Reinforced concrete is concrete that has been strengthened by embedding steel bars or mesh inside it before it hardens. Concrete is excellent at resisting crushing forces (like the weight of a building pressing down), but it cracks easily when pulled or bent. Steel, on the other hand, is excellent at resisting pulling and bending forces. By combining these two materials, engineers create something stronger than either material alone.

The steel reinforcement, called rebar (short for reinforcing bar), is placed in a framework before wet concrete is poured around it. Once the concrete hardens, the two materials work together: the concrete handles compression while the steel handles tension. Think of it like a wooden pencil: the wood protects the graphite, while the graphite gives the pencil its purpose. Similarly, concrete helps protect the steel from rust while the steel helps keep the concrete from cracking under stress.

This invention, developed in the mid-1800s, transformed construction. Reinforced concrete made possible the skyscrapers, bridges, dams, and highways that define modern cities. The Hoover Dam, completed in 1936, contains enough reinforced concrete to pave a highway from San Francisco to New York City. Without reinforced concrete, many of the structures around you, from your school building to the parking garage downtown, couldn't exist in their current form.