Molecular dynamics analysis of moisture effect on asphalt-aggregate adhesion considering anisotropic mineral surfaces

The objective of this work is to analyze the effect of moisture on the bonding and debonding behaviors between asphalt and aggregates based on molecular dynamics (MD) simulation, considering the anisotropic characteristics of mineral surfaces. Full atomistic models adopted for MD simulations were constructed using the 12-component asphalt model and two types of representative minerals, alpha-quartz and calcite. Anisotropic wettability was studied by simulating the dynamic processes of a water nano-droplet spreading on anisotropic mineral surfaces. An improved energy ratio (ER) considering the residual adhesion between asphalt and aggregates in a moist state was established to evaluate the moisture susceptibility of asphalt mixtures. It was found that (1) anisotropic mineral surfaces have a significant influence on the bonding properties and moisture susceptibilities of asphalt mixtures; (2) the concentrated hydroxyl groups on the hydroxylated alpha-quartz surfaces significantly increase surface hydrophilicity and reduce the resistance to water damage; (3) freshly-cleaved calcite surfaces contribute the most to moisture susceptibility, while un-hydroxylated alpha-quartz surfaces contribute the least among the mineral surfaces studied. This simulation work provides insights to better understand the moisture damage mechanisms of asphalt mixtures at a microscopic level.