New Study Identifies Physical Mechanism Of Salt Scaling: Cracking Under Stress

Long after winter passes, its effects can be seen in cracked and deteriorating pavements across the Northern hemisphere. While more than half a century’s


Long after winter passes, its effects can be seen in cracked and deteriorating pavements across the Northern hemisphere. While more than half a century’s research has documented the characteristics of salt scaling Û superficial damage that occurs during freezing in the presence of saline water, reducing concrete’s mechanical integrity Û none of the causes proposed adequately account for all of the attributes cited. Recent research, however, claims to identify the mechanism responsible for such damage: a new study in the Journal of the American Ceramic Society describes salt scaling as the result of stress arising from thermal expansion mismatch between ice (50 ppm) and cementitious media (10 ppm), which puts the ice in tension as the temperature drops. Cracks thus created in moderately concentrated solutions penetrate the concrete substrate, causing its smooth surface layer to be eroded in flakes.

Contrary to past research invoking a chemical explanation for salt scaling, e.g., pore chemistry, a study titled Mechanism for Salt Scaling (by John J. Valenza II and George W. Scherer of the Princeton Institute for the Science & Technology of Materials, Department of Civil and Environmental Engineering, Princeton University) defines the cause as a physical mechanism susceptible to mitigation by increasing concrete’s surface strength. When ice forms a two-layer composite with concrete, leading to a thermal expansion mismatch that places ice in tension as temperature is reduced, mechanical interlocking that occurs upon solidification is facilitated by the roughness of the concrete surface, explains Valenza. Then, as tensile stress rises above the strength of the ice, the ice cracks, and these cracks travel into the surface of the concrete, removing a flake of material.

Salt scaling is a consequence of the fracture behavior of ice, the researchers emphasize. Numerous studies have confirmed that salt scaling does not occur if the pool contains pure water; it becomes serious at concentrations of a few weight percent, and stops at concentrations above about 6 percent by weight. Examination of the mechanical and viscoelastic properties of ice in the context of a layered ice/concrete composite indicates that thermal expansion mismatch will not cause pure ice to crack, whereas cracking is expected in the case of moderately concentrated solutions. At high concentrations, ice does not form a structure sufficiently rigid to create significant stress, so no damage occurs. Thus, maximum damage is achieved with a moderate amount of salt: the most pernicious solute concentration is widely recognized as approximately 3 percent by weight.

Attributing salt scaling to fracture mechanics contradicts previous theories proposing a chemical basis for the phenomenon. Valenza observes, Salt scaling was considered a result of osmotic pressure. An obvious shortcoming of such a suggestion is that it doesn’t account for the fact that moderate salt concentrations Û not pure water, nor highly concentrated solutions approaching 10 percent Û result in the most severe damage.

By contrast, he elaborates, The thermal expansion mechanism accounts for this characteristic through consideration of the effect of brine pockets on the mechanical properties of ice. Because ice does not incorporate any dissociated salt ions in its crystal lattice, salt is contained in brine pockets within the solid. These pockets act as flaws that weaken the ice, rendering it prone to cracking.

Consistent with such findings, the researchers determined that pure ice does not crack: since rapid creep [increase in deformation with time under constant stress] limits tensile stress in the ice layer to a value below its strength, pure ice is incapable of causing scaling. On the other hand, Valenza notes, when ice forms from a moderately concentrated salt solution, the stress rises faster than the strength, so the ice does crack; and, these cracks result in scaling. Higher salt concentrations prevent the ice layer from gaining strength in the temperature range of interest, he adds, so the ice is incapable of imposing any stress in the concrete surface.

By pinpointing thermal expansion mismatch as the mechanism responsible for salt scaling, the researchers provide a basis for supplying the means to prevent this damage. While salt scaling alone will not render a structure useless, the authors assert, it results in accelerated ingress of aggressive elements, such as chlorides, that contribute to corrosion of reinforcing steel and diminished service life. Especially in cold climates, therefore, where salts are regularly used to de-ice roads and walkways, the value of a remedy for salt scaling is not to be underestimated.

Fracture mechanics analysis indicates that cracks in the ice layer will not penetrate a concrete surface of sufficient toughness. Accordingly, the study’s findings suggest that using solid or chemical additives to increase concrete surface strength could produce stronger pavements able to withstand many more winters without repair or replacement. Scaling might also be reduced, the researchers contend, by the inclusion of fibers to hinder crack propagation.