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200 Cha pte r Se v e n

typical finite element analysis based on fracture mechanics. In the fracture mechanics-based
analysis, an artificial crack must be introduced before the load is applied and critical
stresses that contribute most to the macrocrack propagation are identified.

Summary and Conclusions
This chapter presents the VEPCD model as the constitutive model of asphalt concrete
that accounts for the time and temperature dependence, microcracking damage, and
viscoplastic flow. The VEPCD model in tension is applied to four mixtures tested under
the FHWA ALF study, including three polymer-modified mixtures. The model is shown
to accurately predict the behavior under constant crosshead rate tests, random load
cyclic tests, and TSRST tests. Through the characterization and validation process it is
found that the TTS principle with growing damage is applicable to mixtures with both
unmodified and modified asphalt binders. This finding is significant because the testing
requirements necessary to characterize the material are significantly reduced.
The VEPCD modeling in compression is more complicated than in tension due to
the increased contribution of aggregate particles at high temperatures and/or slow
loading rates. The experimental results suggest that the stiffening effect of aggregate
interlocking must be taken into account to model the compression behavior of asphalt
concrete accurately. A compression model based on HiSS yield surface and Perzyna’s
viscoplasticity theory is presented as an alternative in Chap. 11.
Finally, it is demonstrated that the viscoelastic continuum damage model
incorporated into the finite element program (VECD-FEP++) may be used not only to
evaluate pavement responses under repetitive loading, but also to study the complicated
cracking mechanism in asphalt pavements in a realistic manner. It is shown that, as the
asphalt layer thickness increases, the propensity of top-down cracking increases, which
supports field observations.
The NCSU research team is currently developing a three-dimensional VEPCD-
FEP++ program that can be used in predicting asphalt pavement performance, including
fatigue cracking (both top-down and bottom-up), rutting, and thermal cracking.

Acknowledgment
The authors would like to acknowledge the financial support provided by the Federal
Highway Administration.

References
American Association of State Highway and Transportation Officials (1993), “TP10-93
Method for Thermal Stress Restrained Specimen Tensile Strength.” Washington, D.C.
American Association of State Highway and Transportation Officials (2003), “TP-62
Standard Method of Test for Determining Dynamic Modulus of Hot-Mix Asphalt
Concrete Mixtures.” Washington, D.C.
Bazant, Z. P. (1986), “Mechanics of Distributed Cracking,” Applied Mechanics Reviews,
ASME, Vol. 39, pp. 675–705.
Chehab, G. (2002), “Characterization of Asphalt Concrete in Tension Using a
Viscoelastoplastic Model,” Ph.D. dissertation, North Carolina State University,
Raleigh, N.C.