Biochar-immobilized bacteria and superabsorbent polymers enable self-healing of fiber-reinforced concrete after multiple damage cycles

Self-healing under multiple damage cycles is critical to the serviceability of concrete structures. This article explores crack closure and recovery of mechanical and permeability properties after multiple (Le., three) damage cycles by comparing autogenous and bio-based self-healing in concrete using a combination of steel and PVA fibers, superabsorbent polymer (SAP), and bacteria immobilized in biochar. Swelling of SAP upon exposure to water and enhancement of hydration by curing action of SAP led to improved blocking and filling of cracks compared to a control (plain concrete); however, the effectiveness of autogenous crack closure (by only SAP or SAP plus fibers) was limited to narrow surface cracks (< 500 mu m) after the third healing cycle. Microbial calcite precipitation by biochar- immobilized bacteria combined with SAP and fiber was found to offer superior closure for relatively wider surface cracks (> 600 mu m) and internal micro-cracks compared to that attained by the autogenous mechanism alone in control and concrete containing only SAP and fiber. Effectiveness of crack filling by immobilized spores in biochar was found to be consistently higher than concrete with directly added spores and SAP through all three cycles of damage and healing. Precipitation of calcium carbonate crystals in internal cracks and interfacial zones around PVA fiber and aggregate in concrete with biochar immobilized spores resulted in high recovery of strength and permeability compared to the autogenous healing mechanism in control and concrete with SAP and fiber. However, it was found that macro-voids formed by SAP with a larger average particle size and higher swelling capacities affected total permeability and permeability recovery after repeated healing. Overall, we conclude that cementitious systems with biochar-immobilized bacteria, SAP, and fibers can enhance self-healing and impart improved durability to concrete structures.