexist:vibratingscreeds,panvibrators,plateorgridvibratorytampers,andvibratoryrollerscreeds.

Surface Voids Cavitiesvisibleonthesurfaceofasolid.

Surface Water SeeSurfaceMoisture

Swelling Increaseinlengthorvolume.SeealsoContractionandExpansion.

T

Tamper 1)Animplementusedtoconsolidateconcreteormortarinmoldsorforms.

2)Ahand-operateddeviceforcompactingfloortoppingorotherunformedconcretebyimpactfromthedroppeddeviceinpreparationforstrikeoffandfinishing;contactsurfaceoftenconsistsofascreenoragridofbarstoforcecoarseaggregatesbelowthesurfacetopreventinterferencewithfloatingortroweling.

Tamping Theoperationofcompacting freshlyplacedconcretebyrepeatedblowsorpenetrationswithatampingdevice.

Temper Theadditionofwaterandmixingofconcreteormortarasnecessarytobringitinitiallytothedesiredconsistency.SeealsoRetempering.

Tensile Strength Maximumstressthatamaterialiscapableofresistingunderaxialtensile loadingbasedon thecross-sectional areaof the specimenbeforeloading.

Terminal Joint Jointusedincontinuouslyreinforcedconcretepavement(seeCRCP)to transition to another pavement type or to a bridge structure.

Texturing Theprocessofproducingaspecialtextureoneitherunhardenedorhardenedconcrete.

Thermal Expansion Expansioncausedbyincreaseintemperature.

Thermal Movement Changeofdimensionofconcreteormasonryresultingfromchangeoftemperatures.SeealsoContractionandExpansion.

Thermal Shock The subjectionofnewlyhardened concrete to a rapid change intemperaturewhichmaybeexpectedtohaveapotentiallydeleteriouseffect.

Tiebar Baratrightanglestoandtiedtoreinforcementtokeepitinplace;deformed bar extending across a construction joint to preventseparationofadjoiningslabs.

Time of Haul Inproductionofready-mixedconcrete,theperiodfromfirstcontactbetweenmixingwaterandcementuntilcompletionofdischargeofthefreshlymixedconcrete.

Time of Set Timerequiredafteradditionofwatertocementforcementpaste,mortars,orconcretestoattainacertainarbitrarydegreeofhardnessorstrength.

Time of Setting SeeInitialSettingTimeandFinalSettingTime.

TMMB TruckMixerManufacturers’Bureau;mosttruckmixerscarryTMMBratingplates.

Tongue and Groove Ajointinwhichaprotrudingribontheedgeofonesidefitsintoagrooveintheedgeoftheotherside,abbreviated“T&G.”SeealsoKeyway.

Topping 1)Alayerofhighqualityconcreteplacedtoformafloorsurfaceonaconcretebase.

2)Adry-shakeapplicationofaspecialmaterialtoproduceparticularsurfacecharacteristics.

Transit-mixed Concrete Concrete,themixingofwhichiswhollyorprincipallyaccomplishedinatruckmixer.SeeTruck-MixedConcrete.

Transverse Broom Surfacetextureobtainedusingeitherahandbroomormechanicalbroom that lightly drags the stiff bristles across the surface.

Transverse Crack Crack that develops at a right angle to the long direction of themember.

Transverse Joint Ajointnormaltothelongitudinaldimensionofastructure.

Transverse Reinforcement SeeReinforcement,Transverse.

Transverse Tine Surface texture achieved by a hand held or mechanical deviceequippedwitharake-liketiningheadthatmoveslaterallyacrossthewidthofthepavingsurface.

TRB TransportationResearchBoard

Trial Batch Abatchofconcreteusedforestablishingorcheckingproportions.



Trowel Aflat,broad-bladedsteelhandtoolusedinthefinalstagesoffinishingoperationstoimpartarelativelysmoothsurfacetoconcretefloorsandotherunformedconcretesurfaces;also,aflattriangular-bladedtoolusedforapplyingmortartomasonry.

Truck-Mixed Concrete Concrete, themixingofwhich isaccomplished ina truckmixer.

Truck Mixer A concrete mixer suitable for mounting on a truck chassis andcapableofmixingconcreteintransit.SeealsoHorizontal-AxisMixer,Inclined-AxisMixer,andAgitator.

UUltra-Thin Whitetopping Thinlayerofnewconcrete(2-4inches),usuallyhighstrengthandfiber-reinforced,placedoverapreparedsurfaceofdistressedasphalt

Unbonded Concrete Overlay Overlayofnewconcreteplacedontodistressedexistingconcretepavementwithalayerofasphaltorothermediumbetweenthenewandoldconcretesurfacetoseparatethem.

Uncontrolled Crack Acrackthat is locatedwithinaslabawayfromthesawedjoints.

Under-Sanded Aconcretemixture that isdeficient in sandcontent; a conditionassociated with poor workability or finishing characteristics.

Unit Water Content Thequantityofwaterperunitvolumeoffreshlymixedconcrete,oftenexpressedaspoundsorgallonspercubicyard.Itisthequantityofwateronwhichthewater-cementratioisbasedanddoesnotincludewaterabsorbedbytheaggregate.

Unit Weight SeeBulkDensityandSpecificGravity

Unreinforced Concrete SeePlainConcrete

Unsound Aggregate Anaggregateorindividualparticlesofanaggregatecapableofcausingorcontributingtodeteriorationordisintegrationofconcreteunderanticipatedconditionsofservice.

Uplift Beam Beam-like movement detection device used to monitor slab liftduringslabstabilization.

VVibrated Concrete Concrete compacted by vibration during and after placing.

Vibration Energeticagitationofconcreteproducedbyamechanicaloscillating

device at moderately high frequency to assist consolidation andcompaction.

Vibration Limit Thattimeatwhichfreshconcretehashardenedsufficientlytopreventitsbecomingmobilewhensubjecttovibration.

Vibration, External Externalvibrationemploysvibratingdevicesattachedat strategicpositionsontheformsandisparticularlyapplicabletomanufactureofprecastitemsandforvibrationoftunnel-liningforms;inmanufactureofconcreteproducts,externalvibrationorimpactmaybeappliedtoacastingtable.

Vibration, Internal Internalvibrationemploysoneormorevibratingelementsthatcanbeinsertedintotheconcreteatselectedlocations,andismoregenerallyapplicabletoin-placeconstruction.

Vibration, Surface Surfacevibrationemploysaportablehorizontalplatformonwhichavibratingelementismounted.

Vibrator Anoscillatingmachineusedtoagitatefreshconcretesoastoeliminategross voids, including entrapped air but no entrained air, andproduceintimatecontactwithformsurfacesandembeddedmaterials.

Vibratory Plate Compactor Motorized,one-mantoolconsistingofavibratingsquareplatethattransmitsenergytocompactgranularmaterials.

Volume Batching Themeasuringoftheconstituentmaterialsformortarorconcretebyvolume.

WWash (or Flush) Water Watercarriedona truckmixer ina special tank forflushing theinteriorofthemixerafterdischargeoftheconcrete.

Water-Cement Ratio Theratiooftheamountofwater,exclusiveonlyofthatabsorbedbytheaggregates,totheamountofportlandcementinaconcreteor mortar mixture; preferably stated as a decimal by weight.

Water-Cementitious Materials Ratio Theratiooftheamountofwater,exclusiveonlyofthatabsorbedby the aggregates, to the amount of portland cement and othercementitious material (fly ash, pozzolan, etc.) in a concreteor mortar mixture; preferably stated as a decimal by weight.

Water-Gain SeeBleeding

Water-Reducing Admixture A material that either increases slump of freshly mixed mortaror concrete without increasing water content or maintains aworkabilitywithareducedamountofwater,theeffectbeingduetofactorsotherthanairentrainment;alsoknownaswaterreducer.



Water-Reducing Admixture (High Range) Awater-reducingadmixturecapableofproducinglargewaterorgreatflowabilitywithoutcausingunduesetretardationorentrainmentofairinmortarorconcrete.

Weathering Changesincolor,texture,strength,chemicalcompositionorotherpropertiesofanaturalorartificialmaterialduetotheactionoftheweather.

Weight Batching Measuringtheconstituentmaterialsformortarorconcretebyweight.

Welded-Wire Fabric Reinforcement Welded-wirefabricineithersheetsorrolls,usedtoreinforceconcrete.

Well-Graded Aggregate Aggregate having a particle size distribution that will producemaximumdensity;i.e.,minimumvoidspace.

Wet Coveredwithvisiblefreemoisture;notdry.SeealsoDampandMoist.

Wet Process Inthemanufactureofcement,theprocessinwhichtherawmaterialsareground,blended,mixed,andpumpedwhilemixedwithwater;thewetprocessischosenwhererawmaterialsareextremelywetandsticky,whichwouldmakedryingbeforecrushingandgrindingdifficult.

Whitetopping Concreteoverlaypavementplacedonanexistingasphaltpavement.

Whitetopping, Conventional Overlayofnewconcrete,greaterthan4inchesthick,placedonto

existingasphaltpavementwithnoparticularstepstakentoensurebondingordebonding.

Whitetopping, Ultra-Thin SeeUltra-ThinWhitetopping

Wire Mesh SeeWeldedWireFabric

Workability Thatpropertyoffreshlymixedconcreteormortarwhichdeterminesthe ease and homogeneity with which it can be mixed, placed,compacted,andfinished.

Working Crack A crack in a concrete pavement slab that undergoes significantdeflectionandthermalopeningandclosingmovements;Typicallyoriented transverse to the pavement centerline and near a non-functioningtransversecontractionjoint.

YYield Thevolumeoffreshconcreteproducedfromaknownquantityofingredients;thetotalweightofingredientsdividedbytheunitweightofthefreshlymixedconcrete.

Zero-Slump Concrete Concreteofstifforextremelydryconsistencyshowingnomeasurableslumpafter removalof the slumpcone.SeealsoSlumpandNo-SlumpConcrete.


Abbreviated Index

Chapter 1 Introduction

Chapter 2 Basics of Concrete Pavement Design Chapter 3 Fundamentals of Materials Used for Concrete Pavements

Chapter 4 Transformation of Concrete from Plastic to Solid

Chapter 5 Critical Properties of Concrete

Chapter 6Development of ConcreteMixtures

Chapter 7Preparation for Concrete Placement

Chapter 8 Construction

Chapter 9Quality and Testing

Chapter 10Troubleshooting and Prevention

Abra

sion

Abso

rptio

nAc

cele

ratin

g ad

mixt

ures

Aggr

egat

esAi

r ent

rain

ing

adm

ixtur

es

Air-v

oid

syst

emAl

kali s

ilica

reac

tion

Batc

hing

Blee

ding

Cem

ent

Coef

ficie

nt o

f the

rmal

exp

ansio

n

Chem

ical

adm

ixtur

es

D-cr

ackin

g

Crac

king

Dens

ity (U

nit W

eigh

t)

Curin

g

16 12 22

47, 47 55, 27, 27, 27, 36, 27, 47 55 64 4950 59 39, 56 56 48 28 52, 57

79 69, 79 79, (All) 79 82 81 79 83, 100

146 135 114 (All) 134, 109, 141, 106 112 (All) 129 109, 127, 135, 133, 135 118, 144 113 146, 146, 135 133, 148 155 136

188 183 182 174, 174 189 173, 186 (All) 183 187, 176, 181, 184, 174 180 186, 185

228 206, 207 236 231 224 207, 235

246 246 246 246, 248, 246, 246, 269 269 258

Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes

253 265- 258 252, 267 256

192

Abbreviated Index






Chapter 6Development of Concrete Mixtures





Inco

mpa

tibilit

y

Perm

eabi

lity

Dura

bilit

yEn

viron

men

tFie

ld a

djus

tmen

tsFin

ishin

gFly

ash

Free

ze th

awFr

ost r

esist

ance

Grad

ing

GGBF

slag

Hydr

atio

n

Mixi

ng

Mod

ulus

of e

last

icity

Over

lays

Parti

cle

shap

e

18 19 23

36, 32 38, 49 44 34 46 3647 56

130 135, 132, 132 117, 123, 131 146 137 127 151

185, 182 188 176 182 174 174, 193 188

193 194 200

226 204, 220 209 213 210

260 253 252, 251, 250, 256, 252, 268 259 255, 256

95 94 94 (All) 81, 71, 93 97 (All)

Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes

Abbreviated Index











Pozz

olan

s

Plac

emen

tPo

pout

s

Prop

ortio

ning

Recy

cled

wat

erRe

info

rcem

ent

Reta

rdin

g ad

mixt

ures

Salt

scal

ing

Saw

-cut

ting

Setti

ngSh

rinka

ge /

volu

me

Stre

ngth

and

stre

ngth

gai

n

Sulfa

te re

sista

nce

Supp

ort

Supp

lem

enta

ry c

emen

titio

us

mat

eria

ls

17 16 19

49 35 53 61, 55, 38 59 38 38, 31, 57 62 59 50

80 94 79 77, (All) 72 93 78, 94, 80, 99 82, 93

118 133 134 133, 152 114, 125, 116, 132, 156 109, 137 149 135, 139 113-114,

186 (All) 183 186 174, 188 (All) 187

209, 233213 214

255 269 257, 256, 258, 261, 259 263,

192, 196

146, 117-118 151 139, 142

Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes

Abbreviated Index











Tem

pera

ture

Text

ure

Unifo

rmity

Wat

erW

ater

redu

cing

adm

ixtur

es

Wor

kabi

lity

17

47 45 27, 55, 52 58

127, 106, 109 114, 108135, 156 134

181 183 174, 185

149,150,156

256, 257 251-259, 260, 260, 261,269

(All) 79, 99

221

Yes Yes Yes Yes Yes Yes

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ISBN: 978-0-9652310-9-1

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