Using Concrete

Concrete is a particle composite material made from a mixture of cement, water
and aggregates. The cement bonds together the sand and small stones that make
up the aggregate. This makes a cheaper material than using cement on its own.

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The term aggregate applies to any filler material used in concrete. Sand is the finest aggregate used; coarse aggregates include gravel and crushed stone.

Concrete is very strong when subjected to a compressive force, making it a suitable material for the pillars that support a road bridge. It breaks easily when in tension, so it is not suitable for the horizontal surfaces supported by the pillars, parts of which are in compression and parts of which are in tension.

To overcome this, concrete can be pre-stressed which ensures that the concrete remains under compression even when subjected to its normal tensile load. In the manufacture of pre-stressed concrete, the concrete is cast around steel bars that are held in tension.

Concrete that is always in compression in normal use is not pre-stressed, but it can be reinforced by casting the concrete around a steel mesh.

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After the concrete has hardened the tension is released, shrinking the steel and compressing the concrete. The steel both reinforces the concrete, sharing the load, and keeps it in compression even when it is stretched.