Why is curing required for cement concrete

The small inquiry - why does concrete harden under water?

"That is actually not a problem", says Michael Raupach, professor at the Institute for Building Research at RWTH Aachen University. Because concrete needs water. Concrete consists of a mixture of cement, sand and grains of gravel. Water must be added to this mixture to make it hard. However, no air is needed for this. Concrete can therefore set under water.

If it is too dry, water must be added

Air can even be harmful to the concrete. This is why the concrete must always be kept wet during construction work on a house in summer, for example. If it dries too quickly and the water evaporates in the concrete, it leaves tiny passages, so-called capillaries, in the concrete - and these can easily crack.

Special precautions required underwater

Mixing concrete together underwater is actually ideal. But there are special conditions, for example when building bridge piers in the sea or in rivers. The building owners must ensure that the individual components of the concrete do not separate from one another. For example, simply pouring them into the water from above and then lowering them to prepared foundations would not work. The heavy parts, such as the gravel grains, would sink to the bottom faster than the cement - this would not result in the right mixture.

In projects such as building a bridge over a large river, the concrete is therefore brought to the right place with ships and brought under water there with buckets or pipes and then poured into previously prepared foundations.

Special underwater concrete is only required in special cases

It is only difficult for concrete to set under water if the foundations are in bodies of water with a strong current. Then there is a risk that the individual components - such as sand and gravel grains - will be carried away by the current before the concrete can harden. In such cases, special underwater concrete is used, which is constructed in such a way that the binding agent in the building material cannot be washed out by the current.

Status: 08/18/2016, 11:06 am