Snow and Ice Control: Guidelines for Materials and Methods (2004)

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11 CHAPTER 5 STRATEGIES AND TACTICS AND THEIR APPLICATION TO SUPPORT LEVEL OF SERVICE CHOICES As stated in the Introduction, the various roadway snow and ice control strategies used in winter maintenance opera- tions can be classified into four general categories and are discussed in details in this chapter: • Anti-icing, • Deicing, • Mechanical removal of snow and ice together with fric- tion enhancement, and • Mechanical removal alone. ANTI-ICING Roadway anti-icing is a snow and ice control strategy of pre- venting the formation or development of bonded snow and ice to a pavement surface by timely applications of a chemical freezing-point depressant. The tactics employed during anti- icing operations consist of chemical applications that are coor- dinated with plowing. Anti-icing is suitable for use during most weather, site, and traffic conditions. It is particularly beneficial as a pretreat- ment using liquid chemicals for anticipated frost and prefer- ential icing situations. Anti-icing with a liquid chemical is a good strategy when the pavement temperatures are above about 20°F at the onset of a snowfall event. It is not a good strategy when the pavement temperatures are below about 20°F at the onset of a snowfall event, or at any freezing pave- ment temperatures when the snowfall event is preceded by rain. Anti-icing with liquid chemicals is not recommended during freezing rain or sleet events. Anti-icing with solid or prewetted solid chemicals is not a good strategy when the pavement temperatures are below about 15°F at the onset of a winter weather event. The use of chemicals below these min- imum pavement temperatures will require excessive amounts of chemicals to be used as shown in Attachment 1. Anti-icing can be initiated before a winter weather event or very early in the event. By continuing the strategy throughout the event there should be a very rapid recovery or achievement of a satisfactory pavement condition after the end of the event. Anti-icing produces very high within-winter weather event and after-winter weather event LOS. DEICING Deicing is a snow and ice control strategy of removing compacted snow or ice already bonded to the pavement sur- face by chemical or mechanical means or a combination of the two. Deicing is a suitable strategy for most weather, site, and traffic conditions except when the pavement temperatures are below 20°F. Deicing operations can be accomplished at temperature lower than 20°F, but the number of chemical applications and/or chemical application rates will be exces- sive, as shown in Attachment 1, and the time to accomplish deicing will be long. Chemical treatments are usually initi- ated later in a winter weather event and continued well after the end until a satisfactory pavement condition is reached. Deicing usually produces lower within-winter weather event and after-winter weather event levels of service. Deicing usu- ally will require more chemicals than anti-icing to produce the same LOS. MECHANICAL REMOVAL OF SNOW AND ICE TOGETHER WITH FRICTION ENHANCEMENT Mechanical removal of snow and ice together with friction enhancement is a strategy in which abrasives or a mixture of abrasives and a chemical are applied to the plowed or scraped roadway surface that may have a layer of compacted snow or ice already bonded to the pavement surface. This strategy is used to provide an increased level of friction for vehicular traffic, although this increase may be short lived. Abrasives, by themselves, are not ice-control chemicals and will not sup- port the fundamental objective of either anti-icing or deicing. This strategy has been used for many years in most snow and ice situations; however, its only real applications are in very low pavement temperature situations (about 12°F) where chemical treatments are not likely to be effective and on roads having a low traffic volume and a LOS designation. This strategy is sometimes used when agencies run out of chemical deicers. The LOS expectation (within-event and after-end-of-event) for this strategy cannot be high unless there are significant amounts of ice control chemicals in the mixture, unless there

are numerous applications of mixtures containing smaller amounts of chemicals, or unless there are very high applica- tion rates of mixes containing smaller amount of chemicals. This strategy is a viable option for unpaved roads if there is no, or very little, ice control chemical in the mixture. MECHANICAL REMOVAL ALONE Mechanical removal alone is a strategy that involves the physical process of attempting to remove an accumulation of snow or ice by means such as plowing, brooming, or blow- ing without the use of snow and ice control chemicals. This strategy is strictly a physical process. This strategy is suitable for use in a variety of situations. It may be the only treatment rendered on unpaved or very low LOS roads. On higher LOS roads, it is effective when pave- ment temperatures are above freezing and snow is not bond- ing to the pavement. Similarly, when pavement temperatures are lower than about 12°F and snow is not bonding to the pavement, this is an effective treatment. This also may be the final treatment for a winter weather event after ice control chemicals or warming pavement temperatures have loosened any bonded snow or ice from the pavement. TRACTION ENHANCEMENT There are some techniques for enhancing the traction of snow/ice surfaces. Mechanical roughening, grooving, or tex- turing provides a small level of traction and directional sta- bility enhancement. This technique is not suitable for higher volume roads as the effect is short lived. It may provide an option in environmentally sensitive areas with low traffic volume. The most common technique for enhancing friction on a snow/ice surface is to apply abrasive materials such as sand, cinders, ash, tailings, and crushed stone/rock. These materials may be applied straight or with varying amounts of ice con- trol chemical in a mixture. This is thought to make them “adhere” a little better to the surface and last a little longer. A solid form of ice control chemical may be mixed into the abra- sive stockpile and allowed to age (to form a brine and coat some of the abrasive particles). A liquid chemical may be sprayed onto the abrasives while they are being applied to the road or while the stockpile is being created. Another tech- nique for enhancing the “retention” and longevity properties is to spray the abrasives with warm water as they are being dispensed onto the road. This is reported to be quite effective. Traction enhancement’s primary role is limited to lower volume roads, situations where ice control chemicals will not likely “work,” and in environmentally sensitive situa- tions where the use of ice control chemicals must be lim- ited. There are potential environmental impacts associated 12 with the (excessive) use of abrasives just as there are poten- tial environmental impacts associated with the (excessive) use of chemicals in deicing operations. Winter maintenance field personnel, as stewards of the highway and its environ- ment, must be ever vigilant in resisting the temptation to use more snow and ice control materials than are needed for the operational conditions. COMBINATIONS OF STRATEGIES Combinations of strategies are almost always used. Many winter weather events present a variety of weather and pave- ment conditions. To deal effectively with these changes, strategies and tactics need to be adapted. The most common scenario requiring changes is when pavement temperatures fall to a low level during a winter weather event. As ice con- trol chemicals become much less effective in colder temper- atures, agencies often switch from straight chemical treat- ments to treatment with abrasives or mixtures of abrasives and chemicals. Achieving stated LOS goals may require using different strategies and tactics during a single winter weather event. An example is where an agency wants a low within-winter weather event LOS and a high LOS at or after the end of a winter weather event. In this case, an agency may initially do anti-icing with ice control chemical, use only mechanical removal techniques during the event, and utilize deicing at the end of the event. The early anti-icing treatment makes the later deicing treatment more effective. SUMMARY Higher LOS can only be provided if snow/ice is not bonded to the pavement. There are only two mechanisms that will achieve this: (1) the use of ice control chemicals and (2) favor- able pavement temperatures. Using the right amount of chem- ical for the operational, weather, and pavement conditions is the most efficient and effective way to meet most higher LOS goals. Warm pavement temperatures above 32°F will usually not allow light to moderate rates of precipitation to bond. Very cold pavement temperatures, lower than about 12°F, together with dry or powder snow will usually not produce ice/pavement bond. In either case, mechanical removal alone may be all that is necessary to achieve a high LOS. There is always the potential danger of chemical residuals becoming diluted and resulting in a refreeze condition, whether at elevated or low temperatures. The material treatment design process which follows considers the impact of weather and road conditions that occur after a given treatment and before the next scheduled treatment in order to prevent refreezing of chemical solutions.