Why stretching steel inside concrete makes it stronger
The short answer: concrete is great at being squeezed and terrible at being pulled. Prestressing squeezes it on purpose, so that when a load tries to pull it apart, the squeeze has to be used up first.
Compression
Being squeezed. Concrete is extremely strong here: a block can take several thousand pounds per square inch of squeezing before it fails.
Tension
Being pulled apart. Concrete is weak here, roughly a tenth as strong. It cracks readily when stretched.
Watch the mechanism
This is one beam, shown in cross-section. First the cable is stretched and the concrete is squeezed. Then a load bends the beam. Watch what happens to the compression.
The bookcase analogy
Think of a row of books squeezed between two shelf ends. You can't push a book out of line easily because the whole row is already under pressure. Concrete prestressing does the same thing: the steel cable is the shelf end, and the squeeze it applies is what keeps the concrete from being pulled apart by a load.
Step by step
Open each step and watch the diagram again. The page makes sense with all of them closed, but the mechanism is clearest when you follow it in order.
1. Concrete is weak in tension
A plain concrete beam under a load bends slightly. The top edge gets squeezed, but the bottom edge gets stretched. Concrete can't take that stretch, so a crack opens on the underside and the beam fails. This is why plain concrete beams can only span short distances.
2. The cable is stretched before the concrete hardens
In pre-tensioning, the steel cable is pulled tight first, then concrete is poured around it. Once the concrete sets, the cable is released. It wants to spring back to its shorter length, but the concrete grabs it, so instead the cable pulls the concrete inward. That inward pull is a permanent squeeze on the whole beam.
3. That squeeze is a stored counterforce
The concrete is now in compression even before anything rests on it. When a load arrives and tries to stretch the bottom edge, it has to overcome that stored squeeze first. Only after the compression is fully used up does the concrete start to feel tension. Prestressing buys you a whole reserve of strength before the concrete is ever in danger.
4. The result: longer spans, thinner beams
Because the concrete is pre-loaded against the exact failure it would otherwise suffer, a prestressed beam can span much farther than a plain one and can be slimmer while doing it. That's why you see prestressed members in bridges, parking structures, and long floor slabs.
Doesn't the tension hurt it?
It's a fair worry: you're adding stress to a material that hasn't done anything yet. But the key is which kind of stress. The cable is in tension, which is exactly what steel is good at. The concrete is in compression, which is exactly what concrete is good at. Each material gets the job it's strong at. Nobody is asking concrete to be pulled, and nobody is asking steel to be squeezed.