Concrete moisture

Establishing the basics

By Christopher Capobianco

I have written about concrete moisture issues as they pertain to floorcoverings, produced and presented continuing education seminars and inspector training classes, written ASTM standards on the subject and inspected more than a few failures related to concrete issues. It’s a complex subject I could fill pages with, but here is a very basic explanation of why and how to test that may help you understand the subject and explain it to your customers.

The reason for testing new concrete is that it has extra water in the mix to make it easy to transport and place. Once the concrete cures, it starts to dry and all that excess moisture evaporates. By the way, concrete curing is the chemical reaction that bonds all the ingredients together, and concrete drying happens after that. Curing and drying are not the same thing.

In old concrete slabs, especially on or below grade level, there is always moisture moving from the ground towards the slab and there may or may not be a moisture retarder or barrier under the slab to stop it. Plus, external sources like poor drainage next to a building can cause moisture to get into the floor slab. So even in an old building, it’s important to test.

Some people only worry about moisture on new slabs or slabs on or below grade. However, per ASTM F 710: “All concrete slabs should be tested for moisture, regardless of age or grade level.” Some of the worst moisture problems I’ve seen have been in old buildings where there was no vapour retarder under the slab. Or, if above grade concrete is pumped to the upper floors, it can have extra water in the mix, especially lightweight concrete. Also, any slab that dries from one side, like a slab over a metal deck, has a longer drying time. So, always test, if it’s old or new, even if there is an existing floor down that looks in good shape. Don’t assume it’s dry.

Please don’t smell it

In my seminars, I talk about several methods for testing concrete, and three of them should not be used. The “senses” test is used to see, feel, or smell moisture in a concrete slab. It’s not very reliable because moisture is usually invisible and odourless. Taping a sheet of plastic to the floor for a day or two is another old method that does not work. I saw the results myself in a laboratory test where an apparently dry plastic sheet was on a slab that had very high results using other methods.

With plastic sheet tests, dry may not mean dry!  Moisture-meter testing is valuable for various purposes, but one of those purposes is not to make a decision on whether a slab is ready for a floor. Meters are valuable as a spot test for surface moisture, for testing to see if gypsum underlayment is dry enough to install floor coverings.

The two methods that are accepted by the floorcovering industry are the relative humidity (RH) method and the calcium chloride test. The RH test is also known as ASTM F 2170: Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes. With this method, holes are drilled to 40 percent of the slab thickness, a plastic sleeve is inserted and a relative humidity probe is used to measure moisture in the cavity at the bottom of the hole.

This test is thought of as being the best way to predict moisture rising to the surface of the concrete. F 2170 is easily repeated days, weeks or even months after the original test. This is truly the latest technology for moisture testing and is very quickly being recognized by flooring manufacturers throughout the world.

The calcium chloride test, or ASTM F 1869: Standard Test Method for Measuring Moisture Vapor Emission Rate (MVER) of Concrete Subfloor Using Anhydrous Calcium Chloride, has been around over 50 years. This is a good test for surface moisture, but it does not measure deep-down moisture the way the F 2170 test does. The principle is based on the fact that calcium chloride, a type of salt, absorbs moisture.

How to test

Place a dish of calcium chloride under a plastic dome for 2 ½ to three days. By weighing the dish before and after the 60- to 72-hour waiting period, a formula is used to determine the moisture vapour emission rate (MVER) as pounds of moisture vapour per 1000 square feet per 24 hours. The F 1869 test is not to be used over patching or levelling compounds, lightweight concrete, gypsum underlayment, moisture mitigation systems or other coatings. It’s only for bare concrete tested at the same temperature and humidity expected during normal use.

To prepare the test site, you have to use a grinder to prepare a 20-inch square. It is not enough to scrape, sand or wire brush the surface. A light grinding should produce a texture similar to coarse sandpaper. Before placing the test, wait 24 hours if the concrete has a sealer, curing compound, old adhesive or any other type of coating. This allows the surface to dry out if there is any moisture trapped under the coating. If the slab was uncoated for at least 30 days, you don’t have to wait the 24 hours.

That is a very simple outline, and if you want to learn more, it will take a day or two. The International Concrete Repair Institute (ICRI) offers a training and certification program for moisture testing technicians, and The IICRC also offers a substrate/subfloor inspection class.

The industry recommends independent third-party testing, but often times it falls to the installer to do the test, so installers and dealers should know as much as possible about why and how to test. If you don’t test, you are gambling on some expensive failures.

Christopher Capobianco is a flooring expert currently working with Sorest 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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