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18 CHAPTER 8 RECOMMENDED OPERATIONAL GUIDELINES FOR WINTER MAINTENANCE FIELD PERSONNEL This chapter presents operational guidelines for winter SOLID ICE CONTROL CHEMICALS maintenance field personnel on the selection of appropriate snow and ice control materials and associated application Solid ice control chemicals serve a number of functions in rates for various combinations of operating conditions. These snow and ice control operations. They are used in anti-icing, conditions include precipitation type and rate, pavement tem- in deicing, in mixing with abrasives, and in the production of perature, pavement wheelpath area condition, treatment cycle liquid ice control chemicals. time, traffic volume, and ice/pavement bond conditions. The information presented is discussed in terms of snow and ice control strategies, and tactics and their application to support Anti-icing with Solid Ice Control Chemicals LOS choices. It also complements the previous chapters describing the factors influencing the choice of materials, their Solid chemicals have been used for many years in anti- forms, and associated application rates; and the recommended icing operations. They are typically applied early in an event snow and ice control practices. before ice/pavement bond forms and then periodically through- The snow and ice control materials discussed in this chap- out the event. The first application is made when there is just ter are solid chemicals, liquid chemicals, prewetted solid enough precipitation on the roadway to minimize "bounce chemicals, abrasives, abrasive/chemical mixtures, and prewet- and scatter" and displacement by traffic. ted abrasives including abrasive/chemical mixtures. Plowing Dry solid chemicals can be used to pretreat roadways and other mechanical removal methods are necessary to sup- before a snow or ice event if applied at traffic speeds below port LOS goals and allow material treatments to be more 30 mph and with traffic volume less than about 100 vehicles effective. If needed, plowing and other mechanical removal per hour. The prewetting of a solid chemical prior to spread- methods should precede any material applications so that excess snow, slush, or ice is removed and the pavement is left ing can improve the effectiveness of the solid chemical and wet, slushy, or lightly snow-covered when treated. help the granules adhere better to the road surface. In theory, This chapter is intended as a companion and background only a sufficient amount of liquid to wet every particle of a to Attachment 1, which presents specific recommendations dry chemical is required for prewetting. The actual rate to for using road and weather information to make snow and ice achieve this wetting will vary with the particle size distribu- control treatment decisions. Attachment 2 then provides an tion. In practice it has been found that 10 to 12 gallons of a example of how to select a snow and ice control treatment sodium chloride (NaCl) solution will be sufficient for 1 ton using the treatment design procedure in Attachment 1. of dry chemical of coarse gradation (3). Some agencies have The guidance presented in this chapter and in Attachment used three times this quantity so that the material is applied 1 is based upon the results of the three winters of field test- as a slurry in order to reduce losses by traffic action. Prewet- ing of various strategy/tactic combinations by 24 highway ted finer gradations of a solid chemical will also adhere bet- agencies as described in the main part of this report. The ter to the road surface. Prewetted finer gradations of a solid guidance has been augmented with practices developed chemical may be successfully applied at traffic speeds below within the United States, where necessary, for completeness. 40 mph and with traffic volumes below about 250 vehicles State and local highway agencies engaged in snow and ice per hour. control operations on highways, roads, and streets are encour- The role of a gradation size of solid ice control chemicals aged to use the guidance in this document as a starting point during anti-icing operations is discussed earlier under the for their operations. They are encouraged also to modify the section dealing with properties of solid ice control chemicals. recommendations when necessary in order to accommodate Application rate guidance for the use of solid NaCl local experience, specific site concerns, and highway agency with anti-icing operations (unbonded case) can be found in objectives. Attachment 1.
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19 Deicing with Solid Ice Control Chemicals can help optimize the treatment effectiveness for each of these factors. With the exception of very thin ice situations, solid chemi- cals are the most effective treatment for packed/bonded snow and ice. Prewetting dry solid chemical with a liquid ice control chemical further enhances performance. Coarser graded chem- Two-Lane, Two-Way Traffic Highways (One-Lane icals do a better job of deicing thicker snow/ice accumulations. Each Way) Application rate guidance for the use of solid NaCl with deicing operations (bonded case) can be found in Attach- The most effective way to treat this highway is to spread the ice control chemical in about the middle third of the high- ment 1. way. The slope of the highway and traffic will distribute the chemical fairly quickly across the entire pavement. When Mixing Solid Ice Control Chemicals with doing simultaneous plowing operations, care must be taken Abrasives not to plow chemicals off too quickly. The spreader should be set to spread only in the plowed path. If plowing is not The mixing of solid chemicals with abrasives has been a anticipated, spread the entire middle third on the "out" run of popular practice for many years. The primary reason for this an "out and return" route. practice is to keep stockpiles of abrasives from freezing or chunking. The amount of chemicals in stockpiles is usually less than 10 percent by weight. It is also a common practice to mix higher amounts of chem- Multi-Lane Highways icals with abrasives in order to improve LOS. A popular prac- Most agencies spread ice control chemicals on multi-lane tice is to mix equal volumes of abrasives and chemical. This highways at as nearly full width as possible. Care must be mixing ratio will produce a chemical content by weight of taken not to spread beyond the pavement limits. Narrow about 42 percent with most naturally occurring abrasives. bands of material spread near the high edge of each lane are This mixture is used with anti-icing and deicing operations also effective. in some circumstances; however, anti-icing and deicing can be accomplished more effectively and more cost effectively by using straight chemicals. Hills, Curves, and Intersections Producing Liquid Ice Control Chemicals with Because of the higher traction requirements on hills, curves, Solid Ice Control Chemicals and intersections, many agencies use a higher application rate on these special sections than on straight sections of Liquid ice control chemicals are becoming increasingly highway. On lower LOS highways, these are sometimes the popular with highway maintenance agencies. In most cases a only areas that receive treatment. When making special treat- significant part of the cost of the liquid chemical is in the ment at intersections, it is important to carry the treatment transportation from the point of production to the mainte- beyond the point where traffic normally backs up in snow nance facility. As liquid chemicals are typically 50 percent to and ice conditions. 77 percent water, much of the cost is for transporting water. By purchasing solid chemicals and mixing them with water on site, significant savings can often be realized. There are systems for making "brine" that range from site-fabricated Bridges and Other Elevated Structures Not manual operations to commercially available fully automated Resting on Earth systems. Instructions for preparing salt brine are given in the Manual of Practice (3). In the fall, bridges and other elevated structures are likely to be colder than the adjacent pavement on earth. These sit- uations can occur when the structures are cooled by outgoing Application of Solid Snow and Ice Control radiation to the clear night sky even as the air temperature in Chemicals the vicinity is above freezing. At other times in the fall when there is a rapid, severe decrease in air temperature, the ele- The appropriate solid chemical application rate can be vated structures also are likely to be colder than the adjacent selected for the prevailing conditions using the guidance in pavement on earth. It is appropriate to increase the applica- Attachment 1. However, several special factors need to be tion rate on these structures so refreezing will not occur or considered in the operational treatments with solid snow and will occur at about the same time as the surrounding pave- ice control chemicals. The following application techniques ment. Toward spring, when air temperatures are warming,