The healing performance of aerobic-anaerobic composite microbial self-healing recycled concrete under oxygen...

Concrete inevitably develops cracks during service, which seriously impairs the functional performance of structures. Microbially induced calcium carbonate precipitation technology has gained attention for its eco-friendliness, cost-effectiveness, and potential for crack healing. This study used carbonated recycled fine aggregates to immobilize Bacillus cohnii and Nitratireductor indicus for preparing a microbial repair agent. Microbial self-healing recycled concrete was made by controlling the repair agent dosage and recycled coarse aggregate replacement ratio. Macroscopic performance tests were conducted to analyze the compressive, splitting, and flexural strengths of self-healing recycled concrete and assess crack healing effectiveness at various mineralization times. Combined with microscopic tests, the micro-morphology and crystal types of mineralized precipitates under different oxygen contents were analyzed. The findings show that increasing the microbial repair agent dosage reduces the concrete's strength, while significantly improving the crack healing ratio. After 28 days of healing, when the repair agent dosage was 4%, the crack healing ratio for 0.3 mm width cracks in the oxygen-rich zone, transition zone, and oxygen-deficient zone was 100.00%, 93.30%, and 96.70%, respectively. However, for 0.7 mm width cracks, the highest healing ratio of only 38.57% occurred in the oxygen-rich zone. The healing effectiveness decreases with increasing crack width. Regarding the influence of oxygen content, the healing performance is optimal in the oxygen-rich zone, followed by the oxygen-deficient zone, and least effective in the transition zone. Under different oxygen environments, the mineralized precipitates were all calcite-type CaCO₃. This study offers theoretical support for prolonging the service life of recycled concrete.IMPORTANCEBased on the fact that in concrete projects under humid environments, the surface of slender cracks in concrete engineering is relatively high, while the oxygen content in the deep cracks is low, this study immobilizes a mixture of aerobic and anaerobic microorganisms in recycled fine aggregates. By analyzing the crack repair performance of microbially self-healing recycled concrete under conditions where oxygen-rich and oxygen-poor areas coexist, as well as the mineralization mechanism of mixed microorganisms, this research can provide a theoretical basis and technical support for the repair performance of recycled concrete under external loads. The research conclusions are significant for the engineering application of microorganisms and the resource utilization of construction waste. At the same time, it has important practical significance for extending the service life of concrete engineering.