Research on the Rutting Resistance of Asphalt Improved by Nitrogen-Rich Soybean Biochar

Asphalt pavements suffer from progressive deterioration during service life due to aging, while conventional polymer modifiers raise environmental concerns regarding recyclability and volatile organic compound emissions. Functionalized-biochar derived from renewable biomass offers a sustainable alternative through its tunable surface chemistry and porous structure. This study develops and evaluates a nitrogen-rich, surface-functionalized biochar as a multifunctional asphalt binder modifier and elucidates the synergistic roles of inherent nitrogen and post-synthetic functionalization in governing binder performance. To this end, soybean powder was pyrolyzed to prepare the biochar precursor, which was subsequently characterized to determine its suitable pyrolysis temperature and surface properties. These analyses identified 300 °C as the preferable pyrolysis temperature within the tested range of 200–500 °C, maximizing biochar yield and achieving favorable surface physicochemical properties including the iodine adsorption value, oil absorption value, surface functional groups and pore morphology. To improve surface functionality, the biochar underwent a two-step modification: nitric acid oxidation followed by hydroxymethylation, hereinafter referred to as functionalized-biochar. The influence of functionalized-biochar content on asphalt binder performance was evaluated across dosages of 10–20 wt%, and 15 wt% was identified as the recommended content. Asphalt binder modified with 15 wt% functionalized-biochar exhibited improved high-temperature performance, as evidenced by enhanced rutting resistance factor G*/sin δ values obtained from dynamic shear rheometer testing. Aging resistance also improved, reflected in a lower complex modulus aging index and a higher phase angle aging index. At 15 wt% dosage, the softening point increased by 6 °C, and ductility rose by 10.6 cm relative to the base asphalt (1.9 cm), corresponding to a 557.9% improvement. Penetration declined by 17%, while the complex modulus aging index decreased from 3.44 to 1.68. Notably, the experimental program in this study was limited to binder-scale characterization.