Rebar is short for “reinforcing bar,” a thick steel rod embedded inside concrete to make it much stronger. While concrete can handle tremendous squeezing and pressing forces (what engineers call compression), it cracks easily when pulled or bent. Steel has the opposite strength: it excels at resisting pulling and bending forces (called tension). By combining concrete and steel rebar, builders create structures that can handle both types of stress.

If you've seen construction sites, you might have noticed workers creating cage-like grids from rebar before pouring concrete. The concrete flows around these steel bars and hardens into a single, powerful unit. This reinforced concrete can support bridges, skyscrapers, parking garages, and highways. Without rebar, the concrete in these structures would crack under their own weight or crumble under the weight of cars and trucks.

The invention of reinforced concrete in the mid-1800s revolutionized construction, making it possible to build taller buildings and longer bridges than ever before. Today's rebar often has ridges or bumps on its surface, which help it grip the concrete more tightly, creating an even stronger bond between the two materials.