Post Tension Cables

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How do post tension cables work? They aren’t springy but once the concrete sets if you cut one they spring up and it’s not a good time.

In: Engineering
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They are springy, sort of. The cables are laid into the concrete before it is poured. After the concrete is partially (but not fully) set, the cables are tensioned. They are literally *stretched*. This creates “pre-stressed concrete”, which is substantially stronger than standard concrete.

Due to stretching the metal cables, they are under extreme tension. But they are trapped in set concrete and can’t contract to release that tension. Unless you cut or break the concrete. Then all hell breaks loose.

Concrete is really weak if you pull it (tension) and strong if you push it (compression).

The steel is placed while being pulled like a spring (tension) and the concrete sets around it. The steel wants to return to its original shape so it pushes on the hardened concrete (compresses it) providing strenth.

If you remove the concrete, the metal is already pulled like a spring and can release that energy.