This is a very important question. Moisture is a problem with fresh concrete slabs, and if you install your floor in a wet slab you will have a lot of problems in the future.
Variables that affects the concrete moisture include:ambient relative humidity, concrete permeability, amount of water in the concrete when it is placed, slab thickness, presence (and thickness) of a vapor barrier in contact with the slab bottom, and method used to finish the surface.
Here you have the needs depending on the type of floor to be installed:
- Impermeable flooring (Vynil, for example): Internal relative humidity around 85% or surface moisture–vapor emission rates (MVER) of 3-5 pounds of moisture per square-feet over 24 hours
- Wood floors: 75% internal relative humidity and an MVER of 3 pounds of moisture per square-feet over 24 hours (National Wood Flooring Association recommendation)
In practical terms, just remember that to achieve the recommendations it takes time. In normal average conditions (relative humidity around 50% most of the time and temperature above 65F, concrete water-cement ratio of 50%) a 4-inch slab can take up to 3 months to dry before you can apply a floor there. But remember that this 3 month period starts only when the cement is dry.
You have some specific tests for that, that are too technical to describe here. Just to mention, the calcium-chloride test and the internal relative humidity test are tow that can be applied for this evaluation.
One of the most important features is the vapor barrier/vapor retarder. Without a vapor barrier the slab will never dry out. That’s why the construction codes put the obligation to use a 6 mils vapor barrier there. The 10mils and also the 15 mils poly sheet can better control the moisture. The best location for the vapor barrier is in direct contact with the bottom of the slab.
If insulation is used over the concrete slab a 4-month drying time should be respected before installing the EPS (normally the EPS absorbs 0.5% by volume).
To have more detailed information about that you can click here and check the Concrete Construction magazine