Self-Healing Performance of Bacteria-Based Mortar in Marine Environment

Concrete structures in marine environment are vulnerable to various physical and chemical deterioration mechanisms which can vary with exposure zones. The durability of these structures can be further compromised due to cracks as they allow the ingress of chloride and sulfate ions. Considering the critical nature of the cracks under marine environment, this research focused on crack-healing by bacteria-based mortar under submerged and tidal marine environments. An initial experimental program was designed to determine the difference in healing under various marine exposure zones. For this purpose, the autogenous healing performance of conventional mortar was monitored under submerged and tidal marine environments. A bacteria-based mortar was then developed using Halobacillus Halophilus and expanded perlite aggregates as carrier. An experimental program was carried out to select the optimum nutrient precursor for bacteria-based mortar in marine environments, which was shown to be calcium lactate. The performance of bacteria-based mortar was evaluated under submerged and tidal marine conditions. The impact of crack orientation and sustained loading on the autonomous healing performance of bacteria-based mortar were also assessed. Experimental techniques such as visual assessment of healing using image processing, mechanical strength, volume of permeable voids (VPV), scanning electron microscopy (SEM), energy dispersive X-Ray (EDX) and X-Ray diffraction (XRD) were used during the research to assess the performance of control and bacteria-based mortars. Results showed that tidal conditions proved critical regarding healing and resulted in considerably less healing as compared to the submerged marine environment. The lower healing in tidal conditions was due to the deterioration of healing layers under wet-dry cycles. Bacteria-based mortar specimens were able to improve healing in submerged and tidal marine conditions compared to control specimens. However, the performance of bacteria-based specimens proved to be affected by the crack orientation and sustained loading. The orientation dependency was discovered to be a gravity-induced effect where upward-faced and side-faced horizontal cracks developed substantially higher healing than downward-faced and side-faced vertical cracks. The dependence of crack healing performance on crack orientation suggests that results in the literature should be carefully interpreted as the information about the orientation of cracks during healing is rarely reported in earlier studies which might have been unknowingly affected by this phenomenon. Moreover, crack widening under sustained loading deteriorated the healing products formed in the cracks and reduced the autogenous and autonomous healing performance as compared to unloaded specimens. Although bacteria-based mortar specimens were able to improve healing under sustained loading in submerged marine conditions as compared to control specimens, they did not show any significant difference in healing under sustained load in tidal conditions. These results can be quite relevant given that previous studies on the performance of bacteria-based cementitious composites, monitored healing under unloaded conditions and similar results may not be reproduced under actual field loading conditions. Moreover, combined with critical exposure conditions such as tidal environment, bacteria-based cementitious composites may not even improve crack healing under certain conditions of sustained loading