Can Calcium Chloride Pounds be Converted to Relative Humidity Percent?
RH and MVER Tests: Is There a Correlation?
When looking to determine if a concrete slab is dry enough to proceed with a finish, flooring or occupancy, there are several methods commonly specified for testing the relative humidity (or moisture condition) of the slab. A dry slab is never at 0% humidity, but determining the level of moisture still held in the concrete can be the difference between a successful flooring installation and a problem-prone floor system.
The two most frequent test methods specified in the industry today are Moisture Vapor Emission Rate (MVER) testing (with results expressed as pounds/1000 square feet) and relative humidity (RH) testing with in situ probes (with results expressed as a percentage). When faced with the two options, contractors, and other industry professionals often ask, “What is the correlation between RH and MVER?”
Simply stated…there isn’t one.
While it might seem logical that there would be some relationship between the two, the reality is that there is nothing more than an imprecise use of the term “moisture test” that links the two test methods.
Beginning around the 1940s, moisture levels were tested by placing an enclosed amount of desiccant on a slab’s surface for a period of time. Calcium chloride, or CaCl, was the most common desiccant used for this type of moisture testing and it is often referred to as the anhydrous calcium chloride test. Any subsequent change to the weight of the desiccant was thought to indicate the amount of water vapor that had left the slab to be absorbed by the desiccant. That weight was represented as a ratio of the total moisture content within the slab, and is referred to as MVER, expressed in terms of “pounds/1000 square feet.”
Extensive research done by the CTLGroup in the 1990s showed several problems with the CaCl test:
- MVER test kits cannot be calibrated.
- The test measures, at most, the top ½ – ¾ inch of the slab, but not deeper.
- Surface treatment, including trowelling practices, curing agents, ambient conditions and more, can skew MVER test results. In fact, calcium chloride testing has been specifically disallowed for lightweight concrete applications because the lightweight aggregate can impact results for false high or low results.
- The limits set for the test (i.e. 3 lbs/1000 sq ft) were somewhat arbitrarily chosen and published.
- There is no scientific backing for the test method as either a qualitative or a quantitative measure of concrete moisture. (1)
The real difficulty of MVER or CaCl testing lies in the fact that it primarily tests only the surface conditions of the slab. Drying concrete tends to have a gradient effect – moisture levels are higher deeper in the slab. As moisture evaporates from the surface of the drying slab, it then allows additional moisture to rise through the natural capillaries of the concrete in a progressive cycle until the moisture content in the slab reaches a balance with conditions around it. MVER is incapable of providing accurate readings of those internal levels.
A traditional “moisture test,” MVER is still regulated by ASTM F1869. Ultimately, though, it has proven to be an unscientific and problematic test method, plagued by subsequent moisture-related flooring problems.
Deep Down Differences
RH testing for concrete, on the other hand, measures internal moisture levels of a concrete slab by placing sensors, or in situ probes, within the concrete slab itself. Testing that had begun in Sweden and elsewhere in the 1990s demonstrated that for slabs that dried from one side, placing the probes at 40% of the slab depth would give a reading that would reflect accurate moisture conditions of the slab if it were sealed (i.e. a floor covering installed) at that point in time. (For slabs drying from two sides, the correct depth is 20% of the slab depth.)
Based on the realities of moisture vapor’s distribution in drying concrete, RH testing can accurately determine the internal moisture levels, or RH, of the concrete.
Understandably, the difference between MVER and RH testing has had a significant impact on the flooring industry, allowing concrete and flooring installers to make informed decisions when choosing products that will tolerate the actual RH levels of the slab, or allowing them to make remediation choices before elevated moisture levels result in flooring problems. As concrete science changes with the additions of admixtures, new aggregates, and a variety of finish options in demand, RH testing continues to provide accurate concrete moisture measurement for industry professionals.
The Rapid RH® 4.0 EX
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The Rapid RH® 4.0 EX is the latest in field-tested, scientifically-backed, industry-proven RH testing for concrete and flooring professionals.
Howard Kanare, Concrete Floor Moisture Tests, August 2007. http://www.concreteconstruction.net/flooring/concrete-floor-moisture-tests.aspx
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