Reinforced concrete

Concrete is strong in compression and weak in tension by roughly a factor of ten, so a concrete beam fails by cracking on its underside long before it crushes on top. Reinforced concrete puts steel bars where the tension is, and the composite carries both.

Three things have to be true for that to work, and the second is a genuine coincidence. The steel must bond to the concrete, which ribbed bar and the cement paste achieve. The two must expand at nearly the same rate with temperature — and they do, within a few per cent, which is not something anybody arranged and without which the material would tear itself apart through a summer. And the concrete must protect the steel from corroding, which it does chemically: fresh concrete is strongly alkaline, around pH 13, and steel in that environment forms a passive film and does not rust.

Almost every failure of reinforced concrete is the third condition breaking down.

Reinforcement is cut, bent and fixed into position, formwork is erected around it, and concrete is poured and compacted so that it flows around every bar without leaving voids. Cover — the depth of concrete between the bar and the surface — is the single most important dimension on the drawing and the one most often got wrong on site.

Prestressing is the refinement: tendons tensioned before or after the concrete cures put the section into compression, so it does not crack at all under service load. That is what allows the long spans of modern bridges and floor slabs.

More of the built environment than any other structural material: frames, floors, foundations, bridges, dams, tunnels, retaining walls and marine structures. By mass, concrete is the second most used substance on Earth after water, and most structural concrete is reinforced.

Several near-simultaneous claims in the mid-nineteenth century; Joseph Monier's reinforced planters of 1867 and François Hennebique's framing system of the 1890s are the usual markers. The Romans had concrete and no reinforcement, which is why Roman structures are arches, vaults and domes — forms that carry load in compression alone.

The twentieth century built with it on an enormous scale on the assumption that it was permanent. It is not, and the buildings of the 1960s and 1970s are where that is now being paid for.

Cement production is responsible for a substantial share of global carbon dioxide emissions — commonly put at around eight per cent — and roughly half of that is the chemistry rather than the fuel: calcining limestone releases carbon dioxide from the rock itself, and no change of energy source removes it.

The durability problem is the other half of the account. Carbonation slowly neutralises the concrete's alkalinity from the surface inwards, and chloride from seawater or de-icing salt penetrates it; either way the steel's passive film goes, it rusts, and rust occupies several times the volume of the steel it came from. The expansion cracks the concrete off from the inside, which admits more water, which accelerates it. That is why so much post-war concrete infrastructure needs expensive repair, and it is a materials failure rather than a design one.