It's hard to think about winter while leaves are still clinging to the trees, but now's the time to prepare crews and equipment for fighting bad weather.
Last year, 38 states were subjected to snow or freezing rain. Most departments focus solely on the material they're going to use to keep ice from forming, forgetting that the most effective strategy for keeping roads safe incorporates other factors—air temperature, the type and rate of precipitation, and road-surface temperature—as well.
Wet Versus Dry
Many departments equip snow plows with pre-wetting applicators, which shoot liquid onto material as it comes out of the spreader. If the material is an abrasive, such as a salt-sand mix, pre-wet material cuts into existing snow and ice much more effectively than a dry mixture. And because wet material doesn't blow off roads as easily as dry material, pre-wetting often saves departments money in material costs.
Pre-wet material begins to work immediately. Chemicals such as magnesium chloride, calcium chloride, and brine (23.7% salt to water) act as a freeze suppressant, bringing down the freezing level of the roadway to well below 32° F. This is particularly important in areas that have colder temperatures within drier climates.
Pre-wetting the material also reduces the amount of material that bounces off the road by up to 50%. Then, the speed of the vehicle spreading the material determines the amount of material that remains on the road to do its job. The truck should not exceed 20 mph.
Many departments pre-wet their rock salt with brine. This liquid enables the rock salt to work quickly with no adverse effects in cases of humid air or rain, making the mixture appropriate in most coastal regions with a temperature of 20° to 35° F. If brine is used in climates where the temperatures tend to drop well below freezing overnight, it may freeze. This can create black ice.
Magnesium chloride is sometimes used to pre-wet rock salt. The downside to using that material is that if the temperature rises to above freezing, magnesium chloride can make the road become slippery (hygroscopic).
Pre-wet abrasive material can't do its job properly if the entire roadway isn't covered. If material is applied before snow or ice falls on the road, apply a narrow band—about 1 to 6 feet wide—of material to the center of the roadway on its crown. The material will work more quickly and will have more potency.
When it begins to work, the brine will run down from the crown and across the roadway. Traffic will speed the process. If the correct amount of chemical is used and it's been applied correctly, most of the material will be diluted with the resulting water before it enters catch basins.
Proper Calibration And Traveling Speed
The amount of rock salt road departments waste each year because of improper vehicle speed and inaccurate equipment calibration is staggering.
Calibration ensures drivers use the correct amount of material at the right speed by estimating how far they can travel with the amount of onboard material. Most spreaders can be calibrated to distribute in several ways, depending on the type of equipment and desired application rates.
The calibration process typically starts with weighing material that comes out of the spreader or tank over a given period of time. Most computers and spreaders are calibrated together annually, and many organizations make this a requirement to ensure that they don't use too much material. It is best to calibrate equipment every six months.