May 4, 2018
by Jason Spangler
The risk of failure in a concrete floor because of excess moisture is high simply because concrete has moisture. Additionally, the costs of misreading the concrete’s moisture condition, and laying down finished floor products too soon, extend well beyond ugly staining or cracking.
Often, undetected excess moisture shows up long after the project is completed, and this can make the cost to remediate quite significant. Detection of any excess moisture in concrete is best done upfront at the construction stage. This is the easiest and least expensive approach.
The in-situ relative humidity (RH) test, specified in ASTM F2170, Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes, is a reliable, accurate indicator of a slab’s readiness for the flooring installation. Due to a recent update to this standard, RH testing is also faster than other test methods. (For more information on the update, read the report “ASTM update reduces wait time for RH test results in concrete.”)
History of the concrete RH test
In 1929, research by the American Concrete Institute (ACI) indicated the RH of the air around concrete causes the slab to shrink or expand. Two researchers (G. Rupert Gause and John Tucker. Jr) from the National Bureau of Standards and Technology (NIST) drew on this finding and other related research in their 1940 paper, “Method for Determining the Moisture Condition in Hardened Concrete” for the Journal of Research of the National Bureau of Standards. (Visit nvlpubs.nist.gov/nistpubs/jres/25/jresv25n4p403_A1b.pdf to access the full paper.) The team tested the use of an electronic hygrometer to measure the RH within the concrete itself. They found RH varied by depth within the slab—a critical discovery for the development of in-situ RH test methods. In their paper, the researchers also postulated the internal RH of a slab is likely an important determining factor as to when it is safe to apply a finished floor product to the concrete surface.
Another key discovery came in 1957. T.C. Powers, a researcher at Portland Cement Association (PCA) determined the variation in RH values at different depths throughout the slab was the factor “pushing” transmission of moisture through the concrete.
In the 1950s and ’60s, researchers at PCA also developed, tested, and refined versions of hygrometers and RH sensors to measure the moisture condition
of concrete, including employing saturated salt solutions to calibrate the machines. While PCA had the resources to develop these large machines, their size and cost made them quite impractical for most uses in the field.
In 1963, G.E. Monfore, another PCA researcher, developed a smaller version of an RH sensor. In 1971, R.E. Carrier, a civil engineer and a writer for The Construction Specifier, reviewed various testing of the Monfore gauge and other small RH sensors. He concluded, “While much testing remains to be done, it is hoped these gauges may provide a simple, inexpensive, and dependable method for evaluating concrete curing effectiveness.”
Continued testing leads to industry standards
Researchers at universities and industry groups around the world continued to examine the RH test method, both to refine it and make it commercially viable. It was not until the 1980s and ’90s when their work began to yield practical results for the industry.
The large hygrometer developed by PCA in the 1950s was the foundation for additional work in Great Britain. In the 1980s, a British industry association developed and published two industry use standards (BS5325, Code of Practice for Installation of Textile Floor Coverings, and BS8203, Code of Practice for Installation of Resilient Floor Coverings). By this time, the hygrometer was a small probe sealed in an impermeable box on the slab. The probe had to acclimate for 72 hours before taking the first reading. Then the standard required two acceptable readings 24 hours apart before the slab could be said to be ready for the floorcovering. New Zealand associations soon developed their own standards, based on the British ones.
Research done in the 1990s at the Technical University of Lund, Sweden, looked into the depth that would provide the most meaningful information when placing RH sensors and taking moisture measurements in concrete. Typically, a moisture gradient forms, with higher moisture levels deeper in the slab. Prior to a flooring installation, lower levels of moisture are always present near the slab’s surface. This fact makes it critically important to know the specific depth to place RH sensors to get an accurate indication of how much moisture the floorcovering will actually “see” after installation.
The Swedish researchers found the sensors should be inserted at 40 percent depth in a slab drying from one side, and 20 percent depth for a slab drying from both sides. With these precise, scientifically validated metrics to guide the use of the in-situ RH test in concrete, professional standards for use in the construction industry were developed. The Nordtest, employed in Sweden and Finland, was first published in 1995. The Nordtest became the foundation for ASTM International’s initial 2002 publication of ASTM F2170.
Until February 2018, ASTM F2170 standard required a 72-hour wait period before official readings of the RH sensors could be taken. The purpose of this wait was to allow the sensors to equilibrate fully within the test hole. Of course, this meant three full days of waiting before getting the information needed to take a specific action based on the test results.
In 2014, ASTM International commissioned a precision and bias (P&B) study by an independent laboratory to determine, in part, how soon an RH sensor could return an accurate, actionable result. The P&B study tested RH sensors from six different manufacturers. The researchers took readings from all the RH sensors at the one-, two-, and four-hour marks as well as at 24-, 48-, and 72-hour marks. Their analysis of the tests found the readings taken at the 24- and 72-hour marks were statistically equivalent, which is to say they were virtually identical despite minor variations. The pair of readings fell within the acceptable range set out by the RH standard.
Thus, this study demonstrated properly conducted RH testing in the field yields actionable readings at 24 hours. Further studies verified these results, which led the ASTM committee to update the F2170 standard to reflect the latest scientific findings. Now, official, documentable RH readings can be taken at just 24 hours.
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.
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.
Benefits of the updated standard
The 2018 update to the ASTM F2170 standard can shave two full days off any project timeline. However, it is important specifiers mention the in-situ RH test in project documentation. Otherwise, one cannot know for sure the project team will choose the RH testing method and take advantage of the shorter testing window.
GCs and flooring professionals should be excited about the accelerated timeframe the updated RH test standard allows, as it will help them focus their time and resources more efficiently in getting to project completion. The latest RH test research, combined with the publication of the revised ASTM F2170 standard, gives assurance of being able to plan and act based on a 24-hour waiting period while minimizing the risk of a moisture-related flooring failure down the road. The new 24-hour RH test window is a “win-win” for everyone involved. GCs and flooring contractors can improve their utilization of project resources. Architects and specifiers can be confident the right RH test standards get employed for accurate results within a shorter timeframe. Clients can also enjoy beautiful, resilient concrete floors for years into the future.
Jason Spangler has more than 20 years of experience in sales and management in a spectrum of industries. He has successfully launched a variety of products, including the original Rapid RH concrete moisture tests. Spangler works with Wagner Meters as its Rapid RH product sales manager. He can be reached at email@example.com.
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