Reinforced concrete

Why does concrete need steel buried inside it to span a gap?

Concrete is the strongest thing most people ever touch, and a column of it carries a building without complaint. Yet cast a plain concrete beam across a gap, load it, and it snaps — from the bottom, where nothing is pressing down at all. Spanning asks the material for a different favour than standing does. What is it?

Bend a rubber eraser between your fingers and watch its sides. The outer face stretches; the inner face bunches up. A loaded beam does the same, invisibly: the top fibres shorten and the bottom fibres lengthen, with a surface between — the neutral axis — doing neither. A beam is not simply "pushed on." It is squeezed along the top and pulled apart along the bottom, at once.

Now ask what concrete does with each demand. Squeezed, it is superb: a typical structural mix takes something like 30 megapascals. Pulled, it is feeble — roughly a tenth of that, and brittle about it, with almost no warning. That asymmetry comes from what concrete is: its strength lives in hardened cement paste gripping stone, and a grip resists being pressed together far better than being torn apart. Which half of the beam fails first, then? The bottom — exactly where the eraser stretched.

So what would fix it? Something that excels precisely where concrete is worst. Ordinary reinforcing bar yields in tension around 500 megapascals, and yields gradually, sagging visibly before it lets go. Could we build the beam from steel alone? We could, and sometimes do — but a slender steel member buckles sideways long before it is crushed, and bare steel in weather rusts.

Put the two lists side by side and the pairing writes itself: concrete where the squeeze is, steel where the stretch is. In practice that means bars low in the beam, and — where a beam runs continuous over a support — bars near the top there, because the bending reverses and the tension moves upstairs. Stirrups loop around them to catch the diagonal tension near the ends.

But complementary strengths alone do not make a composite; two further conditions must hold, and they are the part usually left out. The materials must move together under temperature. They do, nearly: concrete expands about 10 parts per million per degree, steel about 12. Had those differed by a factor of two, every hot afternoon would have sheared the bond apart. And the steel must survive being buried — which it does, because fresh concrete's pore water is strongly alkaline, pH around 13, and that alkalinity grows a thin passive oxide film that stops the bar corroding.

The first thing to unlearn is that a well-built concrete beam does not crack. It does, by design. Concrete reaches its tensile limit at strains far below what the steel needs to take up load, so the tension zone is finely cracked under ordinary service loads. Engineers do not prevent those cracks; they limit their width, so the cover — the concrete between bar and outside world — keeps doing its chemical job.

Which brings us to the honest failure mode, and it is not the steel snapping or the concrete crushing. It is the loss of that alkaline protection, by two routes: carbon dioxide seeping in and reacting with the calcium hydroxide to form calcium carbonate, dropping the pH — carbonation, advancing slowly inward from the surface — or chloride ions from de-icing salt and sea spray reaching the bar and breaking down the passive film locally even while the pH is still high.

Either way the film goes, the steel rusts, and rust occupies several times the volume of the steel it consumed. That expansion is a wedge working from inside: it cracks the cover, the cracks admit more air, water and salt, and the process feeds itself until the cover spalls off in sheets. Durability is therefore mostly a question of how deep and how impermeable the cover is — not of how strong the concrete tested on the day it was poured.

• How prestressing squeezes the concrete in advance so the tension zone never opens at all.
• Why fire is the acute threat: steel loses strength rapidly with temperature, and cover is its shield.

Every term the collection defines is gathered in the glossary.