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Being on many job sites where moisture testing is being done, I have encountered situations where several types of Relative Humidity (RH) probes have been used with different results.  One particular job site there were two testing companies testing, virtually within feet of each other with a 12% difference in their test results.  So the questions were why the huge difference and which result do you believe?  With my almost half century tenure in the flooring industry I have seen moisture testing evolve from educated guess work to a very scientific approach.

Photo 1 – Study: In July 1996, Dr. Göran Hedenblad, Lund Institute of Technology, Stockholm, Sweden performed experiments measuring the relative humidity within concrete by performing in-situ testing at different depths within a concrete slab. His work showed that there exists a Relative Humidity (RH) gradient within a slab that had its surface exposed, where the RH increased as depth into the slab was increased. He further went on to show that after the concrete slab was covered by a flooring material that has minimal permeability, thereby preventing moisture from migrating out of the concrete surface, the RH gradient equalized to an average RH throughout the slab that is equal to the RH at 40% depth of an uncovered slab drying from the surface only (resting directly on a vapor retarder), or equal to the RH at 20% depth of an uncovered slab drying from both top and bottom. These findings became the foundation and basis for the development and publication of ASTM F-2170, “Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes.” The purpose of the F2170 standard is to provide a standardized practice for which to perform an RH measurement within the concrete. The intent of the standard was to provide a practice that would result in measuring the RH at a specified depth (40% depth for slab drying from top only, 20% depth for slab drying from both top and bottom).

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Basically, the standard describes how to drill a hole and to what depth, how to insert a “liner” into the hole that will allow for sealing off the sidewalls of the concrete, the placement of a seal at the top of the liner to seal off the internal environment, the equilibration of the internal environment relative humidity, and the accuracy of the testing instruments employed in the measurements.

Photo 2 - Drilling the Hole: We first need to determine the thickness of the slab, as the hole to be drilled must be depth specific.  Too shallow will yield a drier result and too deep will yield a wetter result.  The F2170 Standard clearly specifies the “Drill-to-Depth from Top of Slab” as 40% for slab drying from top only, and 20% for slab drying from both sides.

The purpose of drilling to the specified depth is to expose the concrete at that depth, so moisture can migrate out of the concrete to accumulate and equilibrate within a sealed-off measuring environment that will allow subsequent measurement to determine the equilibrated RH level at the specified depth. Since the concrete has an RH gradient throughout the depth of a slab that has an exposed surface, it is paramount to measure the correct RH at the correct depth.

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Next, the type of drill is important. A two-fluted carbide bit will tend to cut a hole that is slightly out of round and will not allow a proper seal of the sleeve.  A four-fluted bit will cut a much rounder hole.

Photos 3 and 4 – Brushing and vacuuming the drilled hole: Once the hole is drilled it must be thoroughly wire brushed and vacuumed.  It is imperative to remove all dust and debris as fine dust will contaminate the sensors and render them inaccurate.

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Photo 5 - Inserting the Liner: The F2170 Standard clearly specifies to “Insert hole liner to bottom of hole. Place rubber stopper in upper end of liner and seal around liner to concrete at concrete surface with joint sealant, caulk, or gasketed cover.” Section 6.2 also defines the hole-liner as, “plastic or non-corroding metal tubes, inside diameter not more than 0.04 in. (1 mm) greater than the probe’s external diameter, of sufficient length to seal the hole to the desired depth.”

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The main purpose of the liner is to isolate the very bottom of the concrete at the bottom of the drilled hole at the specified depth to provide a “measurement environment” to perform the RH testing. Further, the purpose for the liner is to seal the sidewalls of the concrete off from the exposed concrete at the open bottom of the liner, and the seal at the concrete surface to prevent moisture from migrating out of the drilled sidewalls and out the surface, which would provide a “drying out” mechanism for the local concrete slab area under testing.

Photo 6 - Calibration of probes: All Relative Humidity probes must be calibrated for accuracy on a regular basis a minimum of once per year.  Dust and everyday usage can throw off the calibration of probes.  Many parties paying for moisture testing will ask for your calibration certificate.

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Equilibration of probes

The placement of the probe into the sleeve requires the time necessary for the probe to reach equilibrium with the slab.  The time required for these probes to reach equilibrium varies depending upon the type and condition of the probe.  The F2170 Standard clearly specifies the need for the measurement sensor or probe to come to thermal equilibrium with the concrete, “Probe shall be a the same temperature as the concrete before reading,” and for the sleeve environment to come back to equilibrium once the sleeve is “uncapped” and a measurement sensor or probe inserted into the sleeve. The F2170 Standard clearly specifies to “Check for Drift.” The relevant section clearly states that the “meter reading must not drift more than 1% relative humidity over 5 minutes.” Furthermore the Standard states, “Equilibration may take several hours to several days depending on factors such as the initial temperature difference between probe and concrete.”

Photo 7 - “Dead Volume” measurement: Dead volume means the volume of air that is being measured for relative humidity.  Since we know there is an increase of relative humidity gradient as you go down into the slab, is the sleeve allowing a true measurement at the 40 percent level or are the sleeves allowing a diluted measurement?  If the sleeve is allowing relative humidity to be diluted, the Dead Volume of relative humidity to be measured from other levels of the hole is going to be lower than if it was taken at the prescribed 40% level.  Because the current designs of some of the relative humidity measurement systems do not take into account the existence of a humidity gradient within a sealed sleeve and due to the fact that many testers are using the 1% RH Drift per 5 minute equilibration “rule” to determine when a sensor has been adequately equilibrated, concrete slabs are many times being determined to be ready for a floor installation when in fact the concrete slabs actually contain a much higher level of moisture than is appropriate for floor installations.

ASTM F2170 committee members are undertaking an experiment to help identify the best way to modify the F2170 standard, so RH testing instrument manufacturers will have a better definition of a well designed sleeve/sensor system, and flooring inspectors will have a better standard of measurement practice to more accurately determine the true moisture content of a concrete slab. We can expect a new version of the F2170 standard to be published sometime in the near future with appropriate modifications.

The manufacturers and installers of materials that have extreme moisture sensitivity need to have testing results that are accurate.  Epoxy terrazzo, rubber, vinyl tile, wood flooring and adhesive manufacturers need to have accurate measurements.