Diffusive Dynamics and Structural Organization of Moisture in Asphaltic Materials Based on Molecular Dynamics Simulation

Understanding the diffusion of moisture in asphaltic materials is crucial because it yields key insights into the moisture-induced damages of asphalt pavement. In this work, the molecular dynamics simulation technique was adopted to characterize the diffusion and structural properties of moisture (water molecules) in both neat asphalt binder and asphalt mastic. Microstructural changes during the diffusion process, including free volume and hydrogen bond formation, were observed to clarify the mechanism of diffusion behavior at the atomistic level. The concentration dependence of moisture diffusion coefficient was revealed. Water molecules were well dispersed in the model and mainly formed hydrogen bonds with asphalt molecules at low concentrations. However, large water clusters were formed with the predominant formation of water-water hydrogen bonds and acceleration of diffusion at high concentrations. In the asphalt mastic system, silica particles not only introduced more free volume to speed up the diffusion of moisture but also intensified the affinity between moisture and the asphalt mastic system. Moreover, the relationship between the moisture diffusion and structural properties suggested that the diffusion of moisture in asphaltic materials is controlled by both the free volume and the cohesion property among asphalt chains. This relationship could be exploited to obtain information regarding diffusion behavior in the asphaltic material system and select materials to achieve excellent antistripping properties for the asphalt mixture.