2" Topping on Slab on Grade

What is the purpose of putting a topping on a new slab?

Why would you want an unbonded 2" topping?

It is likely that you have no control joints in your 15x12 slab. If that is the case, I would bond the topping by using a roughened surface on the slab and use a latex bonding agent. I would not worry about moisture entrapment.

If you have joints in the underlying slab, make the joints in the topping correspond to the joints in the substrate. Toppings do not follow the 24x rule of thumb because they have much less water and less shrinkage than conventional concrete.

Did you put poly sheet under the grade slab?

You should bond the topping to reduce the probability of cracking.

I read in ACI's manual on slabs on grade (ACI 360, I think), that the joints in the topping slab should match the joints in the parent slab.

We've also done a number of thin topping slabs, and have asked for a shrinkage reducing admixture in the concrete, but I remember reading in the same ACI 360, that you shouldn't use shrinkage reducing admixtures for topping slabs on fresh slabs on grade.

We've always asked for a bonding agent, but make sure to have a conversation with the contractor that they should be careful not to let the bonding agent set before placing the topping slab.

For a 2" thick pour you can use simple (and cheaper) concrete. You will want to match the control joints in your original slab (or the cracks that tell you where the control joints should have been) so mark the walls at those locations. As for trapping moisture, the vapor barrier is intended to do just that but I still agree to not use a bonding agent since bonding your new concrete to the vapor barrier is silly.

I believe the vapor barrier is below the parent slab, not between the parent slab and topping slab.