Synergistic Effects of Waste Tire Rubber and Styrene–Butadiene–Styrene on the Viscoelastic Rheology and Fatigue Mechanisms of Asphalt Binders

Conventional rubber-modified asphalt faces critical limitations, including phase separation and compromised low-temperature flexibility, which typically restrict rubber content to 15–25%. This study investigates the viscoelastic rheology and fatigue life prediction of asphalt binders modified with 20% waste tire rubber, utilizing a chemical–thermal desulfurization process and a synergistic styrene–butadiene–styrene (SBS) composite. Five binders were evaluated: base asphalt (BA), SBS-P, WTR, WTD, and WTC. The WTC exhibited the highest softening point at 89 °C and a ductility of 35 cm, successfully overcoming the traditional stiffness–flexibility trade-off. Dynamic shear rheometer (DSR) tests revealed a 440% increase in complex shear modulus (G*) at 58 °C 10.8 kPa vs. 2.0 kPa for BA and a 13.2% reduction in phase angle (61.2°). Multiple stress creep recovery (MSCR) results showed WTC achieved a 60% recovery rate at 0.1 kPa and maintained 25% at 3.2 kPa, with non-recoverable creep compliance (Jnr3.2) well below the 1.0 kPa−1 threshold for heavy traffic. Bending beam rheometer (BBR) tests confirmed WTC low-temperature suitability, achieving a creep stiffness of 280 MPa and an m-value of 0.58 at −24 °C. Furthermore, viscoelastic continuum damage (VECD) modeling demonstrated that WTC increased yield stress by 80.8% (470 kPa) and extended fatigue life by over two orders of magnitude at low strain (2%) compared to BA. Finally, WTC exhibited excellent storage stability, with a softening point difference (SPD) below 2.2 °C. These findings confirm that the rubber/SBS composite provides a highly durable, storage-stable, and fatigue-resistant binder suitable for long-life asphalt pavements under extreme climatic and heavy-traffic conditions.