The largest part of a concrete mixture is the stony part. This is called the aggregate. The stones may be small or large. Small stones are called gravel and large stones are called pebbles.

The three parts of the concrete can be mixed together using a spade but it is more usual to use a concrete mixer. This is a drum into which the mixture is put. The drum is then set turning and the parts of the concrete mix together.

A useful property of the mixture when it is freshly mixed is that it can be poured. This means that it can be poured into a mould and set into almost any shape.

When the concrete has been poured, the cement forms long crystals between the aggregate. They hold the aggregate together and give the concrete some of its strength. The stones in the aggregate also give the concrete strength.

The strength of concrete can be further increased by putting steel rods in it before it sets. Concrete which is strengthened by steel bars is called reinforced concrete.