Concrete moisture meters

Christopher Capobianco

Christopher Capobianco

They have their place, but ...

I am amazed how often I have been asked about using a concrete moisture meter to decide if it is okay to install a floor. I’d love to say you can, because it’s so fast and easy, but not a single floorcovering or adhesive manufacturer accepts such a reading as a determination of a slab’s readiness to receive flooring. It’s just not accurate enough to be used in that way.

Wood meters? They can be valuable when checking the moisture level of a wood floor and a wood substrate, to see if they are both ready for installation. That is a standard practice in the wood floor industry. However, this same practice does not cross over to concrete floor slabs.

I did some research and searched a number of publications and websites to find some kind of reference to moisture meters as an acceptable method to determine whether concrete slabs are ready to receive flooring. I did not find one mention that said “use a meter to make sure the floor is ready” or anything like that.

Known as electrical resistance meters, these devices are, at best, a spot check of the moisture level at the very top of the slab in an exact location. But even then, be careful. ASTM F710 (Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring), section X.2.4, talks about these meters and how to use them but also has a warning about their use:

     Although a high reading … typically indicates high moisture content, a low reading … does not necessarily indicate more than surface dryness, as the concrete may have a higher moisture content below the surface. Conversely, a concrete with low moisture content but containing metal fibers could cause a high reading.

Readings can be deceptive

This points out the risks of relying on this method exclusively. A low reading can mean the slab is just dry at the surface and moisture below the surface is not detected. That’s risky because, if there is moisture below, it will move upwards. You can count on that. It’s part of the natural process of evaporation – moisture moves from the ground to the sky and moves from a damp place to a dry place. In this case, the concrete slab can be that damp place and the room air is drier. On the other hand, because these meters work on electrical current, metal in the concrete can affect the readings. Most concrete has metal rebar in it, so this could be a problem and you might have a high reading for a slab that is actually quite dry.

Installation May June 2014

A concrete moisture meter being used as a spot check during an investigation of a flooring failure.

So, when DO you use a concrete moisture meter? I use it on inspections when I pull up a floor. The meter gives me a spot check of moisture and often tells me I need to recommend that the client have detailed moisture testing done. I have also heard of cases where meters are used to identify potential hot spots where calcium chloride or relative humidity tests should be placed. Meters are useful for testing the dryness of gypsum underlayment, a widely used floor substrate in high-rise residential construction. In these cases, consult with the gypsum underlayment manufacturer for the type of meter it prefers and the allowable limits for the product. Different manufacturers may have different preferences.

If meters are not recommended, what method should you use? ASTM F 1869 is the calcium chloride test method, also known as the MVER (moisture vapour emission rate) test. It has been around for decades and many adhesive and flooring manufacturers still reference this test, which is widely used but often done incorrectly.

There are three key points to keep in mind. First, the building has to be the same temperature and humidity it will be when the space is occupied (heat or air conditioning must be on). Second, the surface of the concrete has to be cleaned by a gentle grinding before the tests go down. Third, if there is any kind of coating or old adhesive in place, you need to wait 24 hours after grinding before putting the kits down. If all three of those factors are not taken into account, the results may not be accurate.

Only for bare-naked concrete

It’s also important to know that the calcium chloride test is not recommended on lightweight concrete or gypsum concrete, and should not be used over any type of a coating, sealer or moisture-mitigation system. It’s for bare-naked concrete only!  Even then, research has shown that the test has only proven to measure moisture from the top ½ to ¾ inches of the slab. That worries me, because most slabs dry from the top down, so it could be very wet inside but the calcium chloride test would not reveal that.

So, moisture meters and calcium chloride tests may not be accurate. Now what? The relative humidity (RH) method, known as ASTM F 2170, involves drilling holes to measure moisture inside the slab, which is a more accurate way of predicting what will happen in the future. This method is accepted and even preferred by the majority of manufacturers of floorcoverings and adhesives today.

There are several different types of RH testing equipment on the market, and most can give accurate and fairly quick results. Plus, because most RH equipment is reusable, you can test the same spot over and over again, or even leave the equipment in place inside the hole. This way, if you get a high reading, you can monitor the slab and, when the readings come down, the slab is ready.

There is more awareness than ever about flooring failures related to moisture. Ideally, independent testing companies should do the testing. Our industry has been saying that for a long time and the Floor Covering Installation Contractors Association (FCICA) recently issued their updated position on the original floorcovering industry white paper position statement on moisture emission testing, which reemphasizes that opinion. However, I still recommend installers, flooring contractors and flooring retailers be aware of concrete moisture problems and know how to test for them. We no longer have an excuse not to do so and the liabilities for failures are extremely high.

The International Concrete Repair Institute (ICRI) offers a two-day training and certification course for concrete moisture testing technicians. If you are serious about testing correctly, this is a good idea.

Christopher Capobianco is a flooring expert currently working with Forest Hill, Md.-based Spartan Surfaces, a distributor of commercial hard-surface flooring. Chris’s family lineage in floorcovering goes back for generations. His career includes time as a retailer, architectural sales rep, technical support manager, consultant, instructor, columnist and active volunteer in several organizations.

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