Self-Healing Mortar

Introduction

Concrete is a strong, relatively cheap construction material and is presently the most widely used material in the construction industry. The main constituent that contributes to concrete strength is Portland cement. It is estimated that cement (Portland clinker) production alone contributes about 7% of the global carbon dioxide emissions due to the burning of limestone and clay at a temperature of ~1500°C as well as fossil fuel. During this process, calcium carbonate (CaCO₃) is converted to calcium oxide (CaO) and carbon dioxide is released (Worrell et al., 2001). From an environmental standpoint, concrete does not appear to be a sustainable material (Gerilla et al., 2007). Moreover, improperly manufactured concrete may experience a shorter service life as it can easily develop cracks due to excessive loading applications and other environmental causes. Hence, a good quality of concrete is needed which not only prolongs its service life but would also reduce the production of cement.

The major problem faced by concrete structures is cracking. It can cause severe damage such as corrosion of steel reinforcement that can lead to the deterioration of structures. A number of cracks with different morphology and size could appear during construction and the design life of structure (Zhang et al., 2011). Small cracks or micro-cracks need to be repaired before they become a major or macro-crack. There are many types of repair materials and methods that can be selected and used to repair these micro-cracks. However, in certain cases, micro-cracks will still exist in the concrete structure and if they cannot be repaired effectively, the performance of the structure is compromised and its service life will be shortened. The cost of repairing concrete is expensive as it requires raw materials such as cement and skilled labours.

In many concrete structures, tensile forces can cause cracks and these can occur relatively soon after the structure is built. Repairing cracks in conventional concrete structures usually involves applying mortar which is bonded to the damaged surface. Sometimes, the mortar needs to be installed into the existing structure to ensure it does not fall off. The bonding property between host concrete and new repair materials needs to be carefully verified to avoid another problem leading to extra repair cost. Repairs can be particularly time consuming and expensive; often it is very difficult to gain access to the structure to make repairs, especially if they are underground or at a great height. Therefore, the development of new technologies and materials is essentially needed that can automatically repair cracks by giving longevity to the structures. Such materials will eventually reduce maintenance costs and fulfil the need of sustainability.

The self-healing concrete had been investigated since 1996 by Stefan Jacobsen and Erik J. Sellevold from Norway. In their research paper, they investigated the deterioration of self-healing concretes caused by internal cracking. Besides, Nynke ter Heide (2005) from Delft University of Technology had discussed self-healing process of concrete in detail. This led to more investigations conducted by other researchers in the area of self-healing concrete. The occurrence of autogenous healing or self-healing of cracks in concrete was recognized in a previous study by Reinhardt and Jooss (2003). On the other hand, the research of Jonkers and Schlangen (2008) had developed a microorganism that can heal the cracks automatically.

Self-healing concrete refers to the ability of concrete to automatically repair the cracks without any external intervention. In concrete, the most common self-healing agent is bacteria. The bacteria used in some studies are known as microorganisms that can survive in alkaline environment of concrete (pH 13 to 14), provided the air and enzyme are available. However, in this study, a different type of self-healing agent was used to incorporate with the mortar mix which eventually should heal the cracks.