Using bacteria to make self-healing concrete

Concrete is the most important
of all our construction materials. It’s the most widely used material
on earth, after water. If concrete has one flaw, it’s that
it has low tensile strength, and therefore we need to add steel,
to make reinforced concrete. And in turn, the concrete needs to
protect the steel from corrosion. However, should the concrete crack, then that provides a pathway
for harmful substances such as chlorides, carbon dioxide,
and ultimately oxygen and water, to get to the reinforcing steel, cause corrosion, cause rust, and that ultimately leads to
destruction of the concrete. The cracking of concrete is a problem because of the need for repair,
which can be terribly expensive – imagine motorway bridges, for example. So here at the University of Bath,
we’re working in collaboration with the Universities of Cambridge
and Cardiff, all funded by EPSRC, looking at self-healing concrete. We are trying to seal cracks as they form using bacteria which are able to produce limestone. Remarkable. And if we can incorporate those into the cracks or around the cracks that will form, the limestone will then plug those gaps. Concrete is a very hostile environment, and the bacteria which fit the bill can survive in this hostile environment, of high alkaline pH and produce
limestone at the same time. These are the three bacterial cultures which we are working with right now in the lab and they perfectly grow in alkaline conditions. That is what makes them suitable
for their use in concrete, and here you can see they have grown
in a liquid culture medium which is alkaline. It looks cloudy, which indicates that the bacteria
are growing in this medium. These bacteria are now in a medium
which contains calcium, and in the presence of calcium and
carbon dioxide they produce limestone, which is calcium carbonate. The cracks in concrete are then impregnated with bacteria and bacterial food which contains calcium, and these bacteria have used this calcium
with carbon dioxide to precipitate calcite, and you can see that the crack has been
healed by the bacteria. If we could have a self-healing concrete,
that would alleviate the need for repair, which would save considerable
amounts of money. But it would also have an environmental saving – if you remember how much concrete we make, the amount of cement we need to make that concrete contributes between 5-7% of carbon dioxide emissions. So any concrete that we don’t need to make reduces the CO2 emissions produced by this industry. Bacteria wouldn’t survive the harsh conditions and the manufacture of concrete. So we’ve chosen bacteria that produce spores. These are long-lived, really tough – last 100 years in some species. So, we know how to turn them on in culture, we can produce them in bulk, and they’ve got to go into concrete. The problem is, they probably need protection
in some form of capsule. And we’re producing various things to protect them from the
crushing of the concrete, that will keep the bacteria and the food separate, and will crack or dissolve when the crack forms to release those, to produce the limestone. We’re hoping to carry out trials of this
self-healing concrete in November. In the long term, we anticipate self-healing concrete being a specialist material that can be used in
many important structures, particularly underground structures
or underwater structures, where simply repair isn’t an option. And we hope that self-healing concrete will be
a very important material for the construction industry in the years to come.

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