Casting Reinforced Concrete — Marrying Steel and Stone into the Modern Structure

Concrete is wonderful stuff in compression — squeeze it and it holds — but pull on it and it cracks like a biscuit. Steel is the opposite: superb in tension, useless if you try to build a wall of it. In the late 1800s builders like Joseph Monier and François Hennebique realised that embedding steel inside concrete combines the best of both, and reinforced concrete became the material that built the modern world.

The idea works because of three lucky properties. Concrete and steel expand and contract at almost exactly the same rate, so temperature changes do not tear them apart. Wet concrete is strongly alkaline, which coats the steel in a protective film that stops it rusting. And the two grip each other firmly, so a load shared between them is truly shared. Put steel where the structure is stretched and concrete where it is squeezed, and you get a member strong in every direction.

From this came bridges, dams, tunnels, and the skeletons of skyscrapers — shapes impossible in stone or iron alone. The skill is in knowing where the steel must go and in surrounding it completely with sound, well-cured concrete.