Dilution Rate of Solid NaCl Anti-Icers Under Freezing Rain Conditions

ABSTRACT In order to address environmental concerns and control cost, road salt should only be applied at the rate required to prevent or break the bond between snow and pavement. The effectiveness duration of solid salt anti-icers depends upon the pavement temperature, surface storage capacity, rainfall rate, and salt application rate. In order to better understand the likely dilution rate of solid salt under freezing rain conditions, Automated Surface Observing System (ASOS) weather data from 2018 to 2023 were analyzed for the U.S. cities of Minneapolis, Chicago, Cleveland, Indianapolis, Pittsburgh, and Louisville. Freezing rain events were defined as periods spanning 3 hours before the first indication of freezing rain or drizzle to 3 hours after the last indication. The median rainfall rates during these periods in five of the six cities was 0.001 in./hr (0.0025 cm/hr), and the average rainfall rates ranged from 0.014 to 0.029 in./hr (0.036–0.074 cm/hr). Making the simplifying assumptions of ignoring salt loss due to traffic action and that salt will quickly and uniformly dissolve in surface moisture, a first approximation change in the percentage of sodium chloride, freezing point depression, and anti-icing effectiveness duration is calculated for typical freezing rain rates, surface storage capacities, and temperatures. At rainfall rates greater than 0.02 in./hr (0.051 cm/hr), solid salt may be diluted too quickly for practical anti-icing, but at many common rainfall rates, applying pre-wet or treated rock salt to dry pavement may be considered for anti-icing.