Page 26 - MODELING OF ASPHALT CONCRETE

4 Cha pte r O n e

approach allows the accurate evaluation of the effects of changes in layer stiffnesses due
to damage growth on pavement performance. Prediction of multiple performance
characteristics and their interactions is possible in a realistic manner, although the
material models in both tension and compression are needed.
The lack of computing power needed to calculate damage evolution for the entire life
of the pavement forced earlier researchers to develop the two-step approach to pavement
performance prediction, as opposed to the more realistic one-step integrated approach.
However, improvements in computing power and numerical techniques now allow
modelers to implement more powerful material models into the pavement response
model and to predict the pavement performance directly from the integrated model.

Multiscale Model
Two general approaches in mechanics can be used for modeling the changes in the stress-
strain behavior of asphalt concrete: a micromechanical approach and a continuum
approach. In the micromechanical approach, defects that constitute the damage are
described by microscopic geometrical parameters, such as microcrack size, orientation,
and density. These parameters are evaluated through an appropriate microstructural
evolution law, such as the microcrack growth law. Mechanics is then applied typically on
an idealized RVE to determine the effects of the distribution of microdefects on the
macroscopic constitutive parameters, such as the effective stiffness of the damaged body.
Such analyses are, in general, difficult to perform because of the intrinsic complexity of
the microstructure and the micromechanisms and also due to the interactions among the
defects. Therefore, without proper simplifications and assumptions both in modeling
and analysis, the micromechanical approach may fail to provide realistic information
about the macroscopic constitutive framework for modeling the progressive degradation
of the mechanical properties of solids (Park et al. 1996).
On the other hand, in the continuum approach, or so-called continuum damage
mechanics, the damaged body is represented as a homogeneous continuum on a scale
that is much larger than the flaw sizes. The state of damage is quantified by internal
state variables (ISVs) within the context of the thermodynamics of irreversible processes.
That is, the growth of damage is governed by an appropriate damage evolution law.
The choice and interpretation of the ISVs are somewhat arbitrary, and the functional
form of the thermodynamic potential (typically Helmholtz or Gibbs free energy) and
the resulting stress-strain relations are postulated usually on a phenomenological basis.
The stiffness of the material, which varies with the extent of damage, is determined as
a function of the ISVs by fitting the theoretical model to the available experimental data.
The phenomenological continuum damage models thus provide a viable constitutive
framework for the efficient modeling of macroscopic mechanical behavior of materials
with distributed damage without requiring explicit descriptions of microstructural
evolution kinetics (Park et al. 1996).
Recently, significant advancements in the modeling of asphalt concrete have been
made in both micromechanics and continuum damage mechanics. In future models of
asphalt concrete, micromechanical and continuum damage models will be coupled to
describe the behavior and performance of asphalt pavements using the properties of
their component materials (i.e., binder and aggregate). This multiscale model will take
advantage of the strengths of both micromechanics and continuum damage mechanics,
that is, the ability of the micromechanical model to describe mixture behavior using
component material properties and that of the continuum damage model to describe