The innovative bacterial self-healing concrete is a promising solution to improve the sustainability of concrete structures by sealing the cracks in an autonomous way. Regardless of the types of bacterial-based approach, the provision of a suitable incubation environment is essential for the activation of bacteria and thus for a successful self-healing application. However, the research to date has mainly focused on the self-healing process within humid air or water environment. This research aims to investigate the performance of bacterial self-healing concrete within ground conditions which can potentially benefit the development of more sustainable underground concrete structures such as deep foundations, retaining walls and tunnels.
The research method is comprised of a laboratory experimental program with several stages/ phases. In the first stage, control tests were conducted to examine the influence of different delivery techniques of healing agents such as the material of capsules on the healing performance in water. The outputs from this stage were used as a control test to inform the next stages where the fine-grained concrete/mortar specimens were incubated inside the soil. In this stage, three different delivery techniques of the healing agent were examined namely Direct add, Calcium alginate beads and Perlite. The results showed that the crack-healing capacity was significantly improved with using of bacterial agent for all delivery techniques and the maximum healed crack width was about 0.57 mm after 60 days of incubation for specimens incorporated with perlite (set ID: M4). The volume stability of the perlite capsules has made them more compatible with the cement mortar matrix in comparison with the calcium alginate capsule. The results from Scanning Electron Microscope (SEM) and Energy Dispersive X-ray (EDX) indicated that the mineral precipitations on crack surfaces were calcium carbonate.
The second stage investigates the effect of different ground conditions on the efficiency of bio self-healing concrete. This stage presents a major part of the experimental programme and contains three experimental parts based on the types of soils and their conditions where bio self-healing of cement mortar specimens was examined. The first part investigates the effect of the presence of microbial and organic materials within the soil on the performance of self-healing by incubating cracked mortar specimens into sterilized and non-sterilized soil. This part aims to investigate if the existing bacteria in the soil can produce any self-healing.
In the second part, the investigation focused on the bio self-healing in specimens incubated in coarse-grained soil (sand). The soil was subjected to fully and partially saturated cycles and conditioned with different pH and sulphate levels representing industrially recognised classes of exposure (namely, X0, XA1, and XA3). These classes were selected according to BS EN 206:2013+A1:2016 - based on the risk of corrosion and chemical attack from an aggressive ground environment. In the third part, cement mortar specimens were incubated into fully and partially saturated fine-grained soil (clay) with similar aggressive environments as in part 2. The results showed that the indigenous bacteria naturally present within the soil can enhance the mortar self-healing process. For specimens incubated within coarse-grained soil (sand), the reduction in pH of the incubation environment affected the bio self-healing performance. However, for fine-grained soil (clay) the healing ratios of specimens incubated in the same identical exposure conditions were almost similar, with better results observed in the pH neutral condition. The results showed also that the self-healing efficiencies in both the control and bio-mortar specimens were significantly affected by the soil's moisture content. This indicates that the mineral precipitation of calcium carbonate caused by the metabolic conversion of nutrients by bacteria is heavily reliant on the moisture content of the soil. The hydration of un-hydrated cement particles representing the primary source of autogenous healing is also influenced by soil moisture content.
The third stage investigated the use of a non-destructive technique utilising the concrete electrical resistivity to monitor the crack healing performance of specimens incubated within the soil. The results showed that the improvement in electrical resistivity of bio-mortar specimens was remarkably higher in comparison with control specimens. This improvement can be used as an indication of the healing performance of bio-mortar specimens in comparison with autogenous healing in control specimens.
In general, the study suggests that the bio self-healing process can protect underground concrete structures such as foundations, bridge piers, and tunnels in a range of standard exposure conditions and that this is facilitated by the commonly applied bacterial agent Bacillus subtilis or similar strains. However, as the experimental findings indicated the exposure conditions could affect the healing efficiency. Therefore, future work should consider how formulations, application methods, and ground preparation can be optimised to achieve the best possible incubation environment and thus improved protection for underground concrete structures.