why we use steel reinforcement in concrete for construction?

Reinforced concrete is a material that combines concrete and some form of reinforcement into a composite whole. Whilst steel bars, wires and mesh are by far the most widely used forms of reinforcement, other materials are used in special applications, e.g. carbon-filament reinforcement and steel fibers.
Concrete has a high compressive strength but a low tensile strength. Steel, on the other hand, has a very high tensile strength (as well as a high compressive strength) but is much more expensive than concrete relative to its load-carrying ability. By combining steel and concrete into a composite material, we are able to make use of both the high tensile strength of steel and the relatively low-cost compressive strength of concrete.
There are some other advantages to combining steel and concrete in this way which are derived from the characteristics of the materials.(These characteristics are summarised in Table-1-).

The aim of the reinforced concrete designer is to combine the reinforcement with the concrete in such a manner that sufficient of the relatively expensive reinforcement is incorporated to resist the tensile and shear forces which may occur, whilst utilizing the comparatively inexpensive concrete to resist the compressive forces.
To achieve this aim, the designer needs to determine not only the amount of reinforcement to be used, but how it is to be distributed and where it is to be positioned. These latter decisions are critical to the successful performance of reinforced concrete and it is imperative that, during construction, reinforcement be positioned exactly as specified by the designer.
It is important, therefore, that both those who supervise the fixing of reinforcement on the jobsite, and those who fix it, have a basic appreciation of the principles of reinforced concrete as well as the principles and practices of fixing reinforcement.
Like reinforced concrete, prestressed concrete is a composite material in which the weakness of concrete in tension is compensated by the tensile strength of steel – in this case, steel wires, strands, or bars.