Sustainable Bio-Bitumen Formulation Using a Grey Relational Taguchi Approach with Waste Oils and Biochar

Abstract

Pavement binder properties play a crucial role in road durability, yet conventional bitumen relies on depleting crude oil resources. This study explores a sustainable alternative by modifying 60/70 penetration grade bitumen with biochar (BC), waste engine oil (WEO), waste cooking oil (WCO), and phthalic anhydride (PA). A hybrid optimization framework combining Taguchi-based Grey Relational Analysis (GRA) with Principal Component Analysis (PCA) was applied using an L16 orthogonal array to evaluate multiple performance responses. The optimal formulation, consisting of 5% BC, 2% WEO, 1% WCO, and 0% PA, achieved a penetration of 102 dmm, ductility of 63 cm, a softening point of 50 °C, and an elastic recovery of 100%, meeting the standard requirements. While PA individually improved certain properties such as moisture resistance, its combined effect with other modifiers reduced overall performance, likely due to antagonistic interactions or chemical incompatibility. These findings underscore the need for advanced optimization methods that can address both synergistic and antagonistic effects in multi-component systems. The proposed PCA–Grey–Taguchi approach provides a systematic pathway for designing high-performance bio-bitumen, demonstrating the potential of waste-derived modifiers to deliver cost-effective, environmentally sustainable, and technically viable pavement binders for future road infrastructure.