Replies

1. 
   Guest_ | Apr 27, 1999 10:59pm | #1

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   Cables can be used in pre-stressed concrete. By putting the steel in tension during the pour/cure and releasing the steel afterwards, the concrete is always in compression tension from the stretched steel. Often used in concrete beams and bridges. Unusual to do it in a slab because it's easy to just add more thickness for more strength. Could the slab be on unstable ground? A co-worker once designed a slab to hold equipment at landfill (large, uneven settling). The slab was strong enough to hold its load on any three points of support.

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   jcallahan | Apr 27, 1999 11:32pm | #2

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   It's known as a post tensioned slab. The cables are in addition to other metal reinforcement. The cables are tensioned after the slab is poured. Concrete has excellent strength in compression and poor in tension. The technique is often used in concrete bridges where cables are run through the precast segments (after the bridge is errected) and the cables are tensioned (tightened).I've seen it used in residential construction in Gulf Coast area where unstable clay soils (lots of expansion and contraction) cause many slab problems.

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   Guest_ | Apr 28, 1999 12:06am | #3

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   well Roy there both right as strange as it may sound. Have to see your slab to get some idea what you got there

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   jcallahan | Apr 28, 1999 12:41am | #4

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   Bill, you are right ......could be either prestressed or post tensioned. Assumed it was residential, hadn't run into any prestressed home slabs..... sometimes a little presumption is a dangerous thing :).

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   John_Fuhrman | Apr 28, 1999 01:34am | #5

   *Roy: Ditto jcallahan remarks-used frequently in OK where there is a lot of clay (unstable) soils.

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   Roy | Apr 28, 1999 02:42am | #6

   *jcallahan, sorry, should have mentioned it was residential, actually a large condo complex. Builders have big time problems out here (Denver, CO) with expansive soils so the cabling must keep everything together when the soil starts moving.

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   Roy | Apr 29, 1999 05:27pm | #7

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   Recently while browsing around some new construction I observed what appears to be heavy steel cables (about 3/4" dia.) running through the concrete slab foundations. I am curious, are the cables used in place of rebar? Is there some structural advantage to this?

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   Guest_ | Apr 29, 1999 05:27pm | #8

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   We were replacing the front entry to a condo last summer. two guys went to work with jackhammers and a few minutes later the end of the slab seemed to explode, as the cable pulled through the broken slab. concrete schrapnel. we cleaned our cuts and figured prestressed, or poststressed, it was stressed