Control Joints in Structural Slab on grade

[egc]

I am designing a structural slab on grade for an irregular shaped single-story (RCMU) house in Florida on a site with a clay layer. The geotechnical engineer suggested either this system or a pressure grouting approach (contractor kicked this out). The floor plan has several large projections sticking out from the main portion of the house (about 4000 SF of A/C space).

It will be a 6" slab with monolithic turned down footings.

I have used a wrinkler spring analysis approach with stiffer springs at the perimeter and with loose and stiff springs to get a range of bending/bearing stresses.

Concrete specs: 3000 psi, 4-5" slump, vapor barrrier, chairs, wet cure (burlap/plastic).

For crack control would it be better to:

1. Use two layers of rebar in both directions?

2. Usa a single layer of rebar (with 1.5" top cover) in both directions?

3. Should I specify saw-cut control joints that clear the top layer of steel and are arranged where they would be located in a traditional un-reinforced floating slab?

 

[Lutfi]

Hello fellow Florida engineer. I would suggest the following to control cracks in the concrete slab:

1. I suggest a sand blanket of 3 inches thick under the slab
2. Vapor barrier of 6 mills is a good idea
3. Use concrete with strength of 3500 psi to 4000 psi and water cement ratio of 0.45 or less
4. Use of fiber mesh is good to minimize the hair line cracks
5. I would use construction joints and sawed joints. Lesson learned is that try to place these joints in areas so that are not exposed and avoid middle of floors and hallways. I suggest placing them under or near walls. This improves the aesthetics and avoids crack propagation thru tiles etc. Make sure that the depth of saw cut is about ¼ slab thicknesses and fill it with mastic type filler.
6. Definitely wet cure the slab. This is the best method in my opinion.
7. For 6 inch slab and residential loading, rebar in both directions is a bit too much. I would use 6x6 w2.9xw2.9 WWF and place it in the middle of the slab. Use good joint detail and cut half the wire mesh across the saw joint.

You can obtain a copy of PCA's Slabs on grade booklet which contains very useful charts and examples.

I hope this helps.

 

[egc]

Lufti,

Thanks for responding. The slab is monolithic with the turned down edges. It is not a floating slab. Wire mesh is insufficient in this instance and (in my opinion) seldom ends up where you wanted it. The slab also has interior load strips (fireplace and bearing walls).

Is it your opinion that a single layer would be better for crack control?

 

[Lutfi]

I agree with you on the wire mesh is seldom is located properly. I used to use an old DOS program for thickened edge slab design. I used to use #4 or #5 and extend them into the slab. I use single layer of WWF in 6 inch slabs for light industrial application. So to answer your question, single layer if placed and held in place correctly, in conjunction with fiber mesh should perform fine. I would definitely keep an eye on the WWF during concrete placement.

goof luck