Role of Thermo-chemically Modified Millet Husk Silicon Nitride Ceramic and Bagasse Microfiber on Load Bearing Properties of Polyester Composite Rebars

The increasing demand for sustainable and corrosion-resistant reinforcement materials has accelerated the development of fiber-reinforced polymer (FRP) composite rebars for civil engineering applications. In the present investigation, hybrid polyester composite rebars reinforced with 3-aminopropyltrimethoxysilane (3-APTMS)-treated bagasse microfibers (40 wt. %) and annealed foxtail millet husk-derived silicon nitride (Si 3 N 4 ) nanoparticles (1.4 wt. %) were fabricated using the hand lay-up technique. The composite designations were A11 (59 wt.% polyester resin, 40 wt.% treated bagasse fiber, and 1 wt.% Si 3 N 4 ), A12 (58 wt.% polyester resin, 40 wt.% treated bagasse fiber, and 2 wt.% Si 3 N 4 ), A13 (56 wt.% polyester resin, 40 wt.% treated bagasse fiber, and 4 wt.% Si 3 N 4 ), AT21–AT23 (thermally aged counterparts of A11–A13 at 80°C for 14 days), and AW31–AW33 (seawater-aged counterparts of A11–A13 for 14 days). The fabricated rebars were subjected to thermal aging (80°C for 14 days) and seawater aging (14 days), followed by tensile, flexural, impact, Shore-D hardness, flammability, thermal conductivity, and scanning electron microscopy (SEM) analyses. Compared with the control composite (A11), the optimized composite (A13) exhibited 8.2% higher flexural strength, 11.9% higher impact strength, 16.2% higher hardness, and 45.2% higher thermal conductivity, while A12 recorded the highest tensile strength with an 8.9% improvement. Furthermore, the seawater-aged composite AW31 exhibited a 23.7% lower flammability rate than A11, demonstrating enhanced flame resistance. SEM observations confirmed improved fiber–matrix adhesion, uniform Si 3 N 4 nanoparticle dispersion, and limited crack propagation after aging. Overall, the developed hybrid polyester composite rebars exhibited excellent mechanical integrity, thermal stability, flame resistance, and environmental durability, demonstrating their potential as sustainable alternatives to conventional steel reinforcement in long-term civil engineering applications.