Concrete Porosity

The aspect of properly consolidated, quality, hardened concrete that affects its permeability is the nature of the hardened cement paste itself and, as well, the nature of the interfacial zone between concrete “macro components”, (i.e.: aggregates, reinforcing materials, fibers) and its cement paste structure.  The paste-aggregate interfacial zone is known to be different from the cement paste mass in general.  It is usually more porous, richer in Ca(OH)2 (calcium hydroxide), and is more prone to microcracking than the rest of the paste matrix.  The interfacial zone is normally in the order of 50 um in thickness, and can occupy 30 – 50% of the total volume of cement paste in concrete. In comparison to the bulk hydrated cement paste, the paste-aggregate interfacial zone is weaker, more soluble, and can be a least resistant path for migrating moisture and other harmful substances.

It should be noted that although aggregates are porous, their pores are normally discontinuous in a concrete matrix, being completely enveloped by cement paste.  Discrete voids or pores in concrete, including entrained air bubbles that are discontinuous similarly do not contribute significantly to concrete permeability.

Concrete porosity is usually expressed in terms of percentage by volume of concrete.  It is the interconnectivity of pores, rather than total porosity that determines a concrete’s permeability.  A concrete with a high proportion of disconnected pores may be less permeable that a concrete with a much smaller proportion of connected, or continuous pores.  With greater particularity, it is the overall nature of the matrix pore structure that ultimately affects its permeability, sorptivity, and diffusivity.  The size, distribution, interconnectivity, shape, and tortuosity of pores are all determining factors in the overall permeability of a concrete matrix.

Water-proof or water-tight portland cement concrete is not a real possibility.  Instead, our goal in design and formulation of durable concrete mixes is to slow down and minimize the potential for, and rate of, moisture ingress and movement.