The technical support reps at
Stego Wrap say that the air gaps between aggregate underneath the poly is the capillary break, and the poly is the vapor barrier, and concrete has tiny pores that can make it act like a sponge.
With our buildings being sealed so efficiently these days, you can actually get a water vapor pressure differential via your HVAC system. Turn on your air conditioner, and the room is dehumidified. Meanwhile, the soil under the foundation has moisture in it, and in vapor form it eventually gets sucked up into the slab towards the "low pressure weather system" that is your room interior. If you have VCT, some of that water lingers on the underside of the tiles, and today's water-based glues can't stick when the slab stays moist. The next thing you know the building owner is yelling at the contractor that their building tiles are popping up. I've seen this happen in buildings in the Arizona desert in summer!
To prevent this:
1. Use aggregate, not dirt or sand, below the slab to create a robust capillary air break.
2. Put the poly vapor barrier right on top of the aggregate. (This where are reinforced barrier will help prevent tears.) Seal all edges and penetrations; they make special tapes and adhesives for this.
3. Put the floor rebar system on top of the barrier. Do not add a layer of protective sand (explanation to follow below*). A good reinforced vapor barrier will help prevent punctures during construction.
4. Pour the concrete. Keep only as moist as necessary, but don't over-water (e.g. don't run a sprinkler 24/7 on top of the slab).
*The reason you don't want a protective sand layer on top of a well-sealed vapor barrier is that during the concrete pour, the sand will hold a lot of moisture form the concrete mix, but it can't drain downward (because congratulations, you did such a good job of sealing the barrier). It is too easy to over-water the slab during its curing period. If the sand gets soaked in that situation, it may take 12-18 months to dry out enough to glue down flooring.