Page 80 - Mechanics of Asphalt Microstructure and Micromechanics

Microstructure Characterization 73

regular. Bagi (1996) introduced the concepts of material cell and spatial cell. For granu-
lar materials, the different cells may show different advantages. Other well-established
cells, including Delaunay cells, can also be used.
3.4.2 Fabric Tensors and Their Implications
Particle orientation is an important concept for granular materials and weakly bonded
granular materials such as AC at high temperatures. Masad et al. (2002) used the modi-
fied stress tensor (Satake, 1983; Nemate-Nasser and Mehrabadi, 1984; Tobita, 1989) for
granular mechanics. While this relationship may not be valid for AC where restraint
from binder or mastics may be significant, it offers a theoretical guide for understand-
ing particle shape, particle orientation, and particle configuration on material anisot-
ropy and stress concentration. General theories based on Eshelby mechanics may be
more relevant where the matrix constraint can be considered. Certainly the computa-
tional mechanics approach (e.g., the digital specimen and digital test methods in a later
chapter) can also deal with the orientation, shape, and distribution of particles. The fol-
lowing presents one of the approaches for quantifying the particle fabric tensors.

3.4.2.1 A Review on Fabric Quantities
Fabric quantities have been widely used in granular mechanics to describe the particle
spatial geometric relationship. However, advances in characterizing these parameters
in a 3D scenario have been slow until most recently when high-resolution X-ray tomog-
raphy imaging has provided a viable tool to obtain 3D datasets. Different views, includ-
ing the void view and the skeleton view, are used to look at the microstructure of a
granular system or a weakly bonded granular system. The skeleton view emphasizes
the load transfer through particle contacts while the void view emphasizes the weaken-
ing effects of voids to a continuum and void connectivity. The skeleton view better de-
scribes the mechanical behavior of unbonded granular materials. However, recent dis-
coveries by Shashidhar (Figure 3.10) using photoelasticity indicate that load transfer in

Shashidhar.

FIGURE 3.10 Load transfer of a weakly bonded granular material (Shashidhar).