Toward the development of robust self-healing concrete using vegetative microorganisms
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Robust self-healing concrete, which requires less maintenance and repair throughout its service life than ordinary concrete, can be used for the development of sustainable infrastructure. Li and Herbert stated that a true robust self-healing concrete should meet six critical robustness criteria; the self-healing mechanism should 1. possess a long shelf life comparable to the service life of the structure; 2. be pervasive throughout the material; 3. exhibit good quality as indicated by the percentage of recovery provided; 4. be reliable; 5. be versatile in various environmental conditions; 6. be repeatable over the service life of the structure; Although many approaches can be used to promote self-healing in cement-based materials, use of biomineralization (the process by which organisms stimulate the formation of minerals) for this purpose has generated considerable interest. Previous research on biomineralization, specifically microbial-induced calcium carbonate precipitation, suggested that this process can improve durability and remediate cracks in concrete. This thesis presents the results of a multifaceted research program undertaken to evaluate the robustness of microbial concrete containing vegetative Sporosarcina pasteurii. Specifically, the criteria of versatility and quality were assessed. Versatility was evaluated by examining the influence of environmental factors on the polymorph selection process of calcium carbonate precipitated due to the activity of S. pasteurii, and it was determined that calcium concentration and overall ionic strength impacted morphology as did pH and substrate mineralogy. Another aspect of versatility that was addressed was the ability of vegetative S. pasteurii to remain viable and metabolically active when subjected to harsh conditions that might occur inside concrete including heat, high pH, and nutrient depletion. Quality was assessed by comparing properties of biogenic calcium carbonate and synthetic calcium carbonate, and it was determined that the former exhibited greater kinetic and thermodynamic stability than the latter. Quality was further examined by determining the ability of biomineralization to heal flexural cracks in mortar and provide strength recovery. Finally, the chemical constituents of the growth medium for S. pasteurii were optimized to mitigate severe retardation in cement hydration kinetics that has been observed when vegetative bacteria suspended in growth medium are added to cement, which improved the feasibility of microbial concrete.