Normal is relative when using a concrete moisture meter. Concrete will always have varying mixtures, differing aggregate concentrations, and diverse water-to-cement ratios. You're not necessarily looking for a "normal" reading as much as you are comparing the results of multiple readings to get an idea of where the more extreme levels of moisture and dryness reside within a single concrete slab. For flooring installation, ASTM F2659 recognizes compliant concrete moisture meters like Wagner's C555 as a means to perform a "...preliminary evaluation of the moisture condition of concrete, gypsum cement, or other floor slabs and screeds..." In this capacity, the C555 Concrete Moisture Meter could be used used to determine an average or "mean" moisture content within the concrete slab to assist in determining the optimum location for placement of Rapid RH® L6 Smart Sensors.
Concrete Flooring FAQ