The Lone Escape of Concrete Moisture
Concrete moisture migration can become detective work, because moisture permeates concrete slabs in several ways. However, it is important to clarify the popular myth that concrete, once “dried,” is impermeable, sealed from external moisture. Moisture sources ranging from leaking plumbing to ambient relative humidity can impact the moisture content of any building materials which contain water – concrete included.
Moisture content is the amount of water vapor held in a concrete slab and is usually described in terms of relative humidity (RH). From the moment a foundation is poured, concrete moisture moves from the bottom of the slab to the surface. Once there, moisture evaporates at a rate which is determined by the ambient humidity of the slab’s environment. But the initial water of a concrete mix is not the only place moisture can enter a concrete slab.
Successful floor installation rests on the builder’s understanding of moisture movement within a concrete slab. When a slab is poured, the process of drying means that water from the original concrete mixture must move to the surface and evaporate from there. While the standard drying rate for concrete is 1 month of drying for each inch of concrete thickness, once the environment is conducive to drying, that drying rate is by no means as firm as the concrete it represents. Accurate relative humidity (RH) testing is vital to having a complete understanding of the true moisture content of a slab.
Builders have numerous ways of limiting concrete moisture challenges. They must lay down vapor barriers beneath a foundation to diminish the odds of moisture permeating the slab. Concrete can also be mixed with lower proportions of water to enhance the drying properties of concrete moisture content. Installers can apply topical moisture retarders after the slab has been established. However, all stakeholders benefit from recognizing one immutable universal truth: moisture moves.
And even when the slab has “dried” or reached the RH level necessary for installation or finishing, moisture can continue to move out of and into the concrete depending on the difference between the internal and ambient conditions. This moisture movement will continue with any moisture from above or below if the proper vapor barrier is not in place, but can only evaporate away from the surface.
Moisture testing, specifically relative humidity testing with in situ probes, is invaluable because we can target many locations in a slab in order to build a composite picture of concrete readiness or moisture conditions. Our ability to read the moisture content differences informs builders and owners on when to build or install on a fully-formed concrete floor, or where problem areas might be located.
In order to accurately assess the moisture content of an entire slab, Wagner Meters has developed the Rapid RH concrete moisture meter. Rapid RH uses in-situ probes which are placed in holes drilled into the concrete slab. The probes contain Smart Sensors which assess moisture content for that area as read by a hand-held concrete moisture meter. The Rapid RH reads at 40% depth of a slab for a truer picture of the final RH, and for the best possible indicator of how much longer installers must allow so the slab will fall within the RH installation specs for the project. The Rapid RH can also help pinpoint areas that might be subject to moisture intrusion for further testing or for remediation.
There are many avenues for moisture to enter a concrete slab. But with accurate RH testing, you can be sure to show moisture problems the door.