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To ensure accurate moisture readings for your concrete slabs it is necessary to take a hard look at the ASTM F-2170 “Standard Test Method for Determining the Relative Humidity in Concrete Floor Slabs Using In-Situ Probes.”

In order to prevent a moisture-related failure to a moisture sensitive floor covering, it is necessary to have the proper moisture level in the slab. In the process of measuring moisture in a concrete slab there are two types of measurement.  These are: dynamic moisture testing, which measures the moisture emissions moving out of the slab (calcium chloride); and static moisture testing, which is the internal relative humidity moisture content of the concrete slab.  It has been determined by the scientific community that the measurement of static moisture is a more accurate method of testing and should be done at 40-percent the thickness of a slab.   A slab dries from the slab surface to the vapor retarder and creates a drying curve.  When the slab is covered with a flooring material the slab’s moisture equalizes top to bottom (goes into equilibrium); the equilibrium line crosses the drying curve at its apex at 40-percent the thickness of the slab.  Therefore, to ensure the accuracy of the moisture (relative humidity) readings, it is obviously critical that it be measured at that depth.  There are many manufacturers of RH test equipment in the market. 

Photo 1: Various reusable digital meters with re-useable probes.

The reusable probe is inserted into a plastic “sleeve,” which is then placed in a hole that was drilled into the concrete slab.  This method requires more time for the probe to equilibrate when moving from sleeve-to-sleeve (leap frogging).  Care must be taken to keep the probes in calibration, according to Section 8 in ASTM F2170; verifying a used probe’s measurement accuracy requires testing the probe with a salt solution or with a humidity chamber.  This is a must in order to retrieve accurate results.

Photo 2: RH sleeve/sensor.

The second type is the removable sensor, which is placed into the sleeve and allowed to stay there until you are ready to measure the results.  You plug the meter into the sensor and retrieve the results.

The third type is the disposable combination sleeve/sensor type.  This sleeve contains a sensor that is permanently embedded into the concrete.  With the removable sensor the sensor must be inserted into the sleeve and allowed to equilibrate to achieve an accurate reading.  Leaving the sensor in the sleeve is not advisable due to the risk of damage.   With the disposable probe the sensor is in the sleeve, and once the sensor is placed, allowed to equilibrate, you can obtain subsequent readings immediately and anytime thereafter, just by inserting the reader.

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Photo 3: The disposable RH probe kit.

The kit includes a drill bit, Insert tool, vacuum attachment, wire brush, five sensors with protective covers and a digital reader.  Additional sensors are available in packages of five.

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The Installation Procedure

Photo 4: Drilling the hole.

When drilling the hole the depth of the hole is very important.  The depth of the drilled hole is 40-percent the thickness of the concrete slab.  Often times these types of sleeve or sensor inserts lead to inaccurate readings because they do not adhere to the very specific 40 percent depth requirement of ASTM F-2170.

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Photo 5: Wire brushing the hole.

Wire brushing will remove the dust out of the pores of the concrete created by the drilling process.  This will open up the pores of the concrete allowing it equilibrate faster.

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Photo 6: Vacuuming the hole.

The vacuuming of the fine dust out of the drilled hole stops the possibility of contamination of probes and sensors.

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Photo 7: Inserting the sensor.  

The sensor should be inserted with an insert tool.  The insert tool will prevent any damage to the sensor and will allow the reader to be inserted without difficulty.

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Photo 8: Using the protective cover.  

The protective cover has a tube that snaps into the underside of the protective cap; this helps protect the sensor from damage and is not easily knocked out by construction traffic.

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Photo 9: Using the reader.

The reader is designed to drop into a slot in the sensor and then quarter turned clockwise to engage the contact points, giving a digital reading of relative humidity and slab temperature.  It is important not to allow the reader near any temperature extremes that could transfer to the sensor.

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Photo 10: A bottom view of the sensor.

The disposable sensor has its sensor at the bottom of the unit. I am told by the scientific community that having the sensor at the bottom of the unit gives a truer reading at the 40-percent level of the slab and faster results since equilibration of the sensor is already completed.