Page 337 - Mechanics of Asphalt Microstructure and Micromechanics

Digital Specimen and Digital T est-Integration of Microstructure into Simulation 329

properties of AC in terms of resilient modulus, phase angle, and dynamic modulus. In
practice, the interpretation of the test is simplified using elasticity or viscoelasticity. For
an indirect tensile test, the simplified theoretical solutions for the plane stress condition
along the horizontal and vertical diameters (Hondros, 1959) are usually used to obtain
stresses. These stresses, in conjunction with measured strains, are used to calculate the
modulus. However, due to the heterogeneity, the stresses are not uniform as the theo-
retical predictions of elasticity or viscoelasticity. Therefore, the modulus obtained often
demonstrated large scattering. Accounting for the heterogeneity is necessary.
AC mixture shows temperature and time dependency under loading. Viscoelastic
(Schapery, 1984, 1990; Park et al., 1996; Zhang et al., 1997; Lee and Kim, 1998) and
viscoplastic (Perzyna, 1966; Seibi et al. 2001; Schwartz et al., 2002; Collop et al., 2003;
Tashman et al., 2005) material models were introduced into the study of asphalt con-
crete mixture. These models are based on additive strain decomposition. Kichenin et
al. (1996) proposed a model with two-dissipative mechanisms, associating an elastic-
viscous and an elastoplastic model in parallel. This stress-overlay-based model re-
flects the stress transfer between the two mechanisms and is suitable to describe ma-
terials consisting of multiple constituents. However, these continuum mechanics
models have difficulties capturing the material behavior due to the microstructure of
different mixes.
X-ray-imaging-based digital testing was developed to take into account this mate-
rial heterogeneity and stress nonuniformity (Zhang, et al., 2006). In order to study the
viscoplastic material characteristics of asphalt mixture, the two-layer viscoplastic mod-
el was used to characterize the mixture behavior, serving as the basis for the numerical
performance test. This chapter will first present a macroscopic parametric study to eval-
uate the sensitivity of model parameters to the deformation response of the sample. It
then presents microscopic models for indirect tensile testing and dynamic modulus
testing considering the viscous behavior of the asphalt binder or mastics and the phase
configuration of the AC mixture to build a realistic digital specimen and digital tests
that enable the characterization of mixture properties as well as properties of individu-
al materials. Finally, it presents the digital APA test and a perspective on digital mix
design method currently funded by the National Science Foundation.

10.2.1 Digital Indirect Tensile Test
Indirect tensile testing is widely used in evaluating the rutting and fatigue properties
of AC. This section explains the development of the digital indirect tensile test. As
described in the previous chapters and sections, a realistic material model for asphalt
binder should be a viscoplasticity model to represent the contribution of the visco-
plasticity of asphalt mixture. From a micromechanics point of view, asphalt mixture
can be considered a composite including cavity and aggregate inclusions in the binder
matrix. As aggregates are elastic (under normal traffic loading, but not at the contact
points), the viscoplasticity of the mixture is actually from the binder. Therefore, a vis-
coplasticity model of the same structure can be used for both the binder (mastics) and
the mixture, with different material constants. In the following sub-section, a visco-
plasticity model is first tested on an asphalt mixture at the macroscopic scale (homo-
geneous continuum). After its verification, the same model is then used for represent-
ing the constitutive behavior of the mastics of binder in establishing the digital speci-
men and digital tests.