Microstructural and mechanical performance of coconut shell biochar – Modified asphalt for sustainable pavement applications

This study investigates coconut shell biochar (CSB) as a sustainable, bio-based modifier for enhancing the mechanical performance and durability of hot-mix asphalt (HMA). CSB was produced by controlled laboratory pyrolysis, and its chemical and microstructural characteristics were analysed before incorporation into 40/50 penetration-grade asphalt at different dosages. Binder evaluations included penetration, softening point, ductility, and elastic recovery, while mixture performance was assessed using Marshall Stability, flow, volumetric properties, indirect tensile strength (ITS), and tensile strength ratio (TSR). Results indicated a decrease in penetration and an increase in the softening point, indicating a stiffer, more thermally stable binder. Marshall Stability reached 9.2 kN at 12.5 % CSB with a flow of 2.6 mm, while bulk density slightly decreased and air voids increased but remained within 3–5 %, ensuring adequate compaction. VMA and VFB were balanced at 12.5–15 % CSB, and tensile strength and moisture resistance improved up to 15 %, with ITS and TSR values declining at higher dosages. Response Surface Methodology (RSM) confirmed 12.5 % CSB as the optimal dosage, with quadratic models showing high predictive accuracy (R² > 0.95). Microstructural analyses (SEM, EDX, FTIR, XRD), pore structure analyses, and chemical investigations validated these findings, highlighting CSB's potential as an effective asphalt modifier, particularly under India's varied climatic conditions. The improved stiffness and moisture sensitivity performance also suggest practical benefits in terms of durability and reduced binder demand, leading to material savings. Overall findings emphasise CSB’s potential as a sustainable, cost-effective bio-based modifier capable of extending pavement service life under high traffic volumes and diverse climatic conditions.