Concrete is strong in compression but weak in tension. Steel is strong in both. Combine them, and you get reinforced concrete, a material that can span, cantilever, and resist earthquakes. The steel bars, called rebar, are embedded in the concrete before it hardens. They carry the tensile forces that concrete cannot.

The concept seems simple, but the details matter. Rebar must be placed correctly, with enough cover to protect it from moisture and fire. It must be lapped and tied so forces transfer between bars. Stirrups hold longitudinal bars in place and resist shear. The concrete must be consolidated around the steel, with no voids or honeycomb.

Reinforced concrete changed architecture. It made possible the long spans of bridges, the thin shells of mid-century modernism, and the high-rise towers that define modern cities. It also made possible the parking garage, the highway overpass, and the shopping mall.

It is not a perfect material. It cracks. It can corrode. It is heavy. But it is versatile, durable, and widely available. For many structures, it remains the obvious choice.