By Bob Blochinger
Concrete moisture vapor emissions are different than concrete sweating. Although both are related to ambient conditions, the structure of the space, the building materials used and the flooring product installed, a distinct difference remains. Moisture emissions are related to the moisture content within the concrete and its movement to the surface and conditioned space. Concrete sweating is related to dewpoint.
Two separate and distinct issues, as well as causations.
Concrete sweating or “damp” can occur even if a moisture barrier is applied to the surface after mechanical surface preparation. A typical example would be resilient sheet goods with heat-welded seams installed over an applied liquid moisture barrier, after mechanical surface preparation and a self-leveling compound has been applied. The space is under HVAC control with normal operational levels. However, the relative humidity is elevated above 60% and the temperature is in the low 70s. The weight of water in the air, measured in grains per pound, is also elevated.
The moisture emission of the concrete is controlled by the applied barrier, yet the resilient flooring still has physical signs of moisture-related damage including dark spots within the flooring product and accompanying bubbles. Perhaps a seam is compromised, and there is evidence of efflorescence and/or leeching water.
If it is decided to perform destructive testing, caution must be observed as once the bubble is opened, the caustic liquid inside can cause injury to the skin. Since the liquid can also be under pressure, it may squirt up. Whenever destructive testing is performed, a safety perimeter needs to be established. Once completed, the test area should be repaired to eliminate any potential trip hazards.
When the area of concern is opened to perhaps 12 square inches or more, liquid may be visible or just a discolored substrate surface due to moisture concentration and efflorescence (alkalinity/salts) from the self-leveling materials. The liquid moisture barrier, if properly applied, would prevent any moisture vapor emissions from the base concrete.
Concrete sweating can occur on an on-grade or above-grade slab, and with a pan system in multistory buildings. The issue is caused by the ambient conditions within a space reaching dewpoint and is not usually the result of poor installation workmanship.
Dewpoint is the temperature at which air becomes saturated with water vapor and condensation begins to form. Measuring dewpoint, humidity (absolute and relative), temperature and grains per pound (water) within the space is important to understand the issue. These measurements are taken at the floor level, three to four feet up and at the ceiling. Each level will show a different measurement.
Corrective action for this issue is inspection and assessment of the HVAC system. It may be operational; however, it is not working correctly to remove humidity from the enclosed space. The flooring damage will most likely require repair or replacement.
While concrete moisture vapor emissions are also caused by ambient conditions, the moisture comes directly from the concrete slab, whether on-grade or as part of a multistory installation. Concrete will always contain water/moisture in vapor form; the age of the concrete is not typically a factor. Ambient conditions may be right for moisture vapor emissions to travel up and settle to saturation at the top 3/4” of the concrete slab. When the saturation point is reached, out comes liquid water along with the alkalinity (salts) within the concrete. The more elevated the moisture, the more alkalinity comes with it. Alkalinity (pH level) is typically the culprit that will damage adhesives.
Measuring for these conditions comes down to following ASTM F1869, F2170 and/or the mat test. Through testing, visual and physical review, concrete moisture vapor emissions can be assessed and evaluated prior to installation of the flooring product. Concrete sweating, on the other hand, cannot be tested prior to installation as it is a post-installation issue developed through ambient conditions under improper HVAC control.
