Monier put steel where concrete was weak — in tension

Joseph Monier was a French gardener who was not satisfied with the materials available for flowerpots. Clay broke easily and wood weathered and could be broken by the roots of the plant. He began making pots and tubs of concrete, which was cheap and easy to shape, but the concrete alone was not stable enough — it cracked, because concrete is strong when squeezed but weak when pulled.

To make the containers stronger, Monier experimented with embedding an iron mesh in the concrete. He was not the first person to try reinforcing concrete, but he saw the possibilities and promoted it hard, exhibiting the idea at the Paris Exposition of 1867 and taking his first patent on 16 July 1867 for iron-reinforced troughs used in horticulture.

The essential point was that the two materials covered each other's weakness. Concrete is easily procured and shaped and has strong compressive or crushing strength, but it is distinctly weak in tensile or pulling strength. Steel is extremely strong in that pulling direction but is difficult and expensive to work up into custom shapes. So Monier put the steel where the tension was — on the underside of a slab, in a network of small rods — and let the concrete do the compressing.

He kept extending the idea with more patents: iron-reinforced pipes and basins in 1868, building-façade panels in 1869, bridges in 1873 and beams in 1878. In 1875 the first iron-reinforced concrete bridge ever built was constructed at the Château de Chazelet, with Monier as designer.