Moisture Meters

[Blake McFee]

I have heard that a surface moisture meter is not as good a way to measure moisture in concrete as using an internal relative humidity sensor. Could someone clarify this for me?

[rthompson]

Blake,
Correct, surface moisture meters and surface testing measures the very top of the concrete. As concrete dries from the top down, the slab may be wet at the bottom while indicating dry at the surface. Once a slab is covered with a moisture sensitive flooring the slab will go into equilibrium and the moisture will increase at the surface. The use of a RH Sensor will measure the internal moisture at the apex of the drying curve, which is the point where equilibrium crosses.
Please advise any questions
Rayt

[Blake McFee]

Thanks, and how deep should I measure to get to the equilibrium point?

Blake

[rthompson]

ASTM F-2170 recommends 40% the thickness of the slab
Rayt

[rthompson]

Blake,
About 40% the thickness of the slab. 4" slab about 1-1/2 - 1-3/4"
Rayt