Self-healing evolution of asphalt binder modified with polyphosphoric acid and tire pyrolysis oil: Synergistic mechanism under aging

Despite the recognized potential of combining polyphosphoric acid (PPA) with tire pyrolysis oil (TPO) for asphalt modification, the mechanism governing the "competition vs. synergy" equilibrium under aging conditions remains opaque. This study addresses this challenge by establishing a multi-scale "molecular-rheological-macroscopic" framework to elucidate this complex balancing mechanism. Results reveal that PPA primarily governs the elastic reinforcement of the asphalt binder, while TPO effectively regulates the viscous response. Mechanistically, the incorporation of PPA significantly enhances the Cohesive Energy Density (CED) of the system, increasing it from 320.6 J/cm³ in the base asphalt to 343.0 J/cm³ in the modified version. Molecular Dynamics (MD) simulations further demonstrate that while the high diffusivity of TPO molecules rejuvenates the self-healing capacity, PPA-induced asphaltene aggregation initially impedes this process. However, this aggregation is pivotal for constructing a rigid PPA-asphaltene gel network, which endows the asphalt binder with superior long-term aging resistance. Ultimately, macroscopic mixture tests corroborate these mechanisms: the modified system achieved a 207% boost in dynamic stability and a 34% improvement in the aging index. These findings confirm the engineering viability of this composite strategy for developing sustainable, long-life pavements.