How did a gardener's flowerpots lead to reinforced concrete?

Concrete is great at being squeezed and poor at being stretched, and reinforced concrete is the fix: a composite in which concrete's relatively low tensile strength is compensated for by embedded reinforcement with higher tensile strength, usually steel bars. The concrete takes the compression and the steel takes the tension. Put steel bars where a beam will be stretched, and you get a material that can be poured into shape and still carry the pull that plain concrete is poor at resisting.

The idea did not arrive in a flash. Joseph Monier, a French gardener, was dissatisfied with the available materials for durable flowerpots, and in 1867 he was granted a patent for reinforcing concrete flowerpots by mixing a wire mesh into a mortar shell. Remarkably, it is not clear that Monier even knew how much the steel improved the concrete's tensile strength. English builder William Wilkinson, by contrast, placed his reinforcement in 1854 in a way that showed he understood tensile stresses.

François Hennebique made it a system. He realised a floor would be more economic if the iron were used only where the slab was in tension, relying on the concrete in the compression areas. His solution was a concrete slab with steel bars in its bottom face, and in 1892 he patented a whole construction system that joined column and beam into one monolithic piece.

Rebar and concrete turn out to be good partners. When designed correctly, the alkalinity of the concrete protects the steel from corrosion. Ernest Ransome improved the bond by twisting the steel bars. And reinforced concrete soon showed its worth in a famous test: Julia Morgan's concrete bell tower El Campanil survived the 1906 San Francisco earthquake without any damage, which helped launch her career.