Effect of water availability on microbial self-healing of concrete

Microbial-based self-healing is a promising solution for a sustainable concrete. The principle is that carbonate precipitating bacteria and bio-reagents are pre-added into concrete; upon cracking, bacteria will be activated to precipitate CaCO3 to heal the crack. Due to the harsh environment in concrete, encapsulation of bacteria is preferable before incorporation into the matrix. In this study, microcapsules and hydrogels were applied to encapsulate bacterial spores. Water is an essential element for bacterial activities. Therefore, water availability is a key factor to obtain considerable amount of healing. In this work, self-healing behavior of the specimens (with and without bacteria incorporated) were investigated at the conditions of 95%RH and wet-dry cycles (wdc). Two types of wdc were applied: 1) 8h in water and 16h in 60%RH; 2) 1h in water and 11h in 60%RH. Healing efficiency was evaluated by crack closure and the decrease of water permeability. No crack healing was visualized in the specimens stored at 95%RH. The specimens subjected to wdcs had much more crack healing. Water permeability of the specimens with microencapsulated bacteria was about one order of magnitude lower than that of the reference ones. About 40% and 80% of the crack area was healed in the reference and bacterial specimens (at wdc1), respectively. For the ones stored at wdc2, though with a reduced wet period, considerable amount of healing was obtained in the specimens with hydrogel encapsulated bacteria, which had crack healing ratio of 5%~30% and 50%~100% for the reference and bacterial series, respectively. This study indicates that incorporation of carbonate precipitating bacteria into concrete can enhance the self-healing efficiency. The amount of available water and the time during which the water can stay available in cracks are of crucial importance to obtain a sufficient healing. This water availability can be enhanced by encapsulating the bacteria in hydrogels