You’d think that concrete would last forever. After all, it’s pourable stone, so it should hang around as long as the Rock of Gibraltar. But, under the right (or wrong) conditions, concrete decays with alarming speed. To combat this, researchers at the University of Bath in the UK are working on a self-healing concrete that uses bacteria to seal the cracks that lead to decay. In this way, they hope to cut down on maintenance costs and increase the life of concrete structures.

Concrete is one of the most remarkable building materials of the modern age. It’s pourable into an incredible number of shapes, sets like stone, and when combined with iron rebars is immensely strong. Unfortunately, it is much more vulnerable than people think. Proper design, pouring and curing of concrete structures can go a long way, but one tiny crack can set a building on its way to becoming a pile of rubble.

The problem is that concrete, and especially reinforced concrete, is highly vulnerable to water. If the concrete is properly made and the surface remains intact, it’s fine, but if it cracks, it can let water into the interior and that’s where the trouble starts.

In temperate or cold climates, this can lead to periodic freezing and expanding of ice in the crack, forcing it wider like a hydraulic ram. Water can also leach chemicals, resulting in decalcification that leaves the concrete brittle. It can form salts in a process called efflorescence, that causes the concrete to break down into furry crystals. It can also carry in carbon dioxide, sulfates and sulfate-reducing bacteria that cause more damage. If the cracks are deep enough to reach the iron rebar, then it’s game over. The iron will rust, expand, and tear the structure apart in an act of slow-motion self-destruction.

The key is to not let water get into the concrete, and the way to do that is to prevent it from cracking. Cracks can still occur for any number of reasons, however, so in wet climates concrete needs constant maintenance. To cut down on this, the University of Bath in collaboration with lead partner Cardiff University and the University of Cambridge are taking a page from the sulfate-reducing bacteria, but instead of using microbes to destroy concrete, they are finding ways to use bacteria to heal it.

The goal is to create a concrete mix that contains bacteria in microcapsules that will germinate if water enters through a crack. The bacteria will multiply, producing limestone as they go, sealing the crack before the water can do any harm. Dr. Richard Cooper of Bath's Department of Biology & Biochemistry said, “Including bacteria in concrete offers a double layer of protection in preventing steel corrosion. Not only do the bacteria work to plug cracks in the concrete, the process of doing so uses oxygen present which would otherwise be involved in the corrosion process of the steel bars.”

According to the team, the self-healing concrete would not only increase the life of concrete structures, but will also reduce repair costs by half and help to reduce man-made carbon dioxide emissions, of which concrete manufacturing is a major source.

The project is still in early days. The cement used to make concrete is a very hostile environment for most bacteria. It’s extremely alkaline, so the team has to isolate alkaline-tolerant strains. In addition, as concrete hardens and cures, it can crush the microcapsules containing the bacteria. Kevin Paine of the Department of Architecture & Civil Engineering said, “We’re looking at enclosing the bacteria in micro-capsules, along with nutrients and calcium lactate, which the bacteria will convert when water becomes present and use to fill cracks in the concrete.”

The team is currently assessing how well various strains of bacteria survive in concrete over various periods of time. Researchers from Newcastle University have also been exploring the use of bacteria as a healing agent in concrete.