Upcycling of waste plastics, end-of-life tire rubber, and bio-derived oils for circular asphalt pavements: A Systematic review

The asphalt industry faces growing challenges from stringent greenhouse gas (GHG) reduction mandates and diminishing high-quality virgin material reserves. This review paper positions upcycling as the engineered transformation of waste into performance-enhancing additives and positions it as a paradigm shift from traditional recycling practices that often lead to diminished long-term material performance. This systematic review screened 3654 publications and critically analyzed 170 peer-reviewed studies to map the state of knowledge, performance gains, and implementation barriers associated with upcycled additives in flexible pavements. Three dominant waste streams were examined: (i) plastics, (ii) rubber from end-of-life tire (ELT), and (iii) bio-derived oils. Among the most promising techniques identified were plastic-rubber copolymerization processes, controlled devulcanization of ELTs through microwave assistance, and strategic chemical functionalization of bio-oils through epoxidation and transesterification pathways. These innovations address persistent challenges in durability, phase compatibility, and aging while enabling higher waste incorporation rates. However, widespread implementation remains constrained by feedstock heterogeneity, processing infrastructure investments, and regulatory frameworks that are not yet aligned with circular economy principles. This work recommends three critical interventions to accelerate market adoption: (1) standardization of upcycling terminology and testing protocols within existing specification frameworks, (2) integration of circular materials science into civil engineering curricula, and (3) development of procurement policies that properly value life-cycle environmental benefits. Rather than representing an incremental improvement to recycling practices, upcycling fundamentally reconceptualizes waste materials as strategic assets for developing more sustainable and resilient pavement infrastructure.