Reinforced concrete

Reinforced concrete is a composite material made of concrete with steel bars set inside it. The combination is strong under both a squeezing force and a stretching force. Malay: konkrit bertetulang · Chinese: 钢筋混凝土.

The design solves a specific weakness of concrete. Concrete on its own is strong under compression, a squeezing force, but weak under tension, a stretching force, so a plain concrete beam can crack and break when it is bent. Steel, by contrast, is very strong under tension. By casting steel bars inside the concrete, the concrete carries the compression while the steel carries the tension, and the finished beam is strong under both. This is a clear example of a composite: two materials combined so the product is better than either alone.

Example. Reinforced concrete is used for the beams, columns and floors of buildings and for the decks and supports of bridges, where the structure must bear heavy loads that both squeeze and stretch different parts of it at the same time.

Confusion to avoid. Do not confuse compression with tension. Compression is a squeezing or pushing force, which concrete resists well; tension is a stretching or pulling force, which concrete resists poorly but steel resists well. Reinforced concrete is stronger than plain concrete precisely because the steel supplies the tensile strength.

A full answer says why plain concrete alone is not enough: it is strong when squeezed but cracks when stretched, so steel bars are cast in to carry the stretching force. Naming both the force involved and the material that resists it, for each of compression and tension, is what turns a half answer into a complete one under exam conditions.

Reinforced concrete is the go-to composite example in SPM Chemistry, and questions usually want you to link concrete to compression and steel to tension.