Rheological Rebalancing of Guayule Resin–Crumb Rubber Modified Asphalt Binder Using Waste-Derived Used Motor Oil: A Binder-Level Feasibility Study

This study evaluates waste-derived used motor oil (UMO) as a softening and viscoelastic rebalancing agent for a sustainable asphalt binder incorporating guayule resin and crumb rubber modifier (CRM). ARG75(20):25 contained 75% asphalt rubber (20% CRM by asphalt weight) and 25% guayule resin, replacing 37.5% of asphalt (PG 64-22). CRM reinforced high-temperature performance, whereas the guayule-containing ARG system exhibited intermediate- and low-temperature stiffness, preventing PG 64-22 compliance. UMO at 1%, 3%, and 5% by total binder weight was evaluated using rotational viscosity, dynamic shear rheometer, bending beam rheometer, multiple stress creep and recovery (MSCR), black space analysis, and Fourier transform infrared (FTIR) spectroscopy under original, RTFO, and PAV aging. UMO reduced stiffness while maintaining Superpave rutting thresholds. MSCR showed lower Jnr,3.2 at 1% and 3% UMO, and higher R3.2 across dosages than base asphalt, despite increased stress sensitivity. Viscosity decreased, indicating improved flow. Continuous grades shifted from PG 73-16 to PG 70-19, PG 66-21.6, and PG 65-23 at 1%, 3%, and 5% UMO, respectively. FTIR showed attenuated PAV carbonyl development with 5% UMO; sulfoxide development remained comparable. The 3% dosage showed the best balance, whereas 5% UMO alone met standard PG 64-22 criteria, establishing binder-level feasibility for mixture- and pavement-level validation.