Measuring Relative Humidity in Concrete per ASTM F2170
Currently, the relative humidity (%RH) reading that you get can in concrete slabs vary depending on the manufacturer of the instrument that you use. The reason for this seems to be the vertical location of the RH sensor in relation to the bottom of the drilled hole. Everyone seems to agree that holes should be drilled to 40% of the thickness of a slab. That’s the location that best approximates the RH of the entire slab thickness. Sensors placed close to the bottom of the hole will read higher than sensors located higher up—results with current instruments can vary by as much as 8 to 10% RH due to sensor location and other factors of probe shape and internal construction. Members of ASTM committee F-6, which is responsible for F2170, are attempting to agree on probe and sensor geometry so that everyone’s instruments report more similar results. Until there is consensus on this issue you are left to decide which instrument reports the most useful RH. Generally, higher readings are a more accurate indicator of the moisture condition at the bottom of the hole.
Every manufacturer of finished floor products publishes maximum acceptable concrete %RH for their product installations. Some companies specify RH numbers that aren’t attainable for concrete to ever achieve, leading to situations that contractors can’t meet. Often times this leads to contractors voiding manufacturer warranties in order to install products on a timetable required by project owners.
Tell us about your experiences. Do you measure the %RH of your floor slabs? What has been your experience with installing finished floor products?