Fundamental Properties of Asphalts and Modified Asphalts, III

comprised of tens of thousands of different types of molecules that otherwise would need to be
characterized if a totally fundamental approach to the problem were to be considered.
It has also been observed in previous experiments [Robertson et al. 2005, 2006] that different
asphalts tend to form self-ordered microstructural “features” to different degrees with different
crude sources. This was demonstrated at the asphalt-air free surface interface when prepared as
solvent spin-cast, thin-film coatings with thicknesses ranging from 3 microns to as thin as 150
nm. Generally speaking, structures with clearer phase boundaries are observed as the thickness
of these “ultra-thin” films is decreased below 2 microns [Robertson et al. 2005]. Thicker films
tend to exhibit larger structures, particularly the bumble bee shaped structures that appear to
“grow” to a limiting size in 10’s of microns thick films with less distinguishable interface
boundaries. Inevitably, the formation of these microstructures in very thin films of asphalt leads
directly to the development of interfacial grain boundaries between chemically different phases
of materials. These interfacial grain boundaries then constitute discontinuities in the film that
could lead to fracture initiation in actual pavement structures whether they were to rapidly
develop under normal paving conditions or gradually development over time.
The current experimental approaches should not be considered a direct representation or
simulation of pavement structures. Contrarily, in the present research, asphalts are prepared as
thin-films that are similar in magnitude to the average thicknesses of asphalt films estimated for
pavements (e.g., 5-15 microns or 8 to 10-microns on average [Kandhal and Chakaraborty 1996;
Kandhal et al. 1998]). In most cases they are prepared as ultra-thin-films, representing
“theoretical” slices of asphalt very close to and in contact with an aggregate interface. These
systems are then investigated in order to determine the kinetics of microstructure formation as a
function of film-thickness, temperature fluctuation, and method of film preparation (e.g., solvent
spin-coating techniques). This approach has also been adopted due to the difficulty of
experimentally observing asphalt-aggregate interfacial interactions. As a result, thin-film
experimentation on ideal systems combined with computation simulations will be needed to
adequately study these types of systems in order to make recommendations as to how to prolong
the lifecycle of pavements due to fatigue.
Much of the motivation behind the work that has been proposed in this subtask stems from the
desire to know why and how the microstructures that have been observed in these materials, as
observed by atomic force microscopy [Loeber et al. 1996; Pauli and Grimes 2003], could
contribute to pavement performance. Consequently, it is hypothesized that the thermo-kinetic
processes of phase transformations (i.e., molecular order-disorder kinetics) are anticipated to
directly influence the rheological properties of these materials at the mastic thin-film interfaces
in actual pavement structures. These processes include; wax melting-crystallization, potential
for flocculation of asphaltenes, chromatographic interactions of polar and aromatic molecules
with mineral aggregates/filler surfaces, and others that are influenced by fluctuations in
temperature and shear rate. An improved understanding of the correlations between these phase
transformations and rheological properties will help explain the nature of fatigue damage and
subsequent self-healing phenomena brought on during rest periods. In other words, how does the
asphalt chemical composition lead to the order-disorder transitions that produce micro
structuring at interfacial boundaries and thin-film regions? Also, how does this ultimately lead
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