Page 383 - Mechanics of Asphalt Microstructure and Micromechanics

Simulation of Asphalt Compaction 375

and orientate as rigid bodies, the DEM approach provides a viable tool. In recent
years, Fu (2005) and Fu et al. (2007) developed a clustering DEM approach, to include
real particle shapes in the DEM simulations of true 3D behavior of granular materials
for both compression and shearing tests. You and Buttlar (2004, 2005) developed a
micro-fabric DEM approach for 2D simulations. These approaches show great advan-
tages in handling particle movements in large displacements and rotations. These
developments, in conjunction with X-ray computed tomography, allow for accurately
modeling the compaction process and experimentally validating the models. Wang et
al. (1999, 2007) also developed methods to trace particle movements and compute the
effective strains. This approach can be used to assess the particle shape, angularity,
and texture properties, and also provide information for model verification. The
quantification methods (Wang et al., 2004) can be used to calculate the shape, angu-
larity, and texture properties of aggregates for more accurate contact modeling. The
rough contacts, according to contact mechanics, make contact stiffer, and therefore
qualitatively agree with the experimental observations that rough aggregates yield
stiff mixes.
Another advantage of the DEM approach is its ability to handle large deformations.
This is especially useful in modeling the field compaction, where the layer thickness
might be 2.5 to 3.5 times the maximum size of aggregate particles. In that case, indi-
vidual particle movements may significantly affect the compaction effectiveness and
uniformity. The number of particles may not be so large so the computation power may
not be that demanding.
For the gyratory compaction, the digital test and digital specimen (Chapter 10) ap-
proach can also be utilized to model the compaction process. This approach, in conjunc-
tion with the particle identification techniques (Fu, 2005; Fu et al., 2007; Wang et al.,
2008) presents a very powerful tool to experimentally observe how aggregate particles
move during the compaction process.
More recently, a viscoleasticity contact model has been developed and validated
through X-ray images (Wang, 2007). Figure 11.9 illustrates the experimental validation
of the contact model through X-ray tomography imaging, where a binder film between
two aggregate particles under compression load were modeled and tested.

FIGURE 11.9 Validation of a viscoelasticity contact model.