Bases/Subbases for Concrete Pavements: State-of-the-Practice

Tuesday, April 15, 20181:00-2:30 PM ET

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Purpose Provide an overview of the design and construction of bases and subbases for concrete pavements and its impact on pavement performance.

Learning ObjectivesAt the end of this webinar, you will be able to:• Discuss the fundamentals of bases and subbases for concrete

pavements• Understand the design, construction, and cost considerations of bases

and subbases• Understand European practices for base/subbase design and

construction• Understand the performance of various bases at experimental sections

constructed at MnROAD

Bases and Subbases for Concrete Pavements

Transportation Research Board Webinar1:00 PM – 2:30 PM

Tuesday, April 17, 2018

Sam Tyson, P.E.Concrete Pavement Engineer

FHWA Office of Asset Management, Pavements, and Construction

Bases and Subbases for Concrete Pavements

TRB Committee/Webinar Sponsors –• AFD50 – Design and Rehabilitation of

Concrete Pavements• AFH50 – Concrete Pavement Construction

and Rehabilitation

Bases and Subbases for Concrete Pavements

Background: FHWA Publications• Bases and Subbases for Concrete Pavements

FHWA-HIF-16-005, August 2017 (revised)https://www.fhwa.dot.gov/pavement/concrete/pubs/hif16005.pdf

• Precast Concrete Pavement Bedding Support Systems, FHWA-HIF-16-009, November 2015

https://www.fhwa.dot.gov/pavement/concrete/pubs/hif16009.pdf

…..and references cited in those documents.

Bases and Subbases for Concrete Pavements

David Hein, Applied Research Associates, Inc.

• Background• Rigid pavement layer

configurations• Design considerations• Materials• Construction

Bernard Izevbekhai, Minnesota DOT

Webinar OrganizationPresentations – 60 minutes

Question & Answer Period – 30 minutes

• Base drainability• Base stability• Innovative initiatives• Conclusions

Samuel S. Tyson, P.E.Concrete Pavement Engineer

Office of Asset Management, Pavements, and Construction

Federal Highway Administration1200 New Jersey Avenue, S.E. – E73-440

Washington, DC 20590

E-mail: sam.tyson@dot.govPhone: 202-366-1326

Bases and Subbases for Concrete Pavements

6

Outline

7

• Background• Rigid pavement layer configurations• Design considerations• Materials• Construction• Cost• Summary of MnROAD experience• Conclusions

Background

8

• Need for bases and subbases well known for thousands of years

• Romans built over 50,000 miles of roads for troops and supplies

• Recognized the benefits of “protecting” the natural earth subgrade

• Roads such as the Apian Way constructed of multiple layers of stones and sloped to drain water away from the road

Apian Way

9

Early Base/Subbase Thickness

10

• Aggregate base/subbases were very thick until about the 1900s

• Increased use of bound materials

0

10

20

30

40

50

Romans(200 AD)

Telford(Early 1800s)

Macadam(Early 1800s)

Early1900s

Bas

e/Su

bbas

e Th

ickn

ess

(in)

Typical Base/Subbase Thickness(Early European Designs)

Use of Portland Cement Concrete

11

• Portland cement concrete originally used as a base

• Primary benefit was its ability to spread load over a larger area than granular or bituminous bound materials

• Less aggregate used • Non-uniform and low strength, poor consolidation, curing and joint issues

• First used as a wearing surface in the late 1800s

PCC Construction, Early 1900s

12

Pavement Loading

13

• Loads applied to a PCC pavement spread over a large area of the base/subbase and subgrade

• This permits the use of thinner bases for rigid pavements than for flexible pavements

Rigid Pavement Layer Configuration

14

• Rigid pavement surface typically PCC• Base supports construction traffic and to provide uniformity of support to the PCC surface

Rigid Pavement Layer Configuration

15

Concrete Slab (JPCP, CRCP)

Base Course (agg., asphalt, cement)

Subbase (unbound, stabilized)

Compacted Subgrade

Natural Subgrade

Bedrock

Design Considerations

16

Design Considerations

17

• Pavement loading 3 to 5 times heavier than any highway or aircraft loading previously

• Westergaard Design Method chosen based on the Bureau of Public Roads and experience on test roads in California, Texas and Illinois

Design Considerations

18

Subgrade Support

19

Pumping

20

Travel

Saturated support layer

Approachslab

Leave slab

Movement of fines

Fault Joint(or crack)

Wedge of“injected fines”

Frost Heave

21

Soil Expansion

22

Strength and Stiffness

23

P

k = PDeflection

Stress∆

∆

Support for PCC

24

• Base/subbase provides improved protection of the subgrade, a stronger support to the PCC

• The the design thickness of the PCC is not significantly affected by the k-value as the PCC modulus (stiffness) has a high relative stiffness:• PCC ~ 5.000,000 psi

• Base/subbase ~ 30,000 psi• Subgrade ~ 3,000 to 20,000 psi

Base and Subbase Types

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•Unstabilized• Dense graded aggregate base• Open graded aggregate drainage layer

•Stabilized• Cement-stabilized bases• Cement-treated base• Lean concrete base• Cement treated open graded drainage layer

Base and Subbase Types

26

•Stabilized• Asphalt-stabilized bases

• Asphalt dense graded base

• Asphalt-treated base

• Asphalt treated open graded drainage layer

Impact of Stiffness

27

• Stabilized bases contribute to achieving a high level of smoothness for concrete pavements

• A stiffer base layer does not guarantee performance and may cause other problems

• Optimal base strength reduces strains in the pavement and improves load transfer

• If the base is too stiff, it fails to conform to the changes in the shape of the slabs subjected to environmental loading (curling and warping)

Impact of Stiffness

28

• Stresses and deflections increase within the slabs and may cause cracks to develop

• Target compressive strength of CTB should be 300 to 800 psi and LCB, 750 and 1,200 psi

DAYTIME CURLING

NIGHTTIME CURLING

VOID VOID

VOID

Subbase Thickness

29

• Governed by the frost protection desired• Depends on subgrade type, depth of frost penetration, and water near the subgrade

Base Thickness

30

• Depends on support required for the construction equipment and type and condition of the underlying subgrade

• Thicknesses in the range of 4 to 6 inches are most common

Unstabilized Bases

31

• Granular bases, are the most commonly used base types for concrete pavements

• Unstabilized bases exhibit excellent field performance at a lower cost

• Unstabilized bases include crushed stone, sand-gravels, sands, and a variety of wastes and by-products

• Materials should meet requirements of AASHTO M 147

Physical Requirements

32

• Less than 10 percent passing No. 200 sieve• Plasticity Index < 6 and liquid limit < 25• Maximum particle size not exceeding one third of layer thickness

• Los Angeles abrasion resistance (AASHTO T 96) of 50 percent or less

• Permeability of approximately 150 ft/day and not exceeding 350 ft/day

Base/Subbase Gradations

33

Sieve Designation Percent Passing

Inch mm Grading A

Grading B

Grading C

Grading D

Grading E

Grading F

2 in. 50.0 100 100 − − − −

1 in. 25.0 − 75-95 100 100 100 100

¾ in. 9.5 30-65 40-75 50-85 60-100 − −

No. 4 4.75 25-55 30-60 35-65 50-85 55-100 70-100

No. 10 2.00 15-40 20-45 25-50 40-70 40-100 55-100

No. 40 0.425 8-20 15-30 15-30 25-45 20-50 30-70

No. 200 0.075 2-8 5-20 5-15 5-20 6-20 8-25

Cement Treated Base

34

• Typically contain 2 to 5 percent cement• Percent passing No. 200 sieve up to 35 percent• Granular soils with plasticity index of < 10 (A-1, A-3, A-2-4, and A-2-5 soils) may be used

Lean Concrete Base

35

• Also known as econocrete, contains more cement than cement-treated base but less than conventional concrete

• May use lower quality aggregates

Open Graded Drainage Layers

36

• Small percentage passing the No. 200 sieve for untreated

• Asphalt cement contents between 1.6 and 1.8 percent

• Cement treated layers have water to cement ratio of 0.37 and a cement content of 185 to 220 lbs/yd3

Open Graded Drainage Layers

37

• Should only be used when:• Potential for moisture damage to pavement• Medium to heavy truck traffic

• Proper design and construction• Commitment to inspection and maintenance

• Outlets may include edge drains with outlet pipes or “daylighted”

Open Graded Drainage Layers

38

• Daylighted bases well suited for flat grades and shallow ditches

• Permeability of 500 to 800 ft/day (stable)• May require a geosynthetic separator• Bottom should be > 6 in above 10 year storm line• 3 % cross slope• May require maintenance

Open Graded Drainage Layers

39

• Reduced emphasis on treated OGDL layers• Sacrifice high permeability for stability• Potential issues with stability of anchored dowel baskets during hot weather

• Potential for aggregate stripping and “collapse” of the stabilized layer

Recycled Materials

40

• Recycled materials can be a good source of aggregate for base and subbase

• Recycled concrete is the most frequently used

Use of Recycled Materials

41

• Can be very angular and require more compaction effort than virgin aggregates

• Material should be checked for contaminants such as soil, wood, plaster, gypsum, plastic, rubber, etc.

• Some “fines” may stick to the processed coarse aggregates

• Wash the aggregates to reduce potential for leaching and clogging of the drainage system

Recycled Materials

42

• Other recycled materials include:• Reclaimed asphalt pavement• Mill tailings

• Waste rock materials

Construction (Unstablized)

43

• Homogeneous blending of material • Maintain optimum moisture content for compaction

• Minimum, 95% standard proctor (AASHTO T 99) density (98% modified proctor (AASHTO T 180) for heavy traffic roads)

• Trimmed to ± ½ in. of the design profile grade • Avoid any segregation of aggregates• Wet prior to paving concrete

Construction (Cement Treated)

44

• Typically placed using an asphalt or concrete spreader and compacted with rollers

• Time to place, compact, and trim cement treated base is limited to about 4 hours

• Trim to ± ½ in. of the design profile grade • Curing with a light fog spray of water or curing compound (0.15 to 0.25 gal/yd2)

• Prevent bonding of the base to the PCC (thin layer of sand or two coats of wax-based curing compound)

Construction (Lean Concrete)

45

• Similar to conventional concrete• Compressive strength of 750 and 1,200 psi• Does not need joints• Shrinkage cracks will develop but not reflect through the PCC slabs

• Should within ± ¼ in. of the design profile grade

Construction (Lean Concrete)

46

• Untextured surface to prevent bonding to the PCC slabs

• Bond breaker such as two coats of wax-based curing compound

• Do not want to be excessively stiff (if so, notch at PCC joints)

• Current German practice includes 0.2 in. thick polypropylene geotextile interlayer as a bond breaker

Construction (Asphalt Treated)

47

• Identical to the conventional asphalt • Smooth surface• May need to treat to reduce surface temperature during concrete placement

• Should be within ± ¼ in. of design profile grade

Cost Considerations

48

• Consider purpose of the base and locally available materials

• Evaluate using life-cycle cost analysis • Inputs include the material cost and performance• May be difficult to characterize

Relative Costs

49

Base Type Relative Cost

No base/subbase 84

Dense-graded unstabilized 100

Open-graded unstabilized 114

Lean concrete 122

Open-graded asphalt-treated 123

Open-graded cement-treated 124

Lean concrete 122

Dense graded asphalt 135

MnROAD Experience with Bases for Concrete Pavements

50

(Sensor Lead Length ≈DC to Philadelphia)

• Rigid pavements do not necessarily require a strong foundation

• More important that the foundation provides uniform support

• Needed to prevent pumping• Better slab support uniformity than subgrade• More stable working platform for construction equipment

• Control of differential frost heave

Reason for Existence

Some Initial MnROAD Test Cells

52

Class 6: >15 percent of material which shall be crushedClass 5: > 10 percent of material which shall be crushed Crushing: Weight of the material retained on a 3/4-inch sieveMnDOT Standard Spec for Construction 2018

Typical MnDOT Gradations

Traditional Sub-surface Infrastructure

Edge Drains MnDOT Class 5 Aggregate Base

PASB Aggregate Gradation Sieve size % Passing

1½ in (37.5 mm) 100

1 in (25.4 mm) 95-100

¾ in (19 mm) 85-95

3/8 in (9.5 mm) 30-60

No. 4 (4.75 mm) 10-30

No. 8 (2.36 mm) 0-10

No. 30 (600 µm) 0-5

No. 200 (75 µm) 0-3

Class 5 Aggregate Base

Sieve size % Passing1½ in (37.5 mm) -

1 in (25.4 mm) 100

¾ in (19 mm) 90-100

3/8 in (9.5 mm) 50-90

No. 4 (4.75 mm) 35-80

No. 10 (2.0 mm) 20-65

No. 40 (400 µm) 10-35

No. 200 (75 µm) 3-10

Original MnROAD Test Cells

• Geocomposite Joint Drain Class 5 Q Base

Innovative Items

PSAB Drainability, Stability and Interfacial Bond

•Drainable Vs Non Drainable Bases• Importance of drainage / Performance• PCC Roadways

PASBCell 7

Class-5Cell 12

Class-5Non-Drainable

Base

MnROAD Observation: Benefits of Good Drainage

ScouringNon Draining

Base

Drainage Related Degradation

PASBCell 7

Pavement Age 18 years

US 169 7.5 inch JPCP 1.25 in Dia 15 ft panels on MnDOT Class 5. Coring water did not drain in 2 bad cores but drained well in region of 3 good cores

US TH 52 4 lane Divided JRCP 1.0 inch Dowel and 27 ft long Panels. Joints were tight but cores were delaminated

Open versus Dense Graded Base

USTH 59 2 Lane 8 inch thick JPCP 15 ft Panels OGAB. All Joints are in Perfect Condition Minimal Surface Distresses.

USTH 14 2 Lane 8 in JRCP with 1.25 inch Dowels on Class 5. Widened Joints and Incipient Dowel Deterioration.

Open versus Dense Graded Base

Ride Quality PASB vs Non-PASB

62

0

0.5

1

1.5

2

2.5

IRI (

M/K

M)

IRI_LWPCell 6IRI_LWPCell 7 (PASB)

Sustainable Synthetic Subsurface Drainage Initiatives• 7.5 inch recycled aggregate concrete test cell

• Non-skewed joints (15 ft intervals)• In lieu of OGAB subbase• Day-lighted geocomposite joint drainage + unsealed

joints

• Non-woven geotextile interlayer• 3 inch concrete overlay non-woven geotextile interlayer• Non-skewed joints (15 ft intervals)• In lieu of GJD/OGAB/PASB subbase

• Sudden appearance of widespread cracks in August 2012 on an OGAB Section

• Forensic Evaluation

• Cracking pattern atypical of temperature and locked joints

• Only MnROAD concrete cell built on OGAB

• One of the cells with RCC Shoulders

• Crack progression more predictable than 1st appearance

Undermined Test Section

• Reliability Evaluation of Network OGAB Sections

• Remediation

Base Stability

Sinking Support Fundamentals

Very little has been specifically done in relating this phenomenon to jointed plain concrete pavements with or without dowels

Sinking support moment of a propped cantilever of displacement 𝛿𝛿 is given by 𝑀𝑀 = 3𝐸𝐸𝐸𝐸𝐸𝐸

𝐿𝐿2

Sinking Support Concept• For a simply supported beam with sinking support that

the sinking support moment is

M = 6𝐸𝐸𝐸𝐸𝐸𝐸𝐿𝐿2

• Introducing radius of relative stiffness

R = 𝐸𝐸𝐸3

12 1− 𝜇𝜇2 𝑘𝑘

0.25

• From where induced moment for propped cantilever similar to a locked joint and a freely moving joint

M = 3𝑏𝑏𝑘𝑘 1− 𝜇𝜇2 𝐸𝐸𝛿𝛿4

𝐿𝐿2

OGAB Special August, 2012/16

2012

2016

68

Forensics: Stepwise Approach

1.41.51.61.71.81.9

22.12.22.3

IRI (

m/k

m)

306 LWP 306 RWP

406 LWP 406 RWP

Ride Quality History and Joint

DCP Before and After

Stiffer Yet Less Stable !!

Gradations Before & After

Gradation Before and After

MnROAD Undermining Experiment

OGAB Efficacy & Risk

Odds of OGAB showing early distress is 5 times as High OGAB Special increased risk of early failure by 100 (2.5-1) = 150%OR, RR >>1 OGAB early failure is not rare in the networkOR, RR ≈1 Void induced distress may be rare in Test Cell 12

Early Distress

No early Distress Odds Ratio Risk Ratio

NetworkOGAB 25 14 5.18 2.5

No OGAB 10 29

MnROADVoid 12 37 1.26 1.2

No Void 10 39

Innovation – Geocomposite Joint Drain (GJD)

• 7 ½ inch recycled aggregate concrete• GJD at Joints • 4 ½ -7 ½ inch Class 1 (Non drainable)• Existing silt subgrade

75

GJD Load Transfer Efficiency

Gravel/Sand Basis for Stability and Drainability

Innovative Bases: MnDOT Class 5 Q506 606 706 806

11 in. Class 5Qaggregate base

11 in. Class 5Qaggregate base

5 in. FRC11.7 lbs/CY fibers24 ft pave width6 ft (W) x 6 ft (L)

joints

5 in. FRC8 lbs/CY fibers

24 ft pave width6 ft (W) x 6 ft (L)

joints

Clay loam (A-6)subgrade (existing)

Clay loam (A-6)subgrade (existing)

Clay loam (A-6)subgrade (existing)

Clay loam (A-6)subgrade (existing)

3.0 in. agg base(existing)

3.0 in. agg base(existing)

5 in. PCCno fibers

24 ft pave width6 ft (W) x 6 ft (L)

joints

5 in. FRC5 lbs/CY fibers

24 ft pave width6 ft (W) x 6 ft (L)

joints

11 in. Class 5Qaggregate base

11 in. Class 5Qaggregate base

3.0 in. agg base(existing)

3.0 in. agg base(existing)

Conclusions• Rigid pavements do not necessarily require a strong foundation

• The key to achieving the desired performance is uniform support to the concrete slabs

• Primary purpose of base/subbase layers is to prevent pumping

• Base/subbase provides a stable working platform for construction equipment

78

Conclusions• Scouring and bottom joint deterioration are indicative of inadequate subsurface drainage

• Three enemies at work: water, water and water• Traditional bases were not very drainable• PASB, Class 5 Q optimizes drainability, stability and durability

• GJD indicate good initial performance (needs cost-benefit and sustainability evaluation)

79

Conclusions• Sinking support and loss of support are not the same as settlement.

• LAR and DCP: insufficient OGAB aggregate stability prediction. Hydraulic fracture is recommended

• OGAB special section efficacy analysis: Risk ratio >> 1, Odds ratio >> 1 for early failure

80

Conclusions•Best performance obtained by:• Selecting a base/subbase to prevent pumping• Using a material that will remain stable over time• Material that does not exhibit excessive deflections under traffic loading

• Treat the surface to prevent bonding and reduce friction between the PCC and base

• Using specifications that will ensure uniform support

• Construct with grade controls that allow for consistent thickness and smoothness of concrete

81

Questions

82

Today’s Participants

• Sam Tyson, Federal Highway Administration, Sam.Tyson@dot.gov

• David Hein, Applied Research Associates, Inc., dhein@ara.com

• Bernard Izevbekhai, Minnesota Department of Transportation, bernard.izevbekhai@state.mn.us

Panelists Presentations

http://onlinepubs.trb.org/onlinepubs/webinars/180417.pdf

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