Laboratory evaluation of hot mix asphalt containing biochar as partial aggregate substitution

Abstract Biochar is a promising material for mitigating greenhouse gas emissions and is increasingly being incorporated into composite construction materials, such as concrete and asphalt. However, adding sufficient amounts of biochar to asphalt requires substituting mineral aggregates, which significantly alters its properties. Hence, this study explores the viability of partially substituting mineral aggregates with biochar in hot-mix asphalt, designed for use as a base course. Four substitution percentages (0, 2, 4, and 8% by mass of the mineral aggregates) were investigated for their volumetric, mechanical, and hygroscopic behaviour. The physical and mechanical properties exhibited non-monotone evolution with biochar content, as biochar significantly affected both the air void content and the optimum binder content. A biochar content of 2% by mass of the aggregates improved the mechanical performance, as evidenced by a 17% increase in fracture toughness, an average 20% increase in stiffness and a 70% reduction in permanent deformations. The improved performance was attributed to reduced air void content, resulting from potential biochar-proportion-dependent gradation changes in the mixtures. Beyond 4% by mass of aggregates, mechanical performance declined significantly, which can be attributed to both the increasing air void content with increasing biochar content and the lower mechanical strength of biochar compared to mineral aggregates. These findings indicate that biochar can replace up to 4% of the aggregates by mass to achieve greenhouse gas emission goals while maintaining technical feasibility.