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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:

enter image description here

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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.

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This is standard in Brazil. We dump about 50-80mm (depending on the room) of concrete as a "subfloor" slab over compacted soil, afterwards there is a leveling layer of cement+sand (about 20-50mm) and on top of that goes your final floor (ceramic, tile, vinal, carpet, etc). There isn't much movement on the slab per se, but on the joints of the slab with the concrete foundations usually there is fissures. –  Luiz Borges Apr 18 at 18:32
    
But your moisture barrier should be under the concrete slab in the picture. –  DMoore Apr 18 at 18:33
    
here people don't use moisture barrier in this scenario (the foundations and the bottom of the walls are usually waterproof to prevent moisture from climbing on the walls), my thought on a moisture barrier was more as a mean of retaining most of the water on the cement floor so that is cures better. Otherwise, people wet the floor daily for a week or so. –  Luiz Borges Apr 18 at 18:35
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I have laid a lot of cement on concrete. Never had a curing issue. Not exactly sure how the physics of this works or the type of plastic but right away I see you cement drying and then the plastic being ripped with even the slightest movement. Maybe this is one of those cases where it doesn't matter... but again have never added a moisture barrier into the mix. –  DMoore Apr 18 at 18:38
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Cement losing water too fast is usually about climate control and watering, not moisture barriers right? I know that when it is hot in the summer in the US I have seen pouring crews keep a steady amount of water on a slab until it cures. I have also seen wet burlap placed on top of concrete - here is a good link concretenetwork.com/concrete/slabs/curing.htm –  DMoore Apr 18 at 18:52

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