| Abstract | Biotechnology opens exciting possibilities for underground concrete structures, such as tunnels and
underground storage. Bio-self-healing concrete, which uses bacteria to repair cracks automatically,
could enhance the sustainability of these structures by enabling them to self-repair without human
intervention. However, a major challenge in bacteria-based self-healing concrete is ensuring bacterial
survival and activity, particularly in the harsh chemical environment of concrete, which typically has a
high pH that is detrimental to bacterial life.
This research explores a simple yet innovative idea: could the soil surrounding underground
structures provide the bacteria needed for self-healing? If successful, this approach could simplify
the process, making it more practical and cost-effective. The research addresses this question
through a series of lab experiments conducted in three phases, each designed to explore different
aspects of the conditions necessary for soil-driven bio-self-healing.
In the first phase, the focus was on understanding the chemical conditions of cement mortar surfaces
under which bacteria could survive and function. The research sought to identify the factors that
hinder bacterial activity, such as high pH, and to test methods that could improve the conditions for
bacteria to thrive. Specifically, experiments were conducted to assess pH levels, electrical
conductivity, and calcium ion concentrations in water environments around the mortar. The results
revealed that high pH levels resulting from cement mortar leaching hinder bacterial survival. Various
techniques were tested, including introducing supplementary materials like GGBS, using flowing
water, and accelerating carbonation. These methods created a more bacteria-friendly environment,
supporting the growth and activity of bacteria, particularly under conditions mimicking underground
structures exposed to water. This phase laid the foundation for understanding how to optimize the
chemical environment for bacterial activity.
The second phase explored how to supply the necessary nutrients to attract indigenous soil bacteria
to the cement mortar surfaces, thus supporting their activity in the self-healing process. Different
nutrient delivery methods were tested to ensure that the addition of nutrients did not compromise
the concrete’s properties. Nutrient-filled capsules were designed to gradually release nutrients,
facilitating bacterial activity to produce calcium carbonate for crack healing. The results showed that
calcium carbonate capsules had minimal impact on mortar properties due to their small size and low
concentration. In contrast, calcium alginate capsules, which were larger and created voids within the
mortar, weakened the material. Various capsule proportions were tested to find an optimal balance,
ensuring that the mortar strength remained largely intact while still promoting effective self-healing.
This phase answered the research question of how to incorporate nutrients into the mortar without
compromising its mechanical properties.
In the third phase, the performance of the self-healing system was tested in a soil environment to
simulate real-world underground conditions. Cement mortar samples were exposed to different
types of soil as well as water, which acted as a natural source of bacteria and other necessary
elements for self-healing. The success of the self-healing process was measured by observing crack
closure and the formation of calcium carbonate deposits both near the crack surface and deeper
within the mortar. The results demonstrated that the soil environment provided the necessary
conditions for bacteria to activate and repair cracks, simulating conditions typically found in
underground concrete structures. This phase directly addressed the research question by testing the
viability of using natural soil bacteria for self-healing, a cost-effective and sustainable alternative to
adding external bacterial cultures.
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These findings highlight a practical and sustainable approach to self-healing concrete by harnessing
naturally occurring bacteria in organic soil to repair cracks in underground structures. Experimental
results showed that modifying the harsh chemical environment of cement mortar—particularly by
lowering the pH from >13.5 to a range between 9.2 and 10.5 using GGBS replacement (30–50%),
flowing water, and accelerated carbonation—created conditions more conducive to bacterial
viability. Bacterial activity was negligible above pH 11.5, but significantly increased when the pH
dropped below 10.5, with optimal microbial-induced calcium carbonate precipitation (MICP)
occurring near pH 9.5. The use of 50 µm calcium carbonate microcapsules allowed controlled
nutrient release without compromising mechanical strength, unlike larger alginate capsules (>300
µm) that reduced compressive strength by up to 15–20%. Notably, mortar samples exposed to
organic soil demonstrated visible crack closure up to 0.35 mm within 28 days, with calcium carbonate
precipitation reaching 3.1–4.2 g/m² on the crack surfaces, confirming active MICP. Overall, this
research presents soil-driven bio-self-healing concrete as a promising, low-cost, and environmentally
friendly solution to enhance the resilience and service life of underground infrastructure |
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