5 More Myths about Concrete Moisture

Unfinished Concrete Floor Every industry has its folklore and accepted traditions that may or may not be based on facts or best business practices. For concrete, some of these common myths suggest that there are still misconceptions to be found in the flooring and building industry, too…

MYTH #1: Drying time can be estimated accurately.

An industry rule-of-thumb for drying concrete is to allow one month of drying time for each inch of slab depth. Unfortunately, an understanding of the process of moisture movement in a drying slab can very quickly make it clear why any attempt to estimate a slab’s drying time can be a gamble. In order for concrete to dry, a large percentage of the excess moisture in the slab must be able to reach the surface and evaporate. Moisture within a concrete slab follows a number of small pathways, or capillaries, that form as it combines with the initial mix elements–cement, aggregate and any other admixtures. If these pathways are blocked for any reason, or if they are not dispersed well through the slab, the amount of time it takes for the moisture to move out of the slab will be slowed significantly. An overzealous troweling job or sometimes even a curing compound applied to the surface will prevent the moisture from escaping the slab, and that’s only the direct influence on the drying process. Ambient conditions also have a very significant impact on concrete drying times, so any change in relative humidity, air movement, temperature (as it relates to ambient relative humidity) or HVAC impact will ultimately affect the final drying schedule. Only accurate RH testing can monitor the actual moisture conditions inside a concrete slab for dependable results.

MYTH #2: All RH tests are the same.

All RH test methods are the same in that they are based on research that demonstrates how internal concrete moisture measurement (measurement is taken below the slab surface) is proven to be more accurate when testing concrete moisture levels. However, the way each RH testing product is designed can have significant impacts on both accuracy and performance. Most RH test methods use removable sensors that must be inserted into the test hole and then allowed time to re-equilibrate for each and every reading. (Calibrations for each sensor must also be checked no more than thirty days before each use). This type of sensor makes RH testing a time-consuming and tedious process that involves more waiting than testing. It also leads to “leapfrogging”: moving the sensors from one hole to the next, often with insufficient time allowed to let them equilibrate to each new test hole. Only the Rapid RH® 4.0 EX makes use of technological advancements that help ensure both accurate testing and industry compliance. From the Smart Sensors that install directly into the slab for one-time equilibration to the Touch-n-Sense™ powered Easy Reader, the Rapid RH®’s technological innovations ensure proven accuracy, dependability, and affordability. Calibration is also always certified with each Smart Sensor coming with an NIST-traceable certificate of calibration.

MYTH #3: Test hole depth is approximate.


Click the image for a larger view

The target of RH testing is to be able to determine the moisture levels of a concrete slab once the floor covering is applied. Why? Because if the moisture level is too high, it puts the entire floor system at risk, and billions of dollars are spent in repairing moisture-related flooring failures each year.

It’s important to understand how moisture distributes in a drying concrete slab. Moisture levels inside the slab tend to be higher at the bottom of a slab that is unsealed, but will equilibrate (disperse evenly) through the slab once it has been sealed by a floor covering or other technique. Until the slab is sealed, though, the depth of the test hole will definitely impact the accuracy of the RH reading.

That fact is why standards like ASTM F2170 are in place–to ensure that testing happens at the correct depth. For slabs drying from one side, like slabs poured on grade or pan decking, 40% has been proven to be the correct depth for RH in situ test holes. If a slab is drying from two sides, 20% of the slab depth is necessary for accurate RH test results.

Only RH test holes drilled to the correct depth will provide results that reflect true moisture conditions in the slab and let the flooring installer proceed with accurate information about the moisture conditions that will be in contact with the installed flooring once the job is complete.

MYTH #4: Concrete additives don’t impact drying time.

The irony of this concrete myth is that some concrete admixtures are MEANT to impact drying time. Superplasticizers, for example, like lignosulfonate or polymer compounds, are designed to reduce moisture content in concrete slabs so (in theory, at least) drying times can be reduced. Other additives cover a range of intended uses from reducing shrinkage or damp-proofing to extending workability to adding pigments for color applications. Each class of admixture chemically enhances or changes the properties of the concrete batch, and can subsequently impact the actual drying time of the slab. Different admixtures can also impact certain types of moisture test methods. Only RH testing provides legitimate moisture testing results for any drying concrete slab.

MYTH #5: Dust on a job site doesn’t impact RH testing.

Dust and grit may seem inevitable on a job site involving concrete, but it really can impact the accuracy of RH testing if the test hole is filled with residue from the drilling process. By compromising the air volume around the sensor, excess dust or concrete residue limits the ability of the sensor to provide accurate readings. In order to assist in drilling the cleanest test hole possible, each Rapid RH® Flooring Package comes with a wire brush to help dislodge any loose particles in the test hole that might remain after drilling, as well as a vacuum attachment that allows you to clean right to the bottom of the hole. Between readings, the provided orange flush-mount protective cap not only helps identify each hole but also prevents site contaminants from getting into the test hole as well. When planning to install any type of flooring or finishing product over a concrete floor, only RH testing like the Rapid RH® 4.0 EX can provide accurate, affordable and industry-compliant concrete moisture testing. Don’t let myths about concrete moisture testing become costly mistakes for you.

Additional information on Concrete Moisture: 5 Myths about Concrete Moisture

Jason has 20+ years’ experience in sales and sales management in a spectrum of industries and has successfully launched a variety of products to the market, including the original Rapid RH® concrete moisture tests. He currently works with Wagner Meters as our Rapid RH® product sales manager.


  1. Toya says:

    Good Day! I am in the process of having tile laid in a 40 sq ft bathroom. A problem arised when the installer removed the old vinyl and performed the RH test with a Tramex meter, results showed between 5-6%. He said the concrete needed to dry out, so I’ve placed a dehumidifier in the room to absorb the excess moisture. My question is, what is the acceptable range for the meter to read in order for the tile to be installed?

    Thank you…,

    • Jason Spangler says:


      Thanks for the question. Unfortunately, the installer isn’t measuring RH% with a scale like you are referencing. You would need to contact the tile manufacturer or have a discussion with the installer to determine suitability of the concrete moisture level with this device. Good luck.


  2. Darren says:

    Jason thanks for taking the time to make this video and article very helpful for me to understand concrete drying times.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.