Why steel goes in concrete

Concrete is excellent in compression and poor in tension: its tensile strength is only about a tenth of its compressive strength, and it cracks early. Steel is strong in tension. Putting steel bars where a member will be pulled gives a composite material that can bend, span and resist earthquakes.

Two facts make the partnership work. First, the ribs on deformed bars and the cement paste give a strong bond, so the bars and the concrete strain together. Second, steel and concrete expand with heat by similar amounts, so temperature changes do not break that bond. Concrete also protects the steel: its high alkalinity keeps the bar surface passive against rust, provided the cover is adequate and the concrete is dense.

In a beam that sags, the bottom is in tension and needs the main bars there. In a cantilever or over a continuous support, the tension moves to the top. In a column, bars help carry compression and resist any bending, and the ties around them stop the bars buckling and confine the core.

Design codes such as IS 456 treat the cracked section explicitly: concrete in tension is ignored and the steel carries all the tensile force.