This week let's discuss moisture testing of concrete. There are four different methods commonly used to test concrete for moisture. Moisture can be found in concrete in both vapor and liquid forms. For most measurements, we are expected to test for humidity or vapor in the slab.
The laws of thermodynamics tell us that air with high humidity will seek out areas of the air space with lower humidity. When contractors are installing flooring after a water or fire damage incident often there is a lot of liquid moisture in the slab or below the slab. This can cause problems for installers who only test for humidity. When installing floors at a water/fire damage site humidity testing does not measure liquid water in or below the slab.
Let’s look at the moisture testing that is or should be performed on concrete. Prior to installing flooring on it.
First off there is the plastic sheet test. I always liked the plastic sheet test, it's simple on a clean substrate surface. You cover an area (usually about 2ftx2ft) with plastic and tape it down so that it is airtight, leave it in place for a few days and see if the concrete turns wet under the plastic after 24 hours. This is a simple procedure that really gives you a good idea of the state of the slab. It costs almost no money.
However, most flooring manufacturers insist on having relative humidity testing performed on the concrete slab by performing the ASTM f2170 in-situ relative humidity test. This gives a good snapshot of the amount of vapor present in a concrete slab, but their results are not conclusive due to mitigating factors (too numerous to mention here).
Typically flooring these days requires the slab to be below 85% RH, but there are many new vinyl products that allow a slab to be 95-99% relative humidity. This method does not necessarily read the amount of liquid water below, in, or above the slab! So, when dealing with concrete slabs receiving new floor coverings after a water/fire damage incident this type of testing just doesn’t cut it.
In the past several decades, the number one way of performing concrete moisture testing was the calcium chloride test. The formal name for this is ASTM f1869. This testing method is long and arduous. It also revolves around grinding concrete prior to testing, which generates respirable silica dust. In most areas of Canada, respirable silica dust is a recognized hazardous substance that is governed by health and safety regulations. Due to the mess, complexity, long period time it takes to do the test and the associated regulations this test method is no longer performed very often in Canada.
The best way to test concrete prior to installing any type of floor covering would be to test for relative humidity via in-situ and the use of an actual concrete moisture meter. These kind oof moisture meters will measure the actual percentage of water content. Concrete moisture meters will measure the moisture content as a percentage. Some of these concrete meters also have a relative humidity probe attachment, so both humidity and liquid moisture tests can be performed at the same time.
Next week we will look at concrete moisture meter testing in far greater depth via in-situ and concrete moisture meter.
BTW- If you cannot perform the ASTM f2170 test protocol, visit our associate's website for a special deal on this test kit.
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