Potential of Applying Bacteria to Heal Cracks in Concrete

Concrete is a construction material that is used world-wide because of its first-rate properties. However, the drawback of this material is that it easily cracks due to its low tensile strength. As large costs are involved in crack repair, the potential of self-healing of these cracks by means of calcium carbonate (CaCO3) precipitating bacteria was investigated in this study. First, the survival of the bacteria was tested. Next, the optimal concentrations of bacterial cells, urea and Ca 2+ were determined in order to obtain a maximum amount of CaCO3 precipitation. Finally, self-healing of cracks in mortar specimens, by means of bacteria, was investigated. Glass tubes, containing the healing agent were provided inside the mortar matrix. Upon crack occurrence, the tubes break and the healing agent, consisting of a filler material and bacteria, is released into the crack and can cause crack repair. Strength regain up to 60% was thus observed due to self-healing. INTRODUCTION Cracks often occur in concrete because of the low tensile strength of this material. Rapid crack-healing is necessary since it is easier for aggressive substances to ingress into concrete through cracks than through the concrete matrix. It is known that it is costly to inspect, monitor and repair cracks. Moreover, some of the repair methods currently used are not so sustainable [Neville 1996]. Therefore, it would be desirable if concrete cracks could be healed autonomously by releasing healing agents inside the matrix when cracks appear. In this research, an environment-friendly and autonomous crack repair technique is explored. Previous research has shown that Bacillus sphaericus bacteria are able to precipitate calcium carbonate (CaCO3) on their cell constituents and in their micro-environment by conversion of urea (CO(NH2)2) into ammonium (NH4 + ) and carbonate (CO3 2). The bacterial degradation of urea locally increases the pH and promotes the microbial deposition of calcium carbonate in a calcium rich environment. Through this process, the bacterial cell is coated with a layer of calcium carbonate [Dick et al. 2006]. The aim of our research is to use this bio-precipitated CaCO3 to heal cracks in concrete. A calculation showed that precipitation of CaCO3 is not enough to fill wide concrete cracks completely. Therefore, a filling material is needed. In previous research, on manual healing of cracks in concrete by means of bacteria, silica gel was used as filling material [Van Tittelboom et al. 2010b]. Bacteria were mixed into the gel and both were injected into the crack. However, as the gel matrix showed some tiny cracks due to shrinkage and had no strength on its own, polyurethane (PU) was used as filling material in this research. PU with immobilized bacteria has already been reported to be used to repair concrete cracks [Bang et al. 2001]. Bacterial cells were first immobilized into the PU foam. Then, the PU foam was cut into equal-sized pieces. Afterwards, PU foam strips were placed into simulated cracks of mortar specimens. The specimens were then incubated in a urea-CaCl2 medium at room temperature. As a result of CaCO3 precipitation, the regain of compressive strength of the specimens was reported to be obtained. Different from the method described above, in which PU foam with immobilized bacteria was pre-formed and placed into the cracks manually, in our research, PU foam was formed in the crack automatically upon crack appearance. Furthermore, at the same time bacteria were incorporated inside the foam. Bacteria and PU were encapsulated in glass tubes with three compartments which were embedded inside mortar specimens. One compartment of the tubes contained the first component of the PU. The second compartment was filled with the nutrients for bacterial growth, a calcium source and the second component of the PU. The last compartment was filled with the bacterial cells. When a crack appears in the mortar matrix, the glass tubes break and all components mentioned above can flow into the crack and mix together. First, polymerization of the filling agent (polyurethane) is initiated and through this process strength regain is obtained. In a second step, the bacteria, which are dispersed through the filling material can precipitate CaCO3 crystals in the pores of the PU and through this, an additional regain in strength may be obtained. MATERIALS AND METHODS