Development of Debonding-Resistant SBS–Silane Warm Mix Asphalt for Hot and Humid Pavement Conditions with Poor Aggregate Gradation

Asphalt pavements in hot and humid regions such as Southeast Asia are highly susceptible to moisture-induced debonding, especially when WMA is produced using marginal aggregates or less favorable gradation conditions. This study develops an anti-stripping-focused polymer-modified WMA system using SBS and a silane-based liquid additive. This study focuses on evaluating the coupled contribution of SBS-related binder cohesion and silane-related interfacial adhesion under poor gradation conditions, and verifies the selected system through binder-level, mixture-level, durability, and cost-efficiency evaluations. SBS contents of 4.0%, 4.5%, and 5.0% by binder mass were combined with silane dosages of 0%, 0.05%, 0.10%, and 0.15%. The mixtures were evaluated using MSCR, Marshall stability and flow, dry and wet ITS, TSR, Hamburg Wheel Tracking, SCB, and Overlay Test. SBS alone increased dry ITS and Marshall stability, but silane-free mixtures still showed low TSR values of 71.7–73.3%. The optimum mixture, S4.5-Si0.10, achieved a dry ITS of 0.94 MPa, wet ITS of 0.80 MPa, TSR of 85.1%, and Marshall stability of 13.8 kN. MSCR results confirmed that SBS reduced accumulated strain at both 0.1 and 3.2 kPa, while silane did not adversely affect binder deformation resistance. In Stage 2, the optimized SBS–silane mixture under poor gradation reduced Hamburg final settlement by 54.7% compared with the poor-gradation control. SCB work of fracture increased from 1.34 J to 5.20 J, and Overlay Test results confirmed improved load retention. The optimized mixture also reduced the annualized cost index by 27.2%. These findings demonstrate that a balanced SBS–silane WMA system can improve debonding resistance and durability under hot and humid pavement conditions.