Date: March 9, 1998
Author/Local: R. Birch/7292
CC File No. 5801
Vancouver City Council
General Manager of Engineering Services
Use of Road Salt for De-Icing
THAT Council approve the continued use of road salt (sodium chloride).
There is no applicable Council policy.
On October 7, 1997, while approving a recommendation to extend the existing contract for supply of road salt for de-icing, Council requested a report back on substitutes for de-icing salt which may be in use elsewhere, along with comparative costs of using alternatives to de-icing salt. This report is in response to that request.
The City of Vancouver currently uses sodium chloride (road salt) for roadway de-icing purposes. This chemical is spread using truck-mounted spinners both before and during a snowfall. The average consumption of salt over the last 14 years has been about 5,500 tonnes annually, with a maximum of 14,000 tonnes used in 1990/91, and a minimum of 600 tonnes used in 1997/98.
The benefits of de-icing roadways are usually measured in terms of reduced accident rates, reduced delays, and improved ability of the public to make trips when they would like to.
A study of accident rates in New York state documented a freeway accident rate of 4.6 accidents per million vehicle miles (MVM) on snow covered roads, compared to a corresponding figure of 2.3 accidents per MVM for wet roads, and 0.6 accidents per MVM for dry roads. A separate study by Marquette University covering five states measured an 88% reduction in accident rates immediately after the application of de-icing chemicals.
Quantification of the other benefits of de-icing is difficult, and little information is available in current literature. It is generally accepted that the economic and social benefits of applying de-icing chemicals significantly outweigh the costs.
There is much literature available to assist with choosing a de-icing chemical. Three significant research activities have been completed in this decade in the USA. The first, a joint effort of the states of Nevada and California completed in 1992, compiled a comprehensive list of the properties of alternative de-icers. The Transportation Research Board of the U.S. National Research Council compared road salt against the most environmentally friendly alternative, calcium magnesium acetate (CMA). A major initiative was undertaken as part of the Strategic Highway Research Program between 1987 and 1992. Follow-on research from this project continues today.
Industry magazines contain many case studies of application of new methods and strategies throughout North America. Certain industry groups have also established web sites to champion their own de-icing chemical products, but such information must often be taken with a grain of salt.
The following table summarizes the costs and features of the most commonly used roadway de-icing chemicals.
Comparison of Alternative De-Icing Materials
Slower acting than salt. Application rates up to 20% higher than salt.
Usually applied in liquid form. Effective at lower temperatures. May damage concrete.
Usually applied in liquid form. Hygroscopic - attracts water from the atmosphere. Persistent - leaves greasy film. Slightly toxic.
Limited "shelf life".
Choosing an appropriate de-icing material for Vancouver involves comparing the costs and benefits of each of the alternatives.
COSTS OF DE-ICER USE
Apart from the cost to purchase and apply de-icing chemicals, additional public costs are associated with de-icer usage. De-icers have been associated with corrosion, damage to vegetation and degradation of drinking water quality. These are discussed here in Vancouvers context:
Sodium chloride and calcium chloride are both very corrosive to unprotected steel. Magnesium chloride is less corrosive than these two de-icers, but is still moderately corrosive. One supplier has developed a sodium chloride de-icer with anti-corrosion additives. The least corrosive commercially available de-icer is calcium magnesium acetate, which has been found to induce corrosion at a similar rate as pure water.
Although there is no data available to assess the amount of damage caused to vehicles by de-icers in Vancouver, a subjective assessment would be that such damage is relatively minor. De-icer-induced rusting of vehicles is much more visible in central Canadian cities, where usage is much higher. The main cost borne by Vancouverites related to de-icer corrosion would appear to be in the extra cost of the anti-corrosion features which are now designed into new cars. The Transportation Research Board study identified 11 separate design and construction features which have been implemented by the auto industry since 1980. Each of these features adds additional cost to the purchase price of a passenger vehicle. The study estimated that the total extra cost per passenger vehicle of these features is between $100 and $250. It must be noted that even if Vancouver discontinued the use of sodium chloride in favour of a less corrosive alternative material, these costs would continue to be borne by Vancouverites.
De-icer-induced damage to structures is also a relatively minor issue in Vancouver. All of Vancouvers major bridges are protected from corrosion through the use of a waterproofing membrane on the bridge decks. New structures are built with epoxy-coated reinforcing steel, to provide further protection. Bridges which cross salt water bodies are subject to attack from salt water spray, which constitutes a background level of salt exposure to which de-icers provides an additional level of attack. If use of sodium chloride salt was discontinued in Vancouver, the extent to which life of existing structures would be extended is uncertain but maintenance savings would be likely not more than $100,000 per year.
Corrosion has been a concern for parkade structures in the City. The most significant cost experienced so far was the approximately $1.5 million spent in 1996 for repairs and upgrading of the Pacific Centre parkade. The repair work included the installation of a waterproofing membrane, which will significantly reduce future corrosion damage. Other existing structures are experiencing some corrosion damage and will continue to require periodic maintenance, with some requiring major repair similar to Pacific Centre. Based on past experience, we expect a long term average of no more than $200,000 per year for such repairs. This cost will decline in the future as older structures benefit from the installation of protective waterproof membranes. New parkades are built to the standards of the current Building Code, which require the installation of a waterproofing membrane during construction. If the City were to discontinue the use of corrosive de-icers at this time, existing unprotected structures would continue to deteriorate, because the structures are already contaminated with chlorides. Only water is needed in addition to these chlorides to encourage corrosion.
All of the chloride de-icers can have a negative impact on vegetation. The sodium in sodium chloride can also produce toxic effects to vegetation if accumulated in sufficient concentration, while calcium and magnesium have been found to be less toxic than sodium. Calcium magnesium acetate produces calcium and magnesium ions in solution, and is therefore not completely benign. Its toxicity to vegetation is considered low, however.
Park Board staff have advised that they are not able to identify the Citys use of road salt as the cause of ongoing damage to City trees, although they do note that trees in commercial areas have higher mortality rates than trees in residential areas. The Park Board continues to monitor these issues and plans toxicity testing on some of the trees which are experiencing early mortality. Tree species which are known to be salt-resistant are favoured for planting in commercial areas.
Risk Management staff advises that there have been no successful claims for damage to vegetation on private property arising from the use of road salt in the last 15 years. This is not surprising, considering that damage is usually limited to within 15 feet of the road surface, and that Vancouvers climate of high rainfall promotes the rapid elimination of any salt build-up.
3. Water Quality Issues
Contamination of drinking water supplies by chloride de-icers is a significant concern in many areas, but is of course not an issue for Vancouver as a result of our remote watershed. Run-off from melted snow which contains quantities of road salt is handled by the sewerage system and ultimately is discharged into the ocean. Therefore, the water quality issues of the use of road salt in Vancouver are considered to be minimal.
The following table summarizes the financial implications of the alternatives:
Summary of De-Icing Alternatives
Corrosion Inhibited Salt
Calcium Magnesium Acetate
Reduced Corrosion Costs
$0 - $50,000
Net Annual Cost
At current prices ($51 per tonne, including taxes), the City spends an average of $280,000 per year on road salt. Switching from road salt to an alternative material which protects against corrosion (corrosion inhibited salt) would cost approximately $1.8 million per year and only generate an estimated $100,000 per year in savings. Switching to the least corrosive and "environmentally friendly" calcium magnesium acetate would cost $4.95 million per year, and would again save a maximum of $100,000. None of the alternative materials show any promise of being cost-effective compared to road salt.
EXPERIENCE IN OTHER JURISDICTIONS
The general trend among major snow-fighting jurisdictions in the United States and Canada involves the development of "anti-icing" strategies, which reduce the total amount of chemical applied. The state of the art in this area involves the use of integrated weather forecasting, pavement temperature sensing and early application of an anti-icing agent (usually liquid magnesium chloride, calcium chloride or salt brine).
The cost effectiveness of these strategies is dependent on the number of snowstorm events each winter, and the ability of weather forecasters to give adequate advance warning of snowfalls. In Vancouver, we experience relatively few snowstorm events and the accuracy of forecasts has been inconsistent at best. We have not yet found a system which is clearly more economical than our current methods, but remain vigilant for new methods or equipment which make sense for Vancouver.
In summary, the use of road salt in North America causes additional costs to Vancouver residents mainly through higher vehicle purchase prices. Even if Vancouver switched to the least corrosive alternative material, these costs would not be avoided. There would be some reduction in City costs due to extended life of structures, however. Our assessment of the effect on City structures of such a change is that the benefits would be at most about $100,000 per year. Existing parkades which have been damaged by salt-induced corrosion would continue to corrode, even if Vancouver abandoned the use of salt today. The Building Code requires that future parkade structures be built using techniques which mitigate corrosion damage. The environmental consequences of using road salt in Vancouver appear to be small. When one compares the extra $1.4 million per year that it would cost to switch to a less corrosive de-icing chemical, and the limited benefits that such a change would generate, it is apparent that continued use of road salt is advisable.
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(c) 1998 City of Vancouver