Quantifying the Self-Healing Efficiency of Bioconcrete Using Bacillus subtilis Immobilized in Polymer-Coated Lightweight Expanded Clay Aggregates

Concrete is prone to cracking over time, leading to the deterioration of concrete structures. Using the biomineralization capabilities of bacteria, cracks in concrete can be remediated in favorable conditions. In this study, Bacillus subtilis spores were immobilized in three different healing agents, namely lightweight expanded clay aggregates (LECAs), polyvinyl acetate (PVA) fibers, and an air-entraining admixture (AEA). Bacillus subtilis spores, with a turbidity equivalent to a 4 McFarland standard, were used in three different dosages, namely 0.01, 0.1, and 1% (by weight) of cement. Based on the dosage, three groups were developed and each group consisted of a total of nine mixes, which were differentiated based on the method of delivery of the bacterial spores. The specimens were pre-cracked after 7 days, using an embedded steel rod, after being post-tensioned in a universal testing machine. The self-healing efficiency of the concrete was evaluated using ultrasonic pulse velocity testing and surface crack analysis, using ImageJ software, and the self-healing precipitate was analyzed using microstructural tests, namely scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy analysis. The results verified that the self-healing efficiency of the concrete improved with the increase in the bacterial dosage and with an increase in the curing time. LECAs proved to be a promising bacterial carrier, by accommodating the spores and nutrient media over a period of 196 days. PVA fibers helped in bridging the cracks and provided nucleation sites for the bacteria, which enhanced the calcite precipitation. Similarly, the AEA also improved crack healing by encapsulating the spores and sealing cracks up to 0.25 mm, when used in conjunction with LECAs. Furthermore, microstructural tests verified the formation of calcite as a healing product within the cracks in the bioconcrete. The results of this study offer valuable insights for the construction industry, highlighting the ability of bacteria to reduce the deterioration of concrete structures and promoting a sustainable approach that minimizes the need for manual repairs, particularly in hard-to-reach areas.