Page 99 - MODELING OF ASPHALT CONCRETE

Overview of the Stif fness Characterization of Asphalt Concr ete 77

element in determining how much damage will be done to the asphalt concrete by
moisture diffusion. Those asphalts which hold more moisture will experience more
damage due to soaking. Also, based upon the results of recent measurements, those
asphalt-aggregate mixtures in which the work of the water in assisting the fracture of
the interface is the greatest will be the ones that are most damaged by repeated loading
in the presence of moisture.

Fines
The fines in an asphalt concrete mix are all of the particles that are smaller than
0.075 mm. They make up around half of the volume of the mastic. The stiffness of the
mix will be affected significantly by how well the fines bond with the asphalt binder,
the size and size distribution of the fines, how well they are dispersed in the mastic, and
their surface energy compatibility with the asphalt both with and without water. The
size, size distribution, and dispersion all work together to arrest microcracks when they
are small and easier to stop. Microcracks start out as a cloud of many small, dispersed
flaws in the mastic. They grow as strain energy from repeated loading is made available
to the mixture to extend the cracks. If any of these microcracks encounters a fine particle
blocking its path, the particle will act as a crack arrester, and that microcrack will stop
growing. If there are many well-dispersed fine particles in the mastic, many of these
microcracks will be arrested. One of the major effects of microcracks growing in an
asphalt-aggregate mixture is a progressive reduction of stiffness and one of the major
effects of a well-dispersed distribution of fines is to retain the stiffness, precisely because
of its actions in arresting cracks. This also means that the fine particles must bond well
with the asphalt, especially with water present. As discussed before, the adhesive bond
strength between the two is determined by the surface energy characteristics of both the
asphalt and the fine particles.
The same can be said for additives that are expected to improve the stiffness,
strength, and ductility of asphalt-aggregate mix. Regardless of their composition, the
particles of the additive must be small and well dispersed and must bond well with the
asphalt in order to improve the mechanical properties of the mix.

Air Voids
Air voids may be viewed with some accuracy as small particles with zero stiffness. Small,
well-dispersed air voids in the asphalt concrete mixture will provide several benefits to
the mix, including acting as microcrack arresters and providing well-dispersed volumes
for the asphalt to expand into at high temperatures. Too much air will accelerate the
growth of microcracks and too little air will cause bleeding (or flushing) and promote
large plastic deformations. Too much air will also provide ready access of both air and
water into the interior of an asphalt concrete layer and will accelerate aging and moisture
damage. As with the fine particles, the air voids must be small and well dispersed in
order to have its desired effect on the stiffness of the mixture.

Summary
The stiffness of asphalt concrete depends upon numerous factors such as stress state,
temperature, rate of loading, composition, and the mechanical and surface energy
properties of the components of the mixture. It would be impossible to anticipate
empirically how all of these factors will interact to provide an instantaneous value of