Concrete structures usually show some self-healing capacity, i.e. the ability to heal or seal freshly formed micro-cracks. This property is mainly due to two processes: 1. Delayed or secondary hydration of partially or non-hydrated excess cement particles present in the materi- als matrix reacting with crack ingress water, and 2. Formation of calcium carbonate (limestone) due to reaction of atmospheric carbon diox- ide with portlandite minerals (calcium hydroxide) present at the crack surface. This 'autogenous' self-healing capacity of concrete can, at best, seal 0.2 mm wide cracks. At the Delft University of Technology (TU Delft) a new type of self-healing concrete is under development in which bacteria mediate the production of calcium carbonate based minerals. This process of 'autonomous', or enhanced self-healing, can result in sealing of cracks up to 0.8 mm wide, what is more than 4 times the autogenous self-healing capacity of concrete. Sealing of cracks and pores in concrete decreases permeability and ingress of harmful substances such as chloride ions, improves water tightness, and improves frost resistance. The process of self-healing thus better protects embedded steel reinforcement from corrosion and is therefore expected to increase the service life time of construction, while at the same time reducing the need for costly manual inspection and repair actions. In a series of research projects not only self-healing concrete has been developed but also two repair systems based on the same technology to be applied on existing and aged structures. This self-healing repair mortar, and a liquid repair system can be used for struc- tural and durable repair of larger defects, and for regaining water tightness of damaged and cracked concrete structures respectively. In this article the technology of bacteria mediated calcium carbonate formation will be explained, followed by examples of outdoors applications of respectively the self-healing concrete, the repair mortar and the liquid repair system.
