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.”
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