Ice Melt Working Temperature and Melting Capacity

THE EUTECTIC IS A THERMODYNAMIC BOUNDARY AND THE WORKING LIMIT IS AN OPERATIONAL ONE, and confusing them is the most common error in deicer selection. A solution can remain liquid at its eutectic and still take hours to penetrate and undercut a sheet of ice, which is no use to anyone clearing a walk before opening. Sodium chloride's eutectic is about -6 degF and its practical working limit is roughly 15 to 20 -- more than twenty degrees apart -- and A BAG ADVERTISING A VERY LOW TEMPERATURE IS USUALLY QUOTING THE EUTECTIC. Magnesium chloride runs about -28 against a working limit near 0; calcium chloride about -60 against -20 or below. THE CAPACITY COLLAPSE IS WHY COLD EVENTS ARE EXPENSIVE RATHER THAN MERELY DIFFICULT. Rock salt melts roughly 46 lb of ice per pound at 30 degF, about 8.6 at 20, and under 5 at 10 -- so the same job takes on the order of nine times the material from a twenty degree drop, and past a point NO QUANTITY produces the result because the brine that forms refreezes as fast as it is made. A crew responding to a cold event by spreading heavier is spending material to no effect, and it is the most common winter operations mistake there is. The answer is a DIFFERENT CHEMISTRY, not more of the same one. CALCIUM CHLORIDE'S ADVANTAGE IS THERMODYNAMIC RATHER THAN MARKETING. It releases heat as it dissolves, so it warms the brine it is making and keeps the reaction going where an endothermic or neutral product stalls. That is what buys the low working temperature -- and it is also why it is more expensive, more hygroscopic, and more aggressive to concrete and metal. The properties come together and cannot be bought separately. This is a comparison from entered melting capacity data. Capacities are LABORATORY figures measured over a stated time at a stated temperature on a stated ice thickness, and they are not field yields: penetration and undercutting depend on the ice's condition, whether it is bonded, the pavement's temperature and material, traffic working the product in, and dilution by continuing precipitation. A field application is substantially less effective, in a direction and magnitude this cannot quantify. Practical working limits are practice ranges rather than defined quantities and suppliers differ on them. It does not select a product: cost, corrosivity to vehicles and reinforcement, effect on concrete -- particularly young concrete -- tracking indoors, effect on vegetation and on aquatic receiving waters, and the increasing regulatory restrictions on chloride application all bear on the choice and none are in this arithmetic. It does not address blends, pre-wetting, or organic and acetate products, whose behaviour differs from the chlorides. The product supplier's data, the applicable environmental and agency restrictions, and the winter operations manager govern.

ice mass = area x thickness x ice density; product = ice mass / the melting capacity at that temperature; the comparison divides by the capacity at a second temperature.

Published deicer melting capacity and eutectic data. Capacities and the practical working limit are ENTERED laboratory figures: rock salt roughly 46 lb of ice per lb at 30 degF, 8.6 at 20 and under 5 at 10.

A mass calculation and a division by a published capacity.

Estimate. AHJ and licensed professional govern.

Field names used by the API: area_ft2, ice_thickness_in, ice_density_lb_ft3, pavement_temp_f, capacity_lb_ice_per_lb, alt_capacity_lb_ice_per_lb, alt_temp_f, practical_limit_f, eutectic_f, ice_volume_ft3, ice_mass_lb, product_lb, alt_product_lb, product_ratio

• Capacities are laboratory figures stated time, temperature and ice thickness; a field application is substantially less effectivepublished deicer capacity data
• Ice density 57.2 lb per cu ft, entered and adjustablethe density of ice
• Working limits are practice ranges suppliers differ; they are entered rather than definedthe product supplier's data