The effect of sodium chloride solution on the hardness of compacted snow

• A new experimental setup was designed to test hardness of saline snow.
• Solute distribution was studied through micro tomography.
• Small amounts of solution weakens snow considerably.
• A solution weakens the bonds forming between snow crystals.
• Results imply that salt usage might be reduced by preventive salting using solution.

Snow-fall on roads is rapidly compacted by traffic to form a hard crust. To avoid crust formation, sodium chloride (salt) is often applied to the road in a measure called anti-compaction. Salt can however have adverse effects on roadside structures and on the environment. As a result it is desirable to reduce its usage. To safely do so, more knowledge is needed on compaction and anti-compaction processes. An experiment was designed to study how the addition of a sodium chloride solution to snow affects its hardness after compaction. Snow was mixed with a salt solution in amounts from dry to saturated snow and compressed to a dry density of 500 kg/m3, after which the compact hardness was measured with a micro penetration test. The solution was maintained at its liquidus concentration to avoid melting or freezing, and the time scales were kept short to avoid coarsening of the snow. With even small amounts of solution, the snow hardness dropped to 60% of that of dry snow. As the solution content was increased further, the hardness decreased gradually until reaching saturation, where the hardness was a fifth of that of dry snow. This phenomenon is believed to occur because of the weakening of grain–grain bonds due to the lower energy of the ice–solution interface compared to that of the ice–vapour interface and because of grain boundary penetration of the sodium chloride solution. Directly transferred to a road situation, these results indicate that salt in much smaller amounts than what is currently used could weaken snow considerably, provided that the solution is sufficiently distributed to affect a large fraction of the grain bonds. Because the solution at the grain contacts has such a dramatic effect on snow hardness, anti-compaction measures using salt solution could be as effective as those using solid salt. The use of solutions would reduce salt losses from the road, benefiting the environment and contributing to reduced salt usage.