Who built that?
Why squeeze concrete before cars drive over it?
Prestressed concrete overpasses. A true how-it-works story for kids 6–10.
Read along
See those big concrete bridges that cars drive over, called overpasses? Here's a puzzle. Concrete is super strong when you squish it. But when something tries to pull it apart, it cracks!
And when a heavy truck drives across, the bottom of the bridge gets pulled and stretched. So a clever builder from France named Eugène Freyssinet came up with a trick, almost a hundred years ago. Before the concrete is ever used, workers pull long steel cables really tight, like stretching a giant rubber band. Then they let those cables squeeze the concrete hard, like a big tight hug.
The concrete is already squished, so when trucks pull on it later, it doesn't crack! It's like a bridge that's been hugged so tight it forgets to be wobbly. Next time we zoom under an overpass, look up at the long smooth beams. Can you count how many go by before the next one?
Quiz the back seat
What do workers use to squeeze the concrete?
Show the answer
Tight steel cables. Tight steel cables! Some builders even use cables made of super strong carbon fiber instead of steel.
3 fun facts
- Eugène Freyssinet patented prestressed concrete in 1928.
- Stretched tendons squeeze the concrete so it resists pulling forces later.
- Tendons can be steel wires, strands, bars, carbon fiber or aramid fiber.
Questions kids ask
What is prestressed concrete?
It is concrete that gets squeezed by tight cables before it is used, so it can handle pulling forces later.
Who invented prestressed concrete?
A French builder named Eugène Freyssinet patented it in 1928.
Why does concrete need to be squeezed?
Concrete is strong when squished but cracks when pulled, and squeezing it first helps it resist the pull from heavy trucks.
For grown-ups
Written with AI from public sources, then checked claim by claim against the sources below before it was published; stories that don't pass stay off the app and this site. Narration is pre-recorded, and nothing you do on this page is tied to your kids.
Sources
- pci.org/PCI/News-Events/Papers/2008/Buildings-Materials/Eugene-Freyssinet%E2%80%93the-Origins-of-Prestressed-Concrete.aspx?WebsiteKey=5a7b2064-98c2-4c8e-9b4b-18c80973da1e
- aspirebridge.com/magazine/2010Fall/Perspective_Fall10.pdf
- researchgate.net/figure/Eugene-Freyssinet-as-a-young-Engineer-and-in-his-later-years_fig1_266473409
- scienceabc.com/pure-sciences/why-does-concrete-have-great-compressive-strength-but-poor-tensile-strength
- tevema.com/is-concrete-good-in-tension-or-compression
- sciencedirect.com/topics/engineering/prestressed-concrete-bridge
- sciencedirect.com/topics/engineering/prestress
- pci.org/PCI/PCI/Publications/PCI_Journal/Issues/1998/September-October/Behavior_of_Externally_Draped_CFRP_Tendons_in_Prestressed_Concrete_Bridges.aspx
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