Dutch develop self-healing concrete

Presently undergoing testing, researchers demonstrate how a type of bacteria that is present in highly alkaline water and mineral rocks can help repair cracks in concrete automatically without the need for interventions for maintenance

By: Thomas Tjabbes

The demand for efficient solutions in civil construction increases proportionally to the growth of the world’s population that is expected to reach 9 billion people who will need homes by 2025. That's what researchers are thinking about at the Delft University of Technology in the Netherlands. They are developing a type of concrete that promises to significantly reduce the costs associated with repairing and maintaining concrete structures, ensuring greater durability for buildings and, of course, availability of housing. This new development is in the process of being tested and should be released commercially in 2016 as a concrete that heals and renews itself.

The solution is indicated mainly for construction in humid environments where the steel used in construction may suffer deterioration due to exposure to water that can seep into the structure through micro cracks in the concrete. Certain situations that demonstrate the need for such material can be found in tunnels and underground installations, liquid and hazardous waste storage systems, as well as in structures in or near offshore environments where mineral salts in the water are highly corrosive to steel. In countries where weather temperatures may be very low, the technology can be applied to roads where the salt that is used in keeping roads from freezing can penetrate cracks in the concrete and contaminate the structure.

Therefore, this self-renewing or ‘self-healing’ concrete is demonstrated enthusiastically by researchers at Delft. Henk Jonkers, PhD in marine microbiology and the person responsible for the project, says that the key to achieving the proposed goals was the discovery of a type of bacteria, that is present in lakes with high alkalinity and mineral rocks, which occurs naturally and can withstand the very high pH of the environment; the same condition found in cement. Isolating these microorganisms in their habitat, the team studied their properties and found the following: concentrating the bacteria within the concrete mixture at a rate of 100 million bacteria per cubic meter of concrete and adding a reagent, this bacteria is capable of automatically healing cracks of up to 0.5 mm.

According to Jonkers, this result is due to a reaction of the bacteria upon contact with calcium lactate; a component found in milk. The bacteria convert this component into calcium carbonate (limestone) and, thus, fill in the cracked or broken parts. “The curing agent consists of concrete reagent with inactive spores of the bacteria which remain dormant in a protective capsule. When a crack occurs, the protection is broken and the penetration of water activates the bacteria which, in turn, feeds on the material and converts the reagent into limestone”, explains the scientist, pointing out that the process provides a 100% automatic repair of cracks since the capsules are added early on, when the concrete is mixed.

According to the specialist, these capsules are expected to be brought to market as small ‘grains’ (capsules) to be added to the concrete mix. “They remain inactive throughout the mixture and hardening of the concrete and only open when there are cracks throughout the concrete part”, he says.

Furthermore, it was confirmed by the team in charge that the bacteria do not react directly with the components of the mixture and, therefore, can be applied in any type of concrete. Jonkers further anticipates concerns that might arise among future users and assures that the bacterium does not cause damage to the structural steel that is found in concrete constructions. Moreover, he states that the sealing of cracks is precisely meant to protect the steel in the structure from deteriorating.

With these positive prospects, the project has already gained the support of four research institutions in Europe, government agencies involved in construction, contractors, architects and other investors such as suppliers of concrete and chemical additives. Therefore, the team of researchers plans to begin complete testing in open environments as of 2013. “We have already done tests outdoors with a system for impregnation of liquids based on the formation of limestone by the bacteria. The result was that the leak in a concrete ceiling was completely sealed”, he says.

Other tests that are expected to complete the required series and confirm the feasibility of the technology will be performed over the next two years, and Jonkers believes that these tests will prove that the system works in outdoor environments and with different applications of concrete.

As regards cost, he guarantees that the addition of the capsules will not represent more than 2% of the total cost of the works which, in his opinion, means economy throughout the life of the construction due to a reduction in expenses related to maintenance.