Tri-Functional Self-Healing and High-Performance Concrete Incorporating Metakaolin, Bacillus subtilis , and Polypropylene Fibers

This study introduces a high-performance concrete blend designed to address the limitations of conventional concrete, such as cracking, durability issues, and the environmental impact of cement production. The mix incorporates 10% metakaolin (by mass of cement), 3 ml·l−1 Bacillus subtilis culture for microbially induced calcite precipitation (MICP), and 0.3% polypropylene fibers (by volume). Nine mix designs, including control and hybrid-modifier variants, were evaluated for workability, mechanical properties, and durability. Fresh-state workability showed slump values between 50–75 mm, indicating good flow. At 28 days, the hybrid mixes exhibited a 16% improvement in compressive strength, a 21% increase in split tensile strength, and a 36.6% boost in flexural strength compared to the control. Durability tests, including ultrasonic pulse velocity (UPV) and rapid chloride penetration tests (RCPT), confirmed enhanced resistance to ionic ingress. Microstructural analysis using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Energy-Dispersive X-ray Spectroscopy (EDX) revealed a densified microstructure with effective fiber-matrix bonding and in-situ calcite Nano-crystallization. Cyclic UPV monitoring validated the self-healing ability, demonstrated through crack closure by microbial calcite precipitation. This hybrid concrete offers a sustainable solution for durable and eco-friendly infrastructure.