If you’re privvy to the science behind the in-situ method of moisture testing, and why it is the most accurate and researched way of predicting the slab’s moisture condition, this would come as no surprise to you.
ASTM F2420-05(2011)- “Standard Test Method for Determining Relative Humidity on the Surface of Concrete Floor Slabs Using Relative Humidity Probe Measurement and Insulated Hood” has been withdrawn by the standards committee, as of December 2014.
A recap: The “hood” method as it is known (because the Standard’s full title is a mouthful) aimed to capture the RH condition above the slab. AS 1884-2012, Australia’s Resilient Flooring Standard, prescribed ASTM F2170-2012, the in-situ probe at-depth method, to accurately measure the RH% condition of the slab. However, it referenced ASTM F2420-2011 where drilling into the slab is not possible, due to in-slab heating elements or such like.
The problem is, the two methods are very different. Even the standard itself acknowledged that the hood method only captures the RH% condition at the time of the test. The in-situ method is a real predictor of future RH% potential if a sealed floor is placed on that slab.
It was also recognised that the hood method only captured the RH% condition in that portion of the slab. In-situ RH% tests capture the “worst case scenario”, that is, to what RH% the slab would equilibrate to across its surface if it was sealed by a floor.
The hood method was also notorious for drift. While the hood was given the same equilibration period as in-situ (72 hours), the standard itself prescribed a maximum amount of drift in the reading allowable before the reading should be disregarded, and you have to go back later and read it again. Much later. Floor Test Australia has seen this first hand. The drift could be up or down, which also suggests the hood method was more instantly susceptible to environmental changes.
The standards committee actually stated that drift may not settle until up to 70 days after the hoods were placed. We don’t know of many construction jobs which can wait 70 days for a go/no-go on the flooring decision.
Precision and bias tests also showed a large disparity between different hood products in equal environments.
Obviously, the obsolescence of ASTM F2420-05 could be detrimental to those who simply cannot drill into the slab. In these cases, contingencies must be found, such as accurate scanning for in-floor elements to determine drilling locations. It certainly highlights that the need for accuracy in measuring the slab’s moisture.