Steel bars are used in reinforced cement concrete (RCC) to increase the tensile strength cement concrete is also called rebar.
The amount of rebar in typical engineering structures is a small percentage of the amount of concrete. In beams, it is usually about 1% of the cross-section. The rebar in a beam carries the tension forces in bending. In columns up to 6% rebar may be used, partly because the rebar in column also carries some axial forces in addition to tensile forces. When compared to concrete, rebar costs a lot more and hence optimal use of rebar can result in an economical structure.