Self-healing concrete: a novel nanobiotechnological approach to heal the concrete cracks

Abstract

Concrete is one of the world’s most versatile and widely used construction materials due to its unique properties, including high compressive strength, versatility, availability, affordability, simple preparation, fire resistance, excellent thermal mass, compatibility with steel reinforcement bar and the possibility of casting in desired shapes. Despite these advantages, crack formation is the main issue associated with the concrete structures. Low tensile strength, coupled with internal and external stresses, are recognized as the key causes of crack formation in a structure. Although the embedment of reinforcement bars limits the rate of crack growth, it cannot prevent crack initiation in concrete. The initiated cracks accelerate the structure degradation by allowing aggressive fluids and gasses to seep into the matrix. This phenomenon brings about a reduction in concrete service life, increases maintenance costs and, in severe cases, leads to structural failure. With the help of biotechnological pathway, concrete can be designed to have self-healing characteristics to address the above mentioned problems. In this novel approach, a bio-concrete is made by the addition of microorganisms and nutrients into the matrix during the concrete preparation. Once a crack occurs in the concrete structure, the healing agent is activated and the precipitated calcium carbonate (CaCO₃) fills the initiated crack. The resulted CaCO₃ is recognized as the most compatible material with the concrete composition which has efficient bonding capacity with the crack surface. The main goal of this research was, therefore, to design a new generation of viable self-healing mechanism for application in bio self-healing concrete structures. Initially, an investigation was performed to screen the most effective factors, including bacteria, nutrients and operating conditions, on CaCO₃ precipitation and to maximize the production of CaCO₃. Considering the different mechanical and physical properties of CaCO₃ polymorph (calcite, vaterite and aragonite), a morphological qualification based on X-ray diffraction (XRD) was conducted. Since the concrete has a high pH (~12), the capability of microorganisms to produce CaCO₃ in such a condition was investigated. To evaluate the ability of the preliminarily designed bio-agent to induce CaCO₃ in high pH, the concrete environment was simulated using a laboratory fermentor. The results indicate that the proposed bio-agent is able to withstand high pH while decreasing the microbial viability. It was also found that the proposed CaCO₃ production mechanism significantly depends on the presence of air and its effectiveness enhances at a higher level of aeration. This observation shows that the efficiency of the bio self-healing mechanism decreases in the oxygen-limiting areas such as deeper cracks and interior parts of the matrix. To address this issue, possible use of oxygen releasing compounds (ORCs) was investigated. The effects of different ORCs on the concentration and morphology of CaCO₃ were screened, and an optimization study using response surface methodology was performed to further enhance the efficiency of the designed bio-self-healing mechanism in oxygen-limiting conditions. The results demonstrate that the presence of key ORCs at their optimum level can increase CaCO₃ production. Considering the pore size of the concrete matrix, there is a high risk for microorganisms to squeeze and damage upon cement hydration. Furthermore, the exerted shear stress on the bio-agent during the concrete preparation and drying shrinkage as well as the concrete pH can adversely affect the performance of the bio-concrete. Therefore the topic was further explored to minimize the negative effects of direct incorporation of bio-agent into the concrete matrix. It has been proven that the addition of proper nano scale-size metallic particles can improve the properties of the concrete. Considering the unique characteristics of nanoparticles, magnetic iron oxide nanoparticles (IONs) were proposed as a protective vehicle for the bio-agent. Naked and amine-modified IONs were successfully synthesized and characterized by different techniques, including XRD, transmission electron microscopy (TEM), scanning electron microscope (SEM) and Fourier transform infrared spectroscopy (FTIR). The results indicate that the presence of naked IONs has a positive contribution to the production of CaCO₃ and can serve as the carrier for the bio-agent. In the final part of this work, the performance of the designed bio self-healing concrete was investigated using various laboratory tests, including compressive strength, water absorption, drying shrinkage and crack healing observation. The results show that the presence of proposed bio-agents in concrete not only contributes to improving the compressive strength but also results in decreasing the water absorption. To evaluate the self-healing behavior of this technology, several cracks were created in the concrete specimens. The microscopic observation revealed that the bio-concrete possesses a superior crack healing characteristic. The bio-concrete could effectively sense the concrete cracks and the resulted CaCO₃ sealed the damages. This study uncovered several limitations of using bio self-healing mechanism in concrete. Most importantly, it elucidated the potential of applying this novel technology to enhance the concrete durability and mechanical properties by addressing the uncovered issues.