Steel is strong in both tension and compression. Concrete is only strong in compression, and is very fragile in tension. Plus, concrete is heavy--a huge drawback in, say, a bridge deck. And its fairly expensive. And it hides the condition of the steel embedded within it. So what does it bring to the table? Why use it at all?
Pre stressed concrete is held in compression by the steel cables within.
As a composite material, reinforced concrete can be an economical alternative to structural steel in building, bridge and other superstructures. Reinforced concrete can be formed into almost any shape in the field. The bulk of reinforced concrete material - the aggregates - can often be produced local to a project, reducing manufacturing cost. Reinforced concrete weight can be a benefit, resisting loads that cause displacement, uplift and overturning.
Exposed steel requires constant maintenance including inspection and repainting. Steel does not provide a durable driving surface nor a good friction one for stopping unless it is textured, which makes driving on it very noisy. It is not possible to refinish a steel surface as is a concrete surface which can be ground and had several inches of new wear surface in concrete or asphalt added to it. Concrete is much less expensive than steel and the enclosure of steel inside the concrete protects it like a paint job does. Steel is weakened by heat, as was shown by the 9/11 collapses when the applied fire insulation was blasted off the trusses by the crashes, so a burning truck on or under a bridge may damage concrete but leave it standing strong enough to continue handling traffic during repairs, as at overpasses here in Texas where oil tankers crashed and burned.
Steel is strong in both tension and compression. No Steel is strong in tension. Under compression it will bend. Example. Take a steel cable. stretch it (Ok you cant) Now compress it. it bends. Bridges generally need support from below. So you need something thats strong under compression. IE concrete. However concrete can fail if it gets a tension - so pre-tensioned steel rods are cast into it to take any such tension (stretching or bending) forces.
