Effect of bacteria on the self-healing ability of fly ash concrete

Abstract

This study investigated the effect of three species of bacteria namely Sporosarcina pasteurii, Bacillus megaterium, and Bacillus subtilis in four different concentrations on the self-healing property of fly ash concrete. The first two bacteria were acquired as lyophilized ampoules and the third was extracted from alfalfa leaves in the laboratory. Fly ash concrete specimens were made by replacing 20 wt% of cement with fly ash. The specimens were subjected to ultrasonic pulse velocity, impermeability, water absorption, and compressive strength tests. For visual examination, a set of specimens at three days of age were subjected to a load roughly equivalent to 30 % of the fracture load to cause cracking, and the healing process was monitored by light microscopy, Scanning Electron Microscopy (SEM), and Energy dispersive X-ray spectroscopy (EDS). Test results showed that the addition of bacteria caused a considerably greater compressive strength improvement in the specimens that contained fly ash than those without fly ash. The specimens with fly ash and bacteria also showed a greater reduction in ultrasonic pulse transit times than other specimens. The lowest water absorption rate in fly ash concrete specimens, 3.96 %, was measured in the specimen containing Bacillus subtilis with a concentration of 2.8 × 108. Among the studied bacteria, Bacillus megaterium with a concentration of 105 was the most effective in improving compressive strength, providing an 8.8 % strength improvement in the specimens without fly ash and a 39.5 % strength improvement in specimens with fly ash. This concentration of Bacillus megaterium also offered the best improvement in ultrasonic pulse transit velocity and water penetration depth of fly ash concrete specimens. SEM and light microscopy results showed the precipitation of calcium carbonate in the cracks and pores of the concrete, and EDS detected the compounds formed by the cement hydration process and the calcites created by the bacteria.