Aerobic non-ureolytic bacteria-based self-healing concrete: Effects of environmental and exposure conditions

The effects of water-borne contaminants on the durability of concrete are well-known and cracked concrete is more susceptible to ingress of these contaminants. Consequently, there has been significant research to develop concrete that can self-heal cracks; to either reduce their width or eliminate cracks entirely. One approach to autonomic self-healing is the utilization of microbiologically induced calcite-precipitation. This approach uses the metabolic activity of bacteria and biomineral precursors embedded within the concrete to form an inorganic material, usually calcite, as a healing compound.
However, bacteria-based healing of concrete creates several scientific challenges at the biology-concrete technology interface. Some of these are in relation to the germination, survivability, and growth of bacteria in the conditions to which concrete is exposed, and to the internal changes that take place within concrete as it converts from a plastic material at early-age to a dense and solid material in its hardened form.
However, of equal interest are concrete technology challenges associated with the condition of the concrete and the environmental conditions that the concrete is exposed to when the cracks occur. Specific issues of note are whether: (i) the concrete has carbonated, (ii) the nutrients for bacterial growth remain accessible, (iii) there is availability of water and oxygen, and (iv) temperature.
This paper describes research carried out to ascertain the effect of these issues. It describes research in which self-healing concretes were made by encapsulating spores of aerobic non-ureolytic bacteria in aerated concrete granules; with nutrients for bacteria growth either added directly or also encapsulated. These concretes were then exposed to different conditions before cracking (e.g. carbonation and curing duration) and to different conditions after cracking (water availability and temperature). The results show that bacteria-based self-healing can be achieved in a myriad of conditions; however, it is necessary to design bacteria-based self-healing concrete for the specific environment in which it will be used and that an “off-the-shelf” approach is unlikely to work.