Fundamental Properties of Asphalts and Modified Asphalts, III

to crack initiation brought on by fatigue, followed by pavement failure, as a function of
environmental conditions and loading cycles?
It has also been hypothesized that the process of combining asphalt with aggregate at high
temperatures, placing this composite material in a roadway, and then allowing it to cool and cure
may be modeled as a solidification process. Solidification processes are generally considered to
be non-equilibrium thermodynamic processes where fluxes in composition and thermal gradients
are driven by both mechanical and thermodynamic forces (e.g., chemical potential, surface
tension and pressure differentials), thereby constituting mechanisms for order-disorder
transitions that result in the formation of microstructural grain boundaries. These order-disorder
transitions could also be responsible for cohesive failures when, for example, subjected to
temperature fluctuations, shear and compressive loading, and exposure to moisture. Thus, a
thorough investigation of the “chemical-action” of asphalt binder as it exists in pavements is
warranted in order to define the interfacial boundary conditions and kinetic driving mechanisms
that describe the formation and destruction of microstructural phases in asphalt when adopted by
continuum mechanical modeling approaches used to simulate fatigue damage.
Background
In this subtask a somewhat simplified view has been taken to describe the composition of asphalt
thin-films in a “pavement structure.” This is particularly true at the mastic level, which may be
viewed as if one were building sandcastles with asphalt in place of water as the binding agent to
adhere and maintain structural form to the system. While constructing such a system, many
different forces can be considered that could contribute to the strength of the system. These
forces may include the capillarity and adhesion between asphalt and aggregate fine particles, the
viscoelastic response of the asphalt thin-films between aggregate particles to compression and
shear forces, and breakdown of both the cohesive strength of the asphalt and the adhesive bonds
between asphalt and aggregate in the presence of moisture. For the sake of the present
discussion, figure 2-3.1 depicts a volume element slice of an 8-μm thick asphalt thin-film
adhering three aggregate fine particles together in a mastic sub-structure of a pavement structure.
If the presence of the aggregate particles in this system have an influence on the ordering of the
asphalt molecules at the interface (e.g., first monolayer, first 200-nm, first 1.0-μm, etc) at what
distance away from this interface would long-range molecular ordering subside? At what
effective distance away from this interface would the asphalt layer have properties similar to a
bulk phase state? In the present scenario, asphalt “molecules” present at the interface would
experience the strongest interactions with the aggregate surface. Four microns out into the film,
approximately halfway between two of these particles, the asphalt should exhibit properties more
closely resembling that of asphalt in the “bulk” state.
Based on AFM surface imaging it has been observed that thicker thin-films of asphalt (>4.0 μm)
do not differ that much in appearance from films that are 10’s of microns or even 1.0-mm in
thickness. As these films are decreased to