industrial post-tension slab on ground for crack control

[rlewistx]

I have a client who really does not want any visible cracks in his floor as he wants a 'show-room' floor. I am considering post tensioning it and want to get some feedback on others who may have done is for slab on ground crack control. The slab area is approximately 95 feet x 75 feet. 3 sides of the slab perimeter will be exterior wall and one side will abut a slab that continues into office area where I do not have to concern with slab cracking. I will put an expansion joint between the post-tensioned and conventional slab. I will be using a 6" thick concrete slab.

Here is my list of questions:
1. Is this a viable solution for crack control? I recall reading in the past that post tensioning was a good method of crack control, but I have also seen post tension slabs crack, even some large cracks. Post tensioning is not required for expansive soil.
2. When I design post tension slab for expansive soils I limit my interior ribs to 15'-0". Since this is not for a structural foundation, should/could the rib spacing be increased. I'm considering the influence of ribs on the intent of design. Ribs are anchor points in the soil and the more ribs I have, the more anchor points I have. Anchor points resist slab movement so will this negate the closing affect of the post tensioning? I think I need ribs for stiffness, I wouldn't want the slab to buckle under the post tension force. Are ribs detrimental to the whole idea of trying to squeeze the slab back together to limit cracking?
3. The project is in Texas. The 3 sides of the exterior are going to be turned down slab grade beams, typical construction. The most economical way to pour this is to pour the grade beams monolithic with the slab. Perimeter grade beams are anchor point. Seems the same issue as #2 above.
4. How does weather affect this idea? I'm expecting it to be poured in late August, which is still very hot in Texas. The project is out in a wide open lot.

My current intensions is to put a 50% pull on the tendons about 24 hours after pouring concrete. The remaining pull will be made 7-10 days later, after sufficient strength is achieved.

I would appreciate any insight into using this method. I'm also considering using shrinkage compensating concrete as another option. Type K cement is not available in this area, but I have seen some information for an admixture form CTS Cement that purports to turn regular concrete into shrinkage compensating concrete. Has anyone had experience with this product?

Thanks for any assistance you may give.

Rich

 

[hokie66]

1. Yes. Post-tensioned slabs, as reinforced slabs, crack due to restraint. If concrete is not restrained, it will not crack. Bonded PT.
2. No ribs, for no restraint. Keep the thickness constant. The PT needs to be central. Done this a few times, and it didn't buckle. Bonded PT.
3. Don't do it with slab thickenings at the edge. Too much restraint. Keep the slab and walls/footings separate. Bonded PT.
4. Cure. Cure. Cure. Use cooled mixing water to control the fresh concrete temperature.
5. Bonded prestress. The unbonded type you typically use in Texas will not provide the crack control you desire. Also, the surface needs to be as frictionless as possible.

 

[rapt]

Hokie66,
Agreed with everything you said, including your subliminal references to bonded PT .

At 150 thick, you will may have trouble getting anchorages in without a thickening, though I agree a thickening is not desired as it causes friction.

Subgrade treatment after compacting should be 2" of washed sand (not crusher dust) with 2 layers of good quality polythene sheet to reduce friction.

P/A should be relatively high, about 250 - 300psi after allowing for subgrade friction.

And if you do not want the chance of really big cracks, some closely spaced top reinforcement!

And do not put the guarantee in writing!

 

[hokie66]

Agree about the anchorages. The slab soffit may need to be sloped down gradually at the edge, say in a metre from the edge, to accommodate the anchorages, but only to that extent. I was actually cautioning against monolithic thickenings to serve as footings.

300 psi would be my minimum. If it were a larger slab, I would want more.

Disturbing the sand layer when installing the ducts can be an issue. You might have better success with a light bitumen spray than with polythene sheets.

 

[KootK]

I think I need ribs for stiffness, I wouldn't want the slab to buckle under the post tension force.

No need to sweat this. For typical systems, including unbonded PT, the prestress results in no buckling tendency.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.