Freezing of water is accompanied by expansion. A closed container with at least 91.7% of its volume full of water will experience pressure on the walls during freezing. The figure can be thought of as an approximation to the `critical saturation level’, beyond which freezing might be expected to induce damage in a porous brittle material. This simple theory does not adequately explain all aspects of what happens when concrete freezes (see, e.g., Pigeon and Pleau, 1995) but it nevertheless introduces the concept that freezing of a proportion of the pore water in the cementitious matrix, or the moisture in absorbent aggregates, may cause deleterious expansion. Expansion that can be accommodated is not a problem but restraint of movement will give rise to tensile stresses. The induced stresses may exceed the material’s tensile capacity leading to cracking. The deleterious effect is cumulative. A vicious cycle may develop whereby the cracked concrete admits more water to the pore structure during thaw and the volume of ice increases during freeze, leading to greater stresses and cracking.
Vulnerable elements typically include canopies, kerbs, parapets, copings, ledges, and corbels. Vulnerable infrastructure includes pavements, reservoirs, dams, tanks and other hydraulic structures. Special circumstances may also arise in structures such as freezer stores and storage facilities for very low temperature liquids. It is noteworthy that parts of a structure are immune from the problem even in countries where the annual number of freeze-thaw cycles is significant. Protection from rain and prevention of water exposure will keep the water volume below critical levels. Foundations are normally adequately protected by depth of covering. Elements of structure that are most at risk are those that are saturated at time of freezing.
The problems associated with freeze-thaw generally occur in the cementitious matrix of concrete but some porous aggregates are also significant contributory factors, as discussed in Chapter 7. The influence of porous aggregate can, however, be beneficial if it is strong enough to withstand the expansive pressures, while at the same time providing much-needed space to accommodate the ice and redistributed pore water.