Effect and mechanism analysis of usual temperature pitch (a warm-mix asphalt additive) modifiers on asphalt...

To investigate the effect of usual-temperature pitch (USP), a warm-mix asphalt additive, on the microstructural characteristics and modification mechanism of base asphalt and SBS-modified asphalt, five USP dosages were melt-blended with each base binder to prepare USP-modified base asphalt and USP/SBS composite-modified asphalt. The high- and low-temperature rheological properties of the binders were evaluated using temperature sweep tests and bending beam rheometer (BBR) tests. The thermal characteristics of the USP modifier were analyzed by differential scanning calorimetry (DSC), while the phase morphology and functional group evolution of the modified asphalts were characterized using fluorescence microscopy (FM) and Fourier transform infrared spectroscopy (FTIR). The results show that the USP modifier exhibits an endothermic peak at approximately 90 °C, corresponding to a solid-liquid phase transition, which allows it to remain in a liquid state at conventional mixing temperatures. Fluorescence microscopy indicated that the USP modifier was relatively uniformly dispersed in both base asphalt and SBS-modified asphalt, suggesting acceptable compatibility with the asphalt matrices. Rheological tests showed that USP reduced the complex modulus and rutting factor of both asphalts, indicating a reduction in high-temperature deformation resistance. In contrast, USP decreased creep stiffness and increased the creep rate, thereby improving low-temperature flexibility and stress relaxation capacity. FTIR analysis showed no new characteristic peaks after USP addition, indicating that USP was mainly physically blended with the asphalt binders rather than chemically reacted. Overall, USP influences the macroscopic rheological behavior of asphalt binders by changing the internal phase morphology and component proportions through physical blending.