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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


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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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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(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)


(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)