Synergistic biochar-rejuvenator design to mitigate the diffusion–mechanics trade-off in aged asphalt: molecular dynamics insights for pavement recycling

Asphalt recycling performance is frequently constrained by a diffusion–mechanics trade-off: rejuvenators must penetrate the aged binder rapidly, yet excessive softening undermines stiffness and crack resistance. Here, molecular dynamics simulations are used to design a synergistic biochar–rejuvenator strategy that couples aromatic diffusion, microstructural reorganisation, and mechanical/damage responses. Results show that moderate biochar dosage (∼ 5.0 wt % relative to aged asphalt) maximises aromatic penetration and diffusivity, whereas excessive biochar induces aggregation that constricts transport pathways. Microstructurally, biochar promotes polar-fraction redistribution, restores free volume toward the virgin level, and rebalances heavy-fraction associations, providing a mechanistic basis for enhanced transport. Mechanically, biochar acts as a reinforcing skeleton that delays nanovoid coalescence and damage localisation, stabilises micro-crack closure, and accelerates energetic relaxation during healing via strengthened non-bonded interactions. These findings establish an integrated diffusion-structure-mechanics framework and identify a practical dosage window for high-performance, sustainable pavement recycling.