Consider our very own bodies – scar tissue is formed at a cut and bone grows back together when broken. Then consider coral reefs, shells or limestone rocks – all created by calcium carbonate formation and in some processes bacteria are involved.
So, with calcium carbonate formation possible for natural stone – why not bring the same to concrete?
Dutch micro-biologist, Hendrik Marius Jonkers has invented a type of concrete which uses this very biological mechanism to enhance the existing healing property of concrete.
Self-healing concrete specimens
Concrete is made from mixing cement, water, sand and gravel. In time cement and water react to form the binder, gluing the mixture together, creating a strong but brittle material. Concrete has limited capacity to withstand tension, so steel rods are incorporated to take over tensile stresses and in order for the steel bars to activate, the concrete matrix needs to be cracked over the tensile zone - often micro-cracks, not visible to the naked eye. Maximum crack widths are recommended based on aesthetics, durability and functionality, such as ensuring water tightness and preventing leakage.
The natural formation of calcium carbonate type minerals are what blocks the water passage through these cracks, but this inbuilt repair system is relatively limited and uncontrolled. In such cases, external treatment may be necessary, but it is possible to enhance the natural healing properties by introducing a healing agent into the material during production – in this case, a healing agent has been designed to form additional calcium carbonate in a crack.
Bacteria is selected from environments that resemble rock, and the spores are introduced to the concrete which will awake when water and nutrients (which should also be introduced to the concrete) are present and develop into an active colony of bacteria, which then metabolise the nutrients and form calcium based minerals in the process – essentially creating stone inside concrete.
Visual crack closure of healing agent containing specimen during immersion in water (0, 28, 56 days)
Researchers have developed an additive in pellet form, including both the bacterial spores and the necessary nutrients, which are ground to create small flakes that can be easily mixed with the dry concrete constituents.
However, the easiest way to apply bacteria and their nutrients would be by preparing a liquid and spraying it onto a porous or cracked concrete surface, and this method would be more appropriate for repair and maintenance work.
Liquid repair system being spray-applied
Tests have been carried out, whereby concrete samples have been split or bent to create a specific crack width. Water pressure is applied to the crack and the amount of water which passes through is measured. The same water flow is measured after ‘healing’ has taken place.
Outdoor trials are also being carried out on existing structures using the spray system and mortar with bacteria and nutrient-containing particles for patch repair, where the amount of drops of water per minute is counted and compared to control areas without the treatment.
The first trial of structural concrete was carried out by producing part of an Ecuadorian irrigation canal out of concrete with the bacteria and nutrients. The performance of the canal section will be monitored over time.
Cast concrete section of irrigation canal in Ecuador
To learn more about this research, please visit http://www.citg.tudelft.nl/en/research/projects/self-healing-concrete