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Design MethodsDesign Methods
• Highway PavementsAASHTOThe Asphalt InstitutePortland Cement Association
• Airfield PavementsFAAThe Asphalt InstitutePortland Cement AssociationU.S. Army Corps of Engineers
Objectives of Pavement DesignObjectives of Pavement Design
To provide a surface that is:
• StrongSurface strengthMoisture control
• Smooth
• SafeFrictionDrainage
• EconomicalInitial construction costRecurring maintenance cost
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Pavements are Designedto Fail !!
Pavement Design MethodologiesPavement Design Methodologies
• Experience
• EmpiricalStatistical models from road tests
• Mechanistic-EmpiricalCalculation of pavement stresses/strains/deformationsEmpirical pavement performance models
• MechanisticCalculation of pavement stresses/strains/deformationsMechanics-based pavement performance models
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Empirical Empirical vsvs. Mechanistic Design. Mechanistic Design
Wood Floor JoistWood Floor Joist
Empirical “Rule of 2”: d in inches= (L in feet / 2) + 2
Pd
L
Mechanistic: PL4Sbending allowableσ = ≤ σ
1993 Version
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AASHTO Pavement Design GuideAASHTO Pavement Design Guide
• Empirical design methodology• Several versions:
1961 (Interim Guide)19721986
Refined material characterizationVersion included in Huang (1993)
1993More on rehabilitationMore consistency between flexible, rigid designsCurrent version
2002Under developmentWill be based on mechanistic-empirical approach
AASHO Road Test (late 1950’s)AASHO Road Test (late 1950’s)
(AASHO, 1961)
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One Rainfall Zone...One Rainfall Zone...
(AASHO, 1961)
One Temperature Zone...One Temperature Zone...
(AASHO, 1961)
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One Subgrade...One Subgrade...
A-6 / A-7-6 (Clay)Poor Drainage
(AASHO, 1961)
Limited Set of Materials...Limited Set of Materials...
• One asphalt concrete3/4” surface course1” binder course
• One Portland cement concrete (3500 psi @ 14 days)
• Four base materialsWell-graded crushed limestone (main experiment)Well-graded uncrushed gravel (special studies)Bituminous-treated base (special studies)Cement-treated base (special studies)
• One uniform sand/gravel subbase
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1950’s1950’sConstructionConstructionMethods...Methods...
(AASHO, 1961)
1950’s1950’sVehicle Loads...Vehicle Loads...
(AASHO, 1961)
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1.1M Axles1.1M Axles
2 Years2 Years
Time (Months)
Axl
e Lo
ads
(Tho
usan
ds)
Limited Traffic Volumes...Limited Traffic Volumes...
(AASHO, 1961)
1950’s1950’sData Analysis...Data Analysis...
(AASHO, 1961)
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Some Failures...Some Failures...
(Some pavements too!)
(AASHO, 1961)
AASHTO Design Basedon Serviceability Decrease
(AASHTO, 1993)
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What is Serviceability?What is Serviceability?
• Based upon PresentServiceability Rating (PSR)
• Subjective rating byindividual/panel
Initial/post-constructionVarious times afterconstruction
• 0 < PSR < 5
• PSR < ~2.5: Unacceptable
(AASHO, 1961)
Present Serviceability Index (PSI)Present Serviceability Index (PSI)
• PSR correlated to physical pavement measures via PresentServiceability Index (PSI):
2 1/ 2
2
5.03 1.91log(1 ) 1.38 0.01( )
slope variance (measure of roughness)
average rut depth (inches) area of cracking and patching per 1000 ft
PSI SV RD C P
SV
RD
PSI P RP
SC
= − + − − +
=≈
=
=
+
Empirical!
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AASHTO Design Guide (1993)AASHTO Design Guide (1993)
Part I: Pavement Design and ManagementPart I: Pavement Design and ManagementPrinciplesPrinciples
• Introduction and Background
• Design Related to Project Level Pavement Management
• Economic Evaluation of Alternative Design Strategies
• Reliability
AASHTO Design Guide (1993)AASHTO Design Guide (1993)
Part II: Pavement Design Procedures for NewPart II: Pavement Design Procedures for NewConstruction or ReconstructionConstruction or Reconstruction
• Design Requirements
• Highway Pavement Structural Design
• Low-Volume Road Design
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AASHTO Design Guide (1993)AASHTO Design Guide (1993)
Part III: Pavement Design Procedures forPart III: Pavement Design Procedures forRehabilitation of Existing PavementsRehabilitation of Existing Pavements
• Rehabilitation Concepts
• Guides for Field Data Collection
• Rehabilitation Methods Other Than Overlay
• Rehabilitation Methods With Overlays
Design ScenariosIncluded in
AASHTO Guide
(AASHTO, 1993)
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AASHTO Design Basedon Serviceability Decrease
(AASHTO, 1993)
Flexible PavementsFlexible Pavements
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Design EquationDesign Equation
W18 = design traffic (18-kip ESALs)ZR = standard normal deviateSo = combined standard error of traffic and performance prediction∆PSI = difference between initial and terminal serviceability indexMR = resilient modulus (psi)SN = structural number
( ) ( )
( )
( )
10 18 10
10
10
5.19
log 9.36log 1 0.20
log4.2 1.5 2.32log 8.0710940.40
1
R o
R
W Z S SN
PSI
M
SN
= + + −
∆ − + + −
++
Structural Number
(AASHTO, 1993)
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Traffic Traffic vsvs. Analysis Period. Analysis Period
(AASHTO, 1993)
Analysis PeriodAnalysis Period
(Also basis for life-cycle cost analysis)
(AASHTO, 1993)
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Design Traffic (18KDesign Traffic (18K ESALs ESALs))
(AASHTO, 1993)
Design Traffic (18KDesign Traffic (18K ESALs ESALs))
• DD = 0.5 typically
• DL:
(AASHTO, 1993)
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Reliability
(AASHTO, 1993)
Recommended Values forRecommended Values forStandard Error SStandard Error Soo
• Rigid Pavements: 0.30 - 0.40
• Flexible Pavements: 0.40 - 0.50
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Standard Normal Deviate ZStandard Normal Deviate ZRR
(AASHTO, 1993)
Recommended Reliability LevelsRecommended Reliability Levels
(AASHTO, 1993)
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ServiceabilityServiceability
• PSI = Pavement Serviceability Index, 1 < PSI < 5
• po = Initial Serviceability IndexRigid pavements: 4.5Flexible pavements: 4.2
• pt = Terminal Serviceability Index
o tPSI p p∆ = −
(AASHTO, 1993)
Adjustment of Roadbed (Subgrade)
MR for SeasonalVariations
(AASHTO, 1993)
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Structural NumberStructural Number
SN = structural number = f (structural capacity)ai = ith layer coefficientDi = ith layer thickness (inches)mi = ith layer drainage coefficientn = number of layers (3, typically)
1 12
n
i i ii
SN a D a D m=
= +∑
No Unique Solution!
(AASHTO, 1993)
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Layer Coefficient Layer Coefficient aa11: Asphalt Concrete: Asphalt Concrete
(AASHTO, 1993)
Layer Coefficient Layer Coefficient aa22: Granular Base: Granular Base
( )2 100.249 log 0.977
in psi
base
base
a E
E
≅ −
(AASHTO, 1993)
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Layer Coefficient Layer Coefficient aa22: Cement Treated Base: Cement Treated Base
(AASHTO, 1993)
Layer Coefficient Layer Coefficient aa22::Bituminous Treated BaseBituminous Treated Base
(AASHTO, 1993)
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Layer Coefficient Layer Coefficient aa33: Granular Subbase: Granular Subbase
3 100.227(log ) 0.839
in psi
subbase
subbase
a E
E
= −
(AASHTO, 1993)
Quality of DrainageQuality of Drainage
(AASHTO, 1993)
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Drainage Coefficient Drainage Coefficient mmii
mi increases/decreases the effective value for ai
(AASHTO, 1993)
Next Slide
(AASHTO, 1993)
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Traffic Traffic vsvs. Analysis Period. Analysis Period
(AASHTO, 1993)
(AASHTO, 1993)
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Effect of Froston Performance
PSI = Pavement Servicability Index
1 < PSI < 5
“Failure”: PSI < 2+
(AASHTO, 1993)
(AASHTO, 1993)
Frost Heave Rate φ
φ = f (-0.02mm)
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(AASHTO, 1993)
MaximumServiceability
Loss
∆PSImax = f (frost depth, drainage)
Effect ofSwelling on
Performance
PSI = Pavement Servicability Index
1 < PSI < 5
“Failure”: PSI < 2+
(AASHTO, 1993)
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(AASHTO, 1993)
Swell Rate Constant θ
θ = f (moisture supply,soil fabric)
(AASHTO, 1993)VR = f (PI, compaction, thickness)
Maximum PotentialHeave VR
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Rigid PavementsRigid Pavements
Design EquationDesign Equation
W18 = design traffic (18-kip ESALs)ZR = standard normal deviateSo = combined standard error of traffic and
performance predictionD = thickness (inches) of pavement slab∆PSI = difference between initial and terminal
serviceability indicespt = terminal serviceability value
Sc’ = modulus of rupture (psi) for Portland cementconcrete
J = load transfer coefficient
Cd = drainage coefficient
Ec = modulus of elasticity (psi) for Portlandcement concrete
k = modulus of subgrade reaction (pci)
( ) ( )
( )
( ) ( )
( )
10 18 10
' 0.7510
1070.75
8.46 0.25
log 7.35log 1 0.06
log 1.1324.5 1.5 4.22 0.32 log1.64x10 18.421 215.631 /
R o
c dt
c
W Z S D
PSIS C D
p
J DD E k
= + + −
∆
− − + + − + − +
PCC Thickness
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(AASHTO, 1993)
(AASHTO, 1993)
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Design InputsDesign Inputs
W18 = design traffic (18-kip ESALs)
ZR = standard normal deviate
So = combined standard error of traffic and performance prediction
∆PSI = difference between initial and terminal serviceability indices
pt = terminal serviceability index (implicit in flexible design)
All consistent with flexible pavements!
Additional Design InputsAdditional Design Inputs
• S′c = modulus of rupture for concrete
• J = joint load transfer coefficient
• Cd = drainage coefficient (similar in concept to flexiblepavement terms)
• Ec = modulus of elasticity for concrete
• k = modulus of subgrade reaction
Additional inputs reflect differences inAdditional inputs reflect differences inmaterials and structural behavior.materials and structural behavior.
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Modulus of RuptureSc’
(AASHTO, 1993)
Joint Load Transfer Coefficient Joint Load Transfer Coefficient JJ
Pavement Type(no tied shoulders)
J
JCP/JRCPw/ load transfer devices
3.2
JCP/JRCPw/out load transfer devices
3.8-4.4
CRCP 2.9
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Joint Load Transfer Coefficient Joint Load Transfer Coefficient JJ
Additional benefits of tied shoulders:
(AASHTO, 1993)
Drainage Coefficient Drainage Coefficient CCdd
• Two effects:Subbase and subgrade strength/stiffnessJoint load transfer effectiveness
(AASHTO, 1993)
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PCC Modulus of Elasticity PCC Modulus of Elasticity EEcc
• Measure directly per ASTM C469
• Correlation w/ compressive strength:
Ec = 57,000 (fc’)0.5
Ec = elastic modulus (psi)fc’ = compressive strength (psi) per AASHTO T22, T140, or ASTM
C39
Effective Subgrade Modulus Effective Subgrade Modulus kk
• Depends on:Roadbed (subgrade) resilient modulus, MR
Subbase resilient modulus, ESB
• Both vary by season
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Determining Effective Determining Effective k k (See Table 3.2)(See Table 3.2)
• Identify:Subbase typesSubbase thicknessesLoss of support, LS (erosion potential of subbase)Depth to rigid foundation (feet)
• Assign roadbed soil resilient modulus (MR) for each season
• Assign subbase resilient modulus (ESB) for each season15,000 psi (spring thaw) < ESB < 50,000 psi (winter freeze)ESB < 4(MR)
(AASHTO, 1993)
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Determining Effective Determining Effective k k ((cont’dcont’d))
• Determine composite k for each seasonFor DSB = 0: k = MR/19.4For DSB > 0: Use Figure 3.3
• If depth to rigid foundation < 10 feet, correct k for effect ofrigid foundation near the surface (Figure 3.4)
• Estimate required thickness of slab (Figure 3.5) anddetermine relative damage ur for each season
• Use average ur to determine effective k (Figure 3.5)
• Correct k for potential loss of support LS (Figure 3.6)
k = f (MR , ESB , DSB )
Composite Modulusof Subgrade Reaction
(AASHTO, 1993)
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Rigid FoundationCorrection
(AASHTO, 1993)
Relative Damage
ur = f ( k, D)
(AASHTO, 1993)
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(AASHTO, 1993)
Loss of Support, Loss of Support, LSLS
Subbase/subgradeerosion at joints causes
Loss of Support,impairs load transfer.
(AASHTO, 1993)
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Loss of Support
(AASHTO, 1993)
(AASHTO, 1993)
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Consistent with flexible pavement approach!
Next Slide
(AASHTO, 1993)
Traffic Traffic vsvs. Analysis Period. Analysis Period
(AASHTO, 1993)(AASHTO, 1993)
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Joint DesignJoint Design
• Joint TypesContractionExpansionConstructionLongitudinal
• Joint GeometrySpacingLayout (e.g., regular, skewed, randomized)Dimensions
• Joint Sealant Dimensions
Types of JointsTypes of Joints
• ContractionTransverseFor relief of tensile stresses
• ExpansionTransverseFor relief of compressive stressesUsed primarily between pavement and structures (e.g., bridge)
• Construction
• LongitudinalFor relief of curling and warping stresses
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Typical Contraction Joint DetailsTypical Contraction Joint Details
(Huang, 1993)
Typical Expansion Joint DetailTypical Expansion Joint Detail
(Huang, 1993)
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Typical Construction Joint DetailTypical Construction Joint Detail
(Huang, 1993)
Typical Longitudinal Joint DetailTypical Longitudinal Joint Detail
(Huang, 1993)
Full Width Construction
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Typical Longitudinal Joint DetailTypical Longitudinal Joint Detail
(Huang, 1993)
Lane-at-a-Time Construction
Joint SpacingJoint Spacing
• Local experience is best guide
• Rules of thumb:JCP joint spacing (feet) < 2D (inches)W/L < 1.25
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Joint DimensionsJoint Dimensions
• Width controlled by joint sealant extension
• Depths:Contraction joints: D/4Longitudinal joints: D/3
• Joints may be formed by:SawingInsertsForming
Joint SealantDimension
Governed by expected joint
movement,sealant resilience
(AASHTO, 1993)
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Design InputsDesign Inputs
Z αc
(AASHTO, 1993)
Reinforcement Design (JRCP)Reinforcement Design (JRCP)
• Purpose of reinforcement is not to prevent cracking, but to hold tightlyclosed any cracks that may form
• Physical mechanisms:Thermal/moisture contractionFriction resistance from underlying material
• Design based on friction stress analysis(Huang, 1993)
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Dowel Bars: Transverse Joint Load TransferDowel Bars: Transverse Joint Load Transfer
• “…size and spacing should be determined by the localagency’s procedures and/or experience.”
• Guidelines:Dowel bar diameter = D/8 (inches)Dowel spacing: 12 inchesDowel length: 18 inches
Friction StressesFriction Stresses
(Huang, 1993)
Induces tensile stresses in concreteCauses opening of transverse joints
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Applies to both longitudinaland transverse steel reinforcement
(Generally, Ps=0 for L< ~15 feet)
(AASHTO, 1993)
Friction FactorFriction Factor
(AASHTO, 1993)
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Steel Working StressSteel Working Stress
Based on preventing fracture and limiting permanent deformation.
(AASHTO, 1993)
TransverseTie Bars
(AASHTO, 1993)
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TransverseTie Bars
(AASHTO, 1993)