Page 456 - Mechanics of Asphalt Microstructure and Micromechanics

448 C hapter T h ir te en

means for evaluating the moisture sensitivity of asphaltic mixtures, nor does it allow to
evidently determine the effect of mix design parameters on the weakening of the bind-
er-aggregate bond.
Kringos et al. (2007) and Kringos and Scarpas (2008) proposed a 3D elasto-visco-
plastic FE model for mastic to evaluate moisture-induced damage in asphaltic mixes
from quantifying the physical and mechanical processes which lead to moisture dam-
age. Also, a finite element tool that enables the simulation of physical damage due to
water flow and moisture diffusion through asphaltic mixes was developed. The 1-D
schematic of the proposed elasto-visco-plastic material model consists of a single elas-
to-plastic component in parallel with a random number of viscoelastic ones. To simu-
late the water flow through the asphalt mix, the water velocity field was expressed us-
ing the following formula, assuming the validity of Darcy’s Law:

(13-21)

where v = water velocity
p = water pressure
˜
q = water capacity
S = pressure dependent saturation
A diffusion flux similar to the Fick’s law of diffusion was assumed to simulate the
moisture diffusion through the AC material.
J = –D∇(C d ) (13-22)
2
where J = diffusive-dispersive mass flux (M/L T)
2
D = diffusion-dispersion tensor (L /T)
C d = concentration of desorbed mastic (M/L )
3
Numerical models used to simulate fluid flow in 3D microstructures of porous ma-
terials were developed by Kutay et al. (2007) and Masad et al. (2007). From these models
it is possible to create realistic simulations of the material distresses considering the
presence of moisture. The model by Masad et al. (2007) is based on the governing equa-
tions (continuity and Navier-Stokes) in 3D space, for the steady incompressible fluid
flow using finite difference techniques. This way the fluid flow equations are solved
within the actual boundary conditions of the 3D air-void structure.
To establish the fluid flow field within the microstructure, Kutay et al. (2007) used
the Lattice Boltzmann (LB) approach. Through these proposed models, the flow speed
and pressure are determined at all points within the void structure, which means that
at the microstructural level they represent essential inputs for numerical simulations of
moisture damage.

13.3.3 Mesoscale Moisture Damage Mechanisms
13.3.3.1 General Concept
Pore water in the asphalt pavement will produce excess pore water pressure. The excess
pore water pressure will produce tensile stress in the surrounding medium. This addi-
tional tensile stress may reduce the fatigue life and increase the deformation rate. Figure
13.13 illustrates this mechanism through a simple model: a plate containing a small circu-
lar hole.