Interaction behaviours between recycled rubber and warm mix asphalt: insight from experiment and molecular dynamics simulation

Warm rubberised bitumen (WRB) is a promising sustainable bituminous material for pavement engineering in the United States and Europe. However, the molecular-level interactions between crumb rubber (CR) and warm mix asphalt (WMA) remain unclear. This study investigates these interactions using Cole–Cole diagrams, fluorescence microscopy (FM), and molecular dynamics (MD) simulations. Results show that shorter-chain wax additives improve the dispersion of aggregated untreated CR in bitumen, thereby enhancing the uniformity of WRB, while low-temperature conditions are beneficial for WRB storage. Butadiene rubber (BR) exhibits better miscibility with WMA containing both short- and long-chain waxes than natural rubber (NR) and styrene-butadiene rubber (SBR), whereas SBR shows the poorest miscibility. Interaction energies between additives and bitumen are highest for NR and lowest for BR in virgin bitumen (VB) containing short-chain waxes, while BR presents the strongest interaction with VB containing long-chain waxes. Diffusion analysis indicates that long-term aging has limited effects on NR and BR, whereas reduced SBR diffusion results from lower structural stability caused by higher free volume fraction. CR aggregation increases with temperature, with SBR dominating before aging and NR after long-term aging. These findings provide guidance for component and performance optimisation of WRB.