A concrete that ‘heals’ its own cracks has recently been developed and indicates a promising future in reducing the inevitable deterioration of concrete structures and the high maintenance costs involved. Introducing self healing concrete.
Ever since the Colosseum was constructed by the Romans almost 2000 years ago, concrete has been a major part of our built environment. It appears in buildings, bridges, tunnels and roads, and it’s use has skyrocketed in recent years. In the last three years alone, China has used more concrete than the United States has in the last century, with an insatiable building demand that doesn’t look like it’s going to ease anywhere in the near future.
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Despite its strength attributes, concrete is subject to deterioration, with micro-cracks developing during the construction process which can lead to leakage problems and eventually corrosion of steel reinforcements. In worst case scenarios, this can result in collapse, and billions of dollars are spent every year just in the maintenance of concrete infrastructure.
What is Self Healing Concrete?
Self healing concrete is a bio-concrete, created just like regular concrete, but with an extra ‘healing agent’ mixed in. This healing bacteria remains intact throughout the mixing process, but if the concrete cracks and water seeps in, it dissolves and becomes active. The bacteria used is self healing concrete is either Bacillus pseudofirmus or Sporosarcina pasteurii, both of which are found naturally in high alkaline volcanic lakes and can survive up to 200 years without any food or oxygen. These bacteria are activated when they come into contact with water, making them an ideal ingredient in self healing concrete to effectively repair any cracks which may naturally occur.
Who Developed it and Why?
Self healing concrete was developed by microbiologist Hendrik Jonkers of the Delft University of Technology in the Netherlands. It was largely inspired by natural body processes in which bones heal through mineralization, and Jonkers explored the idea of whether this could be replicated in concrete.
Tackling the unique properties and conditions of concrete, it was a challenging process that took Jonkers three years and saw him named as a finalist for the European Inventor Award 2015. Concrete is an extremely alkaline material and finding a bacteria that could survive its dry, stone-like properties and remain dormant before being activated by water was not easy.
Bacillus bacteria was eventually selected because it thrives in alkaline conditions and can produce spores that will survive for decades without any oxygen or food. But not only did the bacteria need to remain active within the concrete, but be able to produce a repair material (limestone) for the concrete when damaged. For this the bacilli needs a food source, such as sugar. But if this was added to the concrete mix, it would result in a softer, weaker substance.
The solution Jonkers came up with was calcium lactate which he set with the bacteria in capsules made from biodegradable plastic. These were added to the wet concrete mixture and only open if water eventually seeps in. This in turn activates the bacteria to germinate and multiply, feeding on the lactate and combining the calcium with carbonate ions to form calcite (limestone), which effectively closes up the cracks and ‘heals’ the concrete.
Jonkers creation is part of an exciting shift towards ‘biological building’, combining natural biological processes into construction materials. He says: “Nature is supplying us a lot of functionality for free – in this case, limestone-producing bacteria. If we can implement it in materials, we can really benefit from it, so I think it’s a really nice example of tying nature and the built environments together in one new concept.”
The Future of Self Healing Concrete
Self healing concrete is currently undergoing its first major trial in the South Wales Valleys at one of Costain’s construction sites, led by a team of researchers from Cardiff University in collaboration with academics from the University of Bath and the University of Cambridge. The project is entitled ‘Materials for Life’ and incorporates three separate pilot programs for self healing concrete technologies within a real-world setting, exposed to real-world elements.
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According to Cardiff University’s principal engineering investigator, Professor Bob Lark:
“Our vision is to create sustainable and resilient systems that continually monitor, regulate, adapt and repair themselves without the need for human intervention. These self healing materials and intelligent structures will significantly enhance durability, improve safety and reduce the extremely high maintenance costs that are spent each year.”
The first technique uses shape-memory polymers to repair large cracks in concrete, with the material transforming into a ‘memorized’ shape when heated with a small current. These could be embedded into concrete to close large cracks, or at least reduce their size significantly.
In the second technique, the researchers are pumping organic and inorganic healing agents through a series of thin tunnels into the concrete, while the third technique uses Jonkers innovation of bacteria and calcium lactate capsules.
Six concrete walls have been cast at the Welsh test site, each containing one of these different technologies. After inducing cracks on the walls, the researchers will monitor the effectiveness of each. One of Costain’s civil engineers on the project states “We will be monitoring properties such as stiffness, permeability and the mechanical damage recovery of the trial walls in comparison with conventional reinforced concrete walls.”
It is hoped that these three technologies will eventually be combined into a single system that could be embedded into concrete when initially set and would effectively be able to repair concrete automatically when damage occurs. This would limit the need for human intervention and eliminate the high costs currently required to maintain, fix and restore concrete structures.
To date, self healing concrete has proven to work effectively and will be on the market this year in three different forms – a self-healing concrete, a repair mortar and a liquid repair spray which can be applied to existing concrete structures. The main set-back to its implementation are the costs involved in its creation compared to traditional concrete, currently being at around twice the cost and making it only feasible for projects where water leakage and corrosion are really problematic. According to Jonker, this is predominantly due to the high costs of calcium lactate, and if the technology could be tweaked with a sugar-based nutrient instead, then the price of self healing concrete would reduce dramatically.