Indigenous Mechanisms of Moisture Movement
Microscopic capillaries within the concrete create the potential avenue for moisture to move through the slab. The differential of temperature & humidity between the moisture source and building interior causes vapor to be drawn out of the slab surface into the building envelope or become trapped under a low-permeable flooring material.
VAPOR PRESSURE, as a Function of Temperature & Relative Humidity
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The above table was created by General Electric in the 1920's and reprinted as long ago as 1954 by the U.S. Housing Authority to explain the effects of pressure upon osmotic movement of moisture, Reprints of this data, as well as all data presented, are available upon request. The shaded area simply illustrates the area of lower pressure relative to the .214 psi of the substrate and is not a feature of the original table. |
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Moisture emits from concrete as a function of higher pressures moving toward lower pressures. In the example shown below the concrete has a static vapor pressure of .214 psi while the room has a static pressure of only .108 psi.
While the static vapor pressure itself of either the concrete or the room atmosphere is unimpressive, the difference between them is significant. For instance, it is a static pressure differential that is responsible for the basic physics of flight.
Faster moving gasses have a lower static pressure than slower moving gasses. When gas (air) is accelerated over an airplane wing creating a pressure differential over the wing, a half million pounds can fly.
The physics of flight are obviously more dramatic than the mechanisms of vapor drive at work in the building envelope. However, the latter has the distinct advantage of time.
The force of vapor pressure isn’t in itself the failure mechanism. There are co-variables of porosity and alkalinity, and related variables of surface profile and installation quality to consider as well. The environmental forces at work in the building envelope are just a vehicle. Logical steps in the design and placement of the slab must be fine-tuned to comply with the physics of building and floor systems today. Otherwise, a floor system failure is simply nothing more than an “accident waiting to happen”. One of the most common accidents occurs in the course of construction itself. Most people do not allow the proper drying of concrete before covering. The vapor pressure differential that promotes drying and creates vapor emission is very effective after the building becomes an envelope, which is usually after the floor system has been installed.
In above grade slabs with water-saturated lightweight aggregate, the accident typically happens several months after the floor has been installed. The excess water in the mix design has dried to compliance with floor covering needs, but the moisture in the aggregates will exit the slab over a long period of time.