Concrete is a strong and durable building material that has been used since 6500 BC. Nearly 2,000 years old, the Colosseum is a concrete structure that still stands today.
Concrete has extremely high compressive strength, but poor tensile strength. Once the load on the unreinforced concrete slab or structure reaches a critical point, tensile cracks will start to appear and the structure will eventually fail. Tensile strength is imparted to the structure by placing rebar and/or steel mesh in the formwork prior to pouring the concrete. So you strengthen the concrete. The rebar transmits tension (tension), push (compression), and sliding (shear) loads between the steel bar and concrete.
Rebar is often deformed (or ribbed) to form a strong bond with the concrete. Since steel and concrete bond so well, they act as a unit to resist the applied loads. The tensile strength of steel is equivalent to 100-140 times the tensile strength of a plain concrete mixture.
Concrete itself is only useful in a few limited applications. However, when reinforced with steel, concrete can be used to build slabs, walls, beams, columns, foundations, frames, and more.