The relative humidity (RH) of a concrete slab is the amount of water vapour inside it. RH will change while a concrete slab is curing, getting lower as more vapour escapes.
Understanding the RH of a concrete slab can help you know when it is safe to install flooring. Having too high relative humidity in your concrete slab can lead to flooring failures.
Relative Humidity vs. Moisture In Concrete
RH is sometimes confused with the moisture content of concrete, but they are not the same thing. The moisture content of concrete tells you how much water in liquid form the concrete contains. The RH is how much water in vapour form the concrete has.
To understand relative humidity better, we can think about a small scale example from normal life:
- In a full glass of water, the moisture content and the RH are 100 percent, like in freshly poured concrete.
- In a half glass of water, the moisture content is 50%, while the RH is still 100% because the air is still completely water-saturated.
- In a mostly empty glass, the moisture content is about five percent, while the RH can still be up to 100% but could be lower.
This same phenomenon happens with concrete. The moisture in concrete is left over from the mixing process or water exposure. The RH is how saturated the air within the concrete is with water vapour.
How Does Water Vapour Escape From Concrete?
During the drying process, concrete forms tiny tunnels called capillaries that allow water vapour to move through it. Water can also enter concrete through capillaries; that’s why you need to have a moisture barrier between the concrete and the earth.
Putting a floor coating onto a concrete slab before the relative humidity gets low enough traps moisture inside it. The water vapour still needs to come out of the concrete and will break through the flooring to do it. RH needs to be at or below 75% to put down flooring in most cases.
How To Test For Concrete RH?
The standard test for RH in a concrete slab is to drill holes 40 percent down into the slab. Then an RH probe is inserted and tests the RH of the slab.
This test gives you accurate results for the RH of the entire slab, not just the surface. Many flooring manufacturers require the ASTM F2170 in-situ RH test to warranty their products.