The American Society for Testing and Materials (ASTM) designed standard ASTM F1869 with calcium chloride test methodology in mind. The industry has used the anhydrous calcium chloride test since the 1940s to measure concrete moisture content by its rate of water vapor emissions. However, ASTM has also recognized relative humidity (RH) testing for concrete moisture content as a viable method for determining concrete slab readiness.
The calcium chloride test produces a moisture vapor emissions rate (MVER): a reading of how much moisture content is released from 1000 square feet of concrete slab over a 24-hour period. The anhydrous calcium chloride test reading forms a surface-based assessment of concrete moisture content in pounds. Calcium chloride testing satisfies ASTM F1869, although as a reminder, it has been disallowed for testing lightweight concrete (see rhspec.com).
Relative humidity (RH) testing assesses the amount of water vapor inside a concrete slab by percentage. In situ probes are inserted into holes drilled in concrete up to 40% depth in order to quantitatively assess moisture content throughout the slab. In the early 2000s, ASTM acknowledged the integrity of relative humidity testing with the creation of standard F2170.
Differences in Materials
Builders may consider the material differences in both test procedures. The calcium chloride test kit contains a salt or other anhydrous mixture, which absorbs water vapor emissions once attached to the surface of a concrete slab. Naturally, the anhydrous calcium chloride test salts are destroyed by the slab’s evaporating moisture content.
Relative humidity probes contain test sensors that are placed inside holes for the purpose of assessing how much moisture content exists at test depth. Even though the relative humidity test probes are not sold as reusable (some are, although they require re-calibration for each test site), the sensors remain intact. In effect, builders establish the ability to repeat – and repeat and repeat and repeat – relative humidity testing at that location where the RH probes were first installed.
Differences in Cost-Efficiency
On material cost comparison, relative humidity testing differs from the calcium chloride test. Both test kits come with all the necessary materials. However, anhydrous calcium chloride test materials must be discarded after each test procedure is complete. Each necessary repeat of the CaCl test means new kit test costs. Testing by relative humidity, however, produces multiple test sites throughout a concrete slab which can be assessed repeatedly.
RH testing like the Wagner Meters Rapid RH saves on total material costs and preparatory work for flooring installers. Smart Sensors continue to digitally represent moisture content levels at the point of installation. Builders may repeatedly assess each test location on site to broaden the scope of relative humidity assessment. This is especially valuable for large commercial construction projects.
Relative humidity testing produces a message that will not self-destruct once complete.