Aashto design

1

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

2

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

3

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

4

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)

5

One Rainfall Zone...One Rainfall Zone...

(AASHO, 1961)

One Temperature Zone...One Temperature Zone...

(AASHO, 1961)

6

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

7

1950’s1950’sConstructionConstructionMethods...Methods...

(AASHO, 1961)

1950’s1950’sVehicle Loads...Vehicle Loads...

(AASHO, 1961)

8

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)

9

Some Failures...Some Failures...

(Some pavements too!)

(AASHO, 1961)

AASHTO Design Basedon Serviceability Decrease

(AASHTO, 1993)

10

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!

11

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

12

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)

13

AASHTO Design Basedon Serviceability Decrease

(AASHTO, 1993)

Flexible PavementsFlexible Pavements

14

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)

15

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)

17

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)

21

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)

25

Traffic Traffic vsvs. Analysis Period. Analysis Period

(AASHTO, 1993)

(AASHTO, 1993)

26

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

30

(AASHTO, 1993)

(AASHTO, 1993)

31

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

33

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)

34

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

35

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)

36

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)

37

Rigid FoundationCorrection

(AASHTO, 1993)

Relative Damage

ur = f ( k, D)

(AASHTO, 1993)

38

(AASHTO, 1993)

Loss of Support, Loss of Support, LSLS

Subbase/subgradeerosion at joints causes

Loss of Support,impairs load transfer.

(AASHTO, 1993)

39

Loss of Support

(AASHTO, 1993)

(AASHTO, 1993)

40

Consistent with flexible pavement approach!

Next Slide

(AASHTO, 1993)

Traffic Traffic vsvs. Analysis Period. Analysis Period

(AASHTO, 1993)(AASHTO, 1993)

41

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

42

Typical Contraction Joint DetailsTypical Contraction Joint Details

(Huang, 1993)

Typical Expansion Joint DetailTypical Expansion Joint Detail

(Huang, 1993)

43

Typical Construction Joint DetailTypical Construction Joint Detail

(Huang, 1993)

Typical Longitudinal Joint DetailTypical Longitudinal Joint Detail

(Huang, 1993)

Full Width Construction

44

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

45

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)

46

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)

47

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

48

Applies to both longitudinaland transverse steel reinforcement

(Generally, Ps=0 for L< ~15 feet)

(AASHTO, 1993)

Friction FactorFriction Factor

(AASHTO, 1993)

49

Steel Working StressSteel Working Stress

Based on preventing fracture and limiting permanent deformation.

(AASHTO, 1993)

TransverseTie Bars

(AASHTO, 1993)

50

TransverseTie Bars

(AASHTO, 1993)