The purpose of reinforcement is to provide additional strength for concrete where it is needed.
Bars, wires, strands, fibers, or other slender elements that are embedded in a matrix such that they act together to resist forces.- ACI Concrete Terminology
Most concrete used for construction is a combination of concrete and reinforcement that is called reinforced concrete. Reinforcement for concrete is provided by embedding deformed steel bars or welded wire fabric within freshly made concrete at the time of casting. The purpose of reinforcement is to provide additional strength for concrete where it is needed. The steel provides all the tensile strength where concrete is in tension, as in beams and slabs; it supplements the compressive strength of concrete in columns and walls; and it provides extra shear strength over and above that of concrete in beams.