Effects of hydrogel-encapsulated bacteria on the healing efficiency and compressive strength of concrete

Microbial-induced calcium carbonate precipitation is a promising technology for self-healing concrete due to its capability to seal microcracks. The main goal of this study was to evaluate the effects of adding hydrogel-encapsulated bacteria on the compressive strength and the self-healing efficiency of concrete. To achieve this objective, 12 sets of mortar samples were prepared, including three different mineral precursors (magnesium acetate, calcium lactate, and sodium lactate), at two concentrations (67.76 and 75.00 ​mM/L), and under two different biological conditions (with and without bacteria). In addition, a set of plain mortar samples was prepared to serve as a control. For each sample set, three mortar cubes and three beams were prepared and subjected to compression and flexural strength tests. From the compression tests, it was found that the sample containing calcium lactate along with yeast extract and bacteria displayed the best results. As for the flexural tests, once cracked, the beams were subjected to 28 ​d of wet/dry cycles (16 ​h of water immersion and 8 ​h of drying), where the bottom crack width was monitored (at 0, 3, 7, 14, 28 ​d of wet/dry cycles). Once the sample with the highest healing efficiency was identified (the one containing calcium lactate and hydrogel-encapsulated bacteria), the study was scaled up to concrete specimens. Two sets of concrete cylinders (consisting of three control samples and three samples with bacteria along with calcium lactate) were tested under compression in order to evaluate the effect of the bacteria-precursor combination on the concrete mechanical properties. The samples that yielded the greatest compressive strength were the ones containing calcium lactate and bacteria, displaying an improvement of 17% as compared to the control specimen. Furthermore, a flexural strength recovery analysis was performed on the concrete specimens revealing that the control showed better flexural strength recovery than the bacteria-containing variant (41.5% vs. 26.1%) after 28 ​d of wet/dry cycles. A healing efficiency analysis was also performed on the cracked samples, revealing that the control displayed the best results. These results are due to the fact that the control specimen showed a narrower crack width in comparison to the bacteria-containing samples.