Diffusion mechanism explorations on the sustainable warm mix asphalt and synchronous rejuvenated SBS-modified asphalt binder using the free volume theory

The blending degree, dominated by the diffusion process between virgin and rejuvenated asphalt, profoundly affects the performance of recycled mixtures. However, research on the blending degree in Warm Mix Asphalt (WMA) containing Styrene-Butadiene-Styrene-modified asphalt (SBSMA) mixtures remains relatively underexplored despite its significant impact on pavement performance. To this concern, a predictive model for characterizing the diffusion behavior of the WMA modified SBSMA and synchronous rejuvenated SBSMA was developed based on the free volume theory, which was validated by the pulsed field gradient nuclear magnetic resonance test. Based on the predicted diffusion coefficients, the influences of WMA additives, rejuvenator types, and temperatures on diffusion behavior were systematically investigated. Results indicated that the diffusion coefficients were affected by the interaction of WMA additives, rejuvenator types, and temperatures. The presence of the WMA additive and the increase in diffusion temperature were able to improve the blending degree by increasing the diffusion coefficient. Rebalancing the unstable colloidal structure of SBSMA can enhance the diffusion coefficient of the WMA modified SBSMA twice than that of untreated one. After further reconstruction of small SBS fragments into the triblock copolymers, the blending between the WMA modified SBSMA and synchronous rejuvenated SBSMA was completed in the form of mutual diffusion, showing the highest diffusion coefficients (10⁻⁷).