when to use PT slab-on-grade?

[abusementpark]

How exactly do you determine when it is best to use a post-tensioned slab for residential construction? Is it simply when the soils report indicates a high shrink/swell potential?

 

[hokie66]

I have never used a post-tensioned slab on grade for small residential construction. Post-tensioned slabs on grade are usually used in industrial structures with the primary objective of eliminating most of the joints.

There would be nothing wrong with using post-tensioned slabs on grade in houses, but the cost would be greater.

Why would post-tensioning help with shrink/swell problems? Increased stiffness of the footing system and articulation of the structure above are the main approaches in limiting damage due to moisture sensitive soils.

 

[abusementpark]

I have heard it being done a lot in houses in SW Louisiana where they have highly expansive soil.

 

[hokie66]

I would be interested to hear more about the LA houses. Do you know if they are stiffened raft slabs? A post-tensioned flat plate may work, but I would think it would have to be at least 8 to 10 inches thick to provide the required stiffness.

 

[rapt]

Hokie66,

You are showing your Australian heritage. Haven't you seen all of the posts on different sites on the large cracks in residential PT slabs in the USA. They are designed using a PTI method that would not be accepted here.

 

[hokie66]

Rapt,

No, I haven't been keeping up with that type information because I don't get involved in houses. But I can imagine. By the way, my heritage is not Australian, I just live here. My wife is the Aussie. I started in the US, and I don't agree with a lot of the things they do either.

 

[abusementpark]

rapt,

You say that there have been reports of large cracks for PT slabs in the USA?? Can you elaborate on what the cause is?

 

[rapt]

Hokie66,

Some of your replies have indicated Australian practice so I assumed you were one. Lucky for you where your wife comes from!!

abusementpark,
I am not into house SOG design either so have not investigated the design method myself but have been told by others that it is lacking.

Personally, I do not think this type of slab is a logical PT option. PT gives good crack control until the concrete cracks, then, especially unbonded PT, gives no crack control at all if the strain is still applied, as would happen with large soil movements. I would be using RC stiffened raft slabs with a lot of ground preparation if it was my house (and I am a PT specialist).
From what I have been told, the P/A levels being used in these slabs is very low and there is not much reinforcement so they tend to crack at fairly low loading and stay cracked and the cracks are big. If the subgrade preparation is not much good, as would often happen with this type of structure, or if the movement is large, which is what you are expecting, then you have no chance of crack control, whereas, with an RC solution, the crack widths will be controlled as long as there is enough reinforcement so that the steel does not yield.

 

[hokie66]

rapt,

Yes, have lived and worked in Australia for last 25 years, so that is why I approach things from the Aussie perspective.

abusementpark,

My concerns with the PTI slab approach mirror those of rapt. I had a look at their site. As I understand it, they are recommending ribbed slabs, with a 4" slab cast integrally with the thickenings. They lightly tension the slabs, and NEARLY ALL OF THIS TENSION MUST BE RESISTED BY THE EDGE THICKENINGS AND PASSIVE SOIL PRESSURE. So there is hardly any prestress in the 4" slab, the tendons which do exist are not bonded, there is little or no deformed bar reinforcement, and when a shrinkage crack forms, the slab is free to slide toward the edge, or at least toward the next rib. One discussion paper I found on the site actually discussed how this happens, but they still recommend the method. Go figure.

Apparently 50% of these type house slabs are in Texas, 25% in California, and the rest scattered across the South, including Louisiana. So they have convinced the cowboys.

 

[csd72]

PT is not my thing, but one thing I have learnt is that it is very expensive to do small PT jobs. The set-up cost is too high and can be more than the actual work cost.

This alone would make it a poor choice for domestic houses.

hokie/Rapt,

I am the other way around (kind of), Australian,english wife, and living in the US.

 

[hokie66]

csd72,

You are forgiven. I thought from some of your posts that you have that sort of broad based experience.

Where posttensioning is done routinely, the set up cost would be reduced. One source I read said that about 40% of the posttensioning tendons in the US go into house slabs. Hard to believe, but obviously the sales people have prevailed.

 

[civilperson]

25% of the Post Tensioned slab on grade houses built in US are in California, (Texas is second). Very cost effective and desirable for lack of visible cracks.

 

[hokie66]

civilperson,

Do you have personal experience with these slabs?

 

[civilperson]

My experience was only observation. The subdivision that I worked on in Orange County California had 2500 lots and I recall that all used PT slab on grade. Approximately 4' spacing of the tendons in a rib under a 5" slab. The tensioning occured five to eight days after the placement of slab and took a three man crew about two hours per lot.

 

[muuddfun]

One reason we use post tensioned slabs on grade here in California is because the code requires that a special foundation be designed if the EI is over 20. The CBC requires that the foundation slab be designed with either the Welded Wire Reinforcing Istitute (WRI) specification procedure for expansive material or the Post Tensioning Istitue (PTI) procedure. See section 1802.3.2, and 1805.8 of the 2007 CBC, and Most large home builders have learned over the years that it is best to just specify the post tensioned slab and be done with it. The lawsuits otherwise cost to much. The cost per unit is not that much higher than a conventional design. What civilperson describes is fairly typical. We even have clients who want pt slabs even if the soils are not expansive, it just saves them from any other problems down the road.

 

[hokie66]

Well, good luck. I still don't see how effective crack control can be provided with the system as I understand it.

 

[rapt]

Muuddfun,

Can we assume from your comments above that because CBC requires it and it is a PTI method then the designer and builder are not responsible for problems as long as it is designed and built in accordance with the PTI procedures.

Civilperson
If it is stressed 5 to eight days after pouring, it will be cracked before they stress it and the cracks will be wide unless there is a lot of normal reinforcement, which I understand is not the case, or it is very low shrinkage concrete (in a house slab!!!) so there goes the crack control!

I am with you on this Hokie66!!

 

[muuddfun]

rapt
No matter what rules you followed there will still be lawsuits if there are problems. The lawyers don't care what you did as long as they have someone with some money to blame. My understanding is that the use of pt slabs has caught on because they are reducing the amount of cracking in the slabs and are performing better than the alternative methods, and thereby reducing the amounts of lawsuits. If it was not reducing the amount and severity of cracking then the lawsuits would have continued and the builders would not be as accepting of the method as they are. My experience is with supplying the geotechnical input parameters for the structurals to do the design, so I am not completely familiar with exactly how the detailing of the tendons and ridge beams are being caried out. From what my boss has told me,he has done some forensic work on situations where pt slabs were used, he has seen them in a situation where the soils were slid out from under the side of a house and the foundation was ok with the house cantileved out 12' over the edge do to the slide. So in some situations it was done well. I guess it really comes down to how they are being designed and built in the field, if shody work is done by either the engineer or the contractors then there will be problems just like any other type of construction

 

[hokie66]

To summarize, we have three US engineers in this thread who report that a lot of these houses in the US are being built with post-tensioned slabs, but do not personally know much about how they are designed or built. And we have two engineers in Australia who are sceptical based on our understanding of how the slabs are built. It would certainly be interesting to hear from someone who knows the details. With that many houses, surely there is someone out there---, but maybe it is a touchy subject.

 

[HN2008]

I did the post tension design for tract homes in California, Nevada and Arizona. I use the PTI software to design and all the building departments from three states have accepted the design. Basically, you can design 5" slab with rib beams spacing about 12'-15' apart or you can design 10" mat slab without footing. We need the soil parameters from the Geotechnical Engineer for the design inputs. The software can do both expansive and compressible soils. The cost is a little bit more than the conventional foundation but the feed back from the builders are great.Less craking, less customer service.