Concrete Moisture Meters vs. Relative Humidity Testing
Concrete moisture meters should never be used to make the final determination as to whether or not a concrete slab is dry enough for a flooring installation. Here’s why:
Do Moisture Meters Work on Concrete
Yes, concrete moisture meters work. But no concrete moisture meter of any type can give consistently accurate concrete moisture meter readings across the different mixes and densities of concrete. Other components (metal reinforcing bar, aggregate size, amount, etc.) can also cause false indications of moisture.
The C555 pinless moisture meter effectively measures moisture in concrete on the top ½ inch of smooth slabs.
Pin-type meters are not practical for measuring moisture in concrete because variable chemical and physical characteristics can cause false moisture readings. This is due to changes in electrical resistance that have nothing to do with moisture.
Additionally, concrete moisture meters were sufficiently accurate (again, they are not). They only measure ‘moisture’ (not relative humidity) in a small area near the concrete slab’s surface. This type of surface ‘moisture’ testing tells us nothing about the conditions down in the slab.
Once a floor covering is placed on a concrete slab, the relative humidity in the concrete slab will equilibrate throughout the thickness of the slab.
Meaning a slab that may have been “dry” (low RH) at the surface (without a floor covering) will see a higher RH at the surface when the floor covering has been installed. This is where the problems occur. The calcium chloride test method (moisture vapor emission) and the UK-based ‘hood’ method (RH) have the same problem, as they are also surface tests.
The calcium chloride test method (moisture vapor emission) and the UK-based ‘hood’ method (RH) have the same problem, as they are also surface tests.
Remember that an uncovered concrete slab will have an RH gradient throughout its thickness until a floor covering is put on top. Under normal conditions, the RH at roughly 50% slab depth will be significantly higher than the surface. Unless the slab has been down for a long time and a vapor retarder is directly underneath the slab.
In reality, the surface of the concrete will more closely reflect the RH in the room or building. This does not indicate the potential for a concrete flooring failure due to the high RH deep in the slab. That RH will migrate to the surface when the concrete can no longer emit water vapor due to the non-permeable floor covering.
Free Download – 4 Reasons Why Your Concrete Is Taking Forever to Dry
Again, most major floor covering and flooring adhesive manufacturers in North America and Europe already have concrete RH specifications in their installation guidelines.
Concrete Moisture Meter Verdict
At best, concrete moisture meters may have some practicality as relative (qualitative, not quantitative) measurement devices for possibly indicating the best placement for accurate, quantitative RH sensors within the concrete. Unlike other non-quantitative test methods, moisture meters for concrete are not final determination tools.
Learn more about RH probes.
As Sales Manager for Wagner Meters, Ron has more than 35 years of experience with instrumentation and measurement systems in different industries. In previous positions, he has served as Regional Sales Manager, Product and Projects Manager, and Sales Manager for manufacturers involved in measurement instrumentation.
Last updated on August 30th, 2022