Fabrication and evaluation of polyester composite rebars reinforced with watermelon rind biochar and nutmeg fibres

This study proposes a sustainable polymer composite rebar reinforced with nutmeg fibre and biochar derived from unused watermelon rinds, aimed at green building applications. Polyester resin was used as the matrix material, combined with cobalt naphthenate accelerator and methyl-ethyl-ketone peroxide hardener to fabricate the composite. Nutmeg fibre of 150 µm diameter and 40 mm length served as reinforcement, while biochar was prepared from watermelon rinds via a multi-stage pyrolysis process at temperatures up to 800 °C, ensuring optimal quality and particle size of 1–3 µm. The rebar composite was developed using a two-part mold with a screw mechanism and polyvinyl alcohol (PVA) as a release agent to ensure precision and prevent leakage during curing. Among the specimens tested, R3 with 2 vol.%biochar demonstrated the best mechanical properties, including the highest tensile strength of 60.9 MPa, flexural strength of 103.95 MPa, and compression strength of 78.75 MPa, indicating optimal fibre-matrix bonding and effective load distribution. Conversely, R5 with 8 vol.%biochar exhibited superior characteristics in terms of density (0.9 ± 0.01 kg/m³), porosity (0.89 ± 0.004%), thermal conductivity (0.63 W/mK), and water absorption (0.003%), attributed to its lightweight, porous structure, efficient heat transfer network, and hydrophobic nature. SEM analysis revealed critical microstructural details, such as fibre breakage, fibre pull-out, cracks, and agglomerated fillers, emphasising the importance of optimizing fibre-matrix adhesion and filler dispersion for enhanced composite performance. This study showcases an innovative pathway for developing high-performance, eco-friendly rebar for sustainable construction.