Crack self healing concrete by native microbial calcium carbonate

Abstract

Inevitable concrete microcracks remain as a challenge to civil engineers as they are considered as a threat to structures durability. One of the most common approach is to incorporate ureolytic bacteria in concrete matrix to hydrolyse urea resulting in the self-healing of concrete cracks through the formation of calcium carbonate. Despite that, the issue revolving around the efficacy of crack self-healing remains important. The existing works are still suffering from a better understanding of the factors affecting the fundamental reactions involved as well as bacterial growth in concrete environment. In this study, a comprehensive investigation was conducted to explore the bacterial growth and the influential factors on the evolution of urea hydrolysis aimed to accurately promote calcium carbonate precipitation inside concrete using native bacteria. Subsequently, native ureolytic bacterium species was isolated, identified by 16S rRNA gene sequencing and deposited in the gene bank database under the accession number of MK357893. The bacterial growth was examined in a condition similar to that of concrete in which modified Luria Bertani (LB) broth was utilised to cultivate the bacteria with static incubation. The ureolytic activity was also investigated at pH values of 7 - 13 as well as different concentrations of urea, calcium and nutrient. The Nessler method and an inductively coupled plasma atomic emission spectroscopy (Agilent 700 ICP-OES) technique were used to measure the evolution of urea hydrolysis and calcium carbonate changes in such conditions. In addition, the extent of microbial activity impact on the compressive strength of concrete incorporated with spores, vegetative cells and urea-vegetative cells solution was also evaluated separately. Similarly, the self-healing of an artificial cracked bio-concrete of 0.4 mm was also monitored and evaluated every two weeks by scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD). In the same context, a system of equations, rationally based on physic-bio-chemical issues, was developed in order to quickly predict a complete understanding of the bio-based healing process. Later, both finite element and finite difference methods were implemented to solve these equations. The results indicated that the bacterium was able to survive as dormant without any reproduction at pH of 12 - 13. While, the optimum bacterial cells concentration was found to be 2 × 107 cells/mL at pH of 9 - 11. In addition, the favoured urea hydrolysis culture conditions were obtained as follows: pH of 9, concentration of calcium ions not exceeding 150 mM, urea concentration of 333 mM and optimum cells concentration of 2 × 108 cells/mL. Subsequent findings also revealed that compressive strength of the concrete incorporated with spores, vegetative cells and urea-vegetative cells was improved by 9%, 10% and 15% compared to that of the control specimens respectively. Moreover, the predicted healing ratio of 0.4 mm crack width was completely achieved after 60 d at the crack mouth, whereas the healing ratio was less than 15% at the deeper part of the concrete surface. This finding was also proved through the experimental work in which the actual crack mouth was fully healed after 70 d. In addition, further studies could be focused on providing a suitable technique to host bacteria for a long term as well as to encourage the bacteria to effectively implement its ureolytic activity inside the concrete matrix.