Interfacial interactions between crumb rubber-polyphosphoric acid composite-modified asphalt and aggregates

Waste crumb rubber and polyphosphoric acid (PPA) were incorporated into asphalt mastic through a staged shearing process under controlled temperatures. Their combined effects were examined using a cross-scale framework that linked surface free-energy measurements, atomic force microscopy, rheological sweeps, and pull-off tests. The combined use of crumb rubber and PPA shifted the asphalt colloidal system from a sol-like state toward a more interconnected gel structure. This transition increased surface roughness, promoted microscale mechanical interlocking, and restrained interfacial slippage. The anti-stripping agent further improved interfacial wetting and polar compatibility, while the fibers supported the mastic skeleton and maintained continuous stress transfer within the material. CRA-PPA-2, containing crumb rubber, PPA, an anti-stripping agent, and lignin fibers, achieved dry pull-off tensile strength (POTS) values of 2.76 and 2.86 MPa and wet POTS values of 2.29 and 2.43 MPa for basalt and diabase, respectively. The corresponding pull-off strength retention ratios (TSR) reached 83.0% and 85.0%, indicating improved resistance to moisture-induced bond deterioration. This combined modification approach effectively alleviated water-induced adhesion loss and interfacial debonding, offering a practical strategy for improving the interfacial durability of asphalt pavements.