Bacterial self-healing and mechanical strength enhancement in concrete: a comparative study of Bacillus subtilis, Bacillus sphaericus, and Escherichia coli

Abstract Cracking in concrete compromises durability and service life by enabling water and aggressive agents to penetrate, leading to accelerated deterioration. This study addresses this problem by investigating the self-healing potential of microbial-induced calcium carbonate precipitation (MICP) as a sustainable approach to enhance concrete performance. The objective is to compare the mechanical, durability, and self-healing behavior of concrete incorporating Bacillus subtilis , Bacillus sphaericus , Escherichia coli , and a novel hybrid combination of Bacillus and E. coli . Five concrete mixes were prepared, each containing 5% bacterial suspension (10 5 cells/ml) and 0.5% urea by cement weight, with a control mix for comparison. Compressive, tensile, and flexural strengths were measured at 7, 28, 56, and 90 days; water absorption, permeability, and scanning electron microscopy (SEM) were used to assess durability and microstructure. Statistical analysis (ANOVA, p < 0.05) confirmed significant improvements over the control. At 90 days, the hybrid mix achieved the highest gains in compressive strength (+ 19.57%), split tensile strength (+ 28.89%), and flexural strength (+ 23.45%), while reducing water absorption by 27% and permeability by 13% compared to the best-performing single-strain mix. SEM analysis revealed extensive crack sealing with dense calcite deposition. Compared to polymer-based self-healing methods reported in literature, the bacterial systems demonstrated comparable crack closure with potentially lower environmental impact. Further research should include life-cycle and cost–benefit assessments, as well as long-term performance evaluation under environmental stressors such as freeze–thaw cycles, salinity, and temperature fluctuations.