Moisture damage mechanism of asphalt mixtures containing reclaimed asphalt pavement binder: A novel molecular dynamics study

Reclaimed asphalt pavement (RAP) stands at the forefront of sustainable practices in road construction and rehabilitation. However, concerns regarding the moisture susceptibility of aged RAP binder have arisen due to its potential hazard to pavement durability. So far, the underlying mechanisms of how moisture infiltrates into the mixture and results in binder stripping remain controversial and scarcely investigated. To address this challenge, a novel molecular dynamics (MD) simulation method has been developed to explore the nanoscale stripping mechanisms, considering the effects of asphalt oxidative aging and rejuvenation. The results indicate that hydrogen bonding interactions between water and the aggregate surface reduce the contact distance compared to the distance between asphalt and aggregate. Under pore water pressure, moisture diffuses into the interface, occupying the van der Waals adhesion sites of asphalt molecules and consequently causing detachment. Asphalt oxidative aging, on one hand, increases the oxygen concentration and hydrophilicity, thereby increasing moisture susceptibility from the energy perspective. On the other hand, it increases the intermolecular friction between aged asphalt molecules, leading to reduced flexibility and age hardening, consequently triggering asphalt premature detachment and diminishing its coating quality on the aggregate surface. Rejuvenators can restore the moisture sensitivity of severely aged asphalt, with a mechanism involving two aspects: firstly, reducing the agglomeration and proportion of polar components in asphalt, thereby weakening its hydrophilicity; secondly, lubricating aged asphalt molecules to restore their flexibility and surface coating properties. Findings from this study contribute to revealing the moisture-induced damage mechanism when recycling RAP binder.