FATIGUE CRACKING

Robert L. LyttonCRACKING IN ASPHALT PAVEMENTS

PAVEMENT PERFORMANCE PREDICTION SYMPOSIUM 2007

WESTERN RESEARCH INSTITUTELARAMIE, WYOMING

JULY 18 – 20, 2007

FATIGUE CRACKING

FATIGUETHERMALLOAD- BOTTOM – UP- TOP – DOWN

STAGES IN LOAD FATIGUEMICROCRACKINGMACROCRACKINGLAB-TO-FIELD SHIFT FACTORSENDURANCE LIMIT

Fatigue Cracking

Interconnected crack pattern in wheelpathStructural failure

Water & air ingress

Fatigue (alligator) cracks

PURPOSES OF FATIGUE CRACKING PREDICTION

• INCENTIVE PAY FOR CONSTRUCTION QUALITY

• PERFORMANCE SPECIFICATIONS• WARRANTY RISK ASSESSMENT• DESIGN RELIABILITY• PAVEMENT ASSET MANAGEMENT

DESIGN APPROACHES• MECHANISTIC EMPIRICAL (ME)

(AASHTO TP8-94, 1996)• MECHANISTIC-EMPIRICAL PAVEMENT

DESIGN GUIDE (MEPDG)(AASHTO, 2005)

• CALIBRATED MECHANISTIC WITH SURFACE ENERGIES (CMSE)(SHRP A – 357, 1993)(WALUBITA, EPPS-MARTIN, GLOVER, CLEVELAND, ET AL)

MACRO CRACKING PHASE

R

R

= A[J ] (PARIS' LAW APPLIES)

c = MEAN CRACK RADIUSc COARSE AGGREGATE RADIUSJ = DISSIPATED PSEUDO-STRAIN RELEASE RATEA, n = PARIS' LAW COEFFICIENT AND

ndcdN

≥

EXPONENT

Modulus Ratio vs Log (Load Repetitions ): Initial Air Voids,

Crack Initiation and Film Thickness Criteria

0

0.2

0.4

0.6

0.8

1

-1 1 3 5 7

Log ( No. Load Repetitions )

Dam

aged

Mod

ulus

/In

tact

M

odul

us

Cohesive Self-Consistent ModelAdhesive Self-Consistent ModelCombined Self-Consistent Model

Cohesive Mixture ModelAdhesive Mixture ModelCombined Mixture Model

MICRO CRACKING PHASE

2

CONSTANT

m = NO. OF CRACKS ( )C = MEAN CRACK RADIUS ( )C COARSE AGGREGATE RADIUSA = CROSS-SECTIONAL AREAt = MEAN BINDER FILM THICKNESS

mct

π=

Α⇓

⇑

≤

MICRO CRACKING PHASE

n-2 1 nn+1 n+1 n +1

Rdc ( ) = A [J ] dN

( ' ) = MEAN CRACK RADIUS

A, n = PARIS' LAW COEFFICIENT AND EXPONENT

c

MODIFIED PARIS LAW APPLIESc

OBSERVED NO. OF LOAD CYCLES TO FAILUREOBSERVED NO. OF LOAD CYCLES TO FAILURE

LOG

,CA

LCU

LATE

D S

TRA

IN

Shift factor

K2fK2l

Log Nf

labfield

FIELD LAB

N f = N x SHIFT FACTORSf

SHIFT FACTORS SCALE•HEALING 1 - 10•ANISOTROPY 1 - 5•PLASTICITY GRADIENT - 3•AGING 1 - 0.1•MOISTURE 1 - 0.2_______________ _______

TOTAL 3 - 150

13

LAB N

• SIZE OF MICROCRACKS• PARIS’ LAW A• “DAMAGED” MODULUS

f

MODULUS OF MATERIALS WITH HOLES IN THEM

Controlled Strain

• Release of Elastic Energy = OAB

Load, P

Displacement, u

A

B

Dissipated Energy

O

Crack diameter = a

Crack diameter = a+Δa

312f f

f s

f

s

E E = 1 - 1 + E E t

E = Modulus of Fluid E = Modulus of Solid

m cA

π⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠

Damaged Modulus/Intact Modulus

Adhesive Formulas

Uniform Strain (Lower Bound)

Damaged Modulus/Intact Modulus

Adhesive Formulas

Uniform Stress (Upper Bound)

1f

3f 2 f

s

E 1 = E E1 + 1 +

E t

m cA

π⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠

Mechanical Testing of Fine Portion of the Mix

X-Ray Tomography

X-Ray Source

Detector

Specimen

Collimator(window)

Dry Conditions

0

0.2

0.4

0.6

0.8

1

1.2

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Number of Cycles

Nor

mal

ized

She

ar M

odul

us, P

a

0.7

0.75

0.8

0.85

0.9

0.95

1

1.05

Nor

mal

ized

Dam

age

Para

met

er

DMA MeasurementsX-ray CT DamageCohesive Model

DMA Fatigue Test Results

0

0.2

0.4

0.6

0.8

1

1.2

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 15000

N

G'/G

Uniform Strain Analysis

Crack growth in uniform strain condition

y = 0.5139x - 15.661R2 = 0.9874

-12

-11.5

-11

-10.5

-10

-9.5

-98.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 9 9.1 9.2

Ln(N)

Ln(c

(N))

Uniform Stress Analysis

crack growth in uniform stress condition

y = 0.5281x - 15.781R2 = 0.9865

-12

-11.5

-11

-10.5

-10

-9.5

-98.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 9 9.1 9.2

Ln(N)

Ln(c

(N))

DMA Paris’ Law A-valueN vs A (Paris' Law) for uniform strain and stress

2.0E-112.5E-11

3.0E-113.5E-114.0E-11

4.5E-115.0E-11

5.5E-116.0E-11

3500 4500 5500 6500 7500 8500 9500

Number of cycles (N)

A (P

aris

' law

par

amet

er)

Uniform Strain Uniform Stress

Marek and Herrin 1968

Dense Graded Mixes

0

500

1000

1500

2000

2500

3000

000E+0 50E-6 100E-6 150E-6 200E-6 250E-6 300E-6

Film Thickness |(m)

Tens

ile S

tres

s (k

N/m

2 )

COHESIVE FRACTURE ADHESIVE FRACTURE

Observed

Tensile Strength vs Failure Strain

0

100

200

300

400

500

600

700

0 100 200 300 400 500

Failure Strain, microstrain

Tens

ile S

tren

gth,

psi

Adhesive Fracture LocusAdhesive Fracture LocusCohesive Fracture LocusCohesive Fracture LocusUndamaged Adhesive ModulusUndamaged Cohesive Modulus

Stress-Strain Curve

0

100

200

300

400

500

600

700

0 50 100 150 200

Strain, Failure Strain, microstrain

Stre

ss, T

ensi

le S

tren

gth

Damaged Modulus Adhesive Fracture Locus

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 2000 4000 6000 8000 10000

No. of loading cycles

Nor

mal

ized

G*

202530354045505560

Phas

e an

gle

(deg

ree)

without RP-Nor. G* with RP-Nor. G*without RP-phase angle with RP-phase angle

Effect of Rest Periods

Healing

1h

hβ

2h

Healing Index (Dimensionless)

Δtp Rest Period, min

Healing Functions

m62.1

LWh

ABh

β

m1

ABh1

202

m1

LWh

11

ΔGΔG011.0h

ΔGE1102.229.0h

ΔGE89.4h

⎥⎦

⎤⎢⎣

⎡=

⎥⎦

⎤⎢⎣

⎡×+=

⎥⎦

⎤⎢⎣

⎡=

0

0.5

1

1.5

2

2.5

0 10 20 30 40 50 60 70 80

ΔGLW h

1h1/min

ergs/cm2

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 20 40 60 80 100

ΔGAB h

2h1/min

ergs/cm2

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.5 1 1.5 2 2.5

ΔGAB h/ΔGLW h

1/min

h β

DPSE, b

y = 3.1603x + 0.8974R2 = 0.9731

y = 1.2503x + 0.3379R2 = 0.9708

y = 0.7111x + 0.0799R2 = 0.9592

0

5

10

15

20

0 1 2 3 4 5 6

Log N

DPS

E (J

/m3 )

Bryan, 0 Months

Bryan, 3 Months

Bryan, 6 Months

Aging↑ ; b↑ (more damage)

Rate of Fracture Damage Accumulation (b)Lab Aging Period @ 60 °C (Months) Bryan Yoakum 0 0.71 0.58 3 1.25 0.95 6 3.16 2.01

y = 2.0146x + 0.5474R2 = 0.9761

y = 0.9536x + 0.2381R2 = 0.9433

y = 0.5800x + 0.0652R2 = 0.9592

0

5

10

15

20

0 1 2 3 4 5 6

Log N

DPS

E (J

/m3 )

Yoakum, 0 Months

Yoakum, 3 Months

Yoakum, 6 Months

DPSE, b

y = 0.6694e0.2486x

R2 = 0.9806

y = 0.5563e0.2075x

R2 = 0.9867

0

1

2

3

4

5

0 2 4 6

Laboratory Aging Period (Months)

Rat

e of

Fra

ctur

e D

amag

e (b

)

Bryan [0 months] Bryan [3 months] Bryan [6 months]

Yoakum [0 months] Yoakum [3 months] Yoakum [6 months]

Aging↑

______

b ↑(more damage)

Binder Surface Energy

• Wilhelmy Plate

Data Acquisition & Calculation

Balance for Force Measurement

Asphalt coated slide dipped in reference liquid

Aggregate Surface Energy

• Universal Sorption Device– Mineralogy– Weathering– PH

Data Acquisition and Automatic Pressure Control

Magnetic Suspension Balance

Sample chamber

Vapor inlet

Testing EquipmentF R SS

Catalog Screening

WP – Wilhelmy PlateMC – MicrocalorimeterUSD USD –– Universal Sorption DeviceUniversal Sorption DeviceAIMS – Aggregate Imaging System

Verification Testing (On Selected Materials)

DMA – Dynamic Mechanical Analyzer (Fine Mix)DCL – Dynamic Cyclic Loading (Full Mix)

System Outline

AsphaltAggregates

Physical Characteristics• Gradation• USD• AIMS

Chemical Property• MC

Chemical Property• WP

Water and Air Diffusion• USD

SizeShapeAngularityTextureSSA

CrushedWeatheredPH Effect

NeatModifiedAgedPH EffectDiffusivity

Materials Catalog

Catalog of Fundamental Material Properties

Wet Conditions

0

0.2

0.4

0.6

0.8

1

1.2

0 500 1000 1500 2000 2500 3000 3500 4000

Number of Cycles

Nor

mal

ized

She

ar M

odul

us, P

a7.00E-01

7.50E-01

8.00E-01

8.50E-01

9.00E-01

9.50E-01

1.00E+00

1.05E+00

Nor

mal

ized

Dam

age

Para

met

er

DMA MeasurementsX-ray CT Damage Cohesive Model

0.000

2.000

4.000

6.000

8.000

10.000

12.000

14.000

16.000

18.000

20.000

0 5000 10000 15000 20000 25000 30000

N

R(N

)

3d4d5d6d7d8d

5 Dry

7 Dry

8 Dry

6 Dry

4 Dry

3 Dry

Mix 7

Mix 8

Mix 6

Mix 4

Mix 3Mix 5

Tested Dry

0.000

5.000

10.000

15.000

20.000

25.000

30.000

0 5000 10000 15000 20000 25000

N

R(N

)

3Wet4wet5wet6wet7wet8wet

4 Wet

7 Wet

3 Wet

8 Wet 6 Wet

5 WetMix 7

Mix 8 Mix 6

Mix 4Mix 3

Mix 5

Tested Wet

Mix 3 had good performance until about 2000 cycles

Effect of Aggregate Angularity on Moisture Damage

Effect of Aggregate Angularity on Moisture Damage

0

1000

2000

3000

4000

5000

6000

7000

Mix 1-G

ranite

Mix 2-G

ranite

Mix 3-Q

uartz

iteMix

4-San

dston

eMix

5-Lim

eston

e

Mix 6-L

ocal

Field S

and

Mix6-S

ands

tone

Mix 7-L

imes

tone

Mix 7-G

ravel

Natural

San

dMix

8-Lim

eston

e

Mix 8-G

ravel

Natural

San

d

DM

A S

peci

men

s A

ngul

arity

Inde

xMix 3

Comparison of Predicted and Observed Mixture Performance

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 1 2 3 4 5 6 7 8 9

Mix Number

Ene

rgy

Rat

io, R

Tota

l

GoodPoor

Limestone

Gravel

Mix Number

Bond

Ene

rgy

Rat

io

Dry

Wet

GG

ΔΔ

Air Void Distribution in Mixes

0

25

50

75

100

0.0% 1.0% 2.0% 3.0% 4.0% 5.0% 6.0% 7.0% 8.0% 9.0% 10.0%

Percent Air Voids

Dep

th (m

m)

A1 A2 B1 B2

Percent Air Voids

Dep

th, m

m

0

0.5

1

1.5

2

2.5

0.6 0.65 0.7 0.75 0.8 0.85 0.9

Average air void size (mm)

r wet/r d

ry a

t 100

0 lo

ad c

ycle

s

A1

A2

B1

B2

Average Air Void Radius, mm

Crac

k Rad

ius

(wet

)/Cr

ack

Rad

ius

(Dry

)

Moi

stur

e Su

scep

tibili

ty I

ncre

ases

A2

A1

B1

B2

All at the Same Percent Air Voids

Development a Procedure for Measuring Total Suction

The Thermocouple Psychrometers were used to measure the total suction in HMA The suction range is 3.67 pF (4.5 bar) to 4.68 pF (47 bar)

Psychrometer’s headThermocouple Psychrometer

The Drying protocol using TCR (60 oC) (Cont’)

1 32

4 5

7

6

98

Mitchell's Simplified Formulation

• Darcy’s Equation:

• Laliberte and Corey's permeability equation:

• Mass balance equation for unsteady flow:

• Simplified diffusion equation:

dxdukv −=

n

hh

kk ⎟⎠⎞

⎜⎝⎛= 0

0

tzyxtxftzyvtzyvQ xxxxx ΔΔΔΔ+ΔΔΔ−ΔΔΔ=Δ Δ+ ),(

tu

xu

∂∂

=∂∂

2

2

α

Comparison with Moisture Damage

0

40

80

120

160

200

0 1 2 3 4

Tim

e , h

rs

0.00E+00

5.00E-06

1.00E-05

1.50E-05

2.00E-05

2.50E-05

Ave

rage

α, c

m2 /s

ec

α

Mix 7Fair

Mix 4 Good

Mix 8Poor

Time

Elapsed Time for the Total Suction to Drop to Psychrometer's Range, and the Average Diffusion Coefficient, α

Concluding Remarks

• Asphalt and aggregates can be individually tested to generate an array of values for all possible mixture combinations

Granite A

Gravel A

Gravel B

Limestone A

Limestone B

Quartzite A

Sandstone A

Sandstone B KAG NAG

PG 64-22 A 0.40 1.05 0.58 0.53 0.74 0.56 1.37 0.88 0.35 0.63

PG 64-22 B 0.65 1.98 1.00 0.86 1.38 0.97 2.54 1.41 0.71 1.08

PG 64-22 C 0.49 1.48 0.76 0.66 1.04 0.74 1.90 1.10 0.51 0.82

PG 64-22 D 0.58 1.38 0.80 0.72 1.03 0.78 1.69 1.10 0.58 0.86

PG 64-22 E 0.61 1.96 0.97 0.83 1.34 0.94 2.57 1.40 0.66 1.05

PG 64-22 F 0.74 2.47 1.18 1.00 1.66 1.14 3.29 1.71 0.81 1.27

PG 64-22 G 0.62 1.92 0.97 0.83 1.34 0.94 2.47 1.37 0.68 1.04

PG 64-22 H 0.47 1.26 0.69 0.62 0.90 0.67 1.63 1.02 0.45 0.75

PG 64-22 I 0.69 2.20 1.08 0.92 1.51 1.05 2.87 1.55 0.75 1.17

PG 64-28 B 0.46 0.93 0.59 0.56 0.70 0.58 1.16 0.83 0.40 0.64

PG 76-22 A 0.72 1.49 0.93 0.87 1.15 0.92 1.79 1.24 0.70 1.00

PG 76-22 B 0.45 1.07 0.62 0.57 0.78 0.61 1.37 0.91 0.40 0.68

PG 76-22 C 0.67 1.24 0.83 0.79 0.99 0.82 1.48 1.09 0.62 0.89

PG 76-22 D 0.60 1.50 0.85 0.77 1.10 0.83 1.88 1.19 0.60 0.92

No

Bond Energy Ratio > x ?

Yes (Good Combination)

Fine Mix Fracture Resistance (DMA)

No (Poor Combination)

Modify Materials:• Change materials• Add anti-strip, lime..

Binder Surface Energy (Wilhelmy Plate)

Aggregate Surface Energy (Universal Sorption)

Material Properties in Catalog?

yes

Add to Catalog

No

Fracture Index < y ?

Full Mix Dynamic Cylic Loading (DCL)

Change Mix Design

Use the Mix

YesFracture Index < z ?

Rutting < w?Yes

No

Fatigue Analysis ModelsMechanistic- Empirical (ME) (AASHTO TP8-94 [1996])

50% stiffness reduction

Calibrated Mechanistic with Surface Energy (CMSE) (Walubita et al. 2006)

7.5 mm microcrack growth

[ ] ESALspiif DesignTrafficNNSFN Q×≥+=

( )[ ]ESALs

kt

f DesignTrafficMTCFkSF

N 2

1 ×≥=−ε

The MEPDG..Fatigue model (AASHTO 2005)

εt = tensile strainE = HMAC moduluski = laboratory determined coefficientsβfi = calibration parameters

Aging & environmental effectsGlobal aging system model (GAM) Enhanced Integrated climatic model (EICM)

( ) ( ) 332211

kktff

ff EkN ββεβ −−=

0.5F0.5F

Deflection

ME MEPDG, CMSE, & CM

F

F

Laboratory Test Setup

Binder & Aggregate Lab TestingTest Schematic Test

ParametersSAFT + PAV*Dynamic shear rheometer (DSR)

Apply sinusoidal shear stress @ various oscillating frequencies & temperatures.0.1 -100 rad/s (60°C)0.1-10 rad/s (20, 40 °C)

G*, G′, G″, δ, ω, DSR function value & slope @ 1 rad/s @ 20 °C

Immersion & withdrawal, 20±2 °C, contact angle (θ)

Vapor pressure & gas adsorbed mass, 20±2 °C

Output

Wilhelmy Plate (WP)(Cheng 2002)Universal Sorption Device (USD)(Cheng 2002)

Surface energy components (Γi

i, Γij ) & fracture & healing bond strengths (ΔGf & ΔGh)

F F

Aggregates

Solvent

Vapor supply

Adsorbed mass

& vapor pressure measurement

Binder

HMAC Mixture Lab TestingTest Schematic Test

Parameters

0.05 in/min @ 20 °CTest time ≅ 5 minutes

Trapezoidal strain-controlled (0.2εf), 60 s loading, 600 s rest, & 10, 20, 30 °C Test time ≅ 25 minutes

Haversine strain-controlled (0.35εf), 1 Hz, 1000, 30 °CTest time ≅ 20 minutes

Output

Tensile Strength (TS)

σt, εf

Relaxation Modulus (RM)

RM Master-Curve, E(t), E1(i), mi, aT.

Uniaxial Repeated Direct-Tension (RDT)

DPSE, b

-200

0

200

0 200 400 600 800 1000 1200 1400

Time, s

Mic

rostr

ain

DPSE

Log N

b

Same specimen (RM & RDT)

HMAC Mixture Test SetupF

F

Example of CMSE/CM Test Setup: Tensile Strength (TS) Testing

Results

RLFL = Lab fatigue life ratio

Laboratory Aging Period

(Months) Parameter Mixture

0 (≅ 0 yrs) 3 (≅ 6 yrs) 6 (≅ 12 yrs)Bryan 1.63 1.65 2.09

aSF [1≤ SFa ≤ 5] Yoakum 2.10 2.08 2.40 Bryan 6.73 4.74 3.07

hSF [1≤ SFh ≤ 10] Yoakum 7.26 4.76 3.81

Bryan 6.31 E+06 2.42 E+06 0.94 E+06

[ ]pi NN + Yoakum 7.88 E+06 4.95 E+06 3.23 E+06

Bryan 1.00 0.38 0.15

[ ][ ]

0pi

ipiLFL NN

NNR

+

+=

Yoakum 1.00 0.63 0.41

HMA Mixtures @ 7±0.5% AVNo. Name Mixture Type Binder + Aggregate

1 A0 Basic, TxDOT Type C 4.6% PG 64-22 + Limestone

2 A1 12.5 mm Superpave 5.3% PG 64-22 + Gravel 3 A2 12.5 mm Superpave 5.8% PG 64-22 + Gravel 4 B1 12.5 mm Superpave (Rut-resistant) 5.6% PG 76-22 [SBS] + Gravel 5 B2 12.5 mm Superpave 6.1% PG 76-22 [SBS] + Gravel 6 C1 12.5 mm Superpave 5.5% PG 76-22 [TR] + Gravel

7 C2 12.5 mm Superpave 6.0% PG 76-22 [TR] + Gravel

8 D 19 mm Superpave (Fatigue-resistant) 5.6% PG 70-22 [SBS] + Igneous

9 E Coarse Matrix High Binder (CMHB)-F

7.9% PG 70-22 [SBS] + Rhyolite

10 F1 12.5 mm Superpave 5.3% PG 76-22 [SBS] + Sandstone

11 F2 12.5 mm Superpave

5.3% PG 76-22 [SBS] + Quartzite

Field Conditions

Five typical Texas pavement structuresPS1,PS2, PS3, P4, PS5With different traffic ESALs, e.g., PS1 = 5 million

Two Texas environmental conditionsWet-warm (WW)Dry-cold (DC)

Design period & Nf predictionOver 20 years design period@ 95% reliability level

Nf (ME)

y = 2E+07e-0.0441x

R2 = 0.8438

y = 2E+07e-0.1485x

R2 = 0.9366

1.0E+05

1.0E+06

1.0E+07

1.0E+08

1.0E+09

0 5 10 15 20 25 30

Aging Exposure Period (Years)

N f

Bryan Yoakum

5.0E+06

Aging↑ Nf ↓Bryan < Yoakum

Nf (CMSE)

y = 1E+08e-0.1358x

R2 = 0.9981

y = 6E+07e-0.16x

R2 = 0.9795

1.0E+05

1.0E+06

1.0E+07

1.0E+08

1.0E+09

0 5 10 15 20 25 30

Aging Exposure Period (Years)

N f

Bryan Yoakum

5.0E+06

Aging↑ Nf ↓Bryan < Yoakum

Nf ComparisonFatigue Average Nf COV of 95% CI

Analysis (1E+06) Ln Nf (%) (1E+06)

Approach BRYN YKM BRYN YKM BRYN YKM

CMSE 3.11 8.41 2.81 3.92 3.08- 6.95 – 9.82

3.21

ME 1.03 8.30 6.87 9.85 0.49 – 2.17 5.41 – 16.74

MEPDG 4.71 6.21 4.66 1.93 – 9.74 2.04 – 15.34

BRYN – Bryan, YKM – Yoakum

ME – mechanistic empirical with bending beam testing

MEPDG – 2002 M-E Pavement Design Guide software

PS#1, WW, Design traffic ESAL’s over a 20-year design life: 5.00 E+06

PAVEMENT DESIGN EQUATION

DESIGN TRAFFIC

12[ var ] n e z iance

fN − Σ= ×

Z - VALUES RELIABILITY LEVELS

0.00 50 %• 0.53 70 %• 1.04 85 %• 1.29 90 %• 1.65 95 %• 2.33 99 %

fvar[ln N ] + var[ln n]

PAVEMENT TRAFFIC LIFE

VARIANCES =

↑ ↑

∑

DESIGNMETHOD

• ME BRYAN 99.2 282• ME YOAKUM 385.1 793• MEPDG - 47.9 51• CMSE BRYAN 29.8 14.7• CMSE YOAKUM 39.0 31.0• TRAFFIC - 10 4.52

f6

6 1 0fN ×MIX MEAN

6

.. 10f

STDDEVN ×

Probability Density of Predicted Traffic Loads

00.020.040.060.08

0.1

0 20 40 60 80 100 120 140 160M illions o f T ra ffic Loads

Prob

abili

ty D

ensi

ty o

f Tr

affic

Expected TrafficCalib rated Mechan istic w/ Su rface Energ iesMEPDG Fatigue LifeMechan istic -Em p irical Fatigue Life

PURPOSES OF FATIGUE CRACKING PREDICTION

• DESIGN RELIABILITY• INCENTIVE PAY FOR CONSTRUCTION

QUALITY• PERFORMANCE SPECIFICATIONS• WARRANTY RISK ASSESSMENT• PAVEMENT ASSET MANAGEMENT

FATIGUE CRACKING

Robert L. LyttonCRACKING IN ASPHALT PAVEMENTS

PAVEMENT PERFORMANCE PREDICTION SYMPOSIUM 2007

WESTERN RESEARCH INSTITUTELARAMIE, WYOMING

JULY 18 – 20, 2007