Concrete Moisture Vapor and Alkalinity Emission

Cause

A moisture related floor failure can be the result of several different contributors.  Most commonly, and the cause for which our products were developed, is the movement of moisture vapor through the concrete slab due to a differential vapor pressure drive. Concrete will contain moisture, either from the water used during the initial placement of the slab, or from additional sources such as ground and irrigation water. 

The movement of the moisture through the concrete slab will be a function of the concrete’s permeability and the vapor pressure drive.  The vapor pressure drive is a force created by the difference in static vapor pressure of the environment within the building envelope and of that within the slab.  Static vapor pressures are calculated based upon the temperature and humidity of each environment.  The greater the difference in static vapor pressures, the stronger the vapor pressure drive.

Permeability of the concrete is going to be dependent upon the mix design and curing methods used during the placement of the concrete.  High quality, low water content concrete that is properly cured is going to yield a less permeable slab than a low quality, high water content concrete mix.  By being less permeable, the concrete slab will not allow moisture to travel through it as easily.  Thus, moisture vapor emission rates will be expectedly lower.

When the concrete is more permeable than the floor covering it, and enough water is available to accumulate between the concrete and floor covering, a floor failure is likely.  Different floor types may fail in different ways, but so long as this unstable condition is allowed to exist, problems are sure to arise.  See the Problems (Effects) page for examples of how a high moisture vapor emission rate can affect different floor coverings.

Additionally, excessive moisture vapor movement through concrete brings with it the very alkali compounds that exist within all concrete.  This alkali material is harmless to concrete, but devastating to most floor coverings.  As this alkalinity is brought to the surface and into contact with the floor covering, it can cause delamination and chemical burning of the floor.

EFFECT   ►