CONCRETE is one of the most commonly used building materials. It is cheap, strong and easy to work with. But, as a short walk through any city centre will prove, it cracks easily. The cracking of concrete pavements is merely a nuisance, but cracks in roads, bridges and buildings are a hazard. A way of making concrete that healed such cracks spontaneously would thus be very welcome. And a team led by Henk Jonkers at the Delft University of Technology in the Netherlands may have come up with one.

The way to stop concrete cracking is to bung up small cracks before they enlarge. That process of enlargement is caused by water getting into a crack, then freezing in cold weather and thus expanding. This freeze-thaw cycle, a common form of erosion of natural rocks, too, weakens a structure directly and also exposes steel reinforcing rods to water, causing them to rust.

When he began his research, Dr Jonkers knew that spraying mineral-producing bacteria onto limestone monuments is often an effective way to stop freeze-thaw in its tracks. The mineral in question is calcium carbonate, the defining ingredient of limestone. He also knew, however, that when applied to concrete, this technique had proved to be just as time-consuming and, indeed, more expensive than traditional repair methods using sticky, water-repellent agents. That led him to wonder if the answer was to incorporate helpful bacteria into concrete from the start.

To find out, he and his team selected various mineral-producing bacterial strains that can handle the highly alkaline environment found in liquid concrete. They added these bacteria, along with calcium lactate, an organic compound that such bacteria convert to calcium carbonate, to different samples and allowed those samples to set. At various intervals, the team powdered the solidified samples, created cultures to test for living bacteria, and ran calculations to determine the number of bacterial cells that had survived solidification. They also examined samples of the concrete for microscopic cracks and to see which minerals had formed.

As they report in Ecological Engineering, Dr Jonkers and his team found that the mineral grains which formed in the cracks of samples of concrete that had been seeded with bacteria were often as large as 80 microns across. That would go a long way towards sealing those cracks and making them waterproof. The equivalent grains in control samples were rarely larger than 5 microns across.

Unfortunately, this study also showed that the bacteria survive for only a few weeks. Beyond that period, the concrete fails to heal. But data from a second study, as yet unpublished, suggest that immobilising the bacteria in particles of clay before they are added to the concrete allows them to live for months, and possibly years. The clay serves both as reservoir for the bacterial food and also as a haven for the bacteria while the concrete hardens. If the process can be scaled up, it may be prove that the best way to preserve concrete is to infect it.