Long term performance of bacteria based self-healing cementitious composites subjected to accelerated and weathering exposure

Abstract

Self-healing based on bacteria is relatively new treatment strategy that focuses on cracks repairing and avoiding damage in concrete structures by using bacteria and their essential nutrients. It has been extensively studied in the past two decades in concrete applications. Many of these studies have focused on the ability of the microbiology induced calcite precipitation (MICP) to remediate the cracks or improve the mechanical properties of cementitious materials. Nevertheless, the durability of cementitious materials is considered one of the most significant problems faced in civil engineering. In order to evaluate the long term durability performance of bacteria based self-healing cementitious materials, in this study, accelerated exposure, i.e. freeze-thaw cycles, wet-dry cycles and outdoor exposure were conducted. A suitable microbial source of calcite has been investigated besides the development of a new encapsulation material that enhances bacterial releasing without affecting cement mortar properties. Effect of bacteria spores encapsulated in calcium alginate-pyrophyllite on mortar properties has also been studied. The effect of accelerated exposure on the mechanical and microstructural properties of self-healing cementitious composites has been extensively examined. The mechanical and microstructural characteristics of bacterial self-healing mortar under outdoor exposure have been investigated. The correlation between the outdoor exposure results and the accelerated test results has been checked to predict the long term performance of bacteria self-healing mortar. It has been found that the Bacillus Sphaericus 57, Bacillus Spharicus 58 and Bacillus Megaterium 33 were suitable urease enzyme producers that capable to hydrolyse urea, generate carbonate ions, thus inducing calcium carbonate precipitation. It has also been proved that after exposing to 30 F-T cycles, the compressive strengths of SP33, SP58 and SP57 have been reduced by 19.4%, 18.37% and 21%, respectively, while the reduction in compressive strength of control mortar was 36.4% percent of the initial strength. After 30 cycles of wetting and drying, bacterial self-healing mortar samples exhibited lower compressive strength than the control. A 21% decline was observed in compressive strength for SP57, whereas for SP58 and SP33 strength dropped by 18.3% and 9.4%, respectively. Under the same environmental conditions, the compressive strength of all bacterial mortar samples increased with the increase of exposure time. The highest compressive strength was achieved by SP33: the compressive strength value reached 53.5 MPa after 18 months of exposure with improvement of 85% comparing to control. Concerning the correlation between the results of the accelerated exposure and those obtained under the outdoor exposure, on the basis of 18 months’ time scale, it was found that the former exposure accelerated the changes in the compressive and flexural strength properties of the bacterial self-healing mortar compared with that in the latter one. Predictions of the long-term behaviour of bacterial mortar under outdoor exposure can be attempted.