Recent studies in the field of concrete materials show that the early cracking criteria in micro-size can occur as soon as the cement matrix becomes hardened. In many ways, these cracks can become macro-size and opened cracks resulting in significant issues for the durability and appearance of concrete structures as water leakage and corrosion. The technique of self-healing using bacteria has recently received attention for its potential applications. However, the effectiveness and the repeatability of this method over a long period have not been clarified. The information on both the survival and the number of bacteria after healing is limited. This paper aims to improve the self-healing ability and repeatability of concrete when using Bacillus subtilis natto. The experimental studies evaluate the effect of biomineralization with lightweight aggregate as the protecting-carrying vehicle, which can control the release of healing fluid through four cracking-healing cycles. The urease activity and the biomineralization of the bacteria with urea as the main carbon source were assessed and the effect of cracking age on the self-healing capacity, associated with the compressive strength improvement was studied. The results obtained from the optical microscope and SEM/EDS analysis indicated the existence of bacteria CaCO 3 forming in concrete after four healing cycles. During long duration, bacterial concentration in concrete was determined by microscopic counting method. Based on experimental results, the restoration of the compressive strength confirmed the high self-healing ability of concrete when using bacteria in lightweight aggregate.

Since concrete with bacteria incorporated in the matrix shows promising results in initial studies, more research has focused on using bacteria for the strength and durability enhancement of concrete. Bacterial concentrations varying from different positions on the crack surface were recorded and evaluated in conjunction with the amount of self-healing product formed. The change in bacteria concentration with the survival time in concrete was investigated and lasted for two years. The degree of mineralization of calcium carbonate from the microbial activity is also closely related to the level of survival and reduction of bacterial concentrations in concrete compared to the initial amount. These processes were tracked and analyzed through phase composition analysis and microstructure analysis. The role of nucleation sites of bacteria for accelerating mineral deposition was also investigated. The change in the content of hydrated cementitious minerals can be seen in groups of samples with different bacteria regarding cracking age (7-90 days). The increase in C-S-H content in the bacterial samples at early cracking age was significant compared with the control group. The effect on healed crack parameters through microscopic observation contributed to supporting and demonstrating the hypothesis of the combination of the formation of the calcium carbonate crystals around the bacterial cell as crystallization nuclei and the promotion of hydration for C-S-H formation.

In the 帥 一 e ・ based desi 帥 r s 啣 cra 曲 g in buildi 皿 ＆ the rellable mathe 瓶 ical model for prediCting shrinkage

No abstract

In recent years, many projects have been carried out to enhance the durability of concrete structure from the influence of cracks. Generally, managing cracks should be a rather preventative method for sustainable development. Based on that, Bacillus subtilis natto-a local bacterium in Japan was found to have the ability to form CaCO 3 , which can be used as healing materials for cracks in the concrete structure. The bacterial biomineralization immobilized in lightweight aggregate was studied to improve the compressive strength by healing the cracks and densifying the structure. Moreover, as an essential parameter for durability enhancement, the water permeability of self-healing concrete through a water-flow system was carried out. Experimental results on the behavior of the materials indicate that Bacillus subtilis natto could lead the compressive strength to 40 % higher than the controls. Promising result in preventing the water through the cracks confirmed the self-healing effect with more potential in larger-scale.