Concrete Moisture Test Methods Revolutionized in Recent Years
The scope of flooring materials and adhesives to be installed atop concrete has significantly expanded over the years, bringing not only greater choices but new moisture-related challenges.
Fortunately, in recent years, we’ve also gained a keener understanding of how best to test for moisture in concrete. Applying stricter scientific scrutiny to the options for testing the moisture condition of a concrete slab has revealed the relative strengths and weaknesses of two key testing methods—the calcium chloride (CaCl) and relative humidity (RH) tests.
As the science has advanced, each test has traced a very different trajectory. While the CaCl test is on the retreat, the RH test has become more accessible and widely used than ever.
The Early Years
In the 1960s, the CaCl test transitioned from a qualitative to a quantitative test. Manufacturers began selling CaCl test kits with specific materials for conducting the test, but also—for the first time—specifications for how to read test results.
Now, instead of flooring installers just eye-balling the crystals to gauge whether enough moisture had left the slab, the CaCl test returned a specific metric. When test results indicated emission rates of less than two pounds of moisture per 1000 square feet over a 24 hour period, the concrete slab was said to be sufficiently dry.
The weakness of the CaCl test, even with this clear metric, was that none of its specifications had any foundation in science. No scientific justification existed for the testing materials in the kit, such as the type of crystals used, or why a moisture emission rate of fewer than two pounds indicated concrete that was dry enough.
During this time, RH testing methods were also being developed. One type of apparatus introduced in the 1960s was the Monfore gauge. It consisted of a miniature (2.5 mm diameter) electronic sensor, using a Dacron thread connected to a fine Advance wire. Though small, the device required a tabletop crowded with electronic equipment to obtain a reading.
A number of laboratories used the Monfore gauge as a standard tool for measuring the moisture condition of the concrete. However, the equipment requirements of the Monfore gauge prevented its widespread commercial use.
Exploring the Science of Concrete and Moisture Condition Testing
The CaCl test grew in popularity in the United States, while most other countries moved in a different direction. Great Britain, New Zealand, Finland, and Sweden all developed RH testing specifications.
In the 1980s, a trade association in New Zealand provided its members with an RH test method using a hygrometer called an Edney Gauge. This gauge was sealed directly on the concrete slab and covered with insulation. It measured the RH of the concrete, but only at its surface. During the 1990s, two British standards were based on a similar type of test.
Research conducted in northern Europe during this time significantly advanced the science of moisture testing. Previous studies had already determined that it is important to measure moisture, not just on the surface of the slab, but RH levels deeper within the concrete.
Realizing the inadequacy of measuring RH only at the slab’s surface, scientists at the Technical University of Lund in Sweden set out to identify the proper depth for placing an RH probe for the most accurate, useful results.
Their research revealed that for slabs poured on grade and drying from one side, the proper depth for conducting an RH test is at 40 percent while 20 percent is the best measurement depth for slabs drying from both sides. These specifications formed the basis for what became known as the Scandinavian Nordtest.
Meanwhile, Back in the United States…
The CaCl test continued to hold sway in the United States. In 1998, the American Society for Testing and Materials elevated it to an ASTM standard, known as ASTM F1869. In practice, it utilized the same metric as the old CaCl test, except a moisture vapor emission rate (MVER) of less than three pounds was to be the new dryness threshold. A number of U.S. trade associations recommended using the ASTM F1869 standard, but just as before, no scientific basis existed for the metric being used.
During the 1990s, scientists at CTLGroup’s laboratories in Illinois performed extensive scientific tests of the MVER method. Their tests, some of them lasting years, clearly demonstrated that the MVER is not a reliable indicator of overall concrete moisture, primarily because it measures conditions only at the slab’s surface. At last, the work of the European and American researchers was converging.
A New Frontier
In 2002, ASTM International developed another standard, known as ASTM F2170, for in-situ RH testing based on the Nordtest. Wagner Meters worked closely with CTLGroup during this time to develop a new RH test kit that would be suitable for flooring contractors, general contractors, inspectors and others to use on construction projects.
Shortly thereafter, in 2005, Wagner Meters introduced its award-winning Rapid RH® test. It revolutionized the industry, making in-situ RH testing not only an accurate and reliable way to test for moisture, but an extremely easy and economical way as well. RH testing of concrete slabs had now become commercially viable.
The trajectories of the RH and CaCl tests officially crossed paths in 2010, when the ASTM disallowed use of its F1869 standard for all lightweight aggregate concrete slabs. In contrast, there’s been a growing recognition in the U.S. of the value in specifying the F2170 standard.
Today, thanks to the Rapid RH® and ten years of innovation by Wagner Meters, architects, designers, engineers, contractors, and inspectors increasingly rely on RH testing. They’ve come to appreciate its significant advantages in accurately assessing the true moisture condition of concrete slabs.
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