When a concrete slab has not completely dried out and an LVT floor is installed, no more moisture can evaporate through the surface. However, over time the dryer top portion of the concrete will pull up the excess moisture from the bottom until the moisture is equally distributed between top and bottom portions of the concrete.
To understand this phenomenon, it is necessary to comprehend the nature of concrete substrates. Concrete is a mixture of cement, water and aggregates. This mixture is pourable and hardens by chemically binding water. But there is always water left after hardening, which will slowly evaporate through the surface. As long as this drying process takes place, the surface of the slab will be dryer than the core. From the surface the concrete appears to be dry, but the problem can come from the bottom of the slab.
That is precisely the reason why the popular in-situ RH probe test was introduced. The test gives quantitative values that can predict what the moisture will be once the slab is covered and sealed off from the outside environment.
In an open slab there could be more moisture at the bottom of the slab than at the top, which makes the use of pinless moisture meters questionable. Pinless meters do not measure deep enough to alert the floor installer there may be a problem-causing moisture condition at the bottom of the slab. If measurements are taken with the in-situ RH probe test and the RH values are within the permissible range, there are no more unpleasant surprises anticipated after the LVT floor covering is installed.
It may sound like a broken record, but it bears repeating: Follow the instructions provided by the manufacturer to ensure the floor stays beautiful and to the warranty stay intact.
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At a depth of 40% in a slab on grade (blue line) and at 20% in a slab drying from both sides, |
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From Floor Covering News by Grete Heimerdinger, Lignomat
→ Click to read more about LVT Not Immune to Moisture Present in Concrete
→ Click to watch a video demonstrating Lignomat’s ASTM F2170 RH in-situ probe test
→ Click to read more about Importance of Measuring Concrete Moisture with In-Situ Probe Test
→ Click to read more about Lignomat’s Unique Design for Taking Measurements Using ASTM F-2170