Wise crack: Self-healing concrete

Self-healing is one of nature’s most remarkable talents. It continues to fascinate us, as doctors make huge leaps in regenerative medicine. But away from the medical field, engineers are hoping to harness nature’s healing power for a very different application - and one that could go deep into our built environment.

Researchers in Belgium and the Netherlands are working on replicating the same processes we use to heal our bodies, within concrete. They claim that within two to four years, they will be able to commercialise a ’biological concrete’ that will have the ability to repair itself under tensile forces.

If they’re successful, it could mean that concrete structures will no longer face deterioration before the end of their service life. It’s a major problem for the construction industry. As an integral part of our infrastructure, concrete is a valuable material, but it suffers from a serious flaw: it develops micro cracks within a matter of weeks when under tension.

This makes the whole structure weak as water can seep in, damaging both the concrete and the building’s steel reinforcement. Structures such as underground basements, motorway bridges and tunnels are particularly vulnerable, and repairs using concrete mortar are expensive and time consuming. Far better would be to use a material that heals itself just as a crack begins to appear.

Existing research has focused on the use of synthetic materials that can seal up cracks as they develop. But the work by Delft and Ghent universities is unique in that they plan to use living bacteria to achieve what they hope will be better results.

At Delft University, Dr Henk Jonkers is developing a biological concrete that uses specially selected bacteria of the genus Bacillus, alongside a combination of calcium lactate, nitrogen and phosphorus, to create a healing agent within the concrete.

If untouched, these agents can remain dormant in the concrete for centuries. But if water begins to seep into the cracks, the spores of the bacteria start to germinate and feed on the calcium lactate. This consumes oxygen, which in turn converts the calcium lactate into limestone that solidifies and seals the surface. The removal of oxygen also improves the durability of the steel reinforcement.

’We use clay pellets that are around 2-4mm wide to make sure that the agents are not activated during the mixing process,’ said Jonkers. ’The problem with this is we have to use relatively high volumes of this porous aggregate within the concrete mix. As a result, you gain self-healing but you lose the strength of the concrete.’

The clay pellets make up 20 per cent of the volume of the concrete that would otherwise be made of a harder material. This is estimated to weaken the concrete by around 25 per cent, which is far too much for applications that require high compressive strength. Jonkers is now working on using a compressed powder instead of pellets that will hold the self-healing agent in less than one per cent of the volume of the concrete.

Meanwhile, researchers at Ghent University are using the micro-organism Bacillus sphaericus with urea as a nutrient source to create calcium carbonate. ’We first discovered the bacterium as it was causing problems closing up water pipes,’ said researcher Dr Nele De Belie. ’We realised the same bacteria could help enhance the durability of concrete.’

Instead of using a porous aggregate to hold the self-healing agent, the Ghent team opted to place the material in a hollow glass capsule with an internal diameter ranging from 0.8 to 4mm. As the concrete cracks, the capsules break, releasing the self-healing agent. This method eliminates the need for porous aggregates and retains the strength of the concrete.

During the course of its research, the team found that the bacteria struggled to fill cracks of more than 300mm. It has since developed a solution that is purely synthetic, by using polyurethane capsules, which foam in moist environments, and an accelerator that shortens the reaction time. Initial tests have shown that the foam can expand 25-30 times more than a bacterial solution. But De Belie hasn’t given up on biological process yet.

’There’s a lot of work still to be done on biological concrete and it has a lot of potential,’ she said. ’You really have to collaborate with other disciplines, and communicate your knowledge, while understanding theirs. It’s one of the biggest challenges of this kind of project, but I think engineers are open to it. When I presented this research in a materials science conference, everyone was overwhelmed that bacteria could work this way.’

As well as the challenges of filling large cracks and maintaining strength, the price of biological concrete could become a barrier to its take-up. Currently its cost is expected to be double that of ordinary concrete. But the researchers at Delft are already on the case. A solution is being developed that uses a sugar-based food nutrient that could bring the cost down to just slightly more than that of concrete used today.

According to Jonkers, despite its difficulties, using bacteria rather than chemicals in this way has its own advantages. ’Some of the chemical additives used, while sealing cracks, could cause the concrete to be more brittle, which will again result in a shorter service life,’ he said. ’Bacteria doesn’t do this and it is also more sustainable than synthetic healing agents.’

With 50 per cent of Europe’s annual construction budget spent on rehabilitation and repair of existing structures, the construction industry is keen for researchers to come up with a viable solution. Delft University predicts that it can get a biological solution on the market within two years. It plans to be undertaking full-scale outdoor testing of self-healing concrete structures later this year, with €420,000 (£370,000) funding from the Dutch government.

Ghent University is continuing to work on its encapsulation process and is hoping to find companies who will commercialise it. De Belie predicts a more conservative time frame of four years before biological concrete comes on the market. It remains to be seen whether a biological or a synthetic solution will break the market first, but it appears that engineers are certainly up to the challenge.