when to use PT slab-on-grade?
when to use PT slab-on-grade?
(OP)
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?





RE: when to use PT slab-on-grade?
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.
RE: when to use PT slab-on-grade?
RE: when to use PT slab-on-grade?
RE: when to use PT slab-on-grade?
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.
RE: when to use PT slab-on-grade?
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.
RE: when to use PT slab-on-grade?
You say that there have been reports of large cracks for PT slabs in the USA?? Can you elaborate on what the cause is?
RE: when to use PT slab-on-grade?
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.
RE: when to use PT slab-on-grade?
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.
RE: when to use PT slab-on-grade?
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.
RE: when to use PT slab-on-grade?
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.
RE: when to use PT slab-on-grade?
RE: when to use PT slab-on-grade?
Do you have personal experience with these slabs?
RE: when to use PT slab-on-grade?
RE: when to use PT slab-on-grade?
RE: when to use PT slab-on-grade?
RE: when to use PT slab-on-grade?
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!!
RE: when to use PT slab-on-grade?
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
RE: when to use PT slab-on-grade?
RE: when to use PT slab-on-grade?
RE: when to use PT slab-on-grade?
These parameters are from a recent job for medium expansive materials.
Soil Information
Liquid Limit (LL) 43
Plastic Limit (PL) 18
Plastic Index (PI) 25
Percent Fine Clay 40
Clay Type Montmorillonite
Expansion Index 69 (MEDIUM)
Summary of Design Parameters
Approximate Depth of Constant Suction:
Center Lift 7 Feet
Edge Lift 7 Feet
Approximate Soil Suction, pF: 3.6
Approximate Moisture Velocity: 0.7 inches/month
Thornthwaite Index:
Center Lift -20
Edge Lift -10
Average Edge Moisture Variation Distance, em:
Center Lift 5.5 Feet
Edge Lift 2.9 Feet
Anticipated Swell, ym:
Center Lift 2.8 Inches
Edge Lift 0.7 Inches
Say for a slab of 30'by 90', what stress would you use on the tendons, and what spacing and layout of ridge beams and tendons would you use? And a bearing capacity of 1500 psf.
RE: when to use PT slab-on-grade?
RE: when to use PT slab-on-grade?
I would want about 1 MPa stress, so 2-12.7mm (1/2") strands in tendons at 1100 (say 3'-6") centres in each direction. So about twice the PT of HN's solution. And all the tendons would be bonded by fully grouting the ducts.
The solution proposed by HN for a ribbed slab sounds about right, but I would not use this with post-tensioned reinforcement. I would use all deformed bars.
Most of the slabs designed with the PTI method will probably result in no complaints, but I would not risk it.
RE: when to use PT slab-on-grade?