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Re: New concrete SOG on existing SOG

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Probably your first step should be to figure out what you need for a slab thickness.  If you are going to put a compressible layer between the two slabs, you can't really say that the new slab is just a topping slab.

If you only need a five in. slab, then 4 in. of a base material would make sense.  You should spec what is normally speced for the base course on industrial slabs on grade - i.e. compactible, trimmable, something that will give you a low coefficient of friction.  Clean sand is not good, it does not hold its compaction when it dries out.  You don't want the base to be overly wet when the concrete it poured - this exacerbates the shrinking/curling problems.

With respect to the putting holes in the existing slab - this is somewhat similar to the question of whether to use a vapor barrier and where to put it.  I'm not sure it is necessary to put holes in the existing slab but if I did so,  I would probably want to core, rather than chip holes,  to avoid the potential for cracking and faulting the existing slab.

Gail Kelley