Preliminary study on in-situ spore germination at different crack depths in bacteria-based self-healing concrete

Bacteria-based self-healing concrete is promising in sustainable constructions, while the germination of carbonate precipitating bacterial spores in crack zone is the start of the healing process, and hence has great impact on crack healing efficiency. In this study, the Ca-alginate hydrogel sheets encapsulated spores were pre-embedded at specific crack depths, and the in-situ germination behavior of bacterial spores was monitored by an optical oxygen microprobe. The influence of crack depth and crack environment (pH and Ca²⁺) on spore germination was extensively examined. The results showed that the oxygen distribution within the crack was heterogeneous, with oxygen easily available at the crack mouth and difficult to diffuse in the deep part. The germination time of the spores at 1/3 and 2/3 crack depth was 1d and 2d, respectively. With two spores containing hydrogel sheets embedded at the same time, the spore germination at 2/3 crack depth was further delayed about 2d. The Ca²⁺ concentration was heterogeneous within the crack, increasing significantly from about 0 mM (surface) to 80 mM (40 mm deep), which had a limited influence on spore germination. The pH distribution was also heterogeneous. The pH value at the crack mouth was about 11 and increased to more than 12.5 at the crack deeper part, indicating that pH might be a controlling factor affecting spore germination at different crack regions. The uneven distribution of oxygen concentration and pH in the crack had a significant influence on the spore germination, and hence affect the healing efficiency at different crack depths.