Importance of Fatigue Performance Testing and Simulation

Jo E. Sias; Eshan V. Dave; Runhua Zhang

University of New Hampshire

Challenges with current design approaches

Current asphalt mixture design procedures based on volumetrics – no

performance check

Aggregate gradation, VMA, VFA

Asphalt binder specs provide an indication of performance but not reliable for

today’s asphalt mixtures

High RAP & RAS mixtures

Modified asphalt binder; Warm mix asphalt; Bio/Synthetic asphalt binder

Differences in asphalt plant production and storage

Why is an asphalt mixture performance test needed?

Volumetric mixture design ≠ good performance

Need “End Result” to directly evaluate the mix performance

Combines the interaction of the aggregate, asphalt binder, and other

additives (RAP, WMA, fibers, etc) with the plant production and storage

(temperature and time) , also aging

Need “Balanced” asphalt mixture: Strong enough for rutting; flexible

enough for cracking!

Asphalt Performance Testing Goals

Identify mixtures prone to performance problems during the mix design

process

Identify potential performance problems during production

Predict performance during mix design and production

Warranties

Performance Specifications

Balanced Mix Design

Evaluate new materials or design tools to improve performance

Performance Based Space Diagram

Performance Based Space is an essential tool to establish the Balanced

Mix Design

A Rutting index is plotted versus a cracking index to evaluate the overall

mixture performance at the same time

Rut

ting

Cracking

Laboratory Mechanical (Performance) Tests at UNH

Complex Modulus (AASHTO T 342)

Direct Tension Cyclic Fatigue Test (AASHTO TP107)

Simplified Viscoelastic Continuum Damage (S-VECD)

Index parameters and pavement evaluation

Semi-Circular Bend (SCB) Test (AASHTO TP 105)

Advanced Fracture Mechanics

Index parameters

Disk-Shaped Compact Tension (DCT) Test (ASTM D7313-13)

Laboratory age conditioning

Simulate field aging of asphalt mix

Direct Tension Cyclic Fatigue (S-VECD AASHTO TP 107)

Test temperature ((PGHT-PGLT)/2)-3°C

# of specimens: typically 4 (different strain levels)

Parameters:

Damage Characteristic Curve (C vs S)

DR: Average reduction in pseudo stiffness up to failure

Semi-Circular Bend (SCB) Test (AASHTO TP 105)

Test Conditions:

Line load control, loading rate = 50 mm/min

Test temperature = 25 deg. C

Parameters:

Fracture Energy (Gf)

Flexibility Index (FI)

Application of the DR Criteria

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5834LM 5828SM 6428SV 6428SM 6428LM 7034LV 7628SM

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STA5d@95°C / 4.0 years12d@95°C / 9.6 years24hr.@135°C

Change with aging

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Application of the FI Criteria

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STA5d@95°C / 4.0 years12d@95°C / 9.6 years24hr.@135°C Change with aging

Pavement Structure Modelling and Performance Prediction

Use of prediction models are essential in predicting the performance of

asphalt mixtures during and at the end of service life, in context of:

Pavement Structure

Climate Condition

Traffic Condition

FlexPAVETM (three-dimensional finite element program ) developed by North

Carolina State University (NCSU) can be used to predict and compare the

fatigue performance of the mixtures

Required inputs are from the laboratory E* test and SVECD fatigue testing

FlexPAVE Software

Fatigue Cracking Prediction by FlexPAVE

4” Asphalt Layer

12” Granular Base

Subgrade

Combination of the material properties (both LVE and fatigue damage characteristics) with the pavement structure, as well as climate and traffic conditions to predict the fatigue performance of the mixtures

Fatigue performance may be better or worse for each individual mixture depending on the trade-off between stiffness and fatigue resistance

Unaged Aged

Fatigue Cracking Prediction by FlexPAVE

4” Asphalt Layer

12” Granular Base

Subgrade02468

101214161820

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amag

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STA5d@95°C12d@95°C24hr.@135°C

Evaluate the effect of pavement structure

Fatigue Cracking Prediction by FlexPAVE

5.5” Asphalt Layer

10.5” Granular Base

Subgrade

Evaluate the effect of pavement structure

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Perc

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STA5d@95°C12d@95°C24hr.@135°C

Fatigue Cracking Prediction by FlexPAVE

Location within pavement structure matters

2” AC

2” AC

12” Granular Base

Subgrade

2” AC (softer)

2” AC (stiffer)

2” AC (stiffer)

2” AC (softer)

Summary

With current evolution of asphalt mixtures (additives, recycling, production

technologies) volumetric measures are no longer sufficient for controlling

performance

Laboratory performance tests are useful tools to evaluate mixtures during

design and production

It is important to consider the combination of rheological and fatigue

properties in context of the pavement structure, climate and traffic

conditions.

Continued Efforts Needed

Continue collection of field performance data for further development,

calibration, and verification of threshold values for laboratory measured

performance indices

Pavement life analysis with the measured different properties of mix and the

predicted performance of the pavement, while also taking aging into account

Acknowledgements

New Hampshire Department of Transportation (NHDOT)

University of New Hampshire Center for Infrastructure Resilience to Climate

(UCIRC)

University of New Hampshire (UNH)

Thank you for your attention!

Viscoelastic Continuum Damage mechanics

Primary outcome of S-VECD testing: Damage (S) versus pseudo-stiffness (material integrity, C) Assumes that in undamaged state the material has an integrity value (Pseudo

stiffness or C) equal to 1 Fatigue: Damage in form of micro-cracks that grows with repeated loading and the

integrity starts to drop

DR Failure Criterion

DR criterion: Amount of drop in material integrity (1-C), per load cycle until failure There is a linear relationship in arithmetic scale between the accumulated

average drop in material integrity (1-C) and Number of cycles to failure. The slope of this line is the DR

Usually the DR value ranges from 0.3 to 0.8 and the higher DR value is considered as better fatigue resistant

Fracture Performance Parameters

Fracture work: Area under Load-Displacement curve

Fracture Energy, Gf: Energy required to create unit fracture surface

Flexibility Index, FI: FI = Gf / m

Fatigue Cracking Prediction by FlexPAVE

4” Asphalt Layer

12” Granular Base

Subgrade

STA 5D

12D 24H

Fatigue Cracking Prediction by FlexPAVE

5.5” Asphalt Layer

10.5” Granular Base

Subgrade

STA 5D

12D 24H