Asphalt binders that perform exceptionally well in resisting both rutting and cracking are highly desirable for withstanding the combined effects of extreme low temperatures and heavy vehicle loads. This work highlights the benefits of softening oils on the cracking performance of styrene–butadiene–styrene-modified asphalt (SBSMA). Additionally, the inherent correlations between cracking-performance parameters of binders and mixtures were thoroughly analyzed. A bio-based oil (bio-oil) and a petroleum-based oil (re-refined engine oil bottom, REOB) were selected as the softening oils. The benefit provided by softening oils was evaluated using various rheological indices, while the adverse effects of oxidative aging on cracking resistance were also considered. The cracking properties at intermediate temperatures were characterized by the modified Glover–Rowe (M G–R) parameter, δ_{8967 kPa}, and fatigue life (N_f). The low-temperature cracking properties of binders were evaluated by stiffness and m-value. The indirect tensile asphalt cracking (IDEAL-CT) test was conducted utilizing the CT-index and post-peak slope to estimate the fracture properties of the mixtures. The oxidative aging of binder and mixture samples was simulated and carried out based on lab aging methods; meanwhile, the carbonyl index obtained from the Fourier transform infrared (FTIR) scanning was used to track and evaluate the aging level of binders. The results show that the cracking performance could be greatly improved by the application of softening oils. Meanwhile, the bio-oils were found to operate with much higher efficiency than REOB, since the oil modification index (OMI) result showed that bio-oils exhibited four to six times the efficiency of REOB, in terms of improving the stress relaxation property. The correlations proved that the cracking-related parameters shared an inherent relationship with R² above 0.85, while these parameters consistently declined as the binder aged. The cracking performance of the mixtures at intermediate temperatures was mainly governed by the fatigue life of the binder, whereas thermal cracking performance was highly associated with the binder’s relaxation property.

Waste plastics have attracted widespread attention in asphalt modification because of their environmental and economic benefits. However, the incorporation of waste plastics may weaken asphalt–aggregate interfacial adhesion, thereby increasing the risk of moisture damage in asphalt pavements. Although liquid anti-stripping agents have been widely used in conventional asphalt systems, their effectiveness and performance evolution in waste plastic-modified asphalt (WPA) remain insufficiently understood. To address this gap, this study systematically investigated the effects of two liquid anti-stripping agents, AJ-1 and AMR-II, on the adhesion, rheological properties, and aging behavior of WPA. Specifically, asphalt–aggregate adhesion was evaluated using water-boiling and binder bond strength tests, rheological properties were characterized by dynamic shear rheometer and bending beam rheometer tests, and aging behavior was analyzed through rolling thin-film oven test, pressurized aging vessel, and Fourier transform infrared spectroscopy. The results show that waste plastics reduce the adhesion performance at the asphalt-aggregate interface, whereas anti-stripping agents compensate for this loss. Compared with AJ-1, AMR-II showed stronger adhesion enhancement, increasing the asphalt residual coating ratio by approximately 1.5%–3.5% and the pull-off tensile strength by 17.4%–28.1%, while the corresponding improvements for AJ-1 were approximately 1.3%–2.7% and 13.0%–25.0%, respectively. As the dosage of both anti-stripping agents increased, the penetration index decreased, the temperature susceptibility increased, the softening point generally decreased, and the ductility increased markedly. Temperature sweep results show that both AJ-1 and AMR-II reduce the high-temperature performance of WPA. According to the bending beam rheometer results, AMR-II also enhances the low-temperature performance of WPA. Aging test results indicate that both anti-stripping agents increase the aging sensitivity of WPA to some extent, but the adverse effect of AMR-II on aging resistance is smaller than that of AJ-1, and AMR-II better preserves the low-temperature ductility and adhesion performance after aging. Overall, this study provides a binder scale evaluation showing that 0.4% AMR-II may offer a more balanced strategy for improving the adhesion and service performance of WPA.