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A SIMPLE TESTING METHOD FOR EVALUATION OF ASPHALT

FATIGUE PERFORMANCE EMBRITLEMENT

A. Andriescu†, A. Copeland*, N. Gibson*, J. Youtcheff*, X. Qi†

†SES Group & Associates, LLC, Turner-Fairbank Highway Research Center, McLean, USA

*FHWA, ,Turner-Fairbank Highway Research Center, McLean, USA

Pavement Performance Prediction Symposium

Laramie, Wyoming

Acknowledgments

Government

U.S. Federal Highway Administration (FHWA)

University

Professor Simon Hesp

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Main Research Product

Test Method to Determine the Essential and Plastic Work of Ductile Fracture and to calculate an approximate Crack Tip Opening Displacement (CTOD) in Asphalt Binders and Mixtures

Method provides possible grading parameters for fatigue cracking (non-linear regime).

Provides the essential work necessary for the formation of fracture surfaces.

Provides the ductile plastic work used in areas away from the crack process zone.

Both the essential and plastic works need to be high to assure good fatigue performance.

Provides the CTOD at failure.3

Fatigue Evaluation History

Pre-SHRP

Oscillating torsion test (Pell, 1962)

Force ductility test (e.g., Anderson and Wiley, 1976)

Elastic recovery test (e.g., Dekker, 1987)

SHRP

Dynamic shear rheometer (DSR) test (Anderson et al., 1994)

Post-SHRP

Time sweep in DSR (Bahia, 1999; Phillips, 1999)

Binder yield energy test (Bahia et al., 2008)

Linear amplitude sweep test (Bahia et al., 2009)

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Problems with Current Binder Fatigue Evaluation

Low temperature and fatigue cracking are high strain fracture processes that cannot be readily captured rheologically.

Fatigue cracking is a non-linear phenomenon.

Micro- and macro-cracking can appear in high stress concentration areas. (Around air voids or large aggregate particles.)

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Binder Grading Through Fracture Mechanics Based Test Methods

Low-Temperature Failure Brittle fracture energy: GIc or Gf

Crack tip opening displacement (CTOD)

Fatigue FailureDuctile fracture energies: we and wp (essential and

plastic works of fracture, respectively)

Approximate crack tip opening displacement:

CTOD (or δt)= wessential/ net section stress at 5mm

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Essential Work of Fracture

Ligament lengths (L): 5, 10, 15, 20 and 25 mm

Sample thickness (B): 6.5 mm

Strain rate = 100 mm/min

T = 25oC

L

40 mm

30 mm

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Data Analysis

Total Work of Fracture:

Wt = We + Wp = LB we + βL2B wp

Total Specific Work of Fracture:

wt = = we + βL wp

Where β is a scaling factor accounting for the shape of the plastic zone.

LB

Wt

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Experimental Set-up

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Experimental Design

Fracture Process Zone

We region: essential for fracture and initiates tearing of neck.

Outer Plastic Zone

Wp region:non-essential for fracture.

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Asphalt Binders Tested

FHWA ALF

PG Grade, ºC

G*sinδ at 25ºC,

MPa

Control (L2)

Airblown (L3)

SBS (L4)

CRTB (L5)

RET (L6)

SB (L9)

72-23

74-28

74-28

79-28

74-31

71-28

5.33

3.58

1.44

2.11

1.16

0.56

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SBS =styrene-butadiene-styrene linear triblock copolymerCRTB = crumb rubber as blended according to a terminal processRET = reactive ethylene terpolymerSB = styrene-butadiene copolymer

Typical Raw DataF

orc

e (N

)

Extension (mm)

00

30

100

Strain Rate = 100 mm/min

T = 25oC

50 150

90

60

L= 5 10 15 20 25

ControlControl

L= 5 10 15 20 25

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Typical Raw DataF

orc

e (N

)

Extension (mm)

00

10

120

Strain Rate = 100 mm/min

T = 25oC

60 180

30

20

L= 5 10 15 20 25

SB diblockSB diblock

L= 5 10 15 20 25

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Typical Raw DataF

orc

e (N

)

Extension (mm)

00

10

200

Strain Rate = 100 mm/min

T = 25oC

100 300

30

20

L= 5 10 15 20

25

SBS linearSBS linear

L= 5 10 15 20

25

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Determination of Essential and Plastic Works of Fracture

wt, kJ m-2

Ligament Length, mm

0 10 20 300

20

40

L2 Control

L3 Airblown

L4 SBS

L5 CRTB

L6 RET

L9 SB

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Ductile vs. Essential Works of FractureBinders from FHWA ALF

e

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10 12 14

β wp, MJ.m-3

w , kJ.m-2

L-2 Control

L-9 SB

L-3 Airblown

L-4 SBS

L-5 CRTB

L-6 RET

BEST

WORST

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Net Section Stresses as a Function of the Ligament Length

=L3 Airblown, =L2 Control, =L5 CRTB, =L4 SBS, =L6 RET, =L9 SB

0

0.5

1

1.5

0 5 10 15 20 25 30

Ligament Length, mm

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L2 Control L3 Airblown L4 SBS L5 CRTB L6 RET

0.0

10.0

20.0

30.0

Binder

7.5 6.8

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8.5

15.7

Crack Tip Opening Displacements

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FHWA Turner-Fairbanks ALF

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Super-Single Loading Wheel(16,000 lbs and 19oC)

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Typical Distress in ALF Lanes

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Fatigue Cracking Rates

0.1

1

10

100

1000

1000 10000 100000 1000000

Number of Load Applications

=L3 Airblown, =L2 Control, =L5 CRTB, =L6 RET, =L4 SBS, =L7 F

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Binder CTOD vs. Fatigue Cracking Rates

0.01

0.1

1

10

1.0 10.0 100.0Cra

ck R

ate

to 2

0 m

, m

m/p

ass

Binder CTOD, mm

L2

L3

L4L5

L6

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0.01

0.1

1

10

0.1 1 10 100

Loss Modulus, MPa

L2

L3

L4

L5L6

Binder Loss Modulus vs. Fatigue Cracking Rates

@ 10rad/s and T = 25ºC

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CTOD values of Recovered Binders

Strain Rate = 50 mm/min

T = 25oC

Lane # Lane 2 (Control) Lane 3 (Airblown) Lane 4 (SBS)

Aging Level Natural Accelerated Natural Accelerated Natural Accelerated

Lane Site S3 S4 S3 S4 S3 S4

CTOD (mm) 8.5 7.2 8.8 7.4 12.6 8.9

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Fracture in ALF Mixtures

Fracture process zone has elliptical shape.Strain Rate=0.5 mm/min, T = 19ºC

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Mixture CTOD vs. Fatigue Cracking Rates

0.01

0.1

1

10

0.1 1.0 10.0 100.0Cra

ck R

ate

to 2

0 m

, m

m/p

ass

Mixture CTOD, mm

L2

L3

L4L5

L6

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Conclusions

EWF test method can be used to provide ductile failure properties of binders and mixtures

No correlation found between CTOD and Loss Modulus, G*sinδ

Binder and Mixture CTOD appears to correlate well with fatigue distress in FHWA ALF trial

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Questions

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Sample after fracture at 0oC under

stable crack growth conditions

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