Concrete is the world's most widely used building material, but there haven't been any fundamental changes in the material, and because of that it is prone to cracks - which means that most structures be substantially reinforced with steel.

This is all about to hopefully change with experimental concrete that self heals - yes, self, heals. The experimental concrete contains limestone-producing bacteria, which are activated by corrosive rainwater working its way into the structure. The new concrete is about to enter outdoor testing, and if successful, we could see new structures made with the concrete that could provide better service life with the concrete, and huge cost savings, too.

The work is being done at Delft Technical University in the Netherlands and is the brainchild of microbiologist Henk Jonkers and concrete technologist Eric Schlangen. If the testing is positive, Dr Jonkers says that they could start the process of commercializing it as soon as 2-3 years from now.

"Micro-cracks" are an expected part of the hardening process, and don't lead to strength loss. Fractures with a width of around 0.2mm are expected, and are completely normal within the concrete industry. But over time, water, as well as aggressive chemicals in it, get into these cracks and corrode the concrete. Dr Jonkers has told the BBC news:

For durability reasons - in order to improve the service life of the construction - it is important to get these micro-cracks healed.

This new testing involves bacterial spores and the nutrients they need to feed on are added as granules into the concrete mix, with water being the missing ingredient they need for the microbes to grow. The spores then remain dormant until rainwater works its way down into the cracks, activating them. The bacteria is harmless, belonging to the Bacillus genus, then feeds on the nutrients, producing limestone.

The bacterial food incorporated into the healing agent is calcium lactate, which is a component of milk. The microbes are used in the granules, and are capable of tolerating the highly alkaline environment that is found in concrete. Dr Jonkers explains:

In the lab we have been able to show healing of cracks with a width of 0.5mm - two to three times higher than the norms state. Now we are upscaling. We have to produce the self-healing agent in huge quantities and we are starting to do outdoor tests, looking at different constructions, different types of concrete to see if this concept really works in practice.

Dr Jonkers explains that the biggest challenge is making sure that the healing agent is strong enough to survive the mixing process. But in order to get to that point, Dr Jonkers says "we have to apply a coating to the particles, which is very expensive". The team are in the process of trying to reduce the cost that the coating process adds to the cost, but Dr Jonkers expects that an improved system will be ready in around six months time.

The outdoor tests will begin shortly after this time, with the team already in discussions with several construction firms that are willing to help out. After this, the concrete will have to be monitored for a minimum of two years to see how it behaves in a real-world setting. The TU Delft researcher adds"then, if everybody's happy, we can think about trying to commercialize the product". This means that even if the healing agent adds an extra 50% to the cost of the concrete, it would more than make up the 1-2% of the total construction cost.

Maintenance is a much bigger percentage of this total cost, so Dr Jonkers expects huge savings over the concrete's service life with the self-healing concrete.