self-healing concrete -- 3/11/15

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Today's selection -- from Stuff Matters by Mark Miodownik. Half of the world's structures are made from concrete, but much of this concrete has not been properly maintained, creating huge and growing infrastructure problems:

"Because despite reinforced concrete's impressive credentials as a building material, it does need care. In fact its vulnerability has the same origin as its strength: its internal structure.

reinforced concrete pre-slab

"In ordinary circumstances, exposed to the elements, the steel that is used to reinforce concrete is prone to rusting. When that steel is encased within concrete, the alkaline conditions create a layer of iron hydroxide on top of the steel, which acts as a protective skin. But during a building's lifetime, arising from normal wear and tear and the expansion and contraction that takes place during winters and summers, small cracks will appear in the concrete. These cracks can allow water inside, water that can freeze, expanding and creating a deeper crack. This type of attrition and erosion is what all stone buildings have to put up with. It is also what mountains have to put up with, which is how they get eroded. To prevent stone or concrete structures being similarly afflicted, maintenance of their fabric needs to be carried out every fifty years or so.

"But concrete can suffer from a more pernicious type of damage. This occurs when lots of water gets into concrete and starts to eat away at the steel reinforcement. The rust expands inside the structure, creating further cracking, and the whole internal steel skeleton can be compromised. It is particularly likely to happen in the presence of saltwater, which destroys the iron hydroxide protection and rusts the steel aggressively. Concrete bridges and roads in cold countries, which are regularly exposed to salt (such as is used to clear snow and ice), are vulnerable to this type of chronic deterioration. Recently London's Hammersmith Flyover was shown to be suffering from concrete decay of this kind.

Concrete cracking as steel reinforcing corrodes and swells. The breakage of material from a surface is called spalling.	Detailed view of spalling.

"Given that literally half of the world's structures are made from concrete, the upkeep of concrete structures represents a huge and growing effort. To make matters more difficult, many of these structures are in environments that we don't want to have to revisit on a regular basis, such as the Øresund Bridge connecting Sweden and Denmark, or the inner core of a nuclear power station. In these situations it would be ideal to find a way to allow concrete to look after itself, to engineer concrete to be self-healing. Such a concrete does now exist, and although it is in its infancy it has already been shown to work.

"The story of these self-healing concretes started when scientists began to investigate the types of life forms that can survive extreme conditions. They found a type of bacterium that lives in the bottom of highly alkaline lakes formed by volcanic activity. These lakes have pH values of between 9 and 11, which will cause burns to human skin. Previously it had been thought, not unreasonably, that no life could exist in these sulfurous ponds. But careful study revealed life to be much more tenacious than we thought. Alkaliphilic bacteria were found to be able to survive in these conditions. And it was discovered that one particular type called B. pasteurii could excrete the mineral calcite, a constituent of concrete. These bacteria were also found to be extremely tough and able to survive dormant, encased in rock, for decades.

"Self-healing concrete has these bacteria embedded inside it along with a form of starch, which acts as food for the bacteria. Under normal circumstances these bacteria remain dormant, encased by the calcium silicate hydrate fibrils. But if a crack forms, the bacteria are released from their bonds, and in the presence of water they wake up and start to look around for food. They find the starch that has been added to the concrete, and this allows them to grow and replicate. In the process they excrete calcite, a form of calcium carbonate. This calcite bonds to the concrete and starts to build up a mineral structure that spans the crack, stopping further growth of the crack and sealing it up.

"It's the sort of idea that might sound good in theory but never work in practice. But it does work. Research now shows that cracked concrete that has been prepared in this way can recover 90 percent of its strength thanks to these bacteria. This self-healing concrete is now being developed for use in real engineering structures. ...