Porosity and permeability are not proportionately linked. Although highly permeable concrete is generally frowned upon, it is interesting that high porosity concrete has the advantage of providing safety valves for ice to expand into, assuming that the concrete is not saturated. This is the basis of air entrainment as a control measure. However, low permeability concrete, derived from low water/cement ratio mixes, is traditionally associated with durable concrete and so it is in the case of freeze-thaw exposure. Such low permeability concretes are beneficial in two respects. The impermeability of the concrete reduces the amount of free water that can enter the pore structure of the hardened concrete from external sources. Additionally the hydrated concrete contains minimal amounts of water that is freezable at normally encountered temperature ranges or available as free water to feed ice formation.
It has already been stated that pore diameter influences the temperature at which water will freeze. The water in the largest pores is the first to form ice at a given temperature. Further drops in temperature cause the water in the capillary network to freeze but the gel pore water remains unfrozen. The volume and proximity of spaces into which the expelled water may escape greatly influences the degree of resistance to damage. Thus a balance is required between high porosity and low permeability.
