Half of everything that gets made gets made from concrete. In terms of usage, it is the single most popular material on the planet – if you want to build a dam, a bridge, an airport or a city, this is the material of choice, yet very few people will admit to liking the stuff. No matter how beautiful they find the Sydney Opera House, how sublime they find the Millau viaduct or how awestruck they are by visiting the Hoover dam, concrete’s image never seems to improve. However, perceptions may now be changing as new forms – such as self-healing concrete – are being brought into production.
Concrete is poured into existence, which is what makes it such versatile and liberating stuff. Build a mould for your structure, pour in the concrete and hey presto, you have something that can last for thousands of years. For example, the dome of the Pantheon in Rome was built from concrete 2,000 years ago.
The hey presto bit is interesting. During the pour, a mixture of cement, stones and water hardens to become synthetic rock. It is not a process of drying out; instead it undergoes a series of chemical reactions via the formation of a gel. Gels are semi-solid and wobbly types of matter. The jelly served at children’s parties is a gel, it doesn’t flow like a liquid because it has an internal skeleton which prevents the liquid moving around. In the case of jelly this is created by the gelatin. In the case of cement the internal skeleton is made up of calcium silicate hydrate fibrils, which are created as a result of the chemical reaction with the water. The gel formed inside the cement is constantly changing and, as the fibrils grow and meet, they mesh together forming bonds and locking in more and more of the water, until it becomes fully solid.
Modern concrete is reinforced with steel rods, which gives it architectural versatility and robustness. You’ll see them sticking out of all construction sites during a concrete pour. Getting their design right is vital if you want a bridge or a dam to be able to withstand bending forces and shocks.
As with any recipe, if you get the ingredients or process wrong, then you get a mess. In the case of concrete though, the problems can go undiscovered, because on the surface all concrete looks roughly the same. This can lead to catastrophe many years after the structure is built. The extent of the devastation due to the 2010 earthquake in Haiti was blamed on shoddy construction and poor quality of concrete: according to government figures, an estimated 250,000 buildings collapsed, killing more than 300,000 people and making a million more homeless.
Given that most of the world’s infrastructure is made from concrete, the upkeep of concrete structures represents a huge and growing effort. Even well-made concrete needs to be looked after, especially if you want it to last sustainably for thousands of years. It would be ideal to find a way to allow concrete to care for itself. Materials scientists have now done just that. Self-healing concrete works by embedding bacillus pasteurii bacteria inside the concrete along with a form of starch which acts as food for the bacteria. Under normal circumstances, these bacteria remain dormant inside the concrete. But if a crack forms, air gets in and the bacteria wake up and start to eat the starch that has been added to the concrete. They grow and reproduce themselves and in the process they excrete the mineral calcite, a form of calcium carbonate. This calcite bonds to the concrete and starts to build up a mineral structure that fills the crack and seals it up. Research now shows that the cracked concrete can recover 90% of its strength with the help of these bacteria.
Another new type of concrete called pervious concrete allows rainwater to flow straight through it, and is designed to protect urban structures from flooding. There is also a self-cleaning concrete that incorporates particles of titanium dioxide which, in the presence of sunlight, causes the degradation of dirt on its surface through a process of photocatalysis. These new materials have the potential to change how concrete looks after itself, and thus how we feel about it.
There are some signs that concrete is being culturally embraced. In the recent British film Locke, Tom Hardy plays John Locke, a construction manager in charge of the largest concrete pour in Europe. Although he is the only actor to actually appear in the film, several other characters are crucial to the plot, and one of them is the concrete itself. The pouring of the material – insistent, demanding and transformative – is metaphorically linked to the imminent birth of a child.
Locke is portrayed as solid, dependable, obstinate and, like concrete, when put under extreme pressure is immovable. The film is well worth seeing, not least because of its nuanced portrayal of this most fundamental of materials.
