Page 447 - Mechanics of Asphalt Microstructure and Micromechanics

Multiscale Modeling and Moisture Damage 439

interface structure to be characterized at different scales and modeled using MD and
RRF. Using MD modeling the tensile and shear behavior (including modulus and vis-
cosity) can be calculated using the statistical mechanics from the state variables in the
MD simulations.

13.2.10.1 Modeling the Stress-Strain-Strain Rate Behavior of Interfaces
In order to develop an efficient model that represents bitumen-rock interfacial interac-
tion, an in-depth study of bitumen molecular structure is needed. Chemical methods
separate asphalt into multiple parts such as asphaltene, resin, and maltene. Solubility
differences and chromatography subdivide an asphalt into asphaltene, polar aromatic,
naphthene aromatic, and saturate components (Roberts et al., 1996). Asphaltenes are
the most viscous and polar components, maltenes are the least viscous and mostly non-
polar, and resins are in-between. In the bitumen model adopted, asphalt mixtures of
reasonable compositions are with different compounds representing each constituent.
Each component is represented using one molecule type. Molecule choice is based on
the measurements by Storm et al. (1994). It contains a moderate-size aromatic core with
small branches. It is taken from NMR studies by Artok et al. (1999). They proposed a
collection of sample molecules that in total represent the statistics of molecular as-
phaltenes. This model is chosen because it represents one of the bonding patterns pres-
ent in asphaltenes.
Once the asphalt mixture structure is built, CVFF-aug force field parameters can be
used to characterize the interatomic interaction. A commonly used potential function
for characterizing intra-connected organic molecules is the CHARMM potential. Bond
energy and torsion energy are built based on a harmonic angle style. Dihedral and im-
proper dihedral energy calculation use CHARRM and CVFF type, respectively. The at-
omistic model of asphalt-quartz interface is depicted in Figure 13.10.
At a fixed temperature, static calculations are performed to minimize energy. This
process also ensures that convergence is achieved, thus relieving any potential overlap
in vdW interactions. In the second step, the molecules system is annealed after heating

FIGURE 13.10 An interface
model for MD simulations.

3 nm
thickness
interface
layer