Concrete with steel bars in it to enlarge the thoughness of the concrete. Mostly used in bridges or other large structures.

Steel is often used to reinforce concrete for several reasons:

  • Steel and concrete perform better in different types of loading. Concrete is very good in compression, and almost worthless structurally in tension. Steel, on the other hand, is very good in tension, and due to ductility, not very good in compression. Therefore, the concrete will tend to bear compressive loads, and steel bears tensile loads.

  • Steel and concrete have pretty similar coefficient of thermal expansion, aka, they expand and contract due to changes in temperature at very similar rates. If this weren't true, significant deformation and stress would occur due to the conflicting forces.

  • Steel and concrete are chemically compatible. None of the common materials used for concrete react strongly with steel. If strong reactions occurred between the materials, for instance if the concrete corroded the steel, the combination would not be nearly as useful.

There you go, a fairly short discussion on the reasons for combining concrete and steel to make stronger, more efficient structures.

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