The Practicality of CaCl – A Test Message That Will Self-Destruct
Building contractors and flooring installers most often choose one of two methods to test for moisture in concrete slabs: either the calcium chloride (CaCl) test or the relative humidity (RH) test. ASTM International, which publishes technical standards for a wide range of materials, has developed standards for both tests. That doesn’t mean the two methods are equally suitable for testing concrete moisture.
ASTM F1869 is one of the standardized methodologies for moisture testing. This method relies on the use of anhydrous CaCl crystals. It is an approach that dates back to the 1940s, but it has no verifiable scientific basis. It is used primarily in the United States and not in other countries.
ASTM F2170 is another ASTM standard for evaluating the moisture condition of concrete. It recognizes in situ RH testing as a viable method for determining the readiness of a concrete slab. Unlike the CaCl test, its scientific basis is clear, in part because of many years of research regarding the most appropriate depth at which to take RH measurements.
Differences in Method
The two testing methods are quite different from one another. The CaCl test measures a moisture vapor emission rate (MVER), expressed in pounds, that indicates how much moisture is released from 1,000 square feet of concrete slab over a 24-hour period. It is important to realize that the anhydrous CaCl test is a surface-based assessment of concrete moisture. It is sensitive to ambient conditions and does not necessarily provide an indication of the moisture condition deeper within the slab. Although ASTM F1869 recognizes CaCl testing, ASTM International has disallowed this test for lightweight concrete (see rhspec.com).
In contrast, the RH test is specifically designed to assess the amount of moisture deeper inside the concrete slab. This is expressed as a percentage of the RH. In situ RH probes are inserted into holes drilled to a specified depth (20% to 40%) of the slab. The probes’ measurements provide a fast quantitative assessment of the overall moisture condition of the slab. In 2002, ASTM International acknowledged the integrity of RH testing by developing the ASTM F2170 standard.
Differences in Materials
Builders and flooring installers should also consider the significant differences in materials used in the two test procedures. The CaCl test kit contains a salt or other anhydrous mixture, which absorbs water vapor after being placed at the surface of a concrete slab. Due to this absorption of moisture, the anhydrous CaCl salts are effectively “destroyed” and cannot be used for subsequent measurements.
RH testing, on the other hand, uses probes that contain RH sensors. These are placed in holes drilled for the specific purpose of assessing the moisture condition of the concrete slab. The RH sensors may be either single-use or reusable, but regardless of which type, they are designed for taking multiple RH readings at each test hole. Contractors, as well as inspectors, have the ability to repeat – and repeat and repeat and repeat – RH testing at every location where the RH probes are installed.
Differences in Cost Effectiveness
This fact contributes to differences in costs. Both CaCl and RH test kits come with all the necessary materials. However, materials in the anhydrous CaCl test kit must be discarded after each test procedure has been completed. An additional material cost is incurred each time the CaCl test is performed or repeated. Because RH tests can be performed repeatedly at each test location using the same probes, obtaining additional measurements does not require any additional cost in materials.
Overall, we have found that RH testing using the Wagner Meters Rapid RH® kits saves on the cost of moisture testing, especially when considering the total cost of both materials and labor (see companion article). And because it is fast and convenient to use Rapid RH® Smart Sensors when getting repeat measurements at each test location on site, it is extremely easy to gain a clear picture of the changing moisture condition of the slab over time. This may be especially valuable when testing moisture at large commercial construction projects.
Unlike CaCl, RH testing produces a message that will not self-destruct once complete.