Like living bone, concrete could soon be healing its own hairline fractures 鈥� with bacteria in the role of osteoblast cells. Worked into the concrete from the beginning, these water-activated bacteria would munch food provided in the mix to patch up cracks and small holes.
Concrete reinforced with steel forms the skeleton of many buildings and bridges. But any cracks in its gritty exterior make it vulnerable: 鈥淲ater is the culprit for concrete because it enters the cracks and it brings aggressive chemicals with it,鈥� says of Delft University of Technology in Delft, the Netherlands. These chemicals degrade both concrete and steel.
Locating and patching cracks in old concrete is a time-consuming business, but rebuilding concrete structures is expensive. Jonkers thinks the solution is to fight nature with nature: he suggests combating water degradation by packing the concrete with bacteria that use water and calcium lactate 鈥渇ood鈥� to make calcite, a natural cement.
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Surviving soda
Unfortunately, most organisms keel over in a pH above 10, which is typical of concrete. To find bacteria that are happy in such an alkaline environment, Jonkers and his colleagues looked to soda lakes in Russia and Egypt where the pH of the water is naturally high 鈥� and found that some strains of Bacillus thrived there.
Moreover, the bacteria can take on a dormant spore state for long periods 鈥� up to 50 years, according to Jonkers 鈥� without food or water. He compares them to seeds waiting for water to germinate.
To keep the spores from activating in the wet concrete mix, and to keep them and their calcium lactate food from affecting the quality of the concrete, Jonkers and his colleagues first set both into ceramic pellets 2 to 4聽millimetres wide and then added them to the concrete.
Only when tiny cracks form in the concrete 鈥� opening up the pellets 鈥� and water seeps inside will the bacteria activate and begin to consume the food that has also been freed. As they feed, they combine the calcium with oxygen and carbon dioxide to form calcite 鈥� essentially pure limestone.
Surface seal
Brajadulal Chattopadhyay of Jadavpur University in Kolkata, India, says the work is 鈥渧ery interesting鈥� and will 鈥渉elp to find a path for the development of real self-healing concrete in the near future鈥�. But he doesn鈥檛 know how the bacteria can produce calcite deep in concrete, where they would be starved of the oxygen they need to metabolise the calcium lactate.
Jonkers argues that it is not necessary to form calcite at such depths: simply sealing off the crack with calcite near the surface is enough to prevent water invading deep enough into the concrete to weaken the structure. 鈥淚f there鈥檚 no water, there鈥檚 not really a fear of damage,鈥� he says.
There鈥檚 also no fear of these bacteria turning into a health threat or ecological problem, he says 鈥� they die in the relatively pH neutral environments preferred by humans.
Jonkers鈥檚 team now aims to reduce the material鈥檚 cost and make sure its strength remains comparable to concrete currently on the market.
Jonkers presented his work at the in Cambridge, UK, today.
