Microstructural synergy and rheological improvement in asphalt binders modified with waste cooking oil and high-elastic modifiers

The increasing demand for sustainable asphalt materials has encouraged the integration of bio-based modifiers to enhance pavement performance while reducing dependency on conventional petroleum-based bitumen. This study investigates the synergistic effects of high-elastic modifiers (HEMs) including HEM-S, HEM-C, and HEM-G combined with waste cooking oil (WCO) on the rheological, microstructural, and chemical properties of asphalt binders. A 70-penetration grade base asphalt was modified using optimized WCO-HEM blends at formulations of 3%W + 5%HEM-S, 5%W + 7%HEM-C, and 7%W + 9%HEM-G by weight. Rheological evaluations demonstrated that WCO significantly improved binder workability by reducing viscosity, while HEMs enhanced elasticity, rutting resistance, and fatigue performance. The 7%W + 9%HEM-G blend exhibited a 20% reduction in non-recoverable creep compliance and a 60% increase in elastic recovery compared to the base asphalt. Microstructural investigations through Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), and fluorescence microscopy (FM) confirmed strong chemical interactions and homogeneous dispersion of modifiers within the asphalt matrix. However, excessive modifier content led to minor phase separation, highlighting the necessity for optimized dosages. Among the formulations, the 5%W + 7%HEM-C blend delivered the most favourable balance between high-temperature rutting resistance, low-temperature flexibility, and storage stability. This study is the first to comprehensively investigate the synergistic interaction between WCO and high-elastic modifiers (HEM-S, HEM-C, HEM-G) using multi-scale microstructural tools (FTIR, SEM, FM) and advanced rheological analysis (MSCR, BBR). The findings provide foundational insights into the formulation of sustainable, performance-optimized asphalt binders.