A "self-healing" concrete that can patch up cracks by itself is ready for outdoor testing, researchers at Delft Technical University in the Netherlands say.

The experimental concrete contains limestone-producing bacteria that are activated by corrosive rainwater working its way into the structure, they said.

Developed by microbiologist Henk Jonkers and concrete technologist Eric Schlangen, the material could potentially increase the service life of the concrete with considerable cost savings as a result, they said.

"Micro-cracks" are an expected part of the hardening process and do not directly cause strength loss, but in time water can seep into the cracks and corrode the concrete, they said.

"For durability reasons -- in order to improve the service life of the construction -- it is important to get these micro-cracks healed," Jonkers told BBC News.

Bacterial spores and the nutrients they will need to feed on are added as granules into the concrete mix, the researchers said, but the spores remain dormant until rainwater works its way into the cracks and activates them, whereupon they feed on the nutrients to produce limestone.