Monitoring the bio-self-healing performance of cement mortar incubated within soil and water using electrical resistivity

In research on self-healing concrete, the restorative performance can be evaluated by a wide range of techniques. However, most of these techniques can be challenging to apply to concrete samples embedded in soil without causing a significant disturbance to the test (as they require removing the samples from the soil, washing off any residue, and examining and returning them). To provide a solution to this issue, we investigated the potential application of an in-situ, non-destructive method utilising electrical resistivity (embedded electrodes). The study was conducted on bio-mortar specimens incubated within saturated soil and water for 11 weeks. The bio-specimens were cast by adding expanded perlite impregnated with Bacillus subtilis and nutrients to the fresh mix. Standard cement mortar (without bacterial agents) was also tested to serve as control specimens. Additional testing (capillary rise and absolute porosity) was conducted under typical conditions to provide context for interpreting the changes in electrical resistivity in relation to the healing process. The bio-mortar showed greater improvements in electrical resistivity (accompanied by a reduction in crack area, water absorption and absolute porosity) than the control mortar. The study demonstrated that the electrical resistivity technique could potentially monitor the self-healing performance of concrete embedded in soil without disturbing the concrete-soil system

Bio-self-healing of cementitious mortar incubated within clay soil

The use of bacteria-based self-healing concrete for sub-structures in ground conditions is an area of increasing interest for enhancing the durability and longevity of infrastructure. In line with this objective, the present study investigates the bio-self-healing performance when a cementitious material is embedded in clay soil with varying chemical exposures and water-saturation regimes. Laboratory experiments were conducted on pre-cracked mortar specimens with Bacillus Subtilis encapsulated in perlite. The specimens were then incubated in the soil with different pH and sulphate levels, representing three exposure classes (based on Eurocodes). The crack healing ratio was evaluated through visual inspection and capillary-water absorption - before and after soil incubation. Findings showed that all inoculated specimens exhibited healing ratios noticeably larger than the control specimens, which mainly experienced small autogenous healing. Of note, the best healing performance was observed when the soil was fully-saturated and pH-neutral. From the design perspective of bio-concrete, this study emphasises the consideration of groundwater regime as well as acidity and sulphate of the ground. This material may be downloaded for personal use only. Any other use requires prior permission of the American Society of Civil Engineers

Bio-protection of cementitious materials below ground: The significance of natural soil environments

This study explores the potential impact of natural soil on concrete crack self-healing in sub-surface structures. Three types of pre-cracked cement mortar samples were prepared for laboratory experiments, with some samples inoculated with bacterial healing agents, others supplemented with nutrients to attract indigenous soil bacteria, and plain mortar served as controls. The samples were placed within saturated soil under two conditions: slightly organic natural soil and sterilised soil. After 100 days, crack closure was evaluated through microscopic inspection, water absorption tests, and SEM-EDX scanning. The results indicated calcite precipitation on crack surfaces across all samples, but with varying ratios of crack closure (16%–81%). Notably, most samples incubated in natural soil exhibited an overall increase (up to 59.4%) in healing ratio compared to those in sterilised soil, highlighting the potential of indigenous soil microorganisms and their microbial activity in enhancing the biogenic mineral precipitation and thus protecting sub-surface concrete structures