Changes to Concrete Standards: How they clarify your choice of test methods

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.

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