From Palm-Industry Residues to Climate-Resilient Roads: Bio-Asphalt Chemistry, Tropical Pavement Distress, RAP Circularity and Engineering Readiness-A Critical Review

Palm-derived resources are increasingly examined as additives for road asphalt, but the designation “bio-asphalt” masks major differences among vegetable oils, fatty derivatives, ash, fibres, clinkers and biochars. This critical qualitative review synthesises recent pavement research through a function-specific framework connecting feedstock processing, binder chemistry, rheology, mixture performance and service conditions in tropical climates. The discussion distinguishes direct oil-palm evidence from analogues involving other waste oils and biomass-derived modifiers. Palm-derived liquids may restore binder mobility or improve workability, but excessive softening threatens rutting resistance under persistent high temperatures. Palm ash, clinker, fibre and biochar can instead modify mastic structure, aggregate skeleton, drainage stability or reinforcement, while introducing risks involving particle variability, stiffness, adsorption and wet adhesion. The review further examines high reclaimed-asphalt-pavement mixtures, ageing, coupled hot-wet traffic exposure, life-cycle assessment and life-cycle cost. Three conceptual Figures, a provisional material-selection and readiness matrix, and a staged qualification protocol are developed. Existing laboratory studies justify selective trials but do not establish generalized field durability or engineering-standard equivalence. Tropical deployment should therefore prioritize residual feedstocks, chemically and physically reproducible processing, multi-distress validation and monitored pilot sections over simple bio-content targets.