Best Practices for Monitoring Concrete Moisture Conditions

On any building project, communication with the various subcontractors is key. When it comes to installing floor coverings over concrete, the contractor and the flooring installer don’t always have the same understanding of the drying requirements that must be in place in order to apply flooring over a concrete slab. Having a better understanding of concrete conditions can keep the project schedule on track.

For example, concrete has been poured and the schedule says flooring can be installed 90 days from the pour date. But the HVAC won’t be on until two months after that date. How does that affect the concrete drying time? How can the drying progress of the concrete slab be accurately monitored to set up a smooth schedule and prevent delays?

Concrete Moisture Conditions
Moisture movement through and out of drying concrete can vary greatly depending on ambient conditions like air humidity, temperature or surface condition when the slab is not at service conditions. For example, if a slab has been power troweled, the slab’s surface loses its ability to release the moisture in the slab and drying time will slow or even halt completely.

According to ASTM standards, the concrete must be at stable service conditions for a minimum of 48 hours to meet the standard for conducting valid moisture (relative humidity) testing (see ASTM F2170 for specific information). The results you get, though, will also depend on the test method used.

Test Methods
There have typically been three major types of tests for measuring moisture content in concrete. Each has a specific use that may or may not be best suited to monitoring a concrete slab as it dries.

Calcium chloride kits have been in use for quite some time but have proven to be unreliable in their results because they are only measuring moisture passage from the concrete at a very shallow depth. Their use is covered under ASTM F1869 but that standard has recently been amended so that calcium chloride is no longer acceptable as a test method for lightweight concrete. Some specs still include calcium chloride kits but this may not meet the ASTM requirements for your project.

Hand held concrete moisture meters work well as a general survey tool but they do not read far enough into the slab for a true overall “moisture” picture. Their electromagnetic field only penetrates about one inch into the slab and can interact with metal (i.e. steel) in the concrete to give a false reading of the moisture conditions. Hand held moisture meters CANNOT be used to determine when to install a floor covering.

Relative humidity testing is conducted through the use of in situ probes that measure relative humidity (RH) within the slab. Their depth-specific application has proven to provide an accurate representation of moisture conditions within the concrete slab. Their very location within the concrete means they are less susceptible to environmental or ambient changes at the surface. And contrary to some opinions, RH testing is not a long involved process.

Some RH testing systems seem complex with repeated equilibration times or cables and meters to carry around. But not all RH systems are cumbersome. For example, with the Rapid RH 4.0 from Wagner Electronics, the sensor is pre-calibrated and is installed directly into the slab where it stays equilibrated. Tracking or monitoring the relative humidity in the slab becomes fast, accurate and, after the initial ASTM-required equilibration period, takes only minutes to ensure that the concrete is truly ready for the flooring application. The pen-style Reader has touch-and-go capabilities so you’re not wasting time at each test hole. When the results indicate the slab is at installation conditions, the probe is simply skim coated into the slab and installation can proceed.

Conclusion
ASTM F-2170 RH testing is the recommended test method for monitoring moisture conditions in a concrete slab because it measures within the slab, and is therefore the best predictor of what the moisture condition will ultimately be at the slab surface after a floor covering or coating is applied. Test methods like calcium chloride kits or hand held surface meters do not get into the slab for a true overall “moisture” picture that is key to successful flooring installations. Understanding the conditions, test method and timing of the moisture conditions in the concrete slab will ensure both you and your flooring installer stay on schedule and on track.

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