These advances will help to translate current lab-scale applications into cost-effective industrial processes and products. The innovation could boost the competitiveness of Europe’s construction industry and save taxpayers’ money by reducing the cost of maintenance of public infrastructure.
The team is exploring three main techniques, says project coordinator Professor Nele De Belie of Ghent University. One of these relies on bacteria, another involves hydrogels, and the third is based on polymers.
Why three-plus techniques rather than a single, effective one? Well, cracks can form for a number of reasons and, depending on these reasons, different types of material are needed to fill them.
Just add water
Bacteria, for example, can deal with the cracks that may appear when concrete hardens and inevitably shrinks. “The bacteria are put in the concrete as spores, the dormant form of the bacteria,” says De Belie. “They are sleeping and waiting until there is a suitable environment to become active.”
Once a crack forms and lets in water, De Belie’s sleeping beauties jump into action. Roused by the smell of nutrients, they start to metabolise and stimulate the precipitation of calcium carbonate, which eventually fills the gap.
This approach currently relies on pure and pricey cultures of bacteria. As part of its drive to upscale the technique, Healcon is finding ways to work with affordable, mixed cultures extracted from waste materials.
Another method is based on the highly absorbent hydrogels used in diapers. The objective here is for hydrogels in concrete, swelling upon exposure to water, to temporarily plug the gap and boost the precipitation of calcium carbonate for long-term repair.
This build-up of calcium carbonate is, essentially, limestone. Unfortunately, limestone is very brittle and works only if the walls of the crack remain stable. In structures where they are likely to move – such as bridges, where vibration must be factored in – elastic materials are needed.
To crack or not to crack
Elastic materials suitable for such repairs include the polymers already used to treat concrete from the outside. Capsules of these can be used in self-repair mechanisms, but designing the process isn’t necessarily straightforward.
“The difficulty,” says De Belie, “is that you have to be able to mix these capsules into concrete. You have a lot of impacts and forces when you mix concrete and the capsules have to survive this, but then later, when they are in the concrete and a crack appears, they do have to break and release their contents.”
This is a tricky goal to achieve, but not beyond the reach of ingenuity. Healcon is notably experimenting with multi-layered capsules and with encapsulation materials that are stronger and more flexible when wet, in fresh concrete, than once the whole thing has dried.
To ensure that the outcomes of this research will be relevant to the market, Healcon has involved end users, mostly ministries of public works, from the start. Most of these, De Belie reports, have expressed an interest in using the new techniques in situations where cracks would be particularly problematic or especially difficult to repair.
Self-healing concrete will be more expensive than regular concrete, but this investment is expected to pay for itself quickly by reducing maintenance costs and making buildings more durable.
According to Healcon estimates, the EU’s bridges, tunnels and earth-retaining walls alone cost some EUR 4 to 6 billion per year in maintenance. If self-healing concrete had been used for all these structures, say the partners, up to EUR 120 million could be saved annually on maintaining this part of Europe’s building stock.