When concrete is freshly poured, every slab has a gradient moisture level– relative humidity (RH) will be significantly higher at the bottom of the slab when compared to the surface of a slab*, or to put it in simpler terms, the slab will be dryer at the surface, and wetter near the bottom. As the slab dries, the moisture from deeper within the slab moves towards the surface, where it can evaporate away.
Once the slab is sealed, water vapor continues to move within the slab until it has the same moisture level throughout– a process known as equilibration. But if a flooring or adhesive is installed before the slab has reached acceptable RH levels for the specified finish, moisture-related problems will result.
Extensive testing begun in the 1990s, and repeatedly confirmed, show that RH readings at 40% of the slab depth* will give the best indicator of the final RH if the slab were sealed at that point in the drying process.