Investigation of Utilizing Bacteria to obtain Self-Healing of Invisible Cracks in Mortar

Abstract

Bio mortar is used to deal with cracks by providing a special type of bacteria along with nutrients to the ingredients of the mortar during the mixing. Once the cracks appear, bacteria in the mortar are activated by oxygen and water and begin calcite precipitation, which automatically fills the cracks inside the mortar without any exterior interference. This research investigated the effects of varying proportions of (type of bacteria and nutrients; concentration of bacteria; bacteria and nutrient percentage) on the properties of self-healing mortar. Investigate the steps of preparation of cell suspensions of bacteria. Investigate obtaining self-healing of invisible cracks in mortar. Forty mortar mixtures were designed and tested. Three types of bacteria were utilized: Bacillus pasteurii DSM 33, Bacillus sphaericus DSM 396, and Bacillus subtilis H50620/9. Two concentrations (2×109 -2×1010) Colony Forming Units per milliliter from DSM 33 and DSM 396 were used. Calcium lactate, calcium acetate, and urea were added as a nutrient to bacteria. Nutrients is added by in proportions ranging from 0.25, 0.50, and 1% of cement weight. Bacterial suspension is added by (0.5, 1, 4, and 10% of cement weight). Bacillus subtilis encapsulated in calcium alginate beads added by (0.25, 0.5, 1, and 4% of cement weight). The specimens were cracked by two different methods, which are visible artificial cracks and invisible natural cracks. Tests were performed for compressive strength and flexural strength. The microstructure analysis of the bacteria mortar has been done utilizing SEM to ensure that calcium carbonate (calcite) has indeed filled the cracks. Thermogravimetric analysis was performed to determine the degree of hydration for bacteria paste. The ANOVA test was conducted on the compressive strength test results by the linear model. The results show all the independent variables affect the dependent variable (the compressive strength). The best results in the increase of compressive strength reached 69.8% and 66.4% at 7 days and 120 days, respectively, in mortar mix which contains 0.5% bacteria DSM 396 with 0.25% calcium lactate. The expected decrease in compressive strength as a result of loading did not occur significantly, that shows the bacteria are working, precipitating calcite at the onset of invisible cracks formation, which assures that self-healing in that case has occurred.