The effect of oxygen and water on the provision of crack closure in bacteria-based self-healing cementitious composites

This study determined the impact of oxygen and water on the crack healing performance of bacteria-based selfhealing cementitious composites (BBSHCCs). Oxygen and water are considered essential to BBSHCCs using nonureolytic bacteria, but in practice, the availability of either, for even short periods of time, cannot be guaranteed in the immediate aftermath of a crack forming. In this study, four healing regimes, (i) daily wet-dry cycle, (ii) weekly wet-dry cycle, (iii) semi-submerged and (iv) a deposition medium, were utilised to compare the impact of oxygen and water on the performance of BBSHCCs. Furthermore, an oxygen-releasing coating (ORC) containing calcium peroxide was evaluated and used in this study to further understand the function of oxygen in bacteriabased self-healing. Results showed that bacteria-based self-healing was only effective in wet conditions that also permit oxygen to penetrate the crack. Where there was a possibility for the composite to dry, after cracking, despite being wet some of the time the self-healing was far less efficient. It was observed that an overdose of nutrients and calcium in the healing environment resulted in early rapid healing, followed at a later stage by the dissolution of the initially formed precipitates. Moreover, it was found that the ORC contributed significantly to the recovery of both crack width and water tightness of samples. These findings will aid the development of appropriate test standards for self-healing cementitious composites and for the design and implementation of such cementitious composites in practice.