If I want to use a cement floor (with broadcast color hardener) over a concrete slab (laid over compact soil), should it adheres to it, or be separated?
The concrete slab is rough, the cement layer over it will be about 40-50 mm thick (just cement and fine sand) and will be formed in squares of about 2x2m.
If I make an adherence bridge (using cement paste and adhesive additive) the cement layer will be an integral part of the concrete slab and moves with it. I see three problems: 1) the cement layer will lose a lot of water the concrete slab during curing; 2) any movement on the concrete slab will show on the surface of the cement; 3) moisture from soil might travel upwards to the floor.
On the other hand, if I use a separating layer (like a plastic film) between the slab and the cement floor I believe (not sure) the curing of the cement will be better, movements on the slab wont show directly to the floor, it might act as a moisture barrier. Is there any downside to it? Will it affect the strenght of the floor in any way?
Below in blue is the cement floor, in gray the concrete slab and in red the separating layer/adherence bridge:
Best Answer
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.