Page 186 - MODELING OF ASPHALT CONCRETE

164 Cha pte r Se v e n

evolution, plastic and viscoplastic flow of the binder, friction among aggregate particles,
and the coupling among these mechanisms. Additional difficulties arise from the fact
that the model must be able to account for the effects of rate of loading, loading time, rest
period, temperature, aging, and stress state so that the resulting model is applicable to a
range of loading and environmental conditions experienced in pavements.
This chapter presents a constitutive model that can describe the deformation behavior
of asphalt-aggregate mixtures under complex loading conditions at a wide range of
temperatures. The modeling strategy adopted is based on (1) the elastic and viscoelastic
behavior of asphalt concrete using the elastic-viscoelastic correspondence principle based
on pseudostrain, (2) the effect of microcracking on the constitutive behavior using
continuum damage mechanics, (3) the plastic and viscoplastic behavior using a time- and
stress-dependent viscoplastic model, and (4) the effect of temperature on the constitutive
behavior using the TTS principle with growing damage. The resulting models are integrated
by the strain decomposition approach to form the VEPCD model. The VEPCD model is
then validated under various loading and temperature conditions. Finally, the viscoelastic
continuum damage (VECD) model is implemented into the finite element program to
model the cracking behavior of asphalt pavements. Implementation of the full VEPCD
model into the finite element program is currently ongoing at North Carolina State
University.

Analytical Framework
The analytical framework of the model presented in this chapter is based on the strain
decomposition principle suggested by Schapery (1999). In his work, Schapery demon-
strated that the total strain can be decomposed into viscoelastic strain and viscoplastic
strain, as follows:
ε = ε + ε (7-1)
Total ve vp
where e Total = total strain
e = viscoelastic (VE) strain
ve
e = viscoplastic (VP) strain
vp
In this formulation, the viscoelastic strain includes both linear viscoelastic (LVE)
strain and strains due to microcracking, and the plastic strain is included in the
viscoplastic strain.
The VEPCD model adopts a stepwise approach, in which the experiment necessary
for the model characterization is designed such that these strain components can be
systematically evaluated from the simplest state to the state that includes more complex
mechanisms. More specifically, the material’s behavior in the simplest state (i.e., LVE
behavior without any cracking or permanent strain) is first modeled by the elastic-
viscoelastic correspondence principle. Then, the effect of microcracking damage is
modeled by applying continuum damage mechanics to the experimental data from low
temperatures and high strain rates where the viscoplastic strain is minimal. The strain
hardening viscoplastic model is applied to the experimental data at high temperatures
and slow strain rates to develop the viscoplastic model. Finally, these models are
combined with the TTS principle with growing damage to allow the prediction of the
material’s behavior at any temperature. The TTS principle with growing damage has