Thursday, August 21, 2014
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Moisture Testing Methods for Concrete Slabs

By Ken Duvall, CIE, CRMI
California Mold Assessment—Mission Viejo, Calif.

There have been growing concerns since the 1950s regarding the acceptable amount of moisture vapor emitting from a concrete-slab sub-floor. The flooring industry long ago adapted guidelines relating to the suitability for various coverings over a concrete sub-floor.

ASTM F1869 defines the standard test method for measuring the vapor emission rate of concrete sub-floor using anhydrous calcium chloride. See referenced standard at

The flooring industry recognizes the need for concrete testing. Why doesn't the environmental industry? Most inspectors and investigators don't recognize a concrete slab sub-floor as what it really is - a giant sponge!  Mr. and Mrs. Homeowner return to their castle after a European vacation to find three inches of water from a broken pipe covering the floor. When their panic subsides, they contact an insurance adjustor who refers them to a disaster cleanup specialist. This company responds in a timely fashion with vacuums, fans and possibly an assortment of dehumidifying equipment. The company may also remediate the water-damaged materials and prepare the interior for new Sheet-rock, floor coverings, baseboards and paint. Often, the services of an environ¬mental hygienist are engaged to prove the successfulness of the remediation project. After "drying out" the interior, new materials are installed, and the home is "back to normal."

Several months pass, and the homeowners begin suffering upper respiratory problems. Suspicious dark patches begin appearing on the lower wall surfaces. The backside of the base¬boards is taking on a life of its own and the new carpet smells. "What's going on now?" they ask.

"What's going on" is the moisture vapor within the concrete slab has begun migrating into the structure. The remediation specialist and environmental hygienist are summoned back by the now coughing and hacking and unhappy homeowners. "What's going on?" is becoming a familiar phrase. The "experts" collectively shrug their shoulders and say, "It was fine when we left."

The real problem is that it wasn't "fine." The concrete slab foundation was doing what it does best: It was holding a tremendous amount of moisture. The hygienist thinks there might be an additional plumbing leak. Didn't he have the system pressure tested? Maybe the neighbor's pool is leaking.

Had the "experts" checked the concrete slab foundation, they would have realized the interior was not ready for reconstruction. A simple application called a calcium chloride dome test would have saved a lot of embarrassment, not to mention cost and a possible lawsuit.

A properly installed moisture barrier over an adequate base will reduce the potential for capillary moisture incursion, hydrostatic pressure incursion and hydrodynamic pressure incursion. But it can also hold moisture in. In the adventure above, moisture was held in the slab. The barrier would not let the water escape into the ground. The dehumidifiers were insufficient in carrying the water vapor away.

If there is concrete, test it! Moisture meters should not be relied upon as a conclusive indicator of dry concrete. They measure only the top surface. A relatively inexpensive accurate examination is the calcium chloride dome test. To conduct this test prop¬erly requires four days, according to ASTM F1869. The first day is to identify, then clean, the surfaces to be tested. Cleaning requires grinding the surface free of any foreign substance. Leave the cleaned areas undisturbed for one day to allow any surface moisture to evaporate. The temperature and humidity should be similar to normal living conditions. Use the HVAC to create these conditions. If not, the concrete slab will be in equilibrium with the ambient air and there will be less vapor emission.

The testing is now ready to commence. The dome test kit consist of a 9"x9"x2" plastic "dome" and a Petri dish that holds a prescribed amount of calcium chloride. The Petri dish is first weighed (5.0 grams is typical), and the label is marked with the weight, time and date. The dish is then opened and placed on the surface. The lid can be taped to the underside of the dome with the accompanying tape (label to the surface). The dome is then applied over the Petri dish onto the cleaned concrete surface using specific tools to ensure an adequate seal. The dome should remain undisturbed for at least 60 hours but no longer than 72 hours. The dome is then removed, and the top is retaped to the Petri dish. The Petri dish is again weighed, and the difference (in grams) is noted. Using a specific formula, a result in pounds of water per 1,000 square feet is obtained. As is true with microbial sampling insufficient samplings will result in insufficient information. For areas up to 1,000 square feet, three tests are required, followed by one additional test for each additional 1,000 square feet (i.e., a 2,000-square-foot area will require four test sites).

A dry slab will test at three to five pounds of moisture. It is important to know what floor covering is going to be installed so the manufacturer's recommendations can be followed. These recommendations will also be sufficient for new construction materials.

This information is general in nature as there are many variables to specific sites. The original concrete composition, porosity, effectiveness of the sub-slab moisture barrier and seasonal conditions all contribute to the accuracy of the test. Testing near cracks, joints and perimeter walls should be avoided.

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