Performance Evaluation of MICP in Crack Repair: Strength and Durability Enhancement Using Different Bacterial Strains

Concrete is fundamentally susceptible to cracking, which creates pathways for aggressive agents, accelerating reinforcement corrosion and reducing service life. Traditional repair methods are ineffective for micro-cracks and generate volatile compounds. Microbially induced calcium carbonate precipitation (MICP) offers a sustainable bio-based alternative that catalyses in situ calcium carbonate (CaCO3) precipitation within cracks, sealing pathways. This study compares two MICP repair systems consisting of Sporosarcina pasteurii (SP, ureolytic) and Bacillus mucilaginosus (BM, non-ureolytic), utilising a dual-viscosity-modifying-agent (VMA) system comprising Welan Gum and Attagel 50 (WA). The repair treatments were applied externally on cracked concrete specimens across crack widths of 0.10–0.80 mm for 16 days. Crack repair effectiveness was evaluated through splitting tensile strength, capillary water absorption, rapid chloride migration (RCM), and X-ray diffraction (XRD). SP + WA exhibited high performance trends compared with BM + WA, achieving tensile strength retention of 59.57–68.95% vs. 56.68–67.15%, capillary water absorption recovery ranges of 58.36–64.81% vs. 51.71–58.02%, chloride resistance recoveries of 63.7–82.1% vs. 50.5–60.0%, and DRCM recoveries of 57.35–77.68% vs. 46.04–61.00% relative to intact controls. Durability recovery efficiency decreased with increasing crack width. XRD confirmed calcite as the sole CaCO3 polymorph in both systems, with sharper peaks in SP + WA indicating differences in calcite crystal characteristics, while BM + WA produced broader peaks similar to a nanocrystalline CaCO3 structure. These findings demonstrate that the dual-VMA-assisted MICP approach potentially improves mechanical and durability properties in cracked concrete, with ureolytic SP demonstrating better repair performance trends.