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.

Currently, research on the modification mechanisms of activated rubber/SBS (styrene–butadiene–styrene) composites and the microscopic processes involved remains limited. To investigate the impact of the rubber activation treatment combined with SBS modifier on asphalt modification, this study employs composite-modified asphalt formulations using either a conventional mix or activated rubber in conjunction with SBS. Infrared spectroscopy (IR) and scanning electron microscopy (SEM) were utilized to analyze the chemical components and microscopic morphology of the composite-modified asphalt following activation treatment. Microscopic analysis revealed that the asphalt stirred for 20 min has a characteristic peak with a wave number of 966 cm⁻¹, while the characteristic peak with a wave number of 700 cm⁻¹ is not obvious. That is, the asphalt sample contains the polybutadiene component and a reduced amount of the polystyrene component. Therefore, it can be inferred that the asphalt sample only contains activated rubber, along with less SBS modifier content. Traditional rubber undergoes significant expansion reactions during the mixing stage, but there are difficulties in degradation, which leave large particles and reduce the proportions of the lightweight asphalt components. However, active rubber and SBS mainly expand and degrade more completely during the shear stage, forming many micro-volume particles in asphalt. Additionally, frequency scanning and multiple creep recovery tests were conducted to evaluate the high-temperature rheological properties of the asphalt. The results indicate that activated rubber, doped at 20%, and SBS, doped at 2%, significantly enhance the high-temperature rheological properties of the composite-modified asphalt compared to base asphalt, exhibiting a 417.16% increase in the complex modulus at 64 °C and 1 Hz. Furthermore, these modifiers interact synergistically to improve modification efficiency.