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?

 

[muuddfun]

So lets see how each of you would design differently for the following conditions, and maybe well get a little closer to what the real differences are in practice.

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.

 

[HN2008]

I have not run the calcuation yet. From the numbers you listed, my estimate would be 5" ribbed slab with 12" wide x 18" deep (below the finished grade) exterior footing and 12" wide x15" deep interior footing spacing 12'-0"+/- both ways. 1/2" dia. Cables should be 4'-0" on center on the 30' direction and 3'-0" on center on the 90' direction.

 

[hokie66]

For a post-tensioned solution, I would avoid the ribbed option and choose the 10" constant thickness slab, placed on a plastic sheet over a sand bed. This would tend to deal with the restraint issues. If a deep beam is required at the edge, it should be separate from the slab and not tied or bonded together.

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.

 

[HN2008]

I agreed with hokie. Yes, you can do the 10" mat slab with 2" thickened edge to cover 2" of sand between the slab and the visqueen. This practice is done in Northern California and Reno areas. Southern Cal does the 5" ribbed mostly.