Self-healing concrete: investigating the synergistic effects of <i>Bacillus subtilis</i> and silica fume

Concrete cracking leads to structural weakness, water ingress, and reduced strength, which can result in costly repairs and maintenance. Self-healing concrete is a novel approach to address this issue by automatically repairing cracks through chemical or biological processes, enhancing the performance of concrete structures. This study explores the effect of varying concentrations of Bacillus subtilis on self-healing of concrete incorporating silica fume (SF) as cement substitution. The Bacillus subtilis was incorporated in concentrations ranging from 4.5 x 107 to 7.2 x 108 CFU/ml, with a constant 8% SF as a replacement for cement. The mechanical properties of self-healing concrete mixes were investigated at 7, 14, 28, 56, and 91 curing days. The SFSHC11 mix exhibited the optimum compressive and splitting tensile strength of 52.3 and 5.33 MPa respectively at 91 curing days. Strength recovery for optimum mix (SFSHC11) was observed to range from 20% and 27% compared to pre-cracked samples, highlighting the effectiveness of calcite precipitation in sealing cracks and restoring the concrete’s ultimate strength. The microscopic crack width analysis of the self-healing mechanism for pre-cracked samples over 28 days revealed that utilisation of Bacillus subtilis in concrete having silica fume significantly enhanced the healing efficiency. The microstructural analysis confirmed the silica-fume in concrete promotes the microbial activity of Bacillus subtilis, which in the presence of oxygen and calcium lactate facilitates the carbonation reactions, leading to the formation of CaCO3. In summary, the utilisation of Bacillus subtilis for self-healing in concrete having a constant 8% of SF is a novel, advanced, and autonomous technique for crack repair, offering significant potential for enhancing the structural integrity of concrete.