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Concrete Floors and Moisture

Facilities Engineering

Unsightly, even hazardous, floor damage can often stem from a fairly simple root cause:  Moisture in concrete. If concrete slabs aren’t given the appropriate amount of time to dry, flooring can stain, bubble, warp – or, more worryingly, develop mold or cause a safety issue.

Moisture testing can help prevent future damage to flooring, particularly in new construction. Perhaps the most unexpected issue with moisture sensitive flooring in new construction is the amount of time it can add to your construction schedule.

In existing buildings, this testing is useful to determine exactly why there are defects in the flooring. Poor groundwater drainage, for instance, could end up intruding through the slab on grade and lead to myriad forms of damage to the flooring, or the concrete slab itself.

Common Tests

The two most common tests ECS is requested to perform for moisture testing are Calcium Chloride Testing and measuring Relative Humidity using probes.  ASTM International has standards for both of these tests as well as less common moisture testing methods, such as the Plastic Sheet Method. 

ASTM F 1869 Vapor Emission Testing using Calcium Chloride – This test relies on Calcium Chloride testing kits that are typically prepared and analyzed by a laboratory. To perform this test, an area of slab must be prepared and conditioned for a minimum of 24 hours prior to installation of the testing kit. The testing kit is weighed and installed where it remains sealed for 60-72 hours. The kit is re-weighed at the time of collection and sent to the laboratory.    The intent of the test is to provide a quantitative determination of the of moisture vapor emitted from the slab. 

ASTM F 2170 Relative Humidity Testing using Probes – This test relies on probes which are drilled into the concrete slab and left in place for 24 hours to allow the slab to reach moisture equilibrium. After 24 hours, a digital reader is inserted into the probe.  The probes may be left in place longer and additional readings may be taken at future times.

The ASTM for both tests provides instructions for the required number of testing locations. Three tests are required for the first 1,000 square foot area and at least one additional test is required for each additional 1,000 square feet. Both tests are vulnerable to construction traffic on active job sites, although the RH probes are typically less likely to be disturbed.

Choosing a Test

Test selection is usually determined by the manufacturer of the flooring being installed, who provides limits to either the Vapor Emission Rate or the Relative Humidity in their product specifications or installation guide.

New Construction

In new buildings, flooring issues caused by moisture in concrete slabs are often the result of hasty construction without proper drying time and testing prior to applying floor finishes. The effect proper drying time can have on a construction schedule is significant. Specific drying times depend on the water-cement ratio of the mix design and other factors, but it is not uncommon to need over 6 months to achieve a relative humidity of 90%. Working with your contractor and design team prior to construction and choosing low water-cement ratio concrete mixes may be the most efficient method to expedite the drying process.

Many industry organizations have released guidelines to address this important issue, such as the American Concrete Institute (ACI) 302.2R “Guide for Concrete Slabs that Receive Moisture- Sensitive Flooring Materials” and the Portland Cement Association (PCA) Engineering Bulletin 119 “Concrete Floors and Moisture.”

We hope that this “Lessons Learned” is helpful to you in your next project.  ECS is available to provide moisture testing on your next flooring installation project, or assist you in determining the cause of an existing flooring problem.

For more information, please contact ECS or your structural engineer.  We hope this “Lessons Learned” will be helpful to you in planning your next project.

ECS Corporate Services, LLC