Page 401 - Mechanics of Asphalt Microstructure and Micromechanics

CHAPTER12

Characterization and Modeling

Anisotropic Properties of

Asphalt Concrete

12.1 Introduction
Asphalt concrete (AC) is a bonded granular material. Its internal structure is anisotro-
pic, which could be due to the anisotropic particle and void shape, particle orientation
distribution, anisotropic compaction (restrain and force pattern applied during the
compaction), and compaction-induced anisotropic distribution of contact normals. The
characterization and modeling of the anisotropic properties of soils have been widely
explored in geomechanics and geotechnical engineering (Kuwano et al., 2000; Penning-
ton et al., 2001; Adu-Osei et al., 2001; Roy et al., 2002; Li and Dafalias, 2002). However,
few studies have focused on the characterization and modeling of aggregate base (Tu-
tumluer et al., 2001; Seyhan and Tutumluer, 2002), AC (Masad et al., 2002; Wang et al.,
2005a; Li and Wang, 2008) and pavement analysis (Boulbibane et al., 1999; Wang et al.,
2005b). It would be worthwhile to evaluate the degree of anisotropy and its effect on
material response because the characterization and analysis methods for isotropic and
anisotropic materials are quite different.
In general, whether a material is isotropic or anisotropic affects the selection of ma-
terial characterization methods, response models, and distress models. For example, if
the axial stiffness and the lateral stiffness of a gyratory specimen are significantly differ-
ent from those of a field specimen, the deformation characteristics obtained using gyra-
tory specimens may not well represent those of field specimens. This chapter presents
a comparison between the stress field of an isotropic pavement and that of an anisotro-
pic pavement through analytical solutions and finite element (FE) simulations, and a
method to characterize the orthotropic material properties using a cubical device. Stud-
ies on other anisotropic properties of AC, including permeability and rutting, are also
summarized in this chapter.

12.2 Orthotropic Elasticity
AC is a viscoelastoplastic material. In pavement analysis and especially stress analysis,
it is often treated as an isotropic elastic material. This treatment has its advantages in

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