Concrete has a split personality: it's fantastically strong when you squeeze it, but it cracks easily the moment you stretch it. That matters the instant you make a beam. Press down in the middle of a concrete beam and the top gets squeezed, which concrete loves, but the bottom gets stretched, and that's where plain concrete cracks and snaps. The fix is simple and brilliant: lay steel bars along the underside, exactly where the stretching happens. Steel is superb at being pulled, so it takes over the stretching while the concrete handles the squeezing, and together they hold huge loads. Load a plain beam in the simulator and watch its underside crack, then switch on the rebar and load it again.
Most people think concrete is simply a strong material that fails when there is not enough of it. In fact concrete is strong squeezed but cracks when stretched, at only a tenth its compressive strength. A loaded beam stretches along its underside, so that is exactly where the steel must go.
What's actually happening
Concrete looks like the perfect building material, and for one kind of load it is. Squeeze it (load it in compression, the way the stones of an arch press on each other) and it's tremendously strong, which is why you can build a tall column or a thick wall out of it and pile floors on top. But concrete has a glaring weakness that shows up the moment you bend it: it is hopeless under tension, the pulling-apart kind of load. Its tensile strength is only about a tenth of its compressive strength, so wherever concrete is stretched, it cracks, and once a crack starts it runs and the piece fails. On its own, concrete is a one-trick material that only wants to be pushed, never pulled.
The trouble is that almost any horizontal beam gets pulled somewhere. Lay a concrete beam across two supports and load the middle, and it bends: imagine the beam curving into a faint smile. The top edge of that curve is being shortened, compressed, and concrete is delighted. But the bottom edge is being lengthened, stretched, and that is concrete's nightmare. So a plain concrete beam, however thick, cracks along its underside and breaks under a load the same beam could easily have carried in pure compression. The weakness isn't the amount of concrete; it's that the load has created a stretched zone the concrete can't survive.
The solution, worked out in the nineteenth century, is almost absurdly neat: put a material that loves being stretched exactly where the stretching is. Steel is strong in tension and bends without snapping, so embedding steel bars, rebar, along the underside of the beam lets the steel carry the tension while the concrete carries the compression up top. The two become a composite, each doing what it's best at, and the beam now holds many times the load. A crucial bit of luck makes it all possible: steel and concrete expand by almost the same amount when heated, so a hot summer doesn't shear them apart, and the alkaline concrete even protects the steel from rusting. This single pairing (concrete for the squeeze, steel for the stretch) is the backbone of nearly every bridge, building, and motorway on Earth.
Concrete takes the squeeze and steel takes the stretch, and pairing them in the tension zone is the backbone of nearly every bridge and building on Earth.
- 1Cast two small beams of plaster of Paris in a mould, and into one of them lay a length of wire or a few toothpicks along what will be the bottom face.
- 2Once set, rest each beam across two supports and press down in the middle: the plain plaster snaps cleanly along its stretched underside.
- 3The one with wire in the bottom resists far more before it gives, and even when it cracks it holds together — the embedded reinforcement is carrying the tension the plaster couldn't.
Common questions
Load the middle of a beam and it bends into a faint smile: the top is compressed, which concrete loves, but the underside is stretched, which concrete cannot survive, so it cracks there and fails.
Steel is strong and bends without snapping under tension, so it takes the pull while concrete takes the squeeze. A crucial bit of luck is that steel and concrete expand by almost the same amount when heated, so temperature swings do not shear them apart.
Modern beams often have their steel pulled tight before the load arrives, putting the whole concrete into compression first. Even the underside is then being squeezed, so it takes a huge load before any part is ever stretched.