Reinforced concrete structures are composed of two different materials
Plain concrete is a strong material in compression. Compressing a plain concrete cube or cylinder requires a relatively large amount of compressive force before reaching compression failure. However plain concrete is relatively weak in tension (typically can only carry one-tenth (1/10) of its compression strength in tension).
Reinforcing steel has excellent strength in both compression and tension loads but is more expensive than concrete. Therefore reinforced concrete structures are typically designed by engineers such that concrete is mainly utilized for most of the compressive forces and reinforcing steel is utilized for all of the tensile forces and in some cases some of the compressive forces. The design of reinforced concrete structures has been streamlined particularly over the last century for safety as well as economic feasibility. Reinforced concrete structures have had a tremendous track record in some of the most complicated structures including dams, bridges, and high-rise buildings across the globe.