The effect of aqueous solutions on compacted snow hardness
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Snow falling on roads rapidly compacts into a hard crust. The crust is difficult to remove mechanically and it often becomes slippery. To avoid crust formation, chemicals are often applied in a measure termed anti-compaction. A large use of chemicals is problematic though. The most commonly used chloride salts cause vehicle and road side structure corrosion, plant and soil damage along the roads, and fresh water source contamination. Simply replacing such chloride salts with another chemical only raises a new set of environmental issues. Thus, the goal of the Norwegian Public Roads Administration is to reduce the use of chemicals by identifying better and more efficient strategies for anti-compaction. To accomplish this goal safely, however, more knowledge on how chemicals work during snowstorms is required. Studies have shown that chemicals not only melt snow, but they also weaken it. An important factor of this weakening process is the solution that forms due to melting. The work of this thesis has been to closely investigate this phenomenon and determine through what mechanisms a solution affect the mechanical properties of snow. The mechanical properties of snow are largely dependent on the bonding between snow crystals. Thus, the focus of this work has been to, in the laboratory, study how aqueous solutions affect the bonds between snow crystals that are created upon snow compaction. To avoid melting effects, the solutions were at thermodynamic equilibrium with ice. Two different laboratory experiments were developed. Using these two experiments, four different studies were conducted. Each with the aim of studying a different mechanism that might explain the weakening of snow. Two important findings were made in this thesis which may have practical applications • Chemicals are not only dependent on melting to reduce snow hardness, the presence of a chemical solution between grains is sufficient to weaken snow; • A very small amount of solution is required to weaken snow, 2.5 wt.% sodium chloride solution caused a 40% reduction in snow hardness and as the solution content increased the hardness decreased smoothly. Snow will hence not suddenly change its character when a solution becomes too diluted or if the solution content drops below a certain level. As melting is not required to weaken snow, it might be possible to predict its consistency on the road by calculating the solution content from a phase diagram. The ability to predict the properties of the snow, without sudden changes in mechanical properties, could make it possible to allow more accumulation of snow on the road, thereby reducing the amount of chemicals required for a successful anti-compaction measure. Three main mechanisms through which aqueous solutions are believed to weaken snow without melting were identified herein • Reduced ice cohesion, by approximately 40%, through a lowered ice interfacial energy; • The replacement of solid ice bonds with liquid bridges through grain boundary penetration; • Greatly accelerated microstructure coarsening which causes a loss of existing bonds and a reduction in the formation of new bonds. All three mechanisms are dependent on the species of the dissolved chemical. A chemical solution that is efficient at weakening snow should adsorb strongly to the ice surface and possess a low viscosity. The results also demonstrate that small amounts, less than 15%, of additives can have a significant effect on the bonding between snow crystals in compacted snow, indicating that it might be possible to identify additives that increase the effects of standard chloride salts.