Treating Icy Road Conditions
updated: Monday, December 29, 2014
Reducing salt use and keeping streets safe.
The City of St. Louis Park has developed a strategy on salt usage in its winter maintenance plan. According to Environmental Protections Agency standards and the Minnehaha Watershed District, a higher chloride level has been noticed in Minnehaha Creek contributed by salt usage. As a result of increased public sentiment of environmental and costs (taxes) concerns, the City of St. Louis Park has created a plan to reduce the amount of chlorides released into the creek while still providing the traveling public a safe, cost effective driving surface.
Salt tracking equipment: The city closely monitors the amount of salt applied to all streets and parking lots throughout the city. Using AVL (Automated Vehicle Location), the city can monitor how much salt is being applied as well as where it was applied. We can adjust application rates of salt using real time data to be most effective for both costs and environmental concerns.
Proactive prior to snow events by “Anti-Icing”: Applying a diluted liquid (23 percent salt brine) to the streets prior to a snow event. This is the faint white lines you see on the street a day or two before a storm. This has proven to help prevent the bond between the driving surface and the ice buildup or hard pack during a snow event. This allows for easier removal and use of less salt during the storm. This product must be applied to a dry road surface prior to storm. If applied on a wet surface or freezing rain, the solution is diluted out and the product becomes ineffective.
Pre-wetting salt: Wetting the salt provides additional moisture to start the brine making process faster which is what you need to melt snow and ice. Trucks carry between 100 – 180 gallons of salt brine in tanks attached to the truck. By introducing a 23% solution of salt brine into the sander to mix and coat the salt with liquid prior to hitting the street has two benefits:
- Prevents bounce and scatter. Keeps more of the salt in the drive lanes where needed to create the brine vs. bouncing into the gutter where there is no value.
- Jump starting the brine making process to promote melting.
Deicing chemicals work by lowering the freezing point of water.
The most common deicer salt uses moisture from water, snow, or slush on the road surface to create the brine solution that does all the melting. Salt effectiveness is affected by the moisture content of the snow, chemical concentration, time, pavement temperature, weather conditions, type of road surface topography, traffic volume, and width of application which all affect the process of melting snow and ice. The city uses accepted industry guidelines which determine the amount of salt required to create brine for various weather conditions. The concentration of the brine and the temperature of the pavement are the key variables in determining whether and how fast the salt will act. When salt dissolves in water, the resulting brine generates a saturation level of 25 to 26 percent, but the brine is quickly diluted by the snow or ice it contacts. As dilution proceeds, there is less salt to depress the water’s freezing point, so the freezing point will rise. If temperatures continue to fall, the loss of melting power accelerates. As temperatures go down, the amount of salt needed create brine to melt a given quantity of snow and ice increases significantly. Multiple applications of salt may be required to keep the brine solution at 25 percent to continue the melting process.
Little known fact: Salt melts four times as much ice at 30 degrees as it does at 20 degrees. Colder temperatures will take more time and material to get same results.
15 degrees required for salt
The city uses a three pronged approach to prevent or remove snow and ice from the roadways.
Standard Road Salt: Road salt is a great material to use but it is not a cure-all for all things icy and it does not work well on its own. Salt is used city wide to create the brine needed with temperatures down to 15 degrees. The mixing action of vehicle traffic will spread this brine up and down the street to promote excellent melting.
Treated Salt: Used below 15 degrees. It is salt treated with calcium chloride by the supplier to lower the freeze point of water to 0-5 degrees. You still need a heat source (friction by traffic) which will create the brine to promote melting. It is 30 percent more costly than straight salt so it is used on all mains, but only on intersections in the neighborhoods.
Sand & Salt:
Prudent use of a chemical / sand mix is the industry standard treatment for extended low temperature conditions (less than 0 degrees F). The sand provides an immediate improvement in traction and the salt is available to attack the ice should pavement temperatures reach 15 degrees during the day. The concern about using the sand salt mixture is that it is temporary at best for a few hours before vehicle traffic will cause the mixture to migrate to the curb lines after a few cars. Sand is also an environmental concern for the waterways and must be swept up in the spring time to prevent it from entering the storm water system.
Why are some roads bare and others not
Higher volume roads such as interstate or highways have the greatest possibility to create brine needed to melt snow and ice due to the number of vehicles. Major arteries through the cities have lower amount of vehicles but still enough traffic to provide some melting below 15 degrees. Whereas residential streets have fewer than 100 cars a day which is nowhere near enough traffic to create the brine needed to melt the snow and ice. Residential streets will stay hard packed until temperatures climb into the upper teens to low 20’s when the salt can use the sunlight or air temperatures to create the brine needed.
Jeff Stevens | Public Works Operations Manager
Municipal Service Center | 7305 Oxford St. | St. Louis Park, MN 55426
P: (952) 928-2853 | firstname.lastname@example.org