Road salt
Road salt is a salt used mainly as an anti-slip and deicing agent for roads, sidewalks and other transportation surfaces during the winter. The most commonly used form is sodium chloride, though other salts such as calcium chloride and magnesium chloride are also employed, especially in colder temperatures. When used in its solid form, road salt is often pre-wet to accelerate the ice-melting process. Road salt lowers the freezing point of water through freezing-point depression and thusly prevents ice formation, which in turn improves traction and safety for vehicles and pedestrians.
The widespread use of road salts began in the 20th century and is since standard practice in many regions with yearly snowfall, especially in Europe and North America. Millions of tons of salt are applied to road surfaces yearly, making it one of the most extensively used chemical agents in urban infrastructure. It has become popular due to its effectiveness, low cost and ease of application.
Road salts have adverse effects on the environment. Chloride ions from road salt can accumulate in soils, surface waters, and groundwater, where they persist and may harm aquatic ecosystems, roadside vegetation, and drinking water supplies. Road salt also accelerates corrosion of vehicles, bridges, and reinforced concrete, leading to increased maintenance costs. These concerns have prompted research into alternative de-icing materials, improved application techniques, and policies aimed at reducing salt use while maintaining road safety.
History
The use of salt for snow and ice control began in the early 20th century. In Paris, sodium chloride was applied on a large scale to combat black ice, although the practice soon revealed disadvantages such as corrosion of iron structures and damage to the hooves of draft animals. Despite these drawbacks, the method proved effective in improving road safety and was soon adopted in other cities and countries.The use of salt for deicing roads began in the United States in the late 1930s, when New Hampshire experimented with spreading granular sodium chloride on roads in 1938. By the winter of 1941–1942, New Hampshire formally adopted a statewide salt-spreading policy; about 5,000 tons of salt were applied on U.S. highways that season. Before the adoption of road salt in New Hampshire, road maintenance in winter typically relied on plowing and spreading abrasives for traction, with salt only used occasionally.
In the post-war era, as the U.S highway networks expanded and the "bare pavement" standard became dominant, salt usage increased greatly. During the 1950s and 1960s, U.S. salt consumption doubled roughly every five years, climbing from about 1 million tons in the mid-1950s to nearly 10 million tons less than a decade later. Supply sources developed along with the demand, large underground rock-salt deposits supplied much of the salt. The Detroit salt mine became a prominent example as the mines expanded over the 20th century and supported local deicing operations. Detroit itself was among the earliest cities to apply salt to its roads believed to be due to its proximity to the resource.
A parallel development occurred in Europe. In Germany, salt spreading became common in the 1960s as motorization increased and road-safety demands grew. Authorities established salt storage facilities and distributed salt manually from trucks before mechanical spreaders were introduced to improve efficiency. The transition from dry to wet-salt application in the early 1970s represented a further advancement, as saturated salt adhered better to road surfaces and was less susceptible to wind displacement.
In later decades, salt use stabilized, and attention has shifted to optimizing application methods and mitigating collateral impacts. More recently, occasional supply constraints and environmental concerns have spurred experimentation with reduced-salt strategies and alternative deicers.
Spreading
Road salt is applied to road surfaces primarily to prevent the formation of ice or to melt existing snow and ice. Application methods and quantities vary depending on weather conditions, road type, traffic volume, and local policies.Deicing and anti-icing
Two main strategies are used in winter road maintenance. De-icing involves applying road salt after snow or ice has formed on the roadway. The salt lowers the freezing point of water, allowing ice to melt and preventing refreezing under certain temperature conditions. Anti-icing is a preventive approach in which salt or salt brine is applied to dry or wet pavement before snowfall or freezing rain occurs. This method is intended to inhibit ice from bonding to the road surface and can reduce the total amount of salt required. Anti-icing is generally considered more efficient than de-icing when applied under appropriate weather forecasts.Application methods
Road salt can be applied in various forms, mainly as dry rock salt, pre-wetted salt and brine. Dry rock salt consists of solid salt crystals spread directly onto the roadway. It is the most traditional and widely used form of road salt, despite being less effective at lower temperatures and being prone to being blown off of the roadway by traffic.Road salt and brine are generally spread using a winter service vehicle called a salt spreader. Salt spreaders are typically added to trucks, loaders, or in the case of brine, tankers. The salt is stored in the large hopper on the rear of the vehicle, with a wire mesh over the top to prevent foreign objects from entering the spreading mechanism and hence becoming jammed. The salt is generally spread across the roadway by an impeller, attached by a hydraulic drive system to a small onboard engine. However, until the 1970s, it was often spread manually either by workers shoveling salt from trucks or by smaller wheelbarrow-like vehicles, the latter still being used today for personal use. Some older spreading mechanisms still require it to be manually loaded into the impeller from the hopper.