Enhancing HiMA Classification: Quantitative Approach Based on SBS Content, Polymer Structure, and Performance Metrics

Highly modified asphalts (HiMAs) are an advanced form of polymer-modified asphalts (PMAs), specifically engineered to endure high-stress pavement conditions. By incorporating increased doses of styrene butadiene styrene (SBS) polymers, HiMA demonstrates superior resistance to rutting, fatigue, and thermal degradation compared with conventional SBS-modified PMAs. Traditional characterization methods, such as fluorescence microscopy (FM), primarily focus only on phase reversal for defining HiMA, which limits their ability to fully capture the asphalt binder’s overall performance under real-world conditions. In contrast to the typically used FM for identification of polymer dose beyond which HiMA can be defined for a given base binder, this study employs conventional and rheological tests to evaluate the phase reversal mechanism in SBS-modified binders. Three SBS structures—linear (LSBS), radial (RSBS), and branched (BSBS)—were incorporated into a viscosity-graded binder, VG40, at varying doses (2%–12% by weight of VG40). To study the combined effect of SBS structure and dose, storage stability, penetration, softening point (SP), performance grading (PG), and frequency sweep tests were conducted. Additionally, the effect of short-term aging was studied using the short-term aging index (STAI) and the multiple stress creep and recovery (MSCR) test. For practical applications, high-temperature workability characteristics were examined using steady shear flow (SSF) testing. Fluorescence microscopy (FM) was used to correlate formation of a HiMA network in PMAs with the physical test properties. Based on the results and analysis, it was concluded that phase reversal in the PMAs, which leads to development of a HiMA network, occurs beyond 8% SBS content. Both dose and structure were found to be statistically significant variables influencing the mechanical properties of the PMAs under unaged and short-term aged conditions. Among the three SBS structures, LSBS was found to be the most effective in producing a thermally stable HiMA for high-demand pavement applications. Use of 8%–10% LSBS is recommended considering workability.