A study on bacterial self-healing concrete encapsulated in lightweight expanded clay aggregates

Concrete is a widely used construction material susceptible to cracking due to internal and external stresses, affecting the long-term strength and durability of the structure. Self-healing concrete appears to be a promising solution to address this issue. For concrete, self-healing is naturally achievable to a certain extent by utilising unhydrated cement particles. However, to heal significant internal and external cracks over a long period, we need to develop new techniques to reach a practical level of self-healing. This study investigates autonomous self-healing concrete using Microbially Induced Calcite Precipitation (MICP). The concrete mix is created by incorporating Lightweight Expanded Clay Aggregate (LECA) encapsulated with Lysinibacillus sphaericus and Bacillus coagulans bacteria. Encapsulation is used to prolong the life of the bacteria and shield them from the highly alkaline environment of the concrete. A bacterial concrete mixture was made by replacing 20% coarse aggregate by volume with the bacteria-encapsulated LECA. The properties of self-healing concrete specimens were compared with conventional concrete specimens of grade M30. After curing, the cracked bacterial specimens underwent a 14-day wet and dry cycle to examine how well the bacteria healed the cracks. Then, the durability property was analysed based on water permeability and chloride penetration tests. The microstructure of concrete was examined using SEM and EDX analysis. Bacillus coagulans and Lysinibacillus sphaericus bacteria specimens showed favourable results compared to conventional concrete for hardened and durability tests performed. The two bacteria-encapsulated concrete also exhibited positive crack healing, and the macrocracks in the range of 0.1 to 0.5 mm were healed. A salt analysis confirmed the presence of Calcium Carbonate (CaCO₃) in the healed portion of the concrete. Lysinibacillus sphaericus bacteria showed slightly better results than Bacillus coagulans, and both bacteria can be used for self-healing concrete applications.