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
0
0.2
0.4
0.6
0.8
1
5834LM 5828SM 6428SV 6428SM 6428LM 7034LV 7628SM
DR
STA5d@95°C / 4.0 years12d@95°C / 9.6 years24hr.@135°C
Change with aging
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5834LM 5828SM 6428SV 6428SM 6428LM 7034LV 7628SM
DR
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io
5d@95°C/STA12d.@95°C/STA24hr.@135°C/STA
Application of the FI Criteria
0
0.2
0.4
0.6
0.8
1
5834LM 5828SM 6428SV 6428SM 6428LM 7034LV 7628SM
FI R
atio
5d@95°C/STA
12d@95°C/STA
24hr.@135°C/STA
(b)
0
10
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5834LM 5828SM 6428SV 6428SM 6428LM 7034LV 7628SM
Flex
ibili
ty In
dex
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
0 50 100 150 200
Per
cen
t D
amag
e
Month
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
0
1
2
3
4
5
6
7
8
0 50 100 150 200
Perc
ent D
amag
e
Month
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