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Impact on other quantitative test methods
The trajectory of scientific research into the RH test method has consistently contributed toward our understanding of how moisture moves through concrete and what metrics best serve as accurate, relevant indicators of the slab’s true moisture condition.
In contrast, research into other quantitative methods of moisture measurement in concrete has shown they are not reliable in providing one very critical data point: the amount of moisture the finished floor will “see” after installation, when the slab is effectively sealed and moisture can no longer evaporate from the slab’s surface. One key factor is surface-based methods do not properly account for the moisture gradient typically existing prior to the installation. (Read Concrete Floors and Moisture by H.M. Kanare, published in 2005 by Portland Cement Association [PCA].) Indeed, this was the crux of the research findings that came out of Sweden in the 1990s. Another problem is results can be significantly influenced by ambient conditions.
In the case of one popular surface-based quantitative test, the moisture vapor emission rate (MVER) test (also known as the calcium chloride test), research has shown it yields a high rate of false results, even on concrete that has been in place for years. (For more information, visit www.constructionspecifier.com/moisture-measurement-in-concrete-floor-slabs.)
The MVER test measures vapor emission rates in terms of pounds released over a 24-hour period per 93 m2 (1000 sf) of concrete. The ASTM F1869, Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride, requires waiting 60 to 72 hours before obtaining test results. This means a properly done RH test can now provide actionable metrics in one-third the time it takes to conduct the MVER test. Of course, it is important to always follow manufacturer’s installation guidelines that may or may not require additional types of tests to warrant installations.
The RH test, as it is used today, is fast, easy to use, reliable, and highly accurate. To the extent that challenges exist, one is determining the thickness of the slab to know the proper depth at which to place RH probes in post-tension slabs and/or slabs with hydronic tubing.
Photos courtesy Wagner Meters
How to take full advantage of the revised standard
In his work, the author has consistently seen concrete moisture test results taken after one hour track closely (within three to five percent) the readings taken at the 24-hour mark. Now, no one can actually move forward at the one-hour mark, but RH test results at one hour often give sufficient indication of what to expect. Whichever way the readings are trending, users can immediately discuss, plan, and prepare for the project’s anticipated next steps.
Trending in the right direction
If readings show RH levels are trending toward meeting manufacturer and project specifications, all resources needed for the project’s next steps can be brought together, including scheduling the flooring contractor for the days needed, and ensuring the flooring and other materials are onsite and ready for use. In this way, the flooring installation can begin without unnecessary delays.
Trending in the wrong direction
On the other hand, if readings show the RH percentage is not within the manufacturer’s specification, discussions can begin in earnest about the possibility of utilizing mitigation measures. Project stakeholders can explore options based on what the readings indicate about the concrete’s moisture condition, the environment of the slab, and other variables. If the decision is to use mitigation products, preparations can begin immediately, instead of waiting 72 hours.
No matter how the RH results are trending, being able to take action after waiting 24 hours instead of 72 hours is a game changer for moving projects forward in a timely, cost-effective, and safe manner. If test results show moisture levels are sufficiently low to ensure a successful installation one will gain maximum benefit from the new, shorter timeframe for RH testing. Even if things are not going according to plan, one still benefits since the project team now has an additional 48 hours to put in place possible mitigation steps for the excess moisture. Either way, the RH test gives valuable information about slab moisture condition faster than other test methods.
The best way to take full advantage of these benefits is to specify the use of ASTM F2170 directly in the project documentation. This way one can avoid giving the general contractor (GC) or the flooring contractor the latitude to choose an alternate moisture test method.
How is it important to shave two days off a concrete moisture testing time frame by using one RH test over another type of test when it will take a minimum of 30 to 90 days for concrete to dry to a level that it is even worthwhile to start testing. I agree the RH test is the most reliable concrete moisture test, but it hardly matters in the overall construction schedule that I can get reliable results in one day instead three, when it is going to take 30 to 90 days before it is practical to even start testing. It is more importan to know when to start tresting. Paul Potts