Page 393 - Mechanics of Asphalt Microstructure and Micromechanics

Simulation of Asphalt Compaction 385

FIGURE 11.18 Illustration of the DEM model.

shape, while Case 3 clusters three balls into a triangle. If the ratio between the largest
dimension and the smallest dimension is considered as a shape factor, then Case 1,
Case 2, and Case 3 would have a shape factor equal to 1, 2, and 1.07, respectively. During
compaction, the particle rotation is one of the important kinematics parameters that af-
fect the compaction of the mixture. It may be intuitively interpreted that the more easily
a particle can be rotated, the less difficult to move the particle into a denser configura-
tion. The results indicate that the ball is easiest to rotate, followed by the triangle and the
rectangular. If this simulation observation can be extended, one of the rational guesses is
that particles of cubical shapes are easier to compact than flat or elongated particles
(these particles may break, however). This is consistent with field observations.
11.3.3.2 Particle Contact Property Effect
For two particles of the same materials, it is understood that the contact normal stiffness
of the two particles would be larger for rougher particles. The contact normal stiffness
could also be larger if the mastics between the two particles are stiffer. By assigning a
larger normal stiffness to the contact pair, the investigations by Wang et al. (2007) indi-
cated that rougher aggregates or stiffer mastics/binder would make asphalt mixture
more difficult to compact.

11.3.3.3 Temperature Effect
When the mixture temperature drops during transportation, the stiffness of the binder/
mastics will increase, resulting in an increase of the normal contact stiffness. The simu-
lation indicates that the lower the temperature, the more difficult to compact. It may
also be deduced that when the surface temperature drops during compaction, forming
a relatively stiffer layer at the surface, the mixture would be more difficult to compact.
It may be summarized that:
• Mixtures with aggregates of elongated and flat particles, larger roughness, and
coarser textures are more difficult to compact.

Case 1: Sphere Case 2: Rectangle Case 3: Triangle
aspect ratio: 1:1 aspect ratio: 2:1 aspect ratio: 1:1

FIGURE 11.19 Particle shape effect.