Relative Humidity (RH) is the amount of water vapor in the air compared to the amount of moisture the air could hold at a given temperature. RH can also be applied to concrete and serves as the most important indicator for when a concrete slab reaches appropriate dryness levels. RH informs construction crews when flooring materials can be safely installed without risk of damages that have cost the construction industry more than $1 billion.
When is a Concrete Slab Sufficiently Dry?
In order for a concrete slab to be considered dry, the RH level typically must reach 80 percent or less. Installing flooring products before this time or without proper moisture mitigation dramatically increases damage risks. The drying process can take several months or longer, pending environmental humidity conditions. In many cases, traditional concrete slabs may never naturally dry to appropriate humidity levels.
How RH in Concrete Construction is Measured
RH tests for concrete slabs have become the industry standard, but must be executed properly in order to generate accurate results. RH readings must be made at 40 percent of a concrete slab’s depth for slabs drying top down. For slabs drying from the top and bottom as seen with a suspended structural slab, RH testing is conducted at 20 percent of slab depth. Once appropriate test depth is determined, holes for sensors must be drilled and probe sleeves subsequently placed. After allowing 72 hours for the test holes to equilibrate, probes are inserted into the holes and are allowed to equilibrate anywhere from 45 minutes to several hours before an official reading is recorded.
Previous concrete moisture detection methods have included a calcium chloride test, a questionably accurate test that measured concrete moisture at the surface of a slab. This test measures MVER (moisture vapor emission rate) and is less reliable compared to RH when it comes to predicting potential long-term moisture issues.
Rapid-Drying Concrete and RH Levels
Rapid-drying concrete will reach appropriate RH levels within 45 days of installing the concrete slabs. Often, the slabs will reach RH levels well below 80 percent within that time, giving construction crews a predictable product that allows them to begin flooring installation on schedule. Conventional concrete can take months to reach acceptable RH levels and might never approach appropriate dryness for flooring installation. Topical moisture mitigation, required for conventional concrete in most cases, isn’t required for rapid-drying concrete.