The significant growth in road infrastructure worldwide over the last decade has resulted in a notable increase in the demand for asphalt binder. However, the utilization of asphalt binder in the road industry poses challenges to environmental sustainability and economic standpoints. The application of vehicular loads and exposure to environmental factors throughout the service life of roads contribute to the deterioration of binder properties, such as hardening and aging, ultimately leading to premature road failure. Therefore, researchers have strived to explore further alternative materials to overcome these challenges, to improve the performance of flexible pavements. Waste Engine Oil (WEO) is one such material that has shown promising effects on asphalt binder. This review aims to conduct an in-depth analysis of previous literature to explore the potential utilization of WEO as a modifier and rejuvenator for asphalt binders. WEO effectively rejuvenates aged asphalt binders, however, the required quantity for rejuvenation varies depending on asphalt characteristics. It was found that the inclusion of the WEO as asphalt modifier significantly affects the high-temperature properties of the WEO-modified asphalt binder. Conversely, WEO addition enhances lower temperature properties, improving thermal and fatigue resistance. Furthermore, the compromise properties of WEO-modified asphalt are enhanced by incorporating various additional additives such as lignin, SBS, polyphosphoric acid and crumb rubber. It was revealed that composite modification can partially substitute 8–15 % asphalt binder, which would be a way forward in cost-effective sustainable construction in the pavement industry. However, additional research is necessary to explore futuristic advancements in WEO modification technology.

Polyphosphoric acid (PPA) is an effective chemical modifier for improving the performance characteristics of bitumen. This study investigated the influence of PPA dosage on the physical, rheological, ageing, and microstructural characteristics of 60/70 penetration grade bitumen. PPA with 115% H₃PO₄ equivalent concentration was incorporated at 0.5%, 1.0%, 1.5%, and 2.0% by weight of bitumen. The binders were evaluated through conventional physical tests, ageing indices, dynamic shear rheology, master curves, Black and Cole–Cole diagrams, field emission scanning electron microscopy (FESEM), and Fourier transform infrared spectroscopy (FTIR). Increasing PPA concentration reduced penetration from 62 to 24 dmm and increased the softening point from 46.5 to 68 °C, while viscosity at 135 °C increased from 0.38 to 1.83 Pa·s. The Penetration Index (PI) and Penetration Viscosity Number (PVN) results also indicated reduced temperature susceptibility. Rheological evaluation showed a pronounced response at approximately 1.0% PPA, where G*/sinδ increased from 1.01 to 12.35 kPa and the high temperature grade increased from Performance Grade (PG)64 to PG 82. Master curves showed a stronger PPA influence at lower reduced frequencies, while Black and Cole–Cole analyses indicated an increased relative elastic contribution. These rheological characteristics remained evident following Rolling Thin Film Over (RTFO) ageing, although the ageing response varied with PPA concentration and the evaluated property. FESEM showed homogeneous PPA incorporation without visible phase separation, while FTIR revealed progressive changes in the phosphate-related spectral region. Among the investigated dosages, 1.0% PPA provided the most balanced performance based on the physical properties, high temperature rheological response, short-term ageing behaviour, and microstructural characteristics evaluated in this study. However, further evaluation of low temperature and long-term ageing performance is required before establishing an overall optimum PPA dosage.