New Concrete Slab over Existing Slab on Grade

[CNCLLC]

I have a building rehabilitation project which consist of an existing skating rink being converted to manufacturing. The existing slab on grade slopes to drains and owner wants to level the floor by pouring an additional 5"+ concrete slab on top of the existing slab on grade.

The literature that I found regarding reinforcing of slab on grades states that the amount of reinforcing is directly proportional to the friction between the concrete and the soil. The existing slab appears to be coated and to minimize costs the owner does not wish to prep the surface. I am concerned that, with minimal friction, the typical slab on grade reinforcing may not be adequate, however, ACI requirements for shrinkage steel seems a bit excessive.

I know that in typical slab on grade construction there are two schools of thought regarding the location of vapor barrier below slab. One has the barrier below a layer of soil and the other has the barrier directly below the slab, so there cannot be any soil friction.

Is this an issue or am I being overly concerned.

 

[Ron]

Yes...you are overly concerned!

This issue you have is one of reflective cracking. The joints in the existing slab will migrate through the new slab if you don't put your joints in the same place. That's if you have a bonded slab overlay.

If you isolate the existing slab from the new slab, you will then be able to place control joints where you want them. You can isolate the slabs by placing a layer of 6 mil polyethylene (Visquene)on the old slab. Get all the wrinkles out, as the wrinkles can be "control" joints, causing cracking where you don't want it.

You can place rebar or wire mesh in the new slab, making sure it is placed as near as practicable to the center of the depth. Sawcut control joints as soon as you saw without raveling the edges...certainly within the same day as placement...do not allow the contractor to wait until the next day to sawcut the joints. Make sure the sawcut depths account for the sloping of the old slab...meaning make the cuts deeper in the thicker areas or figure the needed depth of cut (25% of depth) for the thicker portion and use that throughout.

Since this will be a manufacturing application, I would suggest using steel fiber (not polypropylene) in the slab. Depending on the local authority, you might be able to do away with the mesh by using the steel fiber, keeping in mind that you don't need reinforcement in the slab for bending, just shrinkage control. Joints should be spaced at about 12 feet on center each way. No more than 15 feet, though.