Prestressed Concrete

Freyssinet claims steel "subjected to preliminary tensions" so high they stay tensioned, putting permanent compression into the concrete. Note the order: tension the steel FIRST.

Stand a row of small blocks in a line and squeeze the ends together with your hands. Now you can lift the whole row as one beam — the squeeze holds them together. Let go and they fall apart. That squeeze is prestress.

Cast a thin plaster beam (standing in for concrete), set it, and bend it. It cracks on the tension face at a small load. Record that load — the crack point to beat.

Make a stiff wooden frame with anchor points at each end. A wire will be stretched between them, through the beam mould. This is your prestressing bed.

Run a strong steel wire along the bottom of the beam mould and tension it hard against the jig anchors — as tight as you safely can. Wear glasses; a snapping wire whips.

Pour plaster into the mould around the stretched wire and let it set hard, keeping the wire under tension the whole time.

Once set, cut the wire free of the jig. The wire tries to shorten and squeezes the beam, which bows slightly UPWARD (cambers). You have just put the concrete into permanent compression.

Bend the prestressed beam as in step 3. It carries much more before it cracks — the load first has to cancel the built-in squeeze. Compare the two crack loads.

Cast a beam with a slack (un-tensioned) wire in it — ordinary reinforcement. It cracks early, then the steel holds the crack. The prestressed beam never cracked at that load at all. That is the difference.

Make a very slim prestressed beam and span a wide gap. It holds where a plain plaster beam that thin would snap under its own weight. Prestress buys slenderness and span.

Keep loading the prestressed beam until it does crack — and note that when you unload, the crack closes up again, squeezed shut by the prestress. Crack-free in service is the whole point.

The steel must stay stretched for years as the concrete slowly shrinks and creeps. Only high-strength wire, stretched a long way, keeps enough tension left over after those losses. Ordinary mild steel would go slack. Write down why.

The patent. US 2,172,703, granted 12 September 1939 to Eugène Freyssinet of France (US filing 1938, French priority 1935). This is one of a family of Freyssinet patents; his first prestressing patent was French, in 1928, which is the true birth of the technique. This particular patent extends prestressing to large monolithic, cast-in-place structures, built with light, transportable equipment.

Prestressing is the answer to reinforced concrete's flaw. A reinforced beam only mobilises its steel once the concrete has cracked — so in service it is permanently micro-cracked, which admits water and rusts the steel and limits how slender it can be. Freyssinet's insight was to put the concrete into compression before it ever carries a load: stretch the steel, cast around it, and release, so the recoiling steel clamps the concrete tight (steps 5-7). When a load then tries to stretch the beam's underside, it must first use up that built-in compression, so the concrete never actually goes into tension and never cracks (steps 8-9). Step 2's row of blocks squeezed by hand is the whole idea — compression turns loose pieces into a beam.

The hard part, and why it took a genius to make it work, is the losses. Concrete shrinks as it dries and creeps (slowly deforms) under sustained load for years, and every bit of that shortening lets the steel relax. Early experimenters used ordinary steel, stretched only a little, and their prestress simply leaked away as the concrete moved. Freyssinet's crucial realisation was that you must use very high-strength steel stretched a very long way, so that even after all the shrinkage and creep, plenty of tension — and therefore plenty of compression on the concrete — remains (step 12). He also developed the anchorages and jacks to do it reliably. It is a beautiful case of an idea that is obvious in principle and brutally subtle in practice.

What it built. Prestressed concrete is why concrete can now be slender and span far: it is in long-span bridges, flat floor slabs with few columns, railway sleepers, pressure pipes, storage tanks, stadium roofs and the containment of some structures. Together with Ransome's bond and Portland cement it completes the concrete story — cement makes it, reinforcement lets it take tension after cracking, and prestressing lets it never crack at all. The wires you tensioned in step 5 are, in a real bridge, stressed to tens of tonnes; the principle is identical, and it is squeezing that bridge tight right now.