Engineering-oriented formulation design and synergistic viscosity-enhancing mechanism of a composite modifier for high-viscosity asphalt binder

High-viscosity modified asphalt (HVMA) binder is commonly used in porous asphalt pavements and ultra-thin overlays to resist rutting, raveling, and moisture-induced interface failure. However, the design of high-viscosity modifier (HVM) is still largely empirical, and the synergistic effects of composite modifier system have not been fully clarified. In this study, a prefabricated composite modifier was developed using star-shaped/linear SBS, C9 petroleum resin, and furfural extract oil (FEO). The formulation was selected based on viscosity, rutting factor, elastic recovery, ductility, and BBS pull-off strength. FTIR, DSC, and GPC were used to reveal the viscosity-enhancing mechanism. The results showed that the preferred modifier contained at least 75% star-shaped SBS, with an SBS/C9 petroleum resin mass ratio of 2:1 and a FEO content of 20 wt%. The recommended asphalt binder showed improved high-temperature rheological performance, its rutting factor was about 1.9 times that of conventional SBS-modified asphalt binder. After 72 h of water immersion, all HVA-modified asphalt binders retained relatively high BBS strength, indicating improved resistance to moisture-induced damage. The physicochemical property results indicated that the enhanced performance was not a simple result of increasing modifier content. Instead, it originated from a balanced polymer-resin-oil synergy: star-shaped SBS formed the elastic skeleton, C9 petroleum resin strengthened tackifying and interfacial interactions, and FEO promoted SBS swelling and dispersion. The recommended system achieved higher molecular weight with a narrower molecular-weight distribution, suggesting a more coordinated multicomponent structure. These structural features contributed to the improved viscoelastic response, high-temperature stability, and interfacial durability of the HVMA binder. The prepared composite modifier provides a mechanism-guided approach for improving HVMA performance and helps to promote its broader application in porous asphalt pavements and ultra-thin overlays.