Influence of SBS molecular structure on interfacial adhesion with steel slag and the performance of asphalt mixtures

The present study investigated the influence of SBS polymer molecular structure (L-SBS, B-SBS, HV-SBS, and DB-SBS) on binder–aggregate adhesion and asphalt mixture performance using two aggregate types: steel slag (SS) and natural aggregates (NA). The adhesion characteristics of the different binder–aggregate combinations were evaluated using Surface Free Energy (SFE) analysis, Binder Bond Strength (Pull-Off Tensile Strength, POTS) testing, and Atomic Force Microscopy (AFM). Among all the combinations, the B-SBS–steel slag system exhibited the highest adhesion, demonstrating superior interfacial bonding between the binder and aggregate. The asphalt mixture performance was consistent with the adhesion study, with mixtures prepared using the B-SBS-modified binder outperforming those containing the other SBS polymers. Compared with the conventional L-SBS polymer, the B-SBS copolymer provided significantly enhanced engineering performance across the evaluated parameters. Furthermore, irrespective of the binder type, asphalt mixtures incorporating steel slag aggregates consistently exhibited superior performance compared to those prepared with natural aggregates, owing to the higher hardness, rough surface texture, and improved mechanical interlocking of steel slag. The enhanced performance of the B-SBS-modified mixtures can be attributed to the combined effects of its higher molecular weight, triblock architecture, and long-chain branched molecular structure, which collectively improve elasticity, cohesive strength, and binder–aggregate adhesion. These characteristics make B-SBS particularly suitable for pavement applications subjected to high traffic loads and elevated to intermediate service temperatures. In contrast, the presence of a higher proportion of diblock segments and a lower molecular weight in the DB-SBS polymer resulted in comparatively inferior adhesion and mixture performance. Overall, the findings clearly demonstrate that the molecular architecture of SBS polymers plays a decisive role in governing binder–aggregate interactions and, consequently, the mechanical performance and durability of asphalt mixtures.