Concrete – How to do about cracks in the cement pan for a roll-in shower

Next time, tamp the cement into the corners (using a putty knife, large screwdriver, gloved finger, etc.) after a couple inches is set down. If you wait until the form is almost full, it forms stronger bubbles and they are harder to coax out.

Structurally, the curb is more than strong enough unless you anticipate automobiles driving over it. The major issue is adequate structural support for the finishing material.

If you will be tiling the curb, the thinset is plenty strong enough to support the tile if worked into the gap and corner well. So there would be nothing more to do.

If you will be putting vinyl, etc. over it, the mastic would not be strong enough, so the curb should be smoothed out with concrete filler.

I can't recommend adding a plastic moisture barrier between the pads. Really if you have moisture in the soil then it needs to work its way out. Moisture comes out of concrete all the time - we only see the worst case scenarios.

If you leave the plastic in there then the water will get trapped on the bottom slab. Very good for the short-term. Long-term though this will cause the bottom slab to decay and break down. And since you are building on this, that's not a good thing.

I usually advise in drilling down into existing concrete and installing rebar or a mesh unit to join the floors. To be honest with you I have never installed concrete on top of a floor like the one in your picture. Common sense would tell me there wouldn't be much movement... but if I was giving out a 10 year warranty with my work I would rebar.