Page 24 - MODELING OF ASPHALT CONCRETE

2 Cha pte r O n e

basic principles of mechanics provides relationships between material properties
(chemical or mechanical) and model parameters, which can be used for the selection or
design of better performing binders or mixtures.

Performance Characteristics
Performance of asphalt concrete can be categorized into two major types of distress:
cracking and permanent deformation. Cracking of asphalt concrete can be caused
by mechanical loading from repetitive traffic and/or thermal loading from changes
in temperature. When the asphalt concrete is subjected to repeated loading, whether
it is mechanical or thermal, distributed microstructural damage occurs primarily in
the form of microcracks. A microscopic video image of the cracking area in asphalt
concrete is presented in Fig. 1-1 to display the formation of micro- and macrocracks
under tensile stress. As shown in this figure, microcracks exist ahead of the
macrocrack tip, forming a so-called damage zone. Propagation, coalescence, and
rebonding of these microcracks in the damage zone affect the macrocrack growth
and healing and, thus, the fatigue behavior of asphalt concrete. That is to say, the
modeling of the fatigue behavior of asphalt concrete requires an evaluation of the
effects of both micro- and macrocracks and their interaction on the global behavior
of the mixture.
At high temperatures and/or slow loading rates, the asphalt binder becomes too
soft to carry the load and, thus, the principal type of damage is permanent deformation
due to volume change (i.e., densification) and rearrangement of aggregate particles
caused by shear flow. The degree of aggregate interlocking and anisotropy in asphalt
concrete caused by aggregate orientation under compaction become important
factors in the accurate prediction of the permanent deformation behavior of asphalt
concrete.

FIGURE 1-1 Microscopic surface image of cracking area in asphalt concrete. (Kim et al. 1997,
with permission from International Society for Asphalt Pavements.)