Performance Evaluation of In Situ Prepolymerized TPU/SMC Composite-Modified Asphalt Mixtures with Aggregate Structure Optimization

Conventional thermoplastic polyurethane (TPU)-modified asphalt suffers from poor high-temperature performance, and the influence of aggregate skeleton structure on the performance of TPU-based modified asphalt mixtures remains insufficiently understood. TPU-modified asphalt and SMC-grafted TPU composite-modified asphalt were prepared via in situ prepolymerization, using base asphalt (BA) and SBS-modified asphalt as control binders. Four types of AC-16 asphalt mixtures were designed and systematically evaluated in terms of volumetric properties, low-temperature mechanical behavior, pavement performance, and aging resistance. The coarse aggregate void filling (CAVF) gradation was introduced to compare performance differences among mixtures with identical binders but distinct aggregate skeleton structures. The results demonstrated that TPU/SMC composite modification enhanced the high-temperature performance of asphalt mixtures while maintaining satisfactory low-temperature cracking resistance. The dynamic stability of the composite-modified mixture was over 60% higher than that of the base mixture, while its maximum flexural tensile strain at low temperature exceeded that of the conventional SBS-modified system. After aging, the freeze–thaw splitting strength retention rate of the composite-modified mixture reached 98.5%, and its aging resistance outperformed that of the SBS-modified mixture. CAVF gradation further improved the performance of TSMA mixtures, increasing fatigue life by 22.5% while enhancing rutting and moisture damage resistance. These findings demonstrate that CAVF-based aggregate structure optimization can further improve the pavement performance of TSMA mixtures and provide an experimental basis for material selection and gradation design of steel bridge decks and cold-region asphalt pavements.