A new type of concrete is about to undergo outdoor testing. The material, which contains bacteria that produces limestone, is said to heal its own micro-cracks, according to researchers at Delft Technical University in the Netherlands. "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," said microbiologist Henk Jonkers.