Performance-Based Optimization of Soybean-Based Recycling Agents in SBS-Modified Asphalt Binders and High-RAP

Asphalt binders are complex viscoelastic polymeric materials whose rheological behavior is governed by interactions among their chemical constituents and, in polymer-modified systems, the morphology and stability of the dispersed polymer network. This study developed a performance-based framework for selecting the type and dosage of bio-based soybean recycling agents (RAs) for styrene-butadiene-styrene (SBS)-modified asphalt mixtures containing 30% reclaimed asphalt pavement (RAP). Ten dense-graded asphalt mixtures with a nominal maximum aggregate size of 12.5 mm were evaluated, including a control mixture produced with PG 76-22 (PG 67-22 asphalt binder modified with 3.5% SBS) and no RAP, and nine mixtures containing 30% RAP and three soybean-based recycling agents (RA1, RA2, and RA3) at dosages of 0.5%, 2.0%, and 4.0% by weight of binder. Binder characterization included Superpave performance grading, while mixture performance was evaluated using the Hamburg Wheel Tracking (HWT) test, freeze-thaw-conditioned HWT, Semi-Circular Bend (SCB), IDEAL-CT, IDEAL-RT, and Cantabro abrasion loss tests. Increasing recycling-agent dosage improved cracking resistance but progressively reduced rutting resistance, consistent with the measured decrease in binder viscosity. The magnitude of the softening effect followed the order RA3 > RA2 > RA1, consistent with measured viscosity reductions and HWT rut-depth response. Integrating SCB fracture resistance with the HWT rutting criterion identified acceptable dosage ranges of 1.4-4.0%, 0.5-2.8%, and 0.5-0.7% for RA1, RA2, and RA3, respectively. These results demonstrate that the effectiveness of the evaluated recycling agents, as reflected by binder rheology and asphalt mixture performance, influences the balance between fracture resistance and permanent deformation, providing a performance-based framework for optimizing bio-based recycling-agent selection in high-RAP polymer-modified asphalt mixtures.