Understanding Asphalt Mastic Behavior Through Micromechanics

The mechanical properties of asphalt-mineral filler mastics have long been known to significantly influence the overall performance of paving mixtures. However, reinforcement mechanisms associated with the presence of fillers in asphalt mastics are not well understood. Particulate composite micromechanical models are shown to be a powerful tool for separating various reinforcing mechanisms in asphalt mastics, including volume filling, physiochemical, and particle-interaction reinforcement. The generalized self-consistent scheme model is shown to predict very reasonable baseline reinforcement levels for asphalt mastics, and simplified prediction tools are presented as an alternative to the cumbersome micromechanical solution. An experimental program was conducted to evaluate micromechanical predictions of mastic properties over a broad range of temperatures and filler concentrations. A new equivalent rigid layer modeling technique was developed, which suggests that stiffening effects observed in mastics beyond those due to volume filling may be largely explained by an effective increase in volume concentration of rigid inclusions due to a rigidly adsorbed asphalt layer just 0.02 to 0.10 μm thick. Particle-interaction reinforcement appears to play a smaller role, possibly as a result of the interaction of partially altered asphalt layers, and was observed to be significant only at very high filler contents. More work is needed to better understand the nature of physiochemical reinforcing and to study other possible stiffening mechanisms in mastics such as agglomeration, state of dispersion, and particle-size distribution.