Effects of USP Warm-Mix Modifier on Rheological Properties of PG76-22 Asphalt Binder and Performance of Modified Mixture

The USP modifier is an environmentally friendly warm-mix asphalt additive that can reduce asphalt viscosity and contribute to energy savings and emission mitigation. In this study, the PG76-22 asphalt binder was used as the control material, and a USP-modified PG76-22 asphalt binder was prepared. Microscopic characterization tests, asphalt binder performance tests, and asphalt mixture performance tests were conducted to investigate the effects of the USP modifier on the PG76-22 asphalt binder and its mixtures. The FM observations showed that the USP modifier was relatively uniformly dispersed in the binder without obvious large-scale agglomeration, while the FTIR results showed no new major characteristic absorption peaks after USP modification. These results suggest that no evident chemical reaction was detected under the adopted test conditions. At the binder level, the USP modifier improved the low-temperature ductility of the PG76-22 asphalt binder but reduced its high-temperature deformation resistance, as indicated by a lower rutting factor, increased non-recoverable deformation under high stress, and enhanced stress sensitivity. The LAS results further showed that the fatigue life of the USP-modified asphalt binder was lower than that of the PG76-22 asphalt binder. At the mixture level, USP modification increased the dynamic stability, residual stability, and tensile strength ratio by 6.2%, 5%, and 3%, respectively, and resulted in longer four-point bending fatigue life at the tested strain levels. These results indicate limited improvements in the measured mixture-level performance under the present laboratory conditions. However, the reduced binder-level rutting resistance and LAS fatigue life suggest that USP modification exhibits different effects at the binder and mixture levels, and the mixture-level results should not be directly extrapolated from binder-level rheological performance alone.