Bio-concrete: Unveiling self-healing properties beyond crack-sealing

Do microbes categorically heal the cracks in concrete or only seal the cracks - this study aims to answer, based on a few experimental findings, highlighting the self-healing concepts in cementbased materials. Alkaliphilic spore-forming Bacillus cohnii microbes (105-1010 cells/ml) are used in the cement-concrete (bio-concrete) in addition to varying nutrient amounts. The healing in the form of regaining its structural efficiency is evaluated by its mechanical (compressive, flexural) strength and durability properties. Bio-mineralized calcium carbonate seals 37% of 20-50 nm dimensioned pores and 87% of 50-200 nm dimensioned pores resulting in 43-48% total voids reduction within 28 days of the healing period. Microbes only degrade the nutrients to precipitate healing agents (calcium carbonate) without affecting the C-S-H structure in concrete. Statistical analysis reveals that calcite deposition mostly influences the strength of self-healed bio-concrete whereas experimental findings show 5.8-6.3 wt% calcium carbonate deposition augments the 40-60% compressive strength of bio-concrete compared to reference concrete. The complete-to60% sealing of cracks of 0.02-2 mm width is examined using visual inspection of concrete skins. Microbes reduce porosity, sealing cracks intensifies mechanical strength, and the durability of the concrete validates self-healing rather than only sealing the cracks.