Mechanistic insights into the circular aging-recycling evolution of bio-oil rejuvenated asphalt binders with high RAP content

With the growing demand for sustainable and low-carbon pavement materials, the multiple recycling of reclaimed asphalt binders has attracted increasing attention. However, the molecular and microstructural mechanisms of repeated aging-rejuvenation cycles and their coupling with macroscopic rheology remain insufficiently understood. This study investigated the multiscale evolution of bio-oil-rejuvenated binders containing 60% reclaimed asphalt pavement (RAP) over five aging-rejuvenation cycles. The rejuvenator and binders were characterized using gas chromatography-mass spectrometry (GC-MS), nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), atomic force microscopy (AFM), and dynamic shear rheometer (DSR) tests. The results indicate that the most pronounced recovery occurred in the first three cycles. At the molecular level, rejuvenation reduced aromatic condensation and large molecular size, while recovery weakened in later cycles. At the microstructural level, surface roughness and micro-domain stiffness decreased after rejuvenation, but microstructural restoration remained incomplete, and the fifth cycle showed smoother micro-surfaces with a weaker stiffness-softening effect. At the rheological level, rejuvenation restored the high-temperature viscosity response, while cracking-related indicators increased slightly with continued recycling. Nevertheless, fatigue performance remained improved at low strain, with fatigue life at 2.5% strain reaching up to 3.14 times that of the virgin binder. Pearson correlation analysis further supported clear links between molecular and microstructural evolution and rheological performance. Overall, bio-oil rejuvenation is feasible for multiple recycling under high RAP conditions, although its restoration is most effective in the first three cycles.