Diffusion Simulation of a Multi-Component System with RAP Binder, Virgin Binder, and Rejuvenator

The use of high-content Reclaimed Asphalt Pavement (RAP) mixes is on the rise in the pavement industry due to the significant economic and environmental benefits it offers. However, high-content RAP mixes typically require the use of a rejuvenator in order to combat the issues associated with increased stiffness and brittleness of the aged materials. Rejuvenators attempt to restore properties of the aged asphalt binder to a level that closely resembles that of virgin binder by diffusing into and mobilizing aged binder, therefore, facilitating the mixing and blending of aged and virgin binders. This three-component diffusion process is not well documented in the literature. Consequently, it is crucial to understand this phenomenon and the factors affecting it. To this end, the diffusion process was simulated using Finite Element Method (FEM) with MATLAB and COMSOL software at two different temperatures, 110 and 135 °C, estimating the time taken for each component to reach equilibrium. Parameters for FEM simulations were determined using both experimental testing and values from the existing literature. The simulations considered two cases, constant and variable diffusivity. The variable diffusivity scenario was found to be more realistic, and the results indicate that increasing temperature yields a faster diffusion and, therefore, a shorter time for all three species to reach equilibrium.