Decoupling of permeability and strength recovery in microbial-induced calcium carbonate precipitation (MICP) for concrete crack repair

• mm cracks seal fastest; deeper deposits form in wider cracks. • Clear decoupling between permeability and strength recovery after MICP treatment. • Sealing-dominated mechanism ensures impermeability; weak interface limits load transfer. • MICP is primarily a sealing technique, suitable for non-structural crack repair. Although Microbial-Induced Calcium Carbonate Precipitation (MICP) is a promising technique for concrete crack repair, the relationship between impermeability recovery and mechanical strength remains unclear. This study systematically evaluated the repair performance of MICP on concrete cracks with widths of 0.1, 0.4, 0.7, and 1.0 mm. The results show that MICP substantially delays water ingress, lowers permeability coefficients by more than 99.9%, and partly restores compressive strength. A clear decoupling was observed: the recovery of impermeability markedly outpaced that of compressive strength. To provide functional and depth-resolved evidence beyond surface images, a NaCl trace - conductivity breakthrough test was performed by sampling at the bottom crack outlets, and CaCO 3 coverage on crack walls was quantified along depth using a segmented adhesion-rate analysis. These measurements indicate that impermeability is controlled by sealing the connectivity of seepage pathways, whereas compressive strength requires reestablishing load-transfer integrity, which is constrained by the weak interfacial bond of precipitated calcium carbonate. These findings indicate that MICP functions primarily as an effective sealing approach for non-structural cracks rather than as a solution for structural reinforcement.