Diffusion Mechanism and Efficiency of Rejuvenators with Distinct Molecular Characteristics in Aged Rubber-Modified Asphalt

Rubber-modified asphalt (RMA) is widely used in high-grade pavements for its superior performance and environmental benefits. However, in-service aging inevitably generates substantial quantities of reclaimed RMA requiring recycling. Unlike base asphalt, the presence of rubber particles in RMA introduces complex composition and diverse aging mechanisms, imposing significant challenges on rejuvenation. Specifically, the diffusion efficiency of rejuvenators within the aged rubber–asphalt network is constrained, yet the governing mechanisms and selection criteria for effective rejuvenators remain unclear. To address these gaps, four rejuvenators with distinct molecular characteristics—‍A‍ (saturate oil), B (aromatic oil), C (glycerol), and D (oleic acid)—were evaluated using short-term and long-term aged RMA (S-RMA and L-RMA). A novel diffusion evaluation framework was established, utilizing interlayer rheological gradients and viscosity equilibrium time as quantitative indicators. Furthermore, molecular dynamics (MD) simulations were used to analyze relative concentration distributions, diffusion coefficients, and interfacial energies. The results demonstrate that the proposed macroscopic and microscopic methods yield consistent evaluations. Diffusion is consistently more efficient in S-RMA than in L-RMA, as long-term aging forms a dense cross-linked network that significantly reduces free volume. Among the rejuvenators, diffusion efficiency follows the order A > B > D > C ; notably, however, Rejuvenator-D yields superior performance recovery. Further investigation reveals that diffusion is primarily governed by molecular polarity, steric configuration, and interfacial compatibility. The cross-linked rubber network creates significant diffusion resistance, whereas rejuvenators with flexible linear structures ensure effective permeation, and those with high interfacial compatibility are critical for superior performance restoration. This study provides critical mechanistic insights and a scientific basis for optimizing rejuvenator selection and design for aged RMA.