Reinforced concrete

node · earth · France, then worldwide

Concrete with steel bars embedded where the member goes into tension - the underside of a beam at midspan, the top over a support, the base of a wall. The composite works because the two materials have almost the same coefficient of thermal expansion, so they do not tear apart with temperature, and because the alkaline concrete passivates the steel and protects it from corrosion as long as the cover is sound and uncracked. Joseph Monier, a French gardener, patented iron-reinforced cement tubs and tanks from 1867; the decisive step to a complete structural system - monolithic beams, columns and slabs cast together, with bent-up bars following the tension line - is associated with Francois Hennebique, whose system was patented and licensed widely from the 1890s. Reinforced concrete gives a designer a material that can be moulded into any shape and can take tension, which is why the 20th century can build shells, cantilevers and flat slabs that no masonry tradition could attempt. Its durability failure mode is specific: chloride or carbonation reaches the bar, the bar rusts, rust expands, and the concrete spalls off.