The Steel Inside Modern Concrete Swells as It Rusts and Breaks the Concrete

Modern concrete fails early because rust on the steel inside it breaks the concrete apart.

Concrete is superb at carrying weight pressing straight down, what engineers call compressive strength. That is why it is ideal for pillars. It is weak when it is pulled or bent, what engineers call tensile strength. In the 19th century, builders answered that weakness with reinforced concrete, concrete poured around steel rods known as rebar. One material was meant to carry the load, and the other was meant to take up the stretch.

Concrete seems dense, but it is porous, and tiny cracks form over time. Moisture, air, and deicing salts work their way inward until they reach the steel. Then rust starts.

Rust needs far more room than the steel it replaces. The rust can bulk up to five to 10 times as large as the metal it came from. For every cubic centimeter of steel that turns to rust, up to 10 cubic centimeters of material try to force their way out. That expansion pushes outward with a pressure of three to four megapascals, a unit for force spread over an area.

That push strikes concrete where it is weakest. The ability of concrete to resist being pulled apart is only about 10 percent of its ability to resist being crushed, so it cannot hold back the swelling metal.

When the outer layer cracks open, the damage speeds up. Engineers call this spalling, when concrete breaks and flakes off. With the concrete over it gone, the metal meets still more water and air. Corrosion quickens, expansion grows, and still more of the surface breaks away. Metal meant to add strength ends up driving the breakup.