Mechanisms of salt scaling

A great deal of information has been collected over the years about the phenomenology of salt scaling of concrete, and no single mechanism has been identified that can account for all of the observations. We review the essential facts that any complete theory of scaling must explain, and suggest two mechanisms that seem to be of primary importance. Experiments indicate that damage is worst when the water on the surface of the concrete contains a small amount of solute, regardless of the type of solute. We argue that the role of solute is to create brine pockets in the ice that act as mechanical flaws; when the weakened ice cracks, the crack runs to the surface of the concrete and does superficial damage. An analysis of the stresses created in this way shows that they are more than sufficient to initiate cracks. A second phenomenon, which is present when chloride salts are used, is expansion resulting from precipitation of Friedel's salt. The crystallization pressure from growth of these crystals causes significant expansion of the paste, and results in high stresses on a local scale near the ice/paste boundary. This can substantially weaken the material and facilitate scaling.