Calcium Chloride and RH Testing Revisited

The first consideration of any concrete moisture test method is whether it’s really the best tool for the job. Two test methods, calcium chloride (CaCl) testing and relative humidity (RH) testing, are the most commonly specified test methods in the flooring industry, but how do they compare when put side to side?

Some Practical Considerations: RH vs CaCl Testing

CaCl testing for concrete moisture originated as a “dampness test”. It was meant to be used as an indication that moisture was dispersing from the surface of a drying slab. It was not intended to provide any sort of quantitative indication of the amount of moisture in a concrete slab. As its use became more accepted in the flooring industry, it also was assigned a pounds-per-1000-square-feet expression determined by weighing the desiccant before and after testing. But is it really a practical test method for determining concrete moisture conditions?

RH testing was designed and tested to provide a quantitative measurement of the moisture conditions that is present in a concrete slab. This measurement, taken from the specified depth, is scientifically-proven to be the amount of moisture that will be present on the surface when the slab is sealed with either a low permeable covering or sealer. For a flooring installer looking to prevent callbacks or moisture-related flooring problems, a long-view test method should be imminently more practical.

The test methods themselves also present very different amounts of labor to administer and monitor.

CalCl-TestCaCl testing requires that the floor is cleaned, scraped and ground to make a clean surface that will test equally from site to site. Kits must be opened, desiccant weighed and data recorded, then a dish of the pre-weighed CaCl is placed on the floor and covered with a plastic “dome” during the 72-hour test period. Once the test is complete, the desiccant must be weighed again, data recorded and the test sites cleaned and test materials disposed of.

With RH testing, installation requires drilling and cleaning the test hole, and installing the sensor. Using the Rapid RH® test system, when testing is completed, test holes are easily troweled over with a metal cap in place under the cementitious compound. Sensors stay in place in the test hole and, if the need arises, each test hole can be relocated with the simple use of a metal detector. Data recording can also be simplified by adding the Rapid RH® DataMaster™ app in conjunction with the Bluetooth® Smart Reader for wireless tracking.

Cost-Effective Moisture Testing?

Benefits-vs-costAccording to a cost comparison between CaCl testing and RH testing costs, it was true that RH testing materials were higher in the cost of initial test supplies by about four times. However, it also demonstrated that in labor costs, CaCl testing costs forty-five times as much as RH testing with the Rapid RH®. For a contractor with a budget in mind, that’s a significant difference.

It is worth considering, too, that with CaCl testing, if the seal is jarred at all, or the domes do not seal down, the test is invalid and must be reinstalled. Repeat readings require new test kits each time. Weighing and calculating the desiccant also means that the MVER results take a certain amount of time to calculate for each test spot on the site.

RH testing does not have the same susceptibilities on the job site. Installed inside the slab, the probes are easily protected, and initial readings, capable of informing business-critical decisions, can be almost instantaneous and be repeated as often as necessary. Of course, keep in mind that the ASTM requires 72-hour acclimation to comply with the accepted standard (F2170), and that the sensors must be properly installed at the correct depth, but once in place, they are operational for the duration. What appears as a higher price point for the initial test kits can actually balance out over the course (and cost) of the project.

The Limits of CaCl Testing

It’s not just in real estate that the maxim “location, location, location” is relevant. The location at which each test method is placed really represents the key difference between CaCl and RH testing.

It’s already been pointed out that CaCl tests are limited to the surface layer of the slab. While this limits the testing depth, it also leaves them vulnerable to disturbance. If anything compromises the seal during the 72-hour testing period, the test is invalid. More problematic, however, is the testing that shows CaCl testing is vulnerable to false readings on very high or very low moisture slabs, as well as being susceptible to different pH conditions and ambient conditions.

Reader-Open-KitRH testing does not have the same limitations. Properly installed, the Rapid RH® is isolated within the slab at a depth proven to indicate the final moisture conditions of the slab once the surface is sealed. The ASTM still requires 72-hour acclimation for reporting and documentation, but actionable readings, meaning readings that can inform schedule or materials decisions on the job site, are available in as little as 2 hours.

Future Considerations

In choosing a moisture test method for any concrete slab that forms part of a flooring system, the real question to ask is, “Will this floor stay strong and functional in the future?”

The test method you use will determine whether you can answer the question or not.

CaCl testing, ultimately, gives a limited response in a specific time frame. It may indicate the amount of moisture escaping the slab’s surface at that point in time, but it is incapable of determining the amount of moisture that remains within the slab. Because concrete moisture has a higher concentration lower Top-three-quarters-of-an-inin the slab during the drying process (as the concrete dries, moisture in the slab migrates up to the surface to evaporate away), any surface-based test cannot determine more than the presence of moisture in the top ¾” of the slab. Once the slab is sealed with either a direct finish or with an installed floor system, that moisture will come to a balance within the slab (equilibration) and will therefore also mean that the moisture level in that top ¾” will increase. The danger comes when that rising moisture level during equilibration exceeds the moisture tolerance of the sealant, adhesive or flooring system in contact with the concrete slab.

RH testing, by design, determines the final level of RH if the slab were to be sealed at that point in time and allowed to equilibrate. Testing has proven that placing the in situ probe or sensor at 40% of the slab depth (for a slab drying from one side) will actually correspond to the final moisture level of the slab once it has equilibrated under the sealant or flooring system. As Howard Kanare stated after years of testing, “The bottom line is that RH testing is a far better predictor of whether or not a floor covering or coating on a concrete slab will succeed or fail.” Both from its science-tested development and from field experience, RH testing offers the most accurate insight into the moisture levels of a concrete slab.

In looking at the function, and future realities of both CaCl testing and RH testing, we strongly believe that RH testing answers the ultimate question of any flooring installer or building professional: “Is this concrete dry enough to install a floor that will stay strong and free of moisture-related problems in the future?”

With the Rapid RH® and its field-tested ease of use and accuracy, every flooring professional can answer that question with confidence.

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Jason Spangler

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.

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