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Federal Highway Administration Research and Technology
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|Publication Number: Date: Winter 1996|
Issue No: Vol. 59 No. 3
Date: Winter 1996
With the new year upon us, Bing Crosby may still be crooning for a white Christmas, but state highway officials are once again doing battle with the white stuff. Across the nation, armies of snowplows and salt-bearing trucks struggle to keep highways and local roads open in a relentless battle with Old Man Winter.
For many states, that battle has taken on a decidedly different tone in recent years thanks to the results of a five-year program recently conducted by the Federal Highway Administration (FHWA). Where in the past, states focussed their energies and resources on deicing wintry roads, new technologies have emerged that instead stress preventative anti-icing measures.
Included as part of the five-year, $150 million Strategic Highway Research Program (SHRP) begun in 1987, the anti-icing initiative (H-208) was carried out in nine states during a two-year period. The results of this showed that anti-icing techniques can be used effectively in the United States and depending upon the climate, the type and severity of storms, and the traffic anti-icing procedures could be accomplished with success.
A two-year, post-SHRP, anti-icing test and evaluation project (TE28) was conducted during the winters of 1993-94 and 1994-95 to optimize the effectiveness of anti-icing operations and determine the conditions under which they are effective. Along with the original nine states that participated in the SHRP study, six additional states participated in TE-28. The results of the testing has shown that chemical agents can be used much more effectively to prevent the bonding of the snow or ice to the pavement (anti-icing) than to destroy the bond once formed (deicing). It also showed the vast range of efficient use for the anti -icing operations under most climatical and storm conditions. In some cases anti-icing required less chemicals to be used for the same level of maintenance than deicing procedures. In others, the same amount of chemical used produced an increased level of maintenance as compared with deicing control operations. Anti-icing also reduced the need for abrasives.
Normally, salt is spread on snow- or ice-covered roadways, melting its way to the pavement to debond any snow from the pavement. But there are several problems with such an operation.
First, salt is an exceptionally corrosive agent, eating away at automobiles and the roads upon which they travel.
Second, in large quantities, salt is environmentally toxic. Over the course of a winter season, huge volumes of salt are shoveled or washed into storm drains, roadside lands, etc., creating an environmentally malignant environment.
Third, as salt melts its way to the pavement, snow turns to slush which, with the salt, is plowed aside. Hence, more salt must be placed on the pavement with each pass of the snowplow to continue the deicing action. While salt as a commodity may be inexpensive, economics of scale suggest that the use of salt for a snowstorm in a deicing mode may be expensive and wasteful.
Finally, because a chemical in the deicing mode must in essence play catch-up with the precipitation already on the roads, heavy snowfalls may prove exempt from the chemical's best efforts and can result in a slippery packing of bonded snow on the pavement's surface.
Just as it's better medicine to forego smoking rather than to start and quit later, the SHRP initiative adopted a similar philosophy in dealing with icy highways. The idea was to pursue a preventative anti-icing methodology rather than the conventional, reactive deicing measures. To do so, however, meant that certain new conditions had to be met.
There are three operations for spreading ice control chemicals: using liquid chemicals, using prewetted solid chemicals, and using solids. In the liquid operation, the liquid or solid dissolved in a liquid is sprayed directly onto the pavement using spray-bars. In the prewetted solid operation, the liquid is either sprayed onto the solid at the spreader just before the combined ingredients (prewetted solid) are dispersed onto the pavement, or the liquid is sprayed onto the solid in the truck hopper and the prewetted mix is then dispersed by a spreader onto the pavement. The former prewetting operation, prewetting at the spreader, is preferred along with liquid applications because it works better, it is cleaner, the agents do not cake in the truck, and it does not corrode the truck as much.
Our studies showed that the chemicals now commercially available for snow and ice control have about the same anti-icing effectiveness, especially in liquid form. Considerations as to the chemical of choice should be made by considering other criteria such as chemical cost, corrosion and pavement damage effects, and environmental aspects of using the chemicals.
The anti-icing chemical must be applied to roadways 30 minutes to two hours before the precipitation begins. There are several benefits to the early application methodology. The chemical isn't plowed away with subsequent snow and ice because it's already on the pavement. Reaction time is much faster because it begins to work immediately with the first drops or flakes of precipitation. The agent prevents the snow and ice from bonding with the pavement. The snowplow is used to remove snow (if any is there) from the pavement. As a result, much less of the agent usually needs to be applied than is applied in deicing situations.
Another enormous advantage to employing anti-icing methodologies versus deicing involves the absence of abrasives. Normally, during deicing operations, an abrasive such as sand is also applied to provide additional friction for vehicle tires. This is particularly true when packed snow is present on roadways. But, because anti-icingagents do not allow snow to bond with the pavement, abrasives are not needed. As a result, cleanup costs are significantly lower.
But application of the anti-icing chemical prior to precipitation beginning to fall begs another question: how does one know when it is time to begin anti-icing operations? Highway officials must depend on an array of data from different sources, including the Roadway Weather Information System (RWIS) and national and local weather forecasts. In addition to reporting dew point, humidity, air temperature, and wind velocity and direction, RWIS also reports road conditions via pavement sensors that monitor pavement temperatures and the amount of anti-icing chemicals present on the pavement. Once all of the data is fed into a central database, various modeling techniques can be used to accurately predict the start of ice formation on pavements and the times to start the anti-icing applications.
Detection of the times to reapply the chemical are accomplished with the observation of pavement friction to automobiles. These friction measurements can be determined by a commercially available friction instrument installed in a vehicle connected to its speedometer system. Engineers drive the vehicle on the streets during the storm and measure the friction of the roadway by applying the brakes for a few seconds. Thus, they can determine in this way whether more chemical needs to be applied and when.
This year, three efforts are being conducted to inform more states about anti-icing benefits and to help them adapt this technology to their normal operations. One effort involves four state experts who have conducted successful anti-icing efforts in their own states and are traveling to other states to assist in their anti-icing efforts. A second effort involves several workshops for many states in which five SHRP winter maintenance products including anti-icing will be discussed. The third effort is the determination of the optimization of the conditions under which the five products can be used. Participating states will evaluate each of the products under varying conditions.
It is apparent that through the participation of many states in these efforts a new thinking a revolution in the way snow and ice control is being handled is occurring and will be the standard for winter maintenance procedures in the near future.
Once solid salt was the unchallenged snow and ice control agent of choice used in deicing operations for all highway departments; but now, use of alternative chemicals, anti-icing and other application methods, and consideration of non-conventional operations for snow and ice control are actively being explored.
Dr. Brian Chollar is a research chemist in the Office of Engineering Research and Development at the Turner-Fairbank Highway Research Center in McLean, Va. Dr. Chollar has worked for FHWA for almost 22 years. His areas of expertise are snow and ice chemicals in winter maintenance and asphalt chemistry. He received his bachelor's degree in chemistry from Purdue University and the doctorate in organic chemistry from the University of Minnesota.