SCREENING FOR ALKALINE RESISTANT SPORE FORMING BACTERIA AS CONCRETE HEALING AGENTS
In order to find suitable bacteria as concrete healing agents, we examined a total of 50 bacterial isolates from an alkaline soil sample. These isolates were subsequently tested for sporulation rates, ability to induce calcium carbonate precipitation, tolerance to alkaline conditions, as well as their capacity to heal cracks in mortar samples. Of the 50, two bacterial isolates showed promising results across all these test categories. These isolates were identified as Bacillus horneckiae and B. kochii. Both were able to grow on LB agar at a pH of 10, within 5 days had sporulation rates over 90% on the AR2A agar plates, and precipitated calcium carbonate on B4 agar plates.
Both B. horneckiae and B. kochii had preferences for high alkaline environments. The OD 540nm readings of these two bacteria in pH 9 and 10 LB broths were significantly higher than the readings of their counterparts in pH 8 LB broth after 48 h of incubation. The growth of B. horneckiae and B. kochii in different concentrations of YE broths were tested. These two bacteria both had worse growth in 0.5 and 1% YE broths than in 2% YE broth. The spores of B. horneckiae and B. kochii were also tested for germinations in the same test environments. Results showed that either high pH or low nutrient levels did not have many impacts on spore germinations of these two bacteria.
Calcium carbonate precipitation from these two bacteria were quantified. Bacillus horneckiae and B. kochii reduced approximately 980 and 650 ppm of free Ca2+ ion respectively from a 1/10 LB broth containing 2500ppm of Ca2+ within 7 d and precipitated CaCO3.
The mean viable counts of B. horneckiae and B. kochii decreased 1.2 and 1.5 orders of magnitude respectively in the first 24 h, dropped additional 0.6 and 0.4 orders of magnitude between day 1 and 14, and then, remained constant between day 14 and 28 after being mixed in mortar samples. Healing abilities were tested by incorporating bacterial spores in mortar samples. Cracks up to 0.25 mm were healed in mortar samples containing B. horneckiae or B. kochii spores. All the results suggested that both the bacterial isolates, B. horneckiae and B. kochii, may be used as bacterial healing agents in self-healing concretes.