Concrete Moisture Testing Myths
Before diving into the myths let’s talk about how you test concrete for moisture.
How to Test Concrete for Moisture
ASTM F2170 recognized the use of in situ probes as a means of conducting relative humidity (RH) testing. When testing concrete for moisture, use in situ probes by placing sensors or probes inside the concrete slab itself. In situ probes provide RH measurements at 40% of the slab’s thickness, when drying from one side. For slabs drying from two sides, RH probes should be placed at 20% of the slab’s total depth. In situ measurement provides overall slab moisture levels and the data necessary to make flooring installation decisions. Using RH probes, you’ll need a minimum of 3 tests for the first 1,000 square feet and 1 additional test for each additional 1,000 square feet.
In a time when a myriad of information is available for the building and flooring industries, it’s always interesting (and a little frustrating) when some common misconceptions about moisture testing concrete will cause building professionals to assume they are not at risk for moisture-related problems with their flooring installations.
Top 5 Concrete Moisture Testing Myths
Let’s look at a few of them.
MYTH #1: If the concrete surface is dry, the slab is dry.
For most slabs that dry from one side, the surface is the only escape route for the moisture in a drying concrete slab. As moisture evaporates away from the surface, more moisture can move up through the slab. Therefore, it seems reasonable that if the surface is dry, it could be taken as a sign that there is no more excess moisture in the concrete.
However, it has been demonstrated repeatedly that the surface of the slab cannot serve as an accurate indicator of overall moisture levels.
Concrete is like a sponge absorbing moisture from under your home which can cause moisture to come up through the concrete slab. Typically, this will cause moisture-related flooring problems or cause mold and mildew growth.
Because there are so many variables that can impact the drying rate of a concrete slab–air movement, ambient temperature and relative humidity, troweling techniques, and more–conditions at the surface in no way reflect the presence of moisture within the slab, and leave any test method that only tests the surface to be highly suspect.
The only way to accurately determine when a slab is “dry” is through relative humidity (RH) testing that places a sensor inside the slab away from the surface. RH testing at the correct depth gives a clear picture of the final concrete moisture measurement if the slab were to be sealed at that point in time.
MYTH #2: All floor products have similar moisture tolerances.
At one time, flooring adhesives or some flooring materials were more consistent in their moisture tolerances because most products had a petroleum-derived base in their compositions.
Although moisture-related flooring problems have always plagued the industry, as formulations began to change for specific applications it became increasingly important to be sure that concrete slabs under the flooring installations were dry in order to prevent moisture-related problems down the road.
With the recent advent of the industry’s focus on lowering VOCs (volatile organic compounds) in building products, different adhesives and flooring products on the market today can vary greatly in their tolerances to extended elevated moisture conditions.
For the best results, accurate RH testing should always be performed in order to a) determine the current slab moisture condition, and b) be sure the product specified is compatible with the slab moisture levels, or c) choose an alternate product that meets the current moisture conditions of the slab.
For a quick link to a multitude of manufacturers that specify RH tolerances for their products, visit www.rhspec.com.
MYTH #3: The concrete is “old” so it has to be dry.
Even when a concrete slab has been in service for years, it’s still possible that moisture levels within the slab are high.
Unfortunately, it’s not unusual for an old flooring system to be removed, only to have the newly-installed flooring start to exhibit signs of moisture-related problems.
How is that possible?
Part of the reality with older concrete and flooring systems is that the products used in decades past were inherently more moisture-resistant than many of the products on the market today. The move to lower environmental impact products (like products with lower VOCs) has resulted in the need for more awareness for moisture tolerances in flooring adhesives, sealants, and flooring products.
Hence, an older floor that has never shown signs of moisture problems may still be too high for the RH sensitivities of the new product.
It’s also possible, of course, that an older concrete slab can be facing moisture intrusion from an unidentified source (a compromised vapor barrier, an appliance or plumbing leak) that has raised the slab’s moisture content but not yet reached critical levels for the older flooring.
The best insurance for older concrete slabs is still to do RH testing before installing a new flooring system. The unique design of the Rapid RH® seals a sensor in each test hole for immediate test results. It’s fast, accurate and affordable testing that saves on your bottom line by allowing ASTM-compliant testing at the touch of the Total Reader®.
MYTH #4: The slab is not on grade so I don’t need to do moisture testing.
All concrete is affected by ambient conditions, and all concrete is susceptible to moisture-related complications if not adequately tested before the next stages of construction begin.
There are two significant differences for concrete slabs not on grade:
- Slabs on grade (or on pan decking) have only one surface that moisture can use to exit the slab. Slabs not on grade have two surfaces from which moisture can move out of the slab. This also means that concrete slabs not on grade have two surfaces that can absorb moisture from a humid environment or external moisture source. The need to do accurate moisture testing has little to do with the format of the pour. The correct test method, however, has everything to do with the on-grade or not-on-grade status of the slab.
- For RH testing, studies have shown that for accurately testing final moisture conditions within a slab, 40% is the correct test hole depth for slabs poured on grade or in pan decking (or with one surface from which the moisture will evacuate the slab). For slabs with two surfaces exposed to ambient conditions, the correct test hole depth is 20% of the slab’s thickness.
RH testing for concrete is the only way to ensure against moisture-related problems over time. Only testing done at the appropriate depth will provide accurate and actionable results.
MYTH #5: Surface testing concrete gives a final accurate moisture level.
As was discussed above, any test method that relies on surface conditions is at risk of producing a false reading.
There are three current test methods that only provide results at a slab’s surface:
- The Calcium Chloride Test (or MVER) seals a desiccant material under an impermeable cover and then calculates the MVER rate by weighing the material after a set amount of time. The theory behind this test method is that the desiccant will absorb moisture emitting from the slab. That weight gain will express a ratio of the remaining moisture in the slab. Unfortunately, there is no scientific basis to the test and it has been specifically disallowed for some concrete types.
- The Hood Method places a probe or sensor on the slab surface and again seals it under a moisture-resistant, insulated hood in an effort to mimic the conditions that flooring would be exposed to if it were installed.
- Moisture meters operate by sending an electrical signal into the concrete and measuring its resistance as an indicator of the moisture level in the slab. While they can be a useful survey tool, they typically operate at about ¾” depth and can be susceptible to other elements within the slab: rebar, certain aggregates and more.
Ultimately, understanding the distribution of moisture in a drying slab makes it apparent that surface-based test methods are unreliable at best.
Moisture levels in a drying concrete slab tend to be higher at the bottom of the slab but will equilibrate (disperse evenly) through the slab once it has been sealed by a floor covering or other technique. Any test method that doesn’t measure below the surface into the slab cannot give an overall picture of what moisture levels will become once the flooring is installed.
Non-Surface Testing Is Accurate and Affordable
When planning to install any type of flooring or finishing product over a concrete floor, only accurate Relative Humidity testing, like with the Rapid RH, can provide accurate, affordable and industry-compliant concrete moisture testing. Don’t let some of these common “myths” about moisture testing become a costly mistake for you.
Additional information on concrete moisture testing: Rapid RH Test
5 More Myths about Concrete Moisture Testing
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 L6 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 Total Reader, the Rapid RH’s technological innovations ensure proven accuracy, dependability, and affordability. Calibration is also always certified with each Smart Sensor coming with a NIST-traceable certificate of calibration.
MYTH #3: Test hole depth is approximate.
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 L6 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 testing: Rapid RH Test
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.