Durable concrete

MIT civil engineers have identified what causes concrete to gradually deform, decreasing its durability and shortening the lifespan of bridges and nuclear-waste containment vessels.

The researchers say that concrete creep (the technical term for the time-dependent deformation that occurs in concrete when it is subjected to load) is caused by the rearrangement of particles at the nano-scale.

‘Finally, we can explain how creep occurs,’ said MIT’s Prof Franz-Josef Ulm.

‘We can’t prevent creep from happening, but if we slow the rate at which it occurs, this will increase concrete’s durability and prolong the life of the structures.’

In 2007, Ulm discovered that the basic building-blocks of cement paste at the nano-scale – calcium-silicate-hydrates – are granular in nature and naturally self-assemble at two structurally distinct but chemically similar phases when mixed with water, each with a fixed packing density close to one of the two maximum densities allowed by nature for spherical objects (64 per cent for the lower density and 74 per cent for high).

In his new research, Ulm discovered that concrete creep comes about when these nanometre-sized calcium-silicate-hydrate particles rearrange into altered densities: some looser and others more tightly packed.

He has also shown that a third, denser phase of the calcium-silicate-hydrates can be induced by carefully manipulating the cement mix with other minerals such as silica fumes, a waste material of the aluminium industry.

These reacting fumes form additional smaller particles that fit into the spaces between the nano-granules of the calcium-silicate-hydrates, spaces that were formerly filled with water.

This has the effect of increasing the density of the calcium-silicate-hydrates to up to 87 per cent, which in turn greatly hinders the movement of the granules over time.

‘The addition of silica fumes is one known method in use for changing the density of concrete, and we now know from the nanoscale packing why the addition of fumes reduces the creep of concrete,’ said Ulm.

The researchers have also shown experimentally that the rate of creep is logarithmic, which means slowing creep increases durability exponentially.

They have also demonstrated mathematically that creep can be slowed at a rate that would have a remarkable effect on the durability of concrete structures: a containment vessel for nuclear waste built to last 100 years with today’s concrete, for example, could last up to 16,000 years if made with an ultra-high-density (UHD) concrete.

Ulm stressed that UHD concrete could impact the way that structures are designed and have enormous environmental implications; this is because concrete is the most widely produced man-made material on earth: 20 billion tons a year are made worldwide with a 5 per cent annual increase.

Ulm‘s work was funded in part by the Lafarge Group, a French building-materials producer.