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What is a Post-Tension Slab?

Many modern homes are built on post-tension slab concrete.
Parking garage built with post-tension slab concrete.
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  • Written By: Adam Hill
  • Edited By: Bronwyn Harris
  • Last Modified Date: 16 April 2014
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    Conjecture Corporation
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Even though concrete is a very rigid material, it has a natural weakness when it comes to tension. It is limited with respect to the length of a beam, floor, or bridge that can be made out of it. One way to be able to build these structures with longer spans than would be possible with ordinary concrete is through a technique called pre-stressing. A post-tension slab is a slab of concrete that has been pre-stressed using a specific method to increase the strength of the concrete.

Several methods exist for pre-stressing concrete, with post-tensioning being a very common one. Before a post-tension slab is poured, high-strength steel strands or cables, called tendons, are laid in a tight grid. These help support and give strength to the slab once it has cured. The tendons are sheathed in plastic so that they do not directly touch the concrete. After the grid is made, the concrete is poured, with extra care taken to make sure that the tendons remain at the correct depth.

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The concrete is allowed to cure to about 75% of the way, at which point post-tensioning occurs. Each of the tendons in the post-tension slab is pulled tight, using a hydraulic jack. The tensing of the cables occurs after the concrete has mostly cured, hence the term “post-tension.” The tendons are usually pulled to a tension of 25,000 pounds per square inch (4503 kg per square cm). Once the cables have reached the designated tension, they are anchored in the concrete, and the slab is allowed to fully cure.

Many modern homes are built on a post-tension slab, which serves as an excellent foundation. This method of pre-stressing concrete is especially useful in areas where the soil expands and contracts relative to weather conditions. Apart from residential applications, post-tension concrete opens up the possibility of many construction techniques that otherwise would be impossible. For example, parking garages and stadiums are stronger and cheaper to construct with post-tension concrete.

Using a post-tension slab rather than ordinary concrete often makes good economic sense. Because there is a smaller depth of concrete used to obtain the same end result, construction costs are reduced. This particular advantage has even larger implications for the construction of skyscrapers and office buildings.

When the floor thickness is reduced, so is the weight of the structure. A lighter building means that the cost of building the foundation is reduced. Thinner floors also translate into reduced building height, which means that exterior finishing costs, such as window glass, are lessened.

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Discuss this Article

anon340428
Post 22

Is it necessary to place minimum reinforcement in addition to post tension cable?

anon301688
Post 21

"What would happen if you drilled a hole for a dynabolt and hit one of the tendons"?

At best you expose the tendon to corrosion. At worst you sever it. Both will weaken the structure.

"How does one know if one has a PT slab? Are there any tell-tale signs?" Tendon ends (grouted live ends, slab pans, etc), staples on slab soffit, slender structures.

anon295503
Post 20

I have a post tension foundation that is cracking in the middle of the home working it's way from both exterior walls an soon will meet in the middle. The crack is about a pencil lead in width, but is going through the middle of my 18" tiles cracking each one. I thought the post tension was suppose to prevent this. What can be done to correct this problem?

anon231643
Post 19

Why do we say that a post-tension slab has main steel both ways?

anon205749
Post 18

When did PT start as an option for homes foundations?

anon171689
Post 17

Is there any way I can install an electrical outlet into the floor five feet from the wall of a post tension floor already poured?

anon164491
Post 16

If the distance column to column is 17m wide, can you still use a post tension slab, and on top still have a 10 story high apartment?

anon138282
Post 15

Please could you shed more light on this? I read that post tension becomes economical when the floor span is 7m and above. Why?

senkofamily
Post 14

we want to do a remodel in our kitchen. our house has a post tension foundation. the water supply is coming from the slab and needs to be moved over three feet to the wall. what would be the best approach to our remodel?

anon106560
Post 12

what is the difference between partial and full stressing?

anon106555
Post 10

What is the exact strength for stressing as at the time of stressing no full loading is available.

anon90018
Post 9

how do I execute a pt slab in a multistory building?

anon77658
Post 8

"What would happen if you drilled a hole for a dynabolt and hit 1 of the tendens".

If you hit the cable, and sever it, it is very likely the whole line of the cable will burst, which in turn can cause complete failure of the slab. This is something you must plan very carefully.

"How does one know if one has a PT slab? Are there any tell-tale signs?"

There are no tell tale signs you can see with the naked eye. You would need to scan the slab using a ferro scan machine, which is very costly, or retrieve the construction drawings for the structure.

"seventh day cube test has passed for the PT slab and so the tendon tensioning has been done. Can we deshutter the slab bottoms before grouting the tendons? Please reply immediately."

You would need a structural engineer to calculate if you can de shutter. If it is a multi-storey building, I would de-shutter in small sections and replace the props after the former had been removed; that way the load is transferred to the founds rather than a slab that may not be fully cured. Most concrete can take over a month to be up to design strength, depending on thickness and content.

anon61597
Post 7

seventh day cube test has passed for the PT slab and so the tendon tensioning has been done. Can we deshutter the slab bottoms before grouting the tendons? Please reply immediately.

anon57608
Post 6

How does one know if one has a PT slab? Are there any tell-tale signs?

anon53704
Post 5

Does a post tension slab make for a better foundation that is less likely to have foundation problems in a yazoo clay area?

anon41240
Post 4

What would happen if you drilled a hole for a dyna bolt and hit 1 of the tendens

anon36823
Post 3

Can you cut a loop detector into the surface of a post tentioned slab? the cut would be 4mm wide x 35mm deep. Approx size of the loop detector is 2000mm x 800mm AAI

kalaichelvan
Post 2

If we have BRC (reinforcement) below our stressing tendons (post tensioning), can we do a single stress? Thank you.

kalaichelvan
Post 1

I want to know how is post tensioning stressing basically done. I heard that if we have no reinforcement(BRC)below the tendons you do a initial stressing of 25% after 24hrs of concreting and a final stressing after the concrete after that.

The other thing i want to know is: This is construction of a multistory building. All post tensioning. After stressing, we are allowed 21 days to grout the tendons. Can the following slab be constructed and concreted before grouting the floor below?

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