One of the tools used for preventive maintenance is Direct Liquid Application (DLA). This is where brine (23 per cent salt solution) is sprayed directly on the road surface before a snow event or a risk of frost. The water evaporates leaving the salt residue in the tiny cracks and surface of the roadway. Once snow falls, it mixes with this residue creating brine which spreads across the road surface and prevents the formation of ice. It’s interesting to note that once the water evaporates, the sodium chloride crystalizes on the road surface leaving a white residue which appears to look like a significant amount of salt has been used – when, in fact, much less has been applied by using brine rather than road salt. The appearance can be deceiving.
DLA is a practice used before snow events or in the latter stages when roadways are clear, but wet and may risk freezing up due to dropping air and surface temperatures. It is not effective during snow events when the dry salt is more effective in creating the brine as it mixes with the snow. It can also be used up to 48 hours in advance, to target areas that are prone to early morning frost conditions particularly during the early and late “shoulder” seasons.
As an example, to apply a light application of salt, prior to a snow event on all of the municipality's 3,800 lane km, it would take 361 tonnes of salt at a 95 kg/ lane km rate (lane km is a 3.5m-wide single lane). By comparison, using DLA to pre-treat the same amount of lane km’s would take 76 tonnes of salt to make enough brine to achieve this.
No one strategy covers all the scenarios in any given storm. Depending on the frost depth, weather forecast, current conditions, temperature trends and time of year, each snow event is approached a little differently. Generally, salting is done in early morning to take advantage of rising air and surface temperatures and traffic to help spread the salt and create brine.
The following table illustrates the impact of temperature on the effectiveness of salt.