New tool helps you select the right deicing material
Richard L. Hanneman
Every winter, APWA members apply millions of tons of snow and ice control materials to North American roadways. Choosing which deicer to use involves consideration of price, performance and environmental impact—and politics! Justifying the choice of deicer just got easier.
A new set of research-based Guidelines for the Selection of Snow and Ice Control Materials to Mitigate Environmental Impacts (NCHRP Report 577) includes a user manual, Guidebook to the Decision Tool, Purchase Specification, and Quality Assurance Monitoring Program.
The Guidebook shows every APWA snowfighting agency how to "balance economic value related to cost and performance with potential consequences of use related to environmental and corrosion impacts." It integrates considerations of cost, handling characteristics, ice-melting performance and a vast array of environmental impacts.
Public works directors—and the Councils they report to‐can adjust the weight of each variable to reflect their community values. "Each agency will have unique objectives and priorities that will affect their selections for the decision categories," states the report. The decision tool requires agencies to weight the variables explicitly and includes several subcategories under "environment" and "infrastructure" that fit such customization to local needs. This new tool crunches the numbers dispassionately. The weighting is value-driven, but the process provides full disclosure of the assumptions and, thus, transparency. The final product is an objective, evidence-based recommendation. That objectivity can help public works directors handle the politics they face.
Report 577 examined 42 different deicers, including sodium chloride, calcium chloride, magnesium chloride, calcium magnesium acetate, potassium acetate, organic matter from biomass and abrasives (the latter, of course, not a deicer, but sometimes used as an alternative to using a deicer). Recognizing that the effect of a deicer depends on other variables than simply chemical constituency, the Report also examined application amounts, exposure pathways, chemical-specific impacts and site-specific characteristics.
There is no universal "right answer" on how to assign weights to each variable. Each agency is going to come out differently. Its variables are different. Even neighboring communities which share similar weather patterns and common materials sourcing will have different topography, crews with different skills and preferences, different local ecosystems and, most of all, they will reach unique weightings reflecting their perceived desires of their customers—the roadway users and local voters who provide their funding.
The Report usefully illustrates the process with examples of agency policy weightings. The investigators surveyed snowfighting agencies and determined their current weighting, in practice: 45% cost, 35% performance, 11% environment and 9% infrastructure. They also generated a second example weighting which emphasizes environment and infrastructure impacts. It entirely eliminates costs as a variable, though it retains "performance" at 25%, arguing that "the primary objective of winter maintenance is controlling snow and ice, it is recommended that performance weighting not be assigned a value below 25%." This example assigns the self-imposed minimum 25% for performance and divides the remaining 75% equally, 37.5% for environment and 37.5% for infrastructure. Prices were obtained from Internet-published contracts and the results calculated at temperatures ranging from 30 degrees F to -20 degrees F.
The decision tool calculates a numerical product score that incorporates user priorities and conditions with a technical assessment of potentials for impairment. The agency, of course, is free to accept the model results or make its decision on other criteria, ignoring the "rational" calculation.
Applied to the current average agency weighting, reflecting current preferences for cost-savings and roadway clearing performance, salt comes out best. In the temperature range for most snowfall, 25 degrees to 30 degrees, salt (NaCl) scored 90.4 compared to second-place MgCl (71.1 to 76.4), followed by CaCl (64.8 to 76.1), KA (23.9 to 31.6) and CMA (18.2 to 19.2). Sodium chloride retains its strong preference score of 90.4 down to 15 degrees F (at that temperature, MgCl is 83.6; CaCl, 77; KA, 35.3 and CMA, 18.6). Salt surpasses the acetates down to zero Fahrenheit (we don't recommend NaCl at this temperature ourselves, even though we sell it!) and salt is still preferred to CaCl down to 10 degrees F, though narrowly, 80.8 for NaCl to 78.2 for CaCl (and 91.3 for MgCl). This model, of course, reflects the current preference for cost-savings and roadway clearing performance.
Using the "environment/infrastructure priority" weightings, all four deicers are tightly clustered between 15 degrees and 30 degrees F. Potassium acetate earns top honors at 63.8 to 67.4, narrowly edging out CMA at 63.5 to 63.6; NaCl is third, 62.5 at each temperature; MgCl is 59.2 to 63.1 (edging out salt below 20ø F); while CaCl registers 57.2 to 61.0. For this example, the Report concludes: "NaCl is the preferred product between 22 and 12øF, after which MgCl2 and CaCl2 are preferred," adding: "All materials have similar scores until temperatures reach below 10øF and NaCl and CMA lose performance benefits. Although KA is the preferred product, CMA also scores well at warmer temperatures."
Report 577, five years in preparation, provides a scientifically-sound basis both for examining options and community priorities—and for making a defensible choice of deicer.
Richard L. Hanneman can be reached at (703) 549-4648 or firstname.lastname@example.org.