Oxidative aging mechanism of asphalt binder using experiment-derived average molecular model and ReaxFF molecular dynamics simulation

Oxidative aging in asphalt binders has significant negative impacts on asphalt pavement performance. This study investigated oxidative aging of virgin asphalt binder based on average molecular model through Reactive Force Field (ReaxFF) molecular dynamics simulation, which allows for continuous bond breakage and formation. First, experimental characterizations were used to identify the average molecular model of virgin asphalt. Subsequently, using this model, individual and bulk asphalt molecules were subjected to oxidative aging simulations through ReaxFF molecular dynamics. Moreover, the simulated results were validated by structural parameters and physical properties, including carbonyl index, sulfoxide index, density, glass transition temperature and shear viscosity. Results showed that the proposed average molecular model could characterize the structural changes and physical properties of asphalt binders well. The number and specific locations of sulfoxide groups and ketone groups in each SARA fraction were identified. Sulfoxide groups formed earlier than ketone groups. Oxidative aging reaction can be accelerated by increasing the temperature. Temperature had greater influence on the generation of ketone groups than sulfoxide groups. Initial hydroxyl groups on aromatic rings would be oxidized to ketone groups first, followed by hydroxyl groups on benzyl carbons. Not all benzyl carbons were susceptible to oxidation, and some may not be oxidized to ketone groups. Hydroxyl group was found to be the intermediate oxidation product before the formation of ketone group, and it can be used to establish the short-term aged asphalt model.