Expert: Daniel Humphrey - 5/10/2010

Question
What do I need to be cautious about while adding this structure to the existing concrete slab.I suppose first thing to bear in mind would be the frost level and to join the two i suppose i need some steel bars to connect the two to maintain any changes due to settling.Any ideas would be greatly appreciated. Many thanks.

Upkar

Answer
Upkar,

Yes the two slabs will probably work independently, or at least try to. To drill into the edge of the slab and epoxy rebar will help tie the two together. For outdoor slabs, I insulate with two inches of high density extruded polystyrene panels beneath the slab. This will cut down on the frost heave and usually prevent it entirely.

Putting the slab over a foot of gravel will help also, because the gravel will help keep water from pooling beneath the slab and allow it to settle below the frost line.

Finally, it is a good idea to thicken the slab at the edges to at least 8 inches.

I recommend cutting control joints at least every 64 square feet into the surface of the slab after it has cured a day or two, and then grout the seams with a butyl caulk.

Daniel