There are a number of reasons why RH can remain high in a concrete slab:
If the initial mix ratio (water, cement, admixtures) was high in water, the drying schedule will take much longer to allow that initial moisture content to dissipate.
If ambient conditions do not allow adequate air movement or include low temperatures or high RH levels in the surrounding air, the evaporation process necessary for drying the concrete may be inadequate.
If an adequate moisture barrier has not been installed under the slab, moisture may be accessing the slab from subsurface sources, including groundwater, defective plumbing or inadequate drainage around the slab.
If the slab surface was hard troweled too aggressively, burnishing the concrete surface and the natural capillary pathways that allow moisture to move to the surface and evaporate may have been obstructed, severely limiting the surface’s ability to release moisture from the slab.
Obviously, identifying the cause of the high RH will determine the correct remedial action. Alternate drying methods may be necessary, the slab surface may need grinding, or alternate floor or sealant options should be chosen based on products with a high moisture tolerance. RH testing with the Rapid RH is the best way to get the correct information in order to make an informed decision regarding the situation.