Page 220 - MODELING OF ASPHALT CONCRETE

198 Cha pte r Se v e n

become closer and the air voids collapse, but noticeable aggregate interlocking has
not yet formed. The beginning of Zone 2 indicates the onset of aggregate-to-aggregate
interlocking. As the aggregate particles are locked together, the stiffness of the
mixture increases (i.e., the hardening behavior), as shown in Zone 2. In Zone 3,
aggregate interlocking degrades slowly as the load increases. At the peak stress, the
aggregate particles slip away from each other, which causes the shear failure shown
by the descending stress versus pseudostrain curve in Zone 4.
The mechanistic modeling of the aggregate interlocking mechanism involves triaxial
testing with confining pressure followed by a rigorous formulation of viscoplasticity
with the yield surface and flow rule. Chapter 15 presents one example of this more
rigorous type of model.

Finite Element Implementation of the VEPCD Model
The ultimate goal of developing a constitutive model (e.g., the VEPCD model) of asphalt
concrete is to predict the response and performance of asphalt pavement structures in a
reliable manner. The North Carolina State University research team is currently working
on the implementation of the VEPCD model into the finite element code, FEP++,
developed by Guddati (2001). In the following, some preliminary results from the
VECD-FEP++ are presented in order to shed some light on expectations once the
complete system becomes available.
Figs. 7-24 and 7-25 present the changes in the damage contours as repetitive loading
continues on thin and thick asphalt layers, respectively. In these simulations, a loading
duration of 0.03 second and a rest period of 0.97 second are selected. The thicknesses of
the thin and thick asphalt layers are 3 and 12 in., respectively. The asphalt layer is
represented by the VECD model, and the nonlinear stress-state-dependent model is
used for the aggregate base and subgrade. The pavement structure is modeled by an
axisymmetric finite element model.
The most important observation from Figs. 7-24 and 7-25 is the change in the location
of crack initiation as a function of the asphalt layer thickness. When the thinnest layer
is modeled in Fig. 7-24, the severe damage is found at the bottom of the layer with
negligible damage at the top of the asphalt layer. However, for the thick asphalt layer in
Fig. 7-25, damage initiates from both the bottom of the asphalt layer and directly under
the tire edge, and propagates simultaneously to form a conjoined damage contour. It
can be seen that the intensity of damage under the tire edge is as high as that at the
bottom of the asphalt layer. This conjoined damage contour, shown in Fig. 7-25, supports
the findings from field studies of top-down cracking (Gerritsen et al. 1987).
Also, the conjoined damage contour suggests that the through-the-thickness crack
may develop as these bottom-up and top-down microcracks propagate further and
coalesce together. Gerritsen et al. (1987) report that they found field cores with top-
down cracking for about 10 cm (4 in.), no cracking at all for about 5 cm (2 in.), and
bottom-up cracking for about 10 cm (4 in.) in the same core. The conjoined damage
contour in Fig. 7-25 explains the reason behind this observation. This finding clearly
demonstrates the problem associated with the traditional approach to fatigue
performance prediction in which the tensile strain at the bottom of the asphalt layer is
related to the fatigue life of the pavement.
It should be noted that the VECD-FEP++ model does not need to assume where the
microcracks initiate. This feature of the VECD-FEP++ program is quite different from