Why is steel reinforcement used in concrete?

Steel reinforcement is used because concrete is strong in compression but weak in tension (about 10% of compressive strength). Steel bars placed in tension zones resist tensile stresses that would otherwise cause cracking and failure. The combination works because steel and concrete have similar thermal expansion coefficients, concrete protects steel from corrosion, and bond between steel and concrete allows stress transfer. This creates reinforced concrete (RCC), an efficient structural material.