Microstructure study of WesTrack mixes from X-ray tomography images

Asphalt concrete is a composite comprised of aggregates, air voids (AVs), and microcracks of various sizes and shapes. The property of asphalt mixes is not only affected by volume fractions of these components such as voids in mineral aggregates (VMA, the complementary volume fraction of aggregates) and AV content but also the size and spatial distributions of these components. However, conventional and Superpave volumetric mix designs specify only bulk values of VMA and AV, which may not be able to capture all the critical factors of a mix. The unexpected WesTrack mixes' premature failure may reflect this limitation in design specifications. Results are presented of a study to evaluate statistically the spatial and size distribution of the void systems of the mixes and to find other possible parameters that might be better or complementary to the current parameters to characterize the mixture internal structure and performance. Void systems of three original WesTrack mixes were evaluated using X-ray tomography images and stereology methods. The spatial void content variation and void size distribution for six specimens of three mixes were quantified. Specific damaged surfaces (void surfaces were considered damaged surfaces because they do not transmit forces), damage tensor, mean solid path among damaged surfaces, and mean solid path tensor, which indicate damage level and interaction of the damage, were also quantified. It was concluded that coarse-graded mix is more severely inherently damaged and that this set of parameters gave a performance ranking of the three mixes consistent with field observations. Moreover, quantified tensors are applicable in continuum damage mechanics for fatigue and rutting modeling.