Anti-Icing and Deicing Practices

A primary responsibility of any State Highway Agency (SHA) is keeping roads safe for the traveling public. This responsibility becomes especially acute during inclement winter weather when the accumulation of snow and ice on the surfaces of roads and bridges can result in a dramatic loss of surface friction, creating hazardous driving conditions. SHAs use a variety of means to maintain a safe riding surface, including plowing, the application of abrasives (e.g., sand), and the use of chemicals that depress the freezing temperature of water, thus melting snow and ice. If these chemicals are applied prior to a winter precipitation event, the activity is referred to as anti-icing because it is designed to prevent ice accumulation or assist in ice removal by reducing the bond between the ice and the pavement surface. If applied to remove snow and ice after accumulation begins, the activity is referred to as deicing.

Anti-icing and deicing are largely accomplished through the application of one or more chemicals. These may be solids or liquids, and may be combined with abrasives to help establish traction. Commonly used liquids are aqueous solutions of chlorides (e.g., sodium chloride [NaCl], MgCl2, and CaCl2) or other chemicals such as calcium magnesium acetate (CMA), potassium acetate (KAc), and urea. The effectiveness of these chemicals to melt ice over a range of temperatures is directly related to the type of chemical and its concentration, information that is readily available to winter maintenance personnel.

In the last decade, concerns have been raised regarding the potential for deleterious effects of certain chemical deicers (most notably MgCl2 and CaCl2) on concrete. Chemical deicers based on MgCl2 and CaCl2 are becoming increasingly popular due to their relatively low cost, ease of use, and effectiveness over a broad range of temperatures (Sutter et al. 2008; Taylor, Sutter, and Weiss 2012; Weiss and Farnam 2015). As aqueous solutions, they are commonly applied at relatively high concentrations to dry pavement as anti-icing agents prior to a winter precipitation event, a practice that can result in direct absorption of the solutions at full concentration into the pavement surface and joints. MgCl2 and CaCl2 can also be used as prewetting agent mixed with dry NaCl crystals to help them stick to the pavement surface and in a dry mix with NaCl (typically 20 percent MgCl2 or CaCl2 and 80 percent NaCl) to increase the effectiveness of the deicer.