Understanding, preventing, detecting and correcting moisture in concrete floors

Flooring failures attributed to moisture-related concrete problems are at near epidemic proportions today. Coatings, carpet, vinyl, rubber, wood, laminates and most floor coverings are affected to one degree or another by pH issues and excess water vapor emissions through a concrete slab.

By Christopher Capobianco

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The good news is that testing can detect moisture movement long before a floor covering is installed.

Test when and where?

The short answer is everywhere and always. ASTM F 710 recommends testing concrete slabs for moisture, regardless of age or elevation. Every floor covering manufacturer and adhesive producer says the same thing about old or new concrete from the basement to the penthouse.

To say “It looks dry,” “It feels dry” or “It smells dry” isn’t enough. Taping a plastic sheet to the floor for a day or two also has been proven to be an inaccurate indicator. The easy-to-use electronic meters that test for moisture in concrete yield only a spot test that isn’t a good basis for a global go or no-go decision for a flooring installation.

The floor covering industry recognizes two methods for testing concrete for moisture. The first is the 50-plus-year-old test now known as ASTM F 1869, Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride. This test uses a kit consisting of a dish of calcium chloride that is sealed on a clean section of concrete beneath a plastic dome for three days (Figure 1). Because the salt is hydroscopic, the weight gain can be used to establish the moisture vapor emission rate. Most resilient flooring and carpet can tolerate a maximum MVER of 3 lb. or 5 lb., depending on the product. If done correctly, F 1869 measures moisture emissions from the top 5 cm or so of the concrete. While this is a good test for surface moisture, it doesn’t measure what’s inside the slab.

Figure 1

Figure 1. The weight gain in a sample of calcium chloride correlates with the mass flow rate of water vapor leaving the concrete surface.

Getting that data requires using ASTM F 2170, Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using In-Situ Probes. This is probably the best predictor of future moisture problems (Figure 2). It involves drilling holes in the floor to measure the humidity inside the slab. This is truly the latest technology for moisture testing and is very quickly being recognized by flooring manufacturers throughout the world.

Figure 2

Figure 2. Drilling a hole is the first step in testing with ASTM F 2170.

What if it fails?

A number of measures can accelerate the drying process on a new slab. Get the building’s climate control system up and running — raise the temperature, reduce the humidity, get the air moving. If a curing compound was used on the floor, remove it. Commercial dehumidifiers and drying equipment are used in water damaged buildings and also can used to dry a new concrete slab.

Consider surface-applied vapor retarders to block moisture emission from the slab. These products need to be researched thoroughly before being used. Look at the manufacturer’s reputation, track record, warranty and insurance coverage to be sure that you’ll be covered if the product fails. Check with the floor covering or adhesive manufacturer for additional guidance.

Preventing moisture problems in new floors is all about the proper specification. Using concrete with less water and larger aggregate can make a difference, as can using the cover cure method instead of curing compounds. Always use a vapor retarder between a slab and the ground. Sealing beneath the slab is just as important as sealing above the roof. Placing the concrete beneath some roof-like structure prevents rain exposure and helps the concrete dry faster, as does getting the HVAC system up and running as soon as possible.

Concrete is an ancient material that can last for thousands of years. Placed correctly, concrete floors can accept any floor covering to yield a long-lasting and durable wearing surface in almost any type of space. However, because of the fast-track nature of a lot of today’s construction projects, the installation often is rushed and failures occur. So, it’s important to know what to ask for, how to test, and what to do if you discover a problem.

Christopher Capobianco owns Flooring Answers in Patchogue, Long Island, N.Y. Contact him at www.FlooringAnswers.com and (631) 275-6494.

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