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Bases/Subbases for Concrete Pavements: State-of-the-Practice
Tuesday, April 15, 20181:00-2:30 PM ET
TRANSPORTATION RESEARCH BOARD
The Transportation Research Board has met the standards and
requirements of the Registered Continuing Education Providers Program.
Credit earned on completion of this program will be reported to RCEP. A
certificate of completion will be issued to participants that have registered
and attended the entire session. As such, it does not include content that
may be deemed or construed to be an approval or endorsement by RCEP.
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: [email protected]: 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
25
•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, [email protected]
• David Hein, Applied Research Associates, Inc., [email protected]
• Bernard Izevbekhai, Minnesota Department of Transportation, [email protected]
Panelists Presentations
http://onlinepubs.trb.org/onlinepubs/webinars/180417.pdf
After the webinar, you will receive a follow-up email containing a link to the recording
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