As time goes on and the design and construction industry understands the importance of independent testing, hopefully we will see less testing being done by installers and more done by independent agencies. Often, concrete slab moisture testing is done by “hit or miss,” and testing is done by companies whose primary business is something other than concrete slab moisture testing, i.e., general construction testing laboratories , environmental companies, and flooring contractors. These companies typically are already involved with other aspects of a project, and for convenience are asked to perform this process. Moisture testing typically makes up less then 1 percent of their annual revenue and often the testing process get about the same level of attention.

The Latest Test Methods
Here are six methods for concrete moisture testing. The first three should never be used to make a decision about whether a concrete slab is ready to have a floor installed.

The senses test: (“It looks dry,” “It feels dry” and/or “It smells dry”). Many jobs go forward based on this test method. Unfortunately, you can’t see, feel or smell moisture coming out of a concrete slab, so using this method to decide if a slab is ready is just plain foolish.

The plastic sheet test: Many of us have used a plastic sheet taped to the floor as a way of testing for moisture. Tape down the sheet, come back in a few days and if it’s dry under the plastic, lay the floor. The problem is that this method is influenced by atmospheric conditions and is not accurate. I once saw side by side tests on a slab that had very high readings on the calcium chloride test but the plastic sheet right next to it was completely dry. If you use a plastic sheet, beware! Dry may not be dry!

Moisture meters: I am amazed how often I get calls from someone who gives me a concrete moisture meter reading and asks if it is okay to install a resilient floor. Concrete moisture meters are a “spot check” at a given moment in time, but they give no indication of long term moisture conditions and there are no flooring or adhesive manufacturers who will accept this method as a “go or no go” test for installing resilient flooring over concrete. Meters are useful for testing the dryness of gypsum underlayments, for trouble shooting of concrete related flooring failures, and to identify locations in the concrete that are “wetter” than others so they know where to place their moisture tests.

The following three methods are ASTM Industry standards that have been developed to provide accurate and repeatable results.

Calcium Chloride Test Method. ASTM F 1869, Standard Test Method for Measuring Moisture Vapor Emission Rate (MVER) of Concrete Subfloor Using Anhydrous Calcium Chloride is a test that has been around almost 50 years and is widely used but often done incorrectly. The four most common mistakes when doing the ASTM F 1869 test are:

Wrong conditions: The test must be done when the building is at the same temperature and humidity it will be when the space is occupied. If there is no heat or air conditioning, don’t do the test because the results will not be valid!

No Preparation: The F 1869 method calls for cleaning a 20-inch-by-20-inch square at each test site to be sure you are testing bare concrete. This will remove curing compounds, sealers or old adhesive residue. The people who do testing for a living always grind the surface so the slab is completely clean. Failure to clean the slab may result in a false low reading.

No Waiting: ASTM F 1869 calls for a 24-hour waiting period after cleaning before the test kit is placed so surface moisture that was trapped beneath whatever coating was there can evaporate. Failure to wait 24 hours may result in a false high reading.

Too few tests: For all concrete moisture test methods, go “by the book” and do three tests for the first 1,000 square feet and one test per 1,000 square feet after that.

Even when done correctly, the calcium chloride test has limitations because it only measures moisture vapor emissions from the very top of the slab – less than 1 inch down. Because most concrete slabs dry from the top down, they are often dryer at the top than on the bottom, until you cover it with flooring. When these slabs are covered, the moisture down below moves upward, is blocked by the floor covering and trapped in the adhesive layer, turning it to mush.

Two other methods have been developed that are able to “look deep” into the concrete to see if there is any moisture waiting to come to the top.

Relative Humidity “Probe” Method ASTM F 2170, Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using In Situ Probes involves drilling holes to 40 percent of the slab thickness – usually 2 -3 inches down. This measures moisture inside the slab, which is thought of as a more accurate way of predicting what will happen in the future. Denise Padgett, Sales & Marketing Manager for Wagner Electronics, a manufacturer of handheld moisture measurement devices, explains “Unlike calcium chloride testing, relative humidity probes are less sensitive to fluctuations in ambient air humidity and temperature above the slab.” I’ve noticed that limits for RH tests are starting to show up in manufacturer’s instructions installation manuals, and Ms. Padgett agreed, saying “Many areas within the industry are moving towards RH testing. I’m working with several associations who in the next few months will include RH testing in their installation practices and guidelines.”

Relative Humidity “Hood” Method ASTM F2420 Standard Test Method for Determining Relative Humidity on the Surface of Concrete Floor Slabs Using Relative Humidity Probe Measurement and Insulated Hood is a brand new method here in the United States that has been in use in Europe for a number of years. It was approved as an ASTM standard in 2005. It has not yet started to make its way into manufacturer’s installation instructions but it may in the future so remember you heard about it here first. Like the ASTM F 2170 method, this test also measures relative humidity, but on the surface instead of inside a hole in the concrete. This test seems similar to the calcium chloride test. How is it better?

The hood method measures the amount of free water in the concrete and the calcium chloride kit measures how much vapor is being driven off the surface. One of the reasons humidity testing in concrete is becoming more popular is that these methods are not as sensitive to atmospheric conditions unlike the calcium chloride test kits. The hood method is measuring the equilibrium relative humidity of the concrete in exactly the same way ASTM F 2170 is and is gaining popularity due to the fact no drilling is required.

“Does it have a correlation to the ASTM F 2170 test?”

“They both measure the equilibrium relative humidity. The hood method measures close to the surface and humidity probe tests measure at the 40 percent depth of the concrete. Readings from the hood method tend to be 5 percent lower than with the sleeve method.”
The floor covering industry would do well to stay up on the latest methods for moisture testing. Installers should know these methods in case an employer asks them to do one, and also because there will be opportunities for work from these independent testing agencies as more and more testing is done in the future.

Flooring retailers and contractors need to be aware too, and be more knowledgeable than their customers or the general contractors. If there is moisture related floor covering failure and the floor was not tested before installation, then there is a lot of finger pointing after the fact. Even worse, of the testing was done and not done correctly, then whomever did the testing can be held responsible and the costs are enormous.