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EVALUATIONOFVOIDSINTHEMINERALAGGREGATEFORHMA PAVINGMIXTURES
By
PrithviS.Kandhal
AssociateDirectorNationalCenterforAsphaltTechnology
AuburnUniversity,Alabama
SanjoyChakraborty
GraduateStudentNationalCenterforAsphaltTechnology
AuburnUniversity,Alabama
PaperpublishedintheProceedingsoftheCanadianTechnicalAsphaltAssociation(CTAA),VolumeXLI,
November1996
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DISCLAIMER
Thecontentsofthisreportreflecttheviewsoftheauthorswhoaresolelyresponsibleforthefactsandtheaccuracyofthedatapresentedherein.ThecontentsdonotnecessarilyreflecttheofficialviewsandpoliciesoftheNationalCenterforAsphaltTechnologyofAuburnUniversity.Thisreportdoesnotconstituteastandard,specification,orregulation.
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ABSTRACT
Voidsinthemineralaggregate(VMA),togetherwiththevoidsinthetotalmix(VTM),areconsideredimportantparametersinhotmixasphalt(HMA)mixdesign.ItisbelievedthataminimumVMArequirementisnecessarytoensurethattheHMAmixisnotdeficientinasphaltcement(sothemixisdurable),and/orinVTM(topreventflushingand/orrutting).CurrentVMArequirementsarebasedlargelyupontheworkdoneduring1950to1960.However,theliteraturereviewedaspartofthisstudydidnotindicatetheexistenceofanysignificantrationaldatacorrelatingthedurabilityofHMApavementswiththeminimumVMAvaluesspecifiedformixdesign.
Thisstudywasundertaken(a)toreexaminetherationalebehindtheminimumVMAvalues
currentlybeingused,and(b)toquantifytherelationshipbetweenvariousasphaltfilmthicknessesandtheagingcharacteristicsoftheHMAmix,sothatanoptimumfilmthicknessdesirableforsatisfactorymixdurabilitycouldbeestablished.Theoptimumfilmthicknesscould
thenbeusedtoestablishVMArequirements.Mixespreparedwithasphaltbinderfilmthicknessrangingfromabout4to13microns,weresubjectedtoacceleratedagingusingStrategicHighwayResearchProgram(SHRP)procedurestosimulatebothshortandlongtermaging.Boththeaggregate(RD)andtheasphaltcement(AAM-1)usedinthisstudywereobtainedfromtheSHRPMaterialsReferenceLibrary.Theaged,compactedmixwastestedfortensilestrength,tensilestrainatfailureandresilientmodulus.Theagedasphaltcementwasrecoveredandtestedforpenetration,viscosity,complexbetweenvariousasphaltfilmthicknessesandtheagingcharacteristicsoftheHMAmix,sothatanoptimumfilmthicknessdesirableforsatisfactorymixdurabilitycouldbeestablished.Theoptimumfilmthicknesscouldthenbeusedtoestablishmodulusandphaseangle.Agingindiceswereobtainedfromthesetests,andtherelationshipbetweenfilmthicknessandtheagedmix/agedasphaltcementpropertiesweredeterminedusingregressionanalysis.Fortheparticularaggregate/asphaltcementcombinationusedinthisstudy,itwasfoundthatacceleratedagingwouldoccuriftheasphaltbinderfilmthicknesswaslessthan9-10micronsinanHMAmixturecompactedto8%airvoidcontent.TheminimumVMArequiredtoaccommodatethisoptimumasphaltfilmthicknessand4%airvoidcontent,wasalsocalculated.
KEYWORDS:VMA,voidsinmineralaggregate,hotmixasphalt,HMA,asphaltconcrete,
asphaltpavingmixture,filmthickness,durability.
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EVALUATIONOFVOIDSINTHEMINERALAGGREGATEFORHMAPAVING
MIXTURES
PrithviS.KandhalandSanjoyChakraborty
INTRODUCTION
Theconceptofvoidsinthemineralaggregate(VMA)isconsideredfundamentalinthedesignofdense-gradedhotmixasphalt(HMA)mixtures.ThetermVMAdescribesthatportionofthespaceinacompactedHMApavementorspecimenwhichisnotoccupiedbytheaggregate.CurrentmixdesignproceduresincludingSuperpavearelargelybasedupontheneedforselectingandproportioningthevariousmaterialstomeetcertainvolumetricpropertiessuchasVMA.AminimumVMArequirement(basedonthemaximumnominalsizeoftheaggregate)isusedtoensurethattheHMAmixisnotdeficientinasphaltcement(therebyensuringmixdurability)and/orinthevoidsinthetotalmix(VTM)topreventbleedingorrutting.
TheminimumrequirementsforVMAhavebeenquestionedbymanyresearchersbecausethere
isalackofsignificantresearchdatacorrelatingtheVMAwiththeHMAmixperformanceintermsofdurability.
OBJECTIVES
Thisstudywasundertakentoachievethefollowingobjectives:
1.Reviewavailableliteraturetore-examinetherationalebehindtheminimumVMArequirementscurrentlybeingused.
2.DeterminetheoptimumasphaltbinderfilmthicknessinanHMAmixturetominimizeshortandlongtermagingoftheasphaltbinder,thusprovidingreasonabledurabilityoftheHMAmix.TheoptimumfilmthicknesscanthenbeusedtoestablishminimumVMArequirements.
LITERATUREREVIEW
MinimumVMARequirements
InapaperpresentedtotheHighwayResearchBoardin1956[1],McLeodpointedoutthatthebasiccriteriaforboththedesignandanalysisofpavingmixturesshouldbeonavolumetricbasisandnotonthebasisofweight.Mostspecificationsinthosedaystendedtospecifyarangeofasphaltcontentbyweightalongwithgradingbandsorlimitsfortheaggregate,whichineffectrequiredadesignonthebasisofweight.
McLeod[1]illustratedthevolumetricrelationshipbetweenthetotalasphaltbinder,airvoids
betweenthecoatedaggregateparticles,andthetotalaggregateinacompactedpavingmixture.
HebasedthecompactionrequirementsupontheMarshalltestprocedure,with75blowsoneachsideofthebriquette.HealsorecommendedthattheVMA,whichisthevolumeofvoidsbetweentheaggregateparticles,shouldberestrictedtoaminimumvalueof15%,thevolumeoftheairvoids(withintheVMA)shouldliebetween3and5%,whichinturnrestrictedthevolumeofasphaltcementinthecompactedmixturetoapermissibleminimumof10%byvolume.Therefore,hisproposalforaspecificationofaminimum15%VMA,alongwith5%airvoids,automaticallyestablishesaminimumasphaltcontentofabout4.5%byweight(10%byvolume).Hiscalculationswerebaseduponabulkspecificgravityof2.65fortheaggregateand1.01fortheasphaltcement.Noasphaltabsorptionwasconsideredinthevolumetricanalysis.
AnotherpaperpresentedbyMcLeodin1959[1]totheAmericanSocietyofTestingand
Materials,advocatedtheuseofbulkspecificgravityoftheaggregateforcalculatingboththe
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VMAandtheairvoids.Absorptionoftheasphaltcementintotheaggregatewasalsotakeninto
accountinthevolumetricanalysis.McLeodrecommendedagainthatthelowestpermissibleasphaltcontentinaHMAmixshouldbe4.5%byweight,toensuremixdurability.Thisamountstoabout10%asphaltcementbyvolume.NoHMAperformancedatawerepresentedtosupport
theminimumasphaltcontentof4.5%onwhichtheminimumVMArequirementwasbased.Inthispaper,McLeodalsoproposedarelationshipbetweentheminimumVMAandthenominalmaximumparticlesizeoftheaggregate,whichwasadoptedbytheAsphaltInstitutein1964[3].Hebasedthisrelationshipuponthebulkspecificgravityoftheaggregateandanairvoidscontentof5%forthecompactedmix.However,thebackgrounddataforrelatingtheminimumVMArequirementstothenominalmaximumsizeoftheaggregatewasnotgiven[2].
Duringthelast30yearsorso,mostasphaltpavingtechnologistsdidnotrealizethatthese
minimumVMArequirementswerebasedon5%airvoidcontent(andnot4%airvoidcontentgenerallyusedformixdesign)and75-blowmarshallcompaction.Obviously,theminimumVMArequirementscorrespondingto4%airvoidcontentwouldbe10/0lowerthanthoserecommendedinearliereditionsofAsphaltInstituteMS-2[3].Thiswasrecognizedin1993and
theAsphaltInstituteMS-2wasrevised[4]togiveminimumVMArequirementscorrespondingto3,4,and5%airvoidcontents.TheserevisedminimumVMArequirementshavealsobeenincorporatedinSuperpavemixdesignprocedures.
AsphaltBinderFilmThicknessinDurabilityConsiderations
Itisgenerallyagreedthathighpermeability,highairvoids,andthinasphaltcoatingsontheaggregateparticlesaretheprimarycausesofexcessiveagingoftheasphaltbinderwhichcontributestothelackofdurabilityoftheHMAmixesoftenencounteredinthefield.However,theconceptofan"averagefilmthickness"fordense-gradedasphaltmixturesisnoteasilyunderstood.Howmuchvaliditycanbeassignedtoafilmthickness,calculatedsimplybydividingthetotalsurfaceareaoftheaggregate(obtainedfromitsgradation)bytheeffectiveasphaltcontent?Itishighlyunlikelythatalltheparticlesinamixhavethesamefilmthickness
ofasphaltcoating.Fineaggregateparticlesmayhaveamuchthickercoatingascomparedtothecoarseaggregateparticles,andinfact,forallpracticalpurposes,someveryfineparticlesmightsimplybeembeddedintheasphaltcement/fillermortarsystem.Therefore,theterm"filmthickness"iselusiveanddifficulttodefine.However,forthepurposeofcalculationlaterinthispaper,weshallassumethattheconceptofthe"averagefilmthickness"isindeedvalid,andproceedwiththecalculations.SurfaceareawillbecalculatedusingtheprocedureoutlinedintheAsphaltInstitute'sMS-2[4].Campen,Smith,EricksonandMertz[5]presentedtherelationshipbetweenvoids,surfacearea,filmthicknessandstabilityfordensegradedHMA.Theauthorsrecognizedthatthickerasphaltbinderfilmsproducedmixeswhichwereflexibleanddurable,whilethinfilmsproducedmixeswhichwerebrittle,tendedtocrackandravelexcessively,retardedpavementperformance,andreduceditsusefulservicelife.Onthebasisofthedatatheyanalyzed,averagefilmthicknesses
rangingfrom6to8micronswerefoundtohaveprovidedthemostdesirablepavementmixtures.Theyalsoconcludedthatthefilmthicknessdecreasesasthesurfaceareaoftheaggregateisincreased.However,theasphaltbinderrequirementofamixisnotdirectlyproportionaltoitssurfacearea.Theasphaltbinderrequirementwasfoundtoincreaseasthesurfaceareawasincreased,butataratemuchlowerthanthatguidedbyarelationshipofdirectproportionality[5].
GoodeandLufsey[6]alsodidsomesignificantworkinrelatingasphalthardeningtovoids,
permeabilityandfilmthickness.Theyrecognizedthatthehardeningoftheasphaltbinderinamixwasafunctionofairvoids,filmthickness,temperature,andtime.Onthebasisoftheirworktheyconcludedthataminimumvalueof0.00123for'bitumenindex'(whichcorrespondstoavalueof6micronsofaveragefilmthickness)couldbeincludedasacriterioninallmixdesign
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procedures.The'bitumenindex'wasdefinedaspoundsofasphaltcementpersquarefootof
surfacearea.Theyusedtheconceptofbitumenindextoavoidtheimplicationthatallparticleswerecoatedwiththesameuniformthicknessofasphaltcement.Theirstudyindicatedthatacombinedfactoroftheratiooftheairvoidstothebitumenindexcouldbesatisfactorilyrelated
totheasphaltbinderhardeningcharacteristicsintheHMAmixture.TheysuggestedthattheMarshallmethodofmixdesigncouldbeimprovedbyincorporatingamaximumvalueofvoids-bitumenindexratioinplaceofamaximumvalueofairvoidsalone,andsuggestedavalueof4asthemaximumforthisratio,toensurereasonableresistancetoaging.
KumarandGoetz[7]studiedtheasphaltbinderhardeningasrelatedtoHMApermeabilityand
asphaltfilmthickness.Theystatedthatthebestprocedureforpredictingtheresistanceofhardeningofasphaltbinderinasingle-sizedHMAmixwastocalculatetheratioofthefilmthicknessfactortopermeability.Thefilmthicknessfactorwasdefinedastheratioofthepercentasphaltcontentavailableforcoatingtheaggregatetothesurfaceareaoftheaggregate.Theyindicatedthatfordense-gradedmixtures,theconceptofanaveragefilmthicknessisatbestdubious,ifnottotallyerroneous.Fordense-gradedmixtures,permeabilitywasstatedtobethe
bestmeasureoftheresistancetohardening.However,atthedesignvalueof4%airvoidsasiscommonformostdense-gradedHMAmixtures,theeffectofpermeabilityofthemixwasdeterminedtobequiteinsignificant.
TESTINGPROGRAM
Thistestingprogramwascarriedoutwiththefollowingobjectives:
Toevaluatethechangesinthetheologicalpropertiesoftheasphaltcementduetoaginginrelationtotheasphaltfilmthickness.Bothshortterm(duringHMAproductionandconstruction)andlongterm(duringservicelife)agingwereconsidered.
Todetermineanoptimumrangefortheasphaltfilmthickness,ifpossible,whichwouldminimizeagingoftheasphaltbinder.Thisoptimumfilmthicknesscanthenbe
usedindevelopingminimumVMArequirementsforHMAmixtures.
MaterialUsed
AggregateandasphaltcementsampleswereobtainedfromtheSHRPMaterialReferenceLibrary(MRL).Onlyoneaggregate(SHRPMRLDesignationRD):FrederickLimestonewasusedinthisstudy.Table1givesthephysicalpropertiesofthetotalaggregateobtainedfromSHRPMRL.Table2givesthewashedgradationoftheaggregateusedintheHMAmixture.
Table1.PhysicalPropertiesofRDAggregate(FrederickLimestone)
Property
BulkSpecificGravity
WaterAbsorption,percentL.A.Abrasion(AASHTOT96)
%Wear
FlakinessIndex,percent
SandEquivalent(AASHTOT176)
Value
2.704
0.3
23.4
34.7
69
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Table2.WashedGradationofAggregateUsedintheHMAMix
SieveSize(mm)
12.5
9.5
4.75
2.36
1.18
0.6
0.3
0.15
0.075
PercentPassing
100
8963
4533
2113
85
Anasphaltcement(SHRPMRLDesignationAAM-1)wasusedinthisstudy.ThisasphaltcementwasselectedbecauseithadoneofthehighestpropensitiestoageintheHMAmixbasedontheworkdoneinSHRPA-003AbySosnovskietal[3].ItsphysicalandchemicalpropertiesasobtainedfromSHRParegiveninTable3.
TestProceduresUsed
ThesurfaceareaoftheaggregatewascalculatedusingthesurfaceareafactorsgiveninMS-22
[4].Fortheaggregategradationused(Table2)thesurfaceareawascalculatedtobe27.626ft/lb(5.662m2/kg).
HMAmixtureswerepreparedateachofthefollowingsixeffectiveasphaltfilmthicknesses:3.7,5.6,7.4,9.3,11.1,and13.0microns.Thefilmthicknessesoriginallytargetedforexperimentaldesignwere4,6,8,10,12and14microns.However,certainerrorsinthecalculationswerediscoveredafterthemixeshadactuallybeenprepared.Theactualvaluesofasphaltfilmthicknessesusedwerethenrecalculated.
Avalueof0.20%asphaltabsorptionwasusedfortheRD-AAM-1combinationasdetermined
andreportedin[9].Thisrequiredsixasphaltcontents(byweightofthetotalmix)asfollows:2.2,3.2,4.2,5.1,6.1and7.1percenttoobtainasphaltfilmthicknessrangingfrom3.7to13.0micronasmentionedabove.
AllsixHMAmixtureswerepreparedatthemixingtemperatureof1433C.Thetesting
sequenceforeachmixisgiveninFigure1.ThelooseHMAmixsamplesweresubjectedtoshorttermagingfollowingSHRP#1025
procedures[10].TheprocessinvolvesagingofthelooseHMAmixinaforceddraftovenfor4hoursatatemperatureof135C.Theloosemixisplacedinabakingpanandspreadtoaneventhicknessthatproducedabout21kg/m2.ThisprocedureisdesignedtosimulatetheagingthatthelooseHMAmixundergoesduringtheconstructionphaseofthepavement.ThreesamplesoftheagedHMAmixweresubjectedtoAbsonmethodofrecoveringasphaltbinder.Therecoveredasphaltbinderwastestedforpenetrationat25Candviscosityat60C.Thecomplexmodulus(G*)andphaseangle(*)werealsodeterminedat64CfortherecoveredasphaltcementusingtheDynamicShearRheometer.Thetemperatureof64Cwasusedbecausejustafterconstruction,ruttingfactor(G*/sin*6)iscriticalathighpavementtemperatures.ThistemperaturewouldbeusedfortestingaSuperpavePG64-34binderaftersubjectingittorolling
thinfilmoven(RTFO)whichsimulatesshorttermaging.
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Table3.PropertiesofAsphaltCement(AAM-1)Used
Property
ORIGINALASPHALTCEMENT
SpecificGravity
Viscosityat60C,Pa.s
Viscosityat135C,cSt
Penetrationat25C,0.1mm
Ductilityat4C,cm
SofteningPoint(R&B),C
DynamicShearRheometer(DSR)data:
G*/sin*at64C,kPa
TFORESIDUE
MassChange,%
Viscosityat60C,Pa.s
Viscosityat135C,cSt
RTFORESIDUE
DSRdata
G*/sin*at64C,kPa
PRESSUREAGINGVESSELRESIDUE
DSRdata
G*/sin*at20C,kPa
COMPONENTANALYSIS
Asphaltenes(n-heptane)
PolarAromatics
NaptheneAromatics
Saturates
ELEMENTANALYSIS
Nitrogen,%
Sulphur,%
Vanadium,ppmNickel,ppm
Value
0.993
199.2
56964
4.6
51.71.15
0.00516
394.7
744
2.46
3,200
3.9
50.3
41.91.9
0.50
2.40
60.0
29.0
Five100-mmdiameterspecimenswerecompactedfromeachmixtypeaftershorttermaging.Thecompactedspecimenswerepreparedtogiveatargetairvoidcontentlevelof81%.TheCorpsofEngineersGyratoryTestingMachine(GTM)wasusedforthispurpose.Theresilientmodulus(MR)ofallthecompactedspecimenswasdeterminedat25C.Totalnumberofsamplestested=6(filmthicknesses)x5(replicates)=30.
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Figure1.TestSequenceforEachAsphaltContent/FilmThickness
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The30compactedspecimensweresubjectedtolongtermagingfollowingSHRP#1030
procedures[11].Theprocedureconsistsofplacingthecompactedspecimensonarackinaforceddraftovenfor120hours,andatatemperatureof85C.ThisprocedurewasdesignedtosimulatetheagingthatthecompactedHMApavementundergoesduringits5-10yearsservice
life.Anairvoidcontentof81%incompactedspecimensisusedtosimulatecompactionatthetimeofconstruction.Lowerairvoidcontentsmayalsonotprovideinterconnectedvoidswhichareessentialforthisacceleratedagingtest.
Thefollowingtestswereconductedonthecompactedspecimensafterlongtermaging:
1.Resilientmodulus(MR)at25C.2.Tensilestrength(ST),alongwiththestrainatfailure,at25Cusingastrainrateof50
mm(0.05m)perminute.3.Absonrecoveryofagedasphaltbinderfromall30brokenspecimens.Therecovered
asphaltbindersweretestedforpenetrationat25C,viscosityat60C,complexmodulus(G*)at19C,andphaseangle(6)at19C.
ComplexmodulusandphaseangleweremeasuredusingSuperpavetestprocedures[12].The
temperatureof19Cwasusedbecausethefatiguefactor(G*sin*)iscriticalatmidservicepavementtemperaturesaccordingtoSuperpaveperformancegraded(PG)binderspecifications[12].AfterlongtermagingHMAmixesbecomestiffand,therefore,fatiguecrackingbecomestheprimarydistressofconcernaffectingtheHMAdurability.Thetemperatureof19CwouldbeusedtotestaSuperpavePG64-34binderafteragingitinRTFOandpressureagingvessel(PAV).
ANALYSISOFTESTRESULTS
Table4givesthecompactedHMA'sphysicalproperties(suchasresilientmodulusat25Candtensilestrengthat25C)aftershortandlongtermagingcorrespondingtoasphaltfilmthicknessrangingfrom3.7to13.0micron.Table5givestheconventionalproperties(suchaspenetration
at25Candviscosityat60C)oftherecoveredasphaltbindersaftershorttermandlongtermaging.Table6givesSuperpavebinderproperties(suchascomplexmodulusG*)fortheserecoveredasphaltbinders.
Table4.CompactedHMAPropertiesAfterShortandLongTermAging1
ResilientModulusat25C,MPa
FilmThickness(microns)
3.7
5.6
7.4
9.311.1
13.0
AfterSTA
8,184
6,357
4,027
2,9012,572
1,958
2 AfterLTA3
12,293
9,398
5,240
3,7162,696
2,020
LTA/STARatio
1.50
1.48
1.30
1.281.05
1.0312Allreporteddataareaveragesoffivesamples STA=ShortTermAging3LTA=LongTermAging
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Table5.RecoveredConventionalAsphaltBinderPropertiesAfterAging1
Viscosityat60C,Pa.s ViscosityRatio Penetrationat425C,
0.1mm
RetainedPenetration5
FilmThickness After2 After3 AfterSTAAfterLTAAfterSTAAfterLTAAfterSTAAfterLTA
(microns) STA LTA
3.7 1262.1 4744.4 6.15 23.12 31.3 24.6 50.5 39.7
5.6 809.9 4658.4 3.95 22.70 35.3 25.7 56.9 41.5
7.4 526.1 434.2 2.56 21.19 39.6 27.3 63.9 44.0
9.3 434.6 3940.1 2.12 19.20 43.6 29.0 70.3 46.811.1 276.3 3063.3 1.35 14.93 54.0 33.6 87.1 54.2
13.0 236.7 2897.6 1.15 14.12 56.6 34.3 91.3 55.312Allreporteddataareaveragesofthreesamples STA=ShortTermAging3LTA=LongTermAging 4BasedonviscosityoforiginalasphaltcementmeasuredatNCAT(205.2Pa.s) 5BasedonpenetrationoforiginalasphaltcementmeasuredatNCAT(62)
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Table6.RecoveredSuperpaveAsphaltBinderPropertiesAfterAging1
Complex Complex G*sin*at19C,Modulus,G*at Modulus,G*at Pa
64C,Pa 19C,Pa
FilmThickness(microns)
3.7
5.6
7.4
9.3
11.1
13.0
AfterSTA2
2090
3590
2270
2460
1310
1220
AfterLTA3
2.50E+06
2.25E+06
1.74E+06
1.53E+06
1.22E+06
1.71E+06
AfterLTA3
1.488E+06
1.339E+06
1.068E+06
1.918E+06
1.787E+06
1.094E+0612Allreporteddataareaveragesofthreesamples STA=ShortTermAging3
LTA=LongTermAgingTheconceptofpolynomialregressionhasbeenusedasatooltofittheobserveddatatocurve,
whichquantifytherelationshipbetweentheindependentandthedependentvariables.Theindependentvariable,inmostcases,istheasphaltfilmthickness,againstwhichareplottedthevaluesoftherecoveredasphaltcementpropertieslikepenetration,viscosity,andcomplexmodulus,orthemeasuredpropertiesofthecompactedHMAmix,liketensilestrengthandresilientmodulus.Therelationshipbetweenthemeasuredpropertiesantithefilmthicknesshasbeenquantifiedformixeswhichhavebeensubjectedtobothshortandlongtermaging.AlldependentvariableswhosevaluesaregiveninTables4,5and6wereanalyzed,thedetailedanalysesaregivenelsewhere[13].Aselectednumberofdependentvariablesarediscussedbelow.
CompactedHMAMixPropertiesTheresilientmodulusofthecompactedHMAspecimenswasmeasuredbothaftershorttermagingandafterlongtermaging(Table4).Theloosemixhadbeensubjectedtoshorttermagingbeforecompaction,i.e.,beforethepreparationofthecompactedsamples.Thecompactedsampleswerethensubjectedtolongtermaging.Resilientmodulustestingwascarriedoutat25Cinthediametralorindirecttensilemode.Figures2and3showtherelationshipbetweenthefilmthicknessandmodulusvaluesaftershorttermandlongtermaging,respectively.Quadraticpolynomialregressiongaveanacceptablemodelforthisrelationshipaspresentedbelow:
AfterShortTermAging
Mrst=2069.9-273.15:+10.53:2
R2=0.99035
where,
Mrst=resilientmodulusaftershorttermaging(ksi)(1ksi=6.895Mpa)
:=filmthicknessinmicrons
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AfterLongTermAging
Mrlt=3267.6-456.75:+17.55:2
R2=0.9896
where,Mrlt=resilientmodulusafterlongtermaging(ksi)
Figure2.AsphaltFilmThicknessvs.ResilientModulusafterShortTerm Aging
ItcanbeseeninFigures2and3thatataboutafilmthicknessof11microns,thefittedcurvetendstoflattenoutanddoesnotchangesignificantlywithincreasingfilmthickness.Also,theslopeofthecurvebecomessteeperasthefilmthicknessfallsbelowavalueofabout9to10microns,whichindicatesthatthestiffness(causedbyaging)oftheHMAmixstartstoincreasequiterapidlywithadecreaseinfilmthicknessbelowabout9-10microns.Also,thereisamarkedsimilaritybetweenthecurvesobtainedaftershorttermandlongtermaging.
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Figure3.AsphaltFilmThicknessvs.ResilientModulusafterLongTerm Aging
SomemoreinformationconcerningthechangeinresilientmodulusvalueswithfilmthicknesscanbeobtainedfromthegraphinFigure4wheretheresilientmodulusvaluesaftershorttermaginghavebeenplottedagainstthecorrespondingmodulivaluesafterlongtermaging.Eachpointonthegraphcorrespondstooneparticularasphaltfilmthicknesswhichdecreasesfromlefttoright(becausetheresilientmodulusvaluesincreasewithdecreaseinfilmthickness).Regressionanalysisleadstoalinearrelationshipinthedataasmodeledbythefollowingequation:
Mrlt=-198.12+1.675Mrst
R2=0.998
TheveryhighvalueofR2indicatesthatalinearmodelalmostexactlyrepresentstherelationship.FromthisitcanbeinferredthatbothshortandlongtermagingoftheHMAmixareaffectedinexactlythesamewaybythefilmthickness.ThismeansthickasphaltfilmsminimizetheagingoftheHMAmixesduringconstructionaswellasduringservicelife.
Asexpected,thetensilestrengthat25Cdecreasesandthetensilestrainatfailureincreasesas
theasphaltfilmthicknessintheHMAmix(afterlongtermaging)isincreasedfrom3.7to13.0micron.
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Figure4.ResilientModulusBeforevs.ResilientModulusAfterLTOA
RecoveredAsphaltBinderProperties
ThemeasuredvaluesofviscosityoftherecoveredasphaltaftershorttermandlongtermaginghavebeenpresentedgraphicallyinFigures5and6,respectively.Forsamplessubjectedtoshorttermaging,regressionanalysisleadstotheestablishmentofaquadraticmodeldefinedbythefollowingequation:
Vst=22069-3268.8:+132.8:2
R2=0.988
where,
Vst=viscosityofasphaltcementaftershorttermaging(poises)(1poise=0.1pa.s)
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Figure5.AsphaltFilmThicknessvs.ViscosityAfterShortTermAging
Figure6.AsphaltFilmThicknessvs.ViscosityAfterLongTermAging13
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Ascanbeseenfromthedataforshorttermaging(Figure5),astheasphaltfilmthickness
decreasesbelowavalueofabout9-10micronsthefittedcurvetendstosteepenindicatinganacceleratedrateofincreaseinviscosity.Ontheotherend,thesamecurveisseentoflattenoutatabout11micronsfilmthickness,whichindicatesthatthefilmthicknesshaslesserandlesser
effectontheagingoftheasphaltcementonceitisincreasedaboveavalueofabout11microns.Forsamplessubjectedtolongtermaging,regressionanalysisfailedtoproduceasatisfactorymodelwhichcouldexplainthenatureoftherelationshipbetweenfilmthicknessandagedviscosity.Thus,noequationisavailabletodefinetherelationship.Instead,thepointsonthegraphhavebeenconnectedtogetherbyasmoothcurve.AscanbeseenfromFigure6,theviscosityincreasesatanacceleratedrateoncetheasphaltfilmthicknessdecreasesbelowavalueofabout10microns.Thenatureofthecurvesobtainedwhentheviscosityratio(Table5)isplottedagainstfilmthickness,isaboutthesameasinthepreviouscasesforshortandlongtermagedconditions[13].Theviscosityratioisdefinedastheratiooftheviscosityoftheagedasphalttotheviscosityoftheunaged/originalasphalt.
Similarrelationshipswerealsoobservedbetweenasphaltfilmthicknessandpenetrationor
retainedpenetrationofasphaltcementsaftershortandlongtermaging[13].
Forasphaltcementsubjectedtoshorttermaging,complexmodulusG*andphaseangle*weremeasuredatatemperatureof64C,whereasforlongtermagedspecimens,thetestingoftheasphaltcementwascarriedoutat19C(Table6).Sincethestiffnessoftheasphaltbinderismorecriticalafterlongtermagingratherthanshorttermagingfromthedurability(orresistancetofatigue)standpoint,thecomplexmodulusG*andfatiguefactor(G*sin*)afterlongtermagingwillonlybepresentedhere.TherelationshipbetweenasphaltfilmthicknessandG*at19CafterlongtermaginghasbeenshowninFigure7.AlthoughtheregressionanalysisofG*at19Candfilmthicknessgaveaquadraticmodelfortheshorttermagedasphaltcement[D],alinearmodel(Figure7)betterexpressestherelationshipbetweenasphaltfilmthicknessandG*at19Cafterlongtermaging,asfollows:
G*lt=3158521-176472.6:
R=0.98
where,
G*lt=complexmodulusofasphaltcementsubjectedtolongtermaging(Pa)
Asexpected,valuesofthecomplexmodulusG*decreasewithincreasingasphaltfilmthickness,
bothforshortandlongtermaging.ThisindicatesthatthepresenceofthickerfilmsofasphaltcementintheHMAmixminimizesagingoftheasphaltbinder.However,itisnotapparentfromthefittedcurvesastowhatrangeofasphaltfilmthicknessmightprovetobeoptimuminminimizingasphaltcementaging.
TherelationshipbetweenG*sin*(fatiguefactor)andfilmthicknesshasbeenpresentedinFigure8,forasphaltcementrecoveredfrommixeswhichhadbeensubjectedtolongtermaging.Alinearmodelwasobtainedasfollows:
GSDlt=1848404-98052.5:
R2=0.98
where,
GSDlt=G*sin*(inpascals)forsamplessubjectedtolongtermaging.
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Figure7.AsphaltFilmThicknessvs.ComplexModulus(G*)AfterLongTermAging
Figure8.AsphaltFilmThicknessvs.G*sin *AfterLongTermAging15
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Asisevidentfromthefittedcurve(Figure8),G*sin*increaseswithdecreaseinthefilmthicknessoftheasphaltbinderintheHMAmix.ThisindicatesthatthelowertheasphaltbinderfilmthicknesspresentinaHMAmix,themoresusceptiblethepavementistofatiguecrackinginthelongterm.SHRPhassuggestedavalueof5000KPaastheupperlimitforG*sin*forasphaltcementsubjectedtoacceleratedaginginthePressureAgingVessel(PAV).PAVagingsimulatestheagingthattheasphaltbinderinaHMApavementundergoesafterabout5-10yearsinservice.Asdiscussedearlier,thisstudyemployedaprocedure(alsodevelopedbySHRP)tosimulatelongtermagingofin-servicepavementsusingcompactedHMAsamples,insteadofjustasphaltcement,asisusedinthePAVtest.Therefore,forasphaltcementrecoveredfromcompactedHMAsamplessubjectedtoacceleratedlongtermlaboratoryagingprocedures,5000kPashouldbeconsideredastheupperlimitforG*sin*.AsisevidentfromthedatapresentedinFigure8,themaximumrecordedvalueofG*sin*isabout1500kPa,whichismuchlessthanthelimitingvalueof5000kPa.ThisindicatesthatthePAVagingofasphaltcementismuchmoresevereinthislimitedlaboratorystudythanthatoccurringincompactedHMAsamplesagedinforceddraftovenat85Cfor120hours.
AirVoidstoBitumenIndexRatioAnalysis
Theconceptoftheratiooftheairvoids(percent)tobitumenindex,asameasureoftheagingsusceptibilityofamix(whateverbeitsgradation),wasdiscussedearlier.GoodeandLufsey[6]hadproposedamaximumvalueof4.0forthisratiowhichtheybelievedwouldpreventpavementdistressbyreducingtheagingoftheasphaltfilmcoatingtheaggregate.Mathematically,whattheystatedwas:
Notingthat:
filmthicknessinmicrons=bitumenindex4870
Thepreviousexpressioncanbereducedtoaminimumfilmthicknessrequirement,varyingwith
theairvoidscontentofthegivenmix,asfollows:Atargetvalueof8%fortheairvoidsinthecompactedHMAspecimenswasusedinthepresent
study,inconformancewiththeacceleratedlongtermagingproceduredevelopedinSHRPA003-AProject.Thiscorrespondstoaminimumfilmthicknessrequirementsof9.74microns(about10microns),basedupontheaboveequation.AscanbeseenfromFigures2and3,thecurvesoftheresilientmoduli(forbothshortandlongtermagingconditions)versusfilmthicknesstendtosteepenasthefilmthicknessdecreasesbelowtherangeof9to10microns.Thisindicatesthattherateofagingoftheasphaltcementisacceleratedwhenthefilmthicknessislessthan9-10microns.Thisacceleratedagingratecanalsobeseenintheplotsofviscosityversusfilmthickness(Figures5and6).Thereforeitcanbeconcluded,onthebasisofthepresentstudy,thatamaximumvalueof4.0forthevoids/bitumenindexratioisindeedreasonable,andmightprovetobeabetterspecificationfordesign,atleastasfarastheagingofasphaltcementisconcerned.
Basedonthedatapresentedinthislimitedstudyofoneasphaltcement/oneaggregate
combination,letusassumethattheoptimumasphaltfilmthicknesstominimizeagingis9
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microns.Thevolumeofasphaltcementbindercanthenbecalculatedbasedonthisoptimumfilm
thicknessandsurfaceareaoftheaggregateintheHMAmixused.Thisvolumeofasphaltcementwhenaddedto4%airvoidcontent(generallyusedinHMAmixdesign)shouldthengivetheminimumVMArequiredforthisHMAmixtoensurereasonabledurability.Itisinterestingto
notethattheminimumVMArequirementforthemixusedinthisstudywouldbecalculatedat15.6%baseduponthisprocedureforafilmthicknessof9micronsandanairvoidscontentof4%.AspertherecommendationsoftheAsphaltInstitute[4],thecorrespondingminimumVMAthathastobeprovidedforthismix(withamaximumnominalsizeof12.5mmaccordingtothenewdefinition)is14percent.Thenominalmaximumsizehasnowbeendefinedasonesievesizelargerthanthefirstsievetoretainmorethan10percent.Therefore,theminimumVMArecommendationoftheAsphaltInstitute(alsoadoptedinSuperpavemixdesign)isabout1.5%lessthanthatneededforobtaininganoptimumasphaltfilmthicknessbasedonthisstudy.Thisdifferenceislikelytovaryifdifferentgradationand/ordifferentasphaltcement/aggregatecombinationsareused.ItisalsointerestingtonotethatMcLeod[14]hadrecommendedin1971toincreaseallminimumVMArequirementsinMS-2by2%toobtaingreaterdurability,althoughwithoutprovidinganysignificantsupportingdata.However,itmaynotbepossibleto
increasetheVMAinmanydense-gradedHMAmixtures.CONCLUSIONSANDRECOMMENDATIONS
Thisstudywasundertaken(a)toreexaminetherationalebehindtheminimumVMArequirementscurrentlybeingused,and(b)toquantifytherelationshipbetweenvariousasphaltbinderfilmthicknessesandtheagingcharacteristicsoftheHMAmixsothatanoptimumasphaltfilmthicknessdesirableforsatisfactorymixdurabilitycouldbeestablished.Thefollowingconclusionsweredrawnandrecommendationsmade:
1.TheliteraturereviewedaspartofthisstudydidnotindicatetheexistenceofanysignificantrationaldatacorrelatingtheperformanceofHMApavementswiththeVMAvaluescurrentlyspecifiedforHMAmixdesigns.
2.Therelationshipbetweentheasphaltfilmthicknessandtheagedproperties(both
shorttermandlongterm)oftheHMAmixtures,suchmtensilestrengthandresilientmodulus,wasquantified.AfairlygoodcorrelationwasobtainedbetweentheasphaltfilmthicknessandtheresilientmodulusoftheagedHMAmixtures.Anoptimumfilmthicknessof9-10micronswasindicatedfromthedata,belowwhichtheHMAmix(compactedto8%airvoidcontent)agedatanacceleratedrate.ThisrangeappearstoconcurwiththeresultsobtainedbyGoodeandLufseyintermsofairvoids/bitumenindexratio.
3.Relationshipswerealsoestablishedbetweentheasphaltfilmthicknessandtheagedasphaltbinderproperties(bothshortandlongterm)suchasviscosity,penetration,andcomplexmodulus.Anoptimumfilmthicknessof9-10micronswasgenerallyindicatedfromthedata,belowwhichtheasphaltbinderagedatanacceleratedrate.ThisfilmthicknesscorrespondstoasphaltbindercontainedinanHMAmixcompactedto8%airvoidcontent.
4.TheminimumVMAfortheHMAmixusedinthisstudywascalculatedtobe15.6%toaccommodateanoptimumasphaltfilmthicknessof9micronsand4%airvoids.ThecorrespondingAsphaltInstituteorSuperpaverecommendationforminimumVMAis14%forthismix(maximumnominalsizeof12.5mm).However,itmaynotbepossibletoachievethedesiredVMA(15.6%)insomedense-gradedHMAmixturesofsimilargradation.
5.Theprecedingconclusionsarebasedononlyoneaggregate/asphaltcementcombination.SHRPA-003AandA-003BProjectshaveindicatedthattheagingphenomenonisinfluencedbytheinteractionbetweentheaggregateandtheasphaltcement.Therefore,theoptimumasphaltfilmthicknessindicatedinthisstudyneedstobeconfirmedbyconductingmorestudiesinvolvingdifferentaggregate/asphaltcementcombinations.TheminimumVMArequirementthencouldbebasedontheoptimumasphaltfilmthicknesswhichgivesreasonabledurabilityoftheHMAmix.
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Volume35,(1956).McLeod,N.W.,"VoidRequirementsforDense-GradedBituminousPavingMixtures,"ASTM,SpecialTechnicalPublication252,(1959).MixDesireMethodsforAsphaltConcreteandOtherHotMixTypes,TheAsphaltInstitute,MS-2,May(1984).MixDesignMethodsforAsphaltConcreteandOtherHotMixTypes,TheAsphaltInstitute,MS-2.SixthEdition,(1993).Carnpen,W.H.,Smith,J.R,Erickson,L.G.andMertz,L.R.,"TheRelationshipsBetweenVoids,SurfaceArea,FilmThicknessandStabilityInBituminousPavingMixtures,"Proceedings,AAPT,Vol.28,(1959).Goode,J.F.andLufsey,L.A.,"Voids,Permeability,FilmThicknessvs.AsphaltHardening,"Proceedings,AAPT,Vol.34,(1965).
Kumar,A.andGoetz,W.H.,"AsphaltHardeningasAffectedbyFilmThickness,VoidsandPermeabilityinAsphalticMixtures,"Proceedings,AAPT,Vol.46,(1977).Sosnovske,D.,AbWahab,Y.,andBell,C.,"TheRoleofAsphaltandAggregateintheAgingofBituminousMixtures,"TransportationResearchRecord1386,(1993).Kandhal,P.S.,andKhatri,M.A.,"RelatingAsphaltAbsorptiontoPropertiesofAsphaltCementandAggregate,"TransportationResearchRecord1342,(1992)."StandardPracticeforShortTermAgingofAsphaltConcreteMixtures,"SHRP#1025,(1992)."StandardPracticeforLongTermAgingofAsphaltConcreteMixtures,"SHRP#1030,(1992)."TheSuperpaveMixDesignSystemManualofSpecifications,TestMethods,andPractices,"SHRP-A-379,(1994).Chakraborty,S.,"EvaluationofVoidsintheMineralAggregateforHMAPaving
Mixtures,"M.S.Thesis,AuburnUniversity,(1994).McLeod,N.W.,"DesigningStandardAsphaltPavingMixturesforGreaterDurability,"Proceedings,CanadianTechnicalAsphaltAssociation,Vol.16,(1971).
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