Bacterial Self-Healing Performance of Coated Expanded Clay in Concrete

Concrete crack healing by microbial calcium carbonate producers has been intensively studied for the last 10 years. To overcome the harsh environments of concrete, various carriers have been tested for their ability to protect the healing bacteria. Using expanded clay (EC) as a carrier is attractive because it provides adequate bond strength to the cement composites as well as protection to the bacteria from harsh environments. In this study, self-healing performance by EC was examined using styrene-acrylic emulsion coatings. First, the presence of bacteria (Lysinibacillus boronitolerans YS11) within EC was verified using electron microscopy after the bacteria were immobilized. Although coating has a negative effect on the preservation of bacterial density under normal conditions, the bacterial density was higher for coated EC [$5.0\times {10}^4$ colony forming unit $(\mathrm{CFU})/\mathrm{g}$ of EC] than for uncoated EC ($2.4\times {10}^3\mathrm{CFU}/\mathrm{g}$ of EC) when exposed to a harsh environment (60°C and pH 12) for 48 h. This suggests that coating the surface of EC was successful in protecting bacteria from the environmental stressors. Even though bacteria were within the EC, the bacterial survival rate quickly declined with time inside the mortar. However, the bacterial density was much higher for coated than for uncoated EC at day 28, suggesting that the coating provides the bacteria with excellent protection from the harsh environment within the mortar. The concrete healing rates were 70% for uncoated EC and 75% for coated EC, compared to the healing rate of 50% and 42% for plain mortar and mortar with empty EC, respectively. These results suggest that the healing rate has increased with EC containing bacteria.