The effects of water/cement ratio on slab permeability

 

All slabs are porous as a result of their construction. Serious problems with porosity come from the extra water that was added in order to make a concrete batch a workable material. Concrete recipes have a water/cement ratio that should be carefully considered, as it directly impacts permeability.

The water/cement ratio includes the amount of water added to hydrate the cement powder as well as the amount of water needed to temporarily male concrete a plastic, movable material. A water/cement ratio of 0.30 would hydrate the cement powder, however the concrete mix would resemble modeling clay making it difficult to move and properly place. Therefore, extra water is added in order to produce a plastic, workable material which is easy to place. This extra water, termed “water of convenience” (WoC) needs to dry out of the slab and does so through gravitational and vapor emission-causing forces. This can require several months, depending upon several months, depending upon several other variables.

 

Excess WoC can create a moisture condition that prohibits the immediate installation of a floor covering or coating. It can also create permanent conditions that can allow unacceptable moisture transmission to occur.

 

Excess WoC leaves voids or capillaries inside the slab, which is a perfect environment for containing moisture, just like in the pores of a sponge. With a linear increase in the slab water/cement ratio, comes an exponential increase in permeability. The excess water occupies physical space inside the slab. The more space occupied, the greater the porosity. With an increase in porosity comes an increase in permeability. This allows a higher volume of water vapor to pass from the concrete into the floor surface layer.

 

Pore diameters of about 5 microns as well as the frequency of them, can have a direct impact on the permeability of concrete to moisture sources that would typically intrude upon the slab.

 

Moisture vapor emission is measure of volume, not pressure. The volume of emission depends on vapor retarder effectiveness, slab porosity, surface profile quality and other variables. Vapor pressure differentials between substrates and interiors simply ensure
Special thanks to Dr. Ken Hover at Cornell University for his research into permeability.

Above is an adaptive illustration from Concrete International, May, 1996 and is not meant for research purposes.

moisture will want to move toward the low pressure side if the concrete condition allows it.

 

In order to ensure a low volume of emission, care should be given to create over a properly place vapor retarder, followed by wet curing process. The slab must then be allowed the time to dry.

 

Changing water/cement ratio in concrete can be done by either adding too much water, or by reducing cement content. Design professionals should give careful consideration to the water/cement ratio, in conjunction with compressive strength and slump requirements in specifications.

 

For further information on concrete design and permeability effects, please refer to the resources section for those who can consult with you.

 

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