The Porous and Permeable Nature of Concrete

Concrete is best described as a porous and permeable material. Porosity, which refers to the amount of holes or voids left in concrete, is expressed as a percentage of the total volume of a material. Permeability is an expression of how well the voids are connected. Together, these qualities allow pathways to form that allow the movement of water into, and through, along with the cracking that occurs due to drying shrinkage.

Water chart of permeability size scales Image courtesy of Xypex

Permeability, a broader term than porosity, is the ability of liquid water under pressure to flow through porous material. Permeability is described by a quantity known as the permeability coefficient, commonly referred to as D’Arcy’s Coefficient. The water permeability of a concrete mix is a good indicator of the quality of the concrete for durability reasons. The lower D'Arcy's Coefficient, that is the more impervious the concrete, the higher the quality of the material. Although concrete with a low porosity may be relatively durable, it may still need a waterproofing agent to prevent leakage through cracks.

Despite its apparent density, concrete remains a porous and permeable material that can leak and deteriorate rapidly when in contact with water or solutions containing chemicals such as chlorides, sulfates or other substances. But there are other ways in which water can be transported through concrete.

Vapor Flow and Relative Humidity

Water also migrates through concrete in the form of water vapor which is water held in air as a dissolved gas. The direction of vapor flow occurs s from high vapor pressure, generally the source, to low vapor pressure, by a process of diffusion. The direction of flow can vary based on environmental conditions.

The direction of vapor flow is critical when applying waterproofing treatment in situations where an unbalanced vapor pressure gradient exists. Typical examples include:

• Applying a low vapor permeable membrane, such as a traffic deck coating over a damp concrete surface (even if the very top surface is dry) on a warm day will result in pressure vapor pressure build-up and pin-holing or blistering.
• Applying a coating or sealant to the outside of a building wall may trap moisture into the wall if the sealant is not sufficiently vapor permeable.
• Applying low vapor permeable flooring over a slab-on-grade where there is high subsurface moisture content may result in delamination of the flooring.

Generally, a low vapor permeable sealant or coating should not be placed on the downstream face of a building or structure. Either the vapor pressure or water pressure will act to damage and blister the membrane. Some types of coatings and water permeability reducing admixtures in the concrete accommodate considerable vapor movement, thus allowing them to be placed successfully on the downstream side. Primary examples are cement-based waterproof coatings and water permeability reducing admixtures.

Vapor flow through a foundation wall Image courtesy of Xypex