Optimizing the Self-Healing Efficiency of Hydrogel-Encapsulated Bacteria in Concrete

The main objective of this study is to evaluate and optimize the crack healing efficiency of hydrogel-encapsulated bacteria in concrete applications. To achieve this objective, Bacillus pseudofirmus was evaluated as the bacteria strain, which was combined with three mineral precursors corresponding to magnesium acetate, calcium lactate, and sodium lactate at two concentrations ($67.76$ and $75\mathrm{mM}/\mathrm{L}$). For each combination, three sets of mortar cubes, along with three sets of mortar beams, were prepared. The mechanical properties of these specimens were characterized by compressive and flexural strength tests. Once the beams were cracked, they were subjected to 28 days of wet/dry cycles in which crack width was monitored. Once the wet/dry cycles were completed, the specimens were retested to determine the strength recovery. In the compressive strength and self-healing efficiency tests, mortar samples prepared with calcium lactate at a concentration of $75\mathrm{mM}/\mathrm{L}$ along with bacteria and yeast extract showed the best performance. With respect to flexural strength recovery, no significant difference was noted among the specimens. Microscopic evaluation revealed a high concentration of calcium-rich crystals (i.e., calcium carbonate or calcium hydroxide crystals) indicative of bacterial activities. A scale-up concrete study was performed based on the best-performing mortar samples and then compared to plain concrete. Results from the concrete analysis indicated that the addition of calcium lactate improved concrete’s flexural strength. However, the plain concrete control specimen displayed superior healing efficiency after being subjected to wet/dry cycles.