Self-healing concrete: The surprise in the wastewater

Self-healing concrete through microbial induce carbonate precipitation has brought a lot of optimism for reducing the carbon footprint in the construction industry by promising to eliminate the need for maintenance. However, lab experiments are performed with pure microbial cultures, and the cracked mortars are placed for healing in tap water in a steady environment (i.e., constant temperature), which represent very different conditions of the ones in outdoor applications.
In this study, we tested the healing potential of a selected environmental bacterial strain both in tap water and in a pilot plant facility treating municipal wastewater. Our hypothesis was that healing in the pilot plant will be less efficient, if at all, than in the standard lab conditions due to the competition for nutrients from the numerous bacterial species present the wastewater.
Three sets of samples were tested: plain mortar (ii); mortar with calcium and nutrient (iii); and mortar with calcium, nutrient and bacterial spores (iii). In tap water, samples (iii) had significant healing of the cracks, while there was no healing in samples (i) and (ii). However, when placed in wastewater, samples (ii) and (iii) showed similar level of healing, despite the variable conditions they were exposed to (changing pH, temperature, and wastewater composition). Not only bacteria from wastewater did not outcompete the culture resulted from the pure spores added in mortars, but they seem to be contributing towards healing.
Our results suggest that when considering outdoors applications for self-healing concrete, and in particular in wastewater facilities, one should take advantage of the ubiquitous presence of the environmental bacteria. This has the potential to make the technology cheaper by eliminating the need to produce high quantities of pure spores, currently one of the main limiting steps when planning for large scale applications.