De-Icing with Salt
We are regularly asked to review and assess winter maintenance activities, and whether or not proper de-icing was applied. The most common method for de-icing is using salt, though other chemical de-icers are available. Salt is highly corrosive and can damage concrete structures by degrading rebar. It also has adverse environmental effects when overapplied, since the salt can kill vegetation and contaminate groundwater aquifers. The big advantage to salt is the low cost.
So how does de-icing with salt work? Solid rock salt mixes with water to form a brine. This water can come from snow, ice, rain or humid air. When the moisture is absorbed by the salt, it forms a brine solution. A brine is essentially salty water.
As moisture is absorbed into the salt and the brine forms, the ultimate concentration of 23.3% is reached. As additional moisture is absorbed into the brine from melting snow/ice, the concentration dilutes. Eventually it will dilute to the point where there is so little salt remaining that the brine is basically just water again, and it will re-freeze.
As the concentration of the brine is higher, it is more effective at lower temperatures. At a concentration of 23.3%, salt works at temperatures as low as -21oC. As it gets diluted, the effective temperature for the salt increases. For example, a 10% brine will work at temperatures of approximately -6 o C or warmer and a 3.5% brine will work at temperatures of approximately -1.8oC or warmer.
Salt is not effective at temperatures below -21oC. Realistically, salt has minimal effectiveness between temperatures of approximately -9 oC and – 21oC. It can still be effective if used properly in this temperature range, but does require more regular monitoring/maintaining. Salt is most effective at temperatures above -9 oC.
There are many factors that affect the ability of salt to act as a de-icer; temperature is the most important factor. Spreader rates are important (making sure the right amount of salt is used. Too much is wasteful, too little is ineffective). Also the weather (is it sunny or cloudy? Day or night?), the type of pavement surface (is it asphalt or concrete), the topography (is the area in shade where it is cooler?), traffic volume (to spread around the salt, apply some heat to the solution, etc), and a few other factors.
How is salt used for proper de-icing? Can you just throw salt on the surface and everything will be safe? No, not usually. Salt needs to be re-applied regularly to remain effective, as it is diluting during deicing, and becoming less effective. It shouldn’t be thrown down on top of an unplowed surface with hopes that it will help rid the surface of snow/ice.
Salt helps to break up the bond between the snow/ice and the pavement surface improving traction and making the surface safer. During snowy periods, you should always plow and then salt at regular intervals. The plowing removes a large volume of snow, so that the salt can be more effective. Once the salt has had time to work and break the surface bond, the plow should be back to do another pass and then additional salt applied again. This should be repeated as necessary to get the pavement to a safe and serviceable condition.
The importance of monitoring site conditions is paramount. Local site conditions can have hazards that aren’t noticeable by monitoring weather conditions from an office. Having an onsite presence to see firsthand what is going on and planning accordingly is critical. Inspections can be done by trained equipment operators who are out plowing/de-icing, or by patrollers who are touring around to monitor site conditions and dispatch crews as needed.
In addition to de-icing, there is also anti-icing. De-icing is trying to act after the snow/ice has accumulated. Anti-icing is a more proactive approach where a liquid brine is applied to the roads before a storm. When applied properly, this liquid brine helps to prevent snow from sticking to the ground so that the first pass with the snow plow is more effective at clearing away snow.
Property owners normally contract snow removal and deicing work to contractors who specialize in this work. If they do the proper inspections, plowing and deicing work needed, surfaces such as parking lots and walkways can be kept safe for vehicles and pedestrians, if they fail to do the job properly, ice forms, slip hazards develop and accidents will happen. We help to assess any failures in maintenance that allow these accidents to happen.
