Replies

1. 
   Norman_Williams | Oct 23, 2000 09:58pm | #1

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   Post Tension slabs are nothing new. However the person that is explaining the technology to you has no idea what he is talking about. Post tension slab on grade foundations have been used in Texas for more than 30 years. They are commonly reffered to as waffle slabs. The design incorporates a perimeter beam (footing) and interior beams in a grid pattern.
   Steel strands are laid in a grid pattern. One end is attached to the form. (the dead end) The other end runs wild threw the form. (the live end) After concrete is placed the cables are stressed. Hence the term "POST TENSION" If designed and installed properly it is a fantastic system. It is the foundation of choice in poorer soil conditions. (moderate PVR, PI, normal bearing capacity) To say that it will not crack is absurd. There are many factors that affect foundation performance.
   Reinforcement is secondary. Proper load designs, concrete batch design, placement technic and most importantly moisture control around the foundation have the greatest impact on performance. Should you buy a post tension foundation? That is a question that is impossible to answer without specific information on soils conditions on your site and the design criteria used for the slab. Stay away from the salesman. Talk to the geotechnical and structural engineers about this homes foundation design. Overall we prefer post tension over rebar slabs. Hope this helps.

1. 
   Ed_Hilton | Oct 23, 2000 10:35pm | #2

   *I second Norman. I moved from Dallas to San Antonio, and post tensioned slabs are the norm in both areas. They are not foolproof, though. One of my co-workers in the Dallas area had a cable come loose 2-3 years after construction, and it destroyed the slab in the bedroom. It was a slow failure and no one was hurt, but the reconstruction mess was incredible. You should treat post-tensioning as another option, not the answer to all situations. Basements are unheard of in both areas, so this may be the foundation of choice. We do have a grade-beam footing around the building and about 6-8 ft on center each way; it is 24-30" deep and 12" wide, with rebar as spec'd by the architect. Changes in slab elevations (a step-down living room, for example) can be tough, as the amount of change is limited by the cables. There are work-arounds, consult an architect or competent builder.

1. 
   mark_cadioli | Oct 23, 2000 10:51pm | #3

   *Norman,Are your Waffle slabs using the styrofoam box setup? Are you then running your cables through the voids?Here Down Under, we use waffle pods on Highly reactive soil sites, with moderate success..however Post Tensioning is very very rarely used in residential construction. If it's done right I see no problem with it, although I don't know that there would be any major cost savings. ( I asked an engineer on a commercial site about it once. His comment was on a one only basis it wouldn't be, however if you had a few to do it could be.)kind regardsmark

1. 
   Bob_Walker | Oct 23, 2000 11:54pm | #4

   *Jumping in with a side question: They don't use 'em in my area (NW Ohio) so I've never seen one (at least that I know of.)How would one recognize one in an existing home? What sorts of visual signs might exist of pending or existing problems?Also, how are the cables stressed/tensioned? How is the live end anchored to its end?ThanksBob

1. 
   QED | Oct 24, 2000 12:26am | #5

   *A couple more questions for the experts: Is the theory of this type of slab based on the increased tensile strength of steel (when under tension) combined with the compressive strength of the concrete? Obviously the tensioning is done when the concrete is poured, but before final troweling? Thanks!

2. 
   Ed_Hilton | Oct 24, 2000 01:16am | #6

   *It is usually difficult to detect a post-tensioned residential slab, because after all the work is done and the concrete has cured, the ends of the cables are cut off and the foundation receives a thin coat of cement to hide the honeycomb and the cable end pockets. As for the installation, they come from the factory cut to the approximate length with a flange fastened to the dead end, and a sliding socket 0on the live end. They are tensioned with a hydraulic jack, similar to an automotive bottle jack, and then cone shaped wedges are hammered in place around the cable. The cables are usually sheathed in a pvc material, very similar to a stranded #12 electrical wire, and the contractor usually greases them during installation (very messy).As for the foundation design, since basementts are not built here, everything is above ground, so the house layout starts on the high corner. Foundation boards are layed out, and boards are added to the underside as needed to keep the top level. It's not uncommon to see 6-8 feet of foundation exposed at the low point. For some reason split-level houses are also uncommon. Once the forms are set, cheap laborers start building the fill. It is done using rolls of visqueen cut to about 48" wide. This may be a little difficult to explain, but try to follow. Start by laying the visqueen on the ground where the beams will go, and begin filling the center area, covering about 12" of the edge of the plastic. When the fill gets about 8-10" thick, lay the plastic on top of the fill, add soil to keep it in place, and start another layer of fill. When it's complete, it looks like they have used plastic sandbags to form the walls of the beam trenches. properly compacted, it works well, as the slab is fully supported by either beams or compacted fill.

3. 
   Mark_M | Oct 24, 2000 02:24am | #7

   *I think whoever told you that PT is the way to go in an residential situation is crazy. Any alterations to the slab is a big deal, and a rotohammer bit can easily break a cable, and the result can be deadly.As for QED, the tensioning is done several days after the concrete has hardened.

4. 
   Chris_Robb | Oct 24, 2000 03:51am | #8

   *Post Tension stuff where I'm from in the Northwest (Blodgets neck-o-the woods) is used on a lot of second story stuff, apts., offices, etc. and Mark could'nt be more correct. Drill a hole in one and look out, the thinks will cut you in half. Down here in Phoenix was the first time I'de seen it done in residential stuff and was shocked when I saw the cabinet guy drilling away without a clue, he'd said he had'nt heard of post tention or anything associated with it, so he was given a quick lesson (and the use of my stud finder)before he killed someone. I don't think a lot of people down here understand what there all about, the way holes are punched in the floors as randomly as they are.Just something to think about when you hear the hammer start up!

5. 
   CaseyR_ | Oct 24, 2000 04:21am | #9

   *There is a Post Tensioning organization, although their explanations don't add a lot to what has been said here:http://www.post-tensioning.org/However, if you want to build a post tensioned concrete tennis court, there is some pretty good information at:http://www.ustctba.com/guidelines-tennis/section2h.html

6. 
   Norman_Williams | Oct 24, 2000 04:48am | #10

   *Mark, I have never seen post tension used in conjunction with void box applications. In Texas these are called structural suspended slabs. Piers are drilled deep enough to get below the active zone of the soil and poured. (15' to 35' deep) Next caps are poured on top of the piers to reach the elevation needed to keep the slab from being affected by the soils upward thrust. The next step is to lay the cardboard void boxes in a pattern to create beams. This is accomplished by using void cartons of a different thickness for the main area of the slab. Rebar is used to reinforce the beams. The boxes are used to support the concrete until it is cured. They are left in place. The slab is now resting on the piers. Not the soil below. This creates a void. When the soil takes on water and expands it crushes the void cartons and the slab is undamaged. We see this type of foundation used when the PVR is between 4" and 8". When soil conditions are extreme pier and beam foundations are used. These use the same basic idea. Anchor the piers below the active zone of the soil and suspend the structure above the ground. A PVR between 4" and 8"'s is extreme. However we sometimes see PVR's in excess of 12". Very scary. Do exactly as the engineer recommends. (PVR stands for potential vertical rise, it is a measure of how much the soil will grow when saturated)

7. 
   Tim_Kline | Oct 24, 2000 07:15am | #11

   *b WBA At Your ServiceCasey Thanks for the tennis court info. I'm on it. My backyard awaits.

8. 
   Ed_Hilton | Oct 25, 2000 12:23am | #12

   *Norman - what part of the Lone Star state are you in? I have seen the cardboard void method used for commercial slabs, but never for residential. Probably costs too much. Aslo never have seen 15 ft piers for a house. Don't get me wrong, I'm not questioning your comments, just something that I have not witnessed. Never have heard the term PVR either; usually it's PI (plasticity index) which probably measures the same thing.

9. 
   Norman_Williams | Oct 25, 2000 08:10am | #13

   *Hi Ed, We build in the Dallas area. I had never heard the term PVR until we started building in areas that have a formation known as Eagleford Shale. While the name suggests some sort of rock. Nothing could be further from reality. It is a highly expansive clay. PI is the measure of how much moisture the soil can absorb. PVR is the measure of how much shrink/swell will occur when the soil goes from dry to saturation. Combine high PI's (45 to 85) with high PVR's (4" to 12") and you have Eagleford Shale. I am not a geotech. However I have learned the hard way(with my wallet) what it takes to build a stable foundation in these soil conditions. Straight shaft piers drilled 15' are at the low end for foundations built on this formation. Yes it is expensive. Ask anyone that has had to remove and trash $ 2000.00 worth of void cartons because they did not pour before it rained. There are some other ways to address the problem. One way is to inject water into the soil on and around the building pad. The idea is to swell the soil to it's maximum potential. Next drill the piers and pour the slab. The soil will shrink away from the foundation in severe drought. When it is re-wet it will only swell back to where it was before and the foundation should remain unaffected. Another method is soil ionization. A chemical is injected in to the soil. This chemical binds a molecule in the clay and inhibits it's ability to absorb water. Cost is about $ 2.00 per sq.ft.I'm still researching this method. Some structural guys I spoke to think it only works because it also pre-expands the soil. Don't know for sure how to figure out who is right on that one.

2. 
   Sean_Bullock | Oct 25, 2000 08:10am | #14

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   Recently My wife and I took a tour of a beautiful home here in Tucson, Az. The home was built of straw bale (though it was not apparent)and the builder suggested that all homes should utilize a relatively new method for residential homes in construction of the foundation. He called it Post Tensioned Flooring. He said it was a much cheaper way to go because footings were not necessary. And it would never crack. I was intrigued, but is it really cheaper? And what about practicality?
   Sean B.