Storage Stability, Rheological Performance, and Activation Mechanism of Rapeseed Heavy Oil–Microwave Composite-Activated Crumb-Rubber-Modified Asphalt

Conventional crumb-rubber-modified asphalt (CRMA) often shows high viscosity, storage separation, and limited low-temperature relaxation, whereas existing engineered rubber or single-activation methods do not fully clarify the combined contribution of biomass–oil swelling and microwave treatment. This study develops a rapeseed heavy oil (RHO)–microwave composite activation route for CRMA. Microwave activation, RHO pre-swelling, and their composite treatment were compared by varying rubber size, microwave intensity, and oil-to-rubber ratio. Binder workability, storage stability, DSR/MSCR/BBR rheology, FTIR, SEM, fluorescence microscopy, TGA, and AC-13C mixture performance were evaluated. Microwave activation mainly reduced viscosity and improved rubber dispersion, whereas RHO pre-swelling improved ductility and storage stability. The optimal F84 binder (80-mesh rubber, RHO-to-rubber ratio 1:2, 1.2 kJ/g microwave) reduced 180 °C viscosity and top–bottom softening-point difference by 42.95% and 55.68%, respectively, and increased 10 °C ductility from 10.5 to 19.5 cm relative to inactivated CRMA. Although F84 weakened creep recovery compared with inactivated CRMA, it improved low-temperature relaxation and mixture failure strain (3527.8 µε). The composite route is therefore suitable for CRMA applications prioritizing workability, storage stability, low-temperature cracking resistance, and rubber valorization, while rutting-critical projects require mixture-level verification.