IT’S useful stuff, concrete, but it does have drawbacks. One of the biggest is that it is not as weatherproof as the stone it often substitutes. Salt and ice routinely turn microscopic fractures in its fabric into gaping holes. These let water soak in. And that, in time, can cause the entire structure to fail. The upshot is that it concrete needs constant repair by teams of workmen assigned to fill in the newly formed gaps, which is tedious and expensive. So, all in all, it would be better if the stuff could heal such damage by itself. And that, as he reports in Applied Materials & Interfaces, is exactly what Chan-Moon Chung of Yonsei University in South Korea hopes to get it to do. Self-healing concrete is not a new idea. In 2009 a team at the Delft University of Technology in the Netherlands showed it is possible to mix special bacteria, which release crack-sealing chemicals, into concrete before it is poured. These bacteria do, indeed, keep the concrete healthy—but only while they are alive. Experience shows they last for about a year, so the biological route to self-healing concrete helps only a little.

Dr Chung’s approach, by contrast, is chemical. He and his colleagues knew from laboratory tests that when two substances called methacryloxypropyl-terminated polydimethylsiloxane and benzoin isobutyl ether are mixed in the presence of sunlight, they are transformed into a protective waterproof polymer that sticks readily to concrete. The challenge was to pack these chemicals up in a way that would keep them safe until they were needed, and then release them. The solution the team came up with was to put the healing balm inside tiny capsules made of urea and formaldehyde. These would screen the chemical mixture from sunlight and keep it safely stowed away. They would, however, be weak enough to rupture and release their contents when the concrete near them cracked.

To make these capsules, the team stirred together a solution of water, urea, ammonium chloride and a benzene derivative called resorcinol that encourages capsule formation. They then added methacryloxypropyl-terminated polydimethylsiloxane, benzoin isobutyl ether and formaldehyde, and cooked the mixture for 4½ hours at 55°C. This process caused the urea and the formaldehyde to form, as desired, capsules containing the two concrete-healing chemicals.

To deploy his capsules, Dr Chung mixed them into a liquid polymer, sprayed the mixture on to some concrete blocks, each weighing two-thirds of a kilo, and allowed the resulting film to solidify. He then cracked each block in turn, by applying pressure, and put the blocks out in the sun for four hours.

As he hoped, the cracks in the concrete propagated into the polymer film containing the capsules, and cracked some of them open too, releasing their contents. These then set, on exposure to the sun, into a waterproof layer—a fact he proved by immersing the blocks in water. After 24 hours immersion he weighed the blocks, to see how much water they had soaked up. On average, untreated concrete accumulated 11.3 grams of water. Concrete coated with capsule-free polymer took in 3.9 grams. But concrete covered with a polymer layer containing capsules absorbed just 0.4 grams. The cracks had, just as Dr Chung hoped they would, healed themselves.