Exposed Scored Concrete Slab

[rlewistx]

I’m getting ready to start an office project that will have exposed stained concrete slabs on ground in East Texas. The slab will be scored in a decorative pattern. Since it is exposed and decorative I’m considering post-tensioning the slab to minimize cracking. Post-tensioning is not required from the geotechnical report, but I’m considering it to help resist the shrinkage cracks in the slab.

What I have usually seen for post-tensioned slabs is a waffle style stiffened slab in areas of expansive soils. In our region most foundations are ‘turned-down’ slabs where the exterior grade beams are monolithic with the slab. In this case I’m expecting to put some interior ribs in the slab, just to stiffen it from the compressive stresses, but I don’t think I need ‘full height’ ribs of a traditional stiffened slab. I’m considering a 4 inch slab with 12 inch deep ribs at perhaps 12-15 feet on center. Strands would be 4-5 feet on center. I will also require the post-tension contractor to put 50% of the tension load on 24 hours after pouring and put the remaining load on a week or so later.

I would appreciate any input from engineers who have successfully used this concept, or any other design concepts to assist in minimizing slab cracks in an exposed situation like this. Any recommendations on the amount of post-tensioning needed?

Is anyone aware of printed research on minimizing cracks in exposed decorative using post-tensioning, or other methods of construction?

Thanks for your input.

 

[hokie66]

The ribs and the monolithic perimeter beams will restrain the slab, thus placing the slab in tension as it tries to shrink, making it more likely to crack. The key to reducing cracking is to reduce restraint.

Tensioning to place the slab in compression is only effective in preventing cracking if the force actually gets into the slab, not if it goes into the perimeter beams and stops.

 

[Teguci]

You could cast a mudslab, add a thick vapor barrier (elephant skin) and then place your decorative slab on top. This should decrease your friction between layers and allow your prestressed topping to float.

If you must avoid cracking (construction joints do work), then I'd put in a 4" flat slab with 150 psi prestressing min.