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

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18 CHAPTER 8 RECOMMENDED OPERATIONAL GUIDELINES FOR WINTER MAINTENANCE FIELD PERSONNEL This chapter presents operational guidelines for winter maintenance field personnel on the selection of appropriate snow and ice control materials and associated application rates for various combinations of operating conditions. These conditions include precipitation type and rate, pavement tem- perature, pavement wheelpath area condition, treatment cycle 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 LOS choices. It also complements the previous chapters describing the factors influencing the choice of materials, their forms, and associated application rates; and the recommended snow and ice control practices. The snow and ice control materials discussed in this chap- ter are solid chemicals, liquid chemicals, prewetted solid chemicals, abrasives, abrasive/chemical mixtures, and prewet- ted abrasives including abrasive/chemical mixtures. Plowing and other mechanical removal methods are necessary to sup- port LOS goals and allow material treatments to be more effective. If needed, plowing and other mechanical removal methods should precede any material applications so that excess snow, slush, or ice is removed and the pavement is left wet, slushy, or lightly snow-covered when treated. This chapter is intended as a companion and background to Attachment 1, which presents specific recommendations for using road and weather information to make snow and ice control treatment decisions. Attachment 2 then provides an example of how to select a snow and ice control treatment using the treatment design procedure in Attachment 1. The guidance presented in this chapter and in Attachment 1 is based upon the results of the three winters of field test- ing of various strategy/tactic combinations by 24 highway agencies as described in the main part of this report. The guidance has been augmented with practices developed within the United States, where necessary, for completeness. State and local highway agencies engaged in snow and ice control operations on highways, roads, and streets are encour- aged to use the guidance in this document as a starting point for their operations. They are encouraged also to modify the recommendations when necessary in order to accommodate local experience, specific site concerns, and highway agency objectives. SOLID ICE CONTROL CHEMICALS Solid ice control chemicals serve a number of functions in snow and ice control operations. They are used in anti-icing, in deicing, in mixing with abrasives, and in the production of liquid ice control chemicals. Anti-icing with Solid Ice Control Chemicals Solid chemicals have been used for many years in anti- icing operations. They are typically applied early in an event before ice/pavement bond forms and then periodically through- out the event. The first application is made when there is just enough precipitation on the roadway to minimize “bounce and scatter” and displacement by traffic. Dry solid chemicals can be used to pretreat roadways before a snow or ice event if applied at traffic speeds below 30 mph and with traffic volume less than about 100 vehicles per hour. The prewetting of a solid chemical prior to spread- ing can improve the effectiveness of the solid chemical and help the granules adhere better to the road surface. In theory, only a sufficient amount of liquid to wet every particle of a dry chemical is required for prewetting. The actual rate to achieve this wetting will vary with the particle size distribu- tion. In practice it has been found that 10 to 12 gallons of a sodium chloride (NaCl) solution will be sufficient for 1 ton of dry chemical of coarse gradation (3). Some agencies have used three times this quantity so that the material is applied as a slurry in order to reduce losses by traffic action. Prewet- ted finer gradations of a solid chemical will also adhere bet- ter to the road surface. Prewetted finer gradations of a solid chemical may be successfully applied at traffic speeds below 40 mph and with traffic volumes below about 250 vehicles per hour. The role of a gradation size of solid ice control chemicals during anti-icing operations is discussed earlier under the section dealing with properties of solid ice control chemicals. Application rate guidance for the use of solid NaCl with anti-icing operations (unbonded case) can be found in Attachment 1.

Deicing with Solid Ice Control Chemicals 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- icals do a better job of deicing thicker snow/ice accumulations. Application rate guidance for the use of solid NaCl with deicing operations (bonded case) can be found in Attach- ment 1. Mixing Solid Ice Control Chemicals with Abrasives The mixing of solid chemicals with abrasives has been a popular practice for many years. The primary reason for this 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- icals with abrasives in order to improve LOS. A popular prac- tice is to mix equal volumes of abrasives and chemical. This mixing ratio will produce a chemical content by weight of about 42 percent with most naturally occurring abrasives. This mixture is used with anti-icing and deicing operations in some circumstances; however, anti-icing and deicing can be accomplished more effectively and more cost effectively by using straight chemicals. Producing Liquid Ice Control Chemicals with Solid Ice Control Chemicals Liquid ice control chemicals are becoming increasingly popular with highway maintenance agencies. In most cases a significant part of the cost of the liquid chemical is in the transportation from the point of production to the mainte- nance facility. As liquid chemicals are typically 50 percent to 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 manual operations to commercially available fully automated systems. Instructions for preparing salt brine are given in the Manual of Practice (3). Application of Solid Snow and Ice Control Chemicals The appropriate solid chemical application rate can be selected for the prevailing conditions using the guidance in Attachment 1. However, several special factors need to be considered in the operational treatments with solid snow and ice control chemicals. The following application techniques 19 can help optimize the treatment effectiveness for each of these factors. Two-Lane, Two-Way Traffic Highways (One-Lane Each Way) The most effective way to treat this highway is to spread the ice control chemical in about the middle third of the high- way. The slope of the highway and traffic will distribute the chemical fairly quickly across the entire pavement. When doing simultaneous plowing operations, care must be taken not to plow chemicals off too quickly. The spreader should be set to spread only in the plowed path. If plowing is not anticipated, spread the entire middle third on the “out” run of an “out and return” route. Multi-Lane Highways Most agencies spread ice control chemicals on multi-lane highways at as nearly full width as possible. Care must be taken not to spread beyond the pavement limits. Narrow bands of material spread near the high edge of each lane are also effective. Hills, Curves, and Intersections Because of the higher traction requirements on hills, curves, and intersections, many agencies use a higher application rate on these special sections than on straight sections of highway. On lower LOS highways, these are sometimes the only areas that receive treatment. When making special treat- ment at intersections, it is important to carry the treatment beyond the point where traffic normally backs up in snow and ice conditions. Bridges and Other Elevated Structures Not Resting on Earth 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 radiation to the clear night sky even as the air temperature in the vicinity is above freezing. At other times in the fall when there is a rapid, severe decrease in air temperature, the ele- vated structures also are likely to be colder than the adjacent pavement on earth. It is appropriate to increase the applica- tion rate on these structures so refreezing will not occur or will occur at about the same time as the surrounding pave- ment. Toward spring, when air temperatures are warming,

structure temperatures are likely to be warmer than the sur- rounding pavement. Higher application rates on these struc- tures are not necessary in these situations. Banked or Superelevated Curves The spread pattern should be kept on the high side of superelevated curves. As the chemical goes into solution, the brine will migrate over the remainder of the roadway. Strong Crosswinds When spreading in strong crosswinds situations, the spreader should be kept upwind of the intended spread location. Spreading may not be appropriate on downwind lanes when crosswind speeds are in excess of about 25 mph. Parking Areas Spreading ice control chemicals as evenly as possible over the entire paved area is recommended for parking areas. These areas present a unique opportunity for anti-icing with solid chemicals because traffic generally will not displace them from the surface. Changes in Maintenance Jurisdiction or Level of Service Sometimes, where maintenance jurisdiction or mandated LOS changes, there will be a dramatic change in pavement conditions, including slipperiness. This is a dangerous con- dition as it is usually unexpected. Appropriate signing should be used to alert motorists of the situation, or more correctly, transitioning of the LOS treatment should be used by maintenance. Worst-Case Scenarios The worst cases usually occur when the chemical treatment is quickly overwhelmed (diluted) by excessive amounts of water or ice. Blizzard conditions (i.e., intense snowfall, wind, very cold temperatures) quickly dilute ice control chemicals and render them virtually useless. If the pavement tempera- ture going into and coming out of a blizzard is expected to be below about 12°F, then plowing–only is probably the best strategy. If it is still very cold after the blizzard, abrasives should be used as necessary until warmer temperatures will allow chemical deicing to work. If the pavement temperature throughout and after the blizzard is likely to be above 12°F, 20 a treatment with an ice control chemical before or early in the winter weather event followed by plowing–only throughout the winter weather event, will make deicing at the end of the winter weather event more efficient and cleanup will be accomplished much quicker. Rapidly accumulating freezing rain is another maintenance nightmare. The best strategy is to apply solid ice control chem- icals, at a high rate, in very narrow bands in the high-side wheel path of each lane. This approach should produce a loca- tion in each lane that will provide enough traction to allow vehicles to stop and steer. Getting the Application Right Application rates for ice control chemicals are usually specified in pound-per-lane-mile (lb/LM) or kilogram-per- lane-kilometer (kg/Lkm). Spreaders are usually calibrated to deliver pounds per mile or kilograms per kilometer (the dis- charge rate). It is important to understand that relationship in order to ensure that the proper application rate is being used. The application rate is the number of pounds or kilograms dis- pensed per mile or kilometer (the discharge rate) divided by the number of lanes being treated. Table 8 demonstrates the relationship between discharge rates and application rates. LIQUID ICE CONTROL CHEMICALS Liquid chemicals serve a number of functions in snow and ice control operations. They are used to prewet solid ice con- trol chemicals, abrasives, and abrasive/solid chemical mix- tures to make those applications more effective. Liquid chem- icals are used to pretreat and treat “colder highway spots” for frost, black ice, and localized icing. They are used as a pre- treatment for general storms to facilitate higher LOS in the initial storm phase and to “buy time” until treatments with solid chemicals can be made. They may be used also as a treatment within certain low moisture winter weather events. Liquid chemicals should generally not be used for freezing rain and sleet events nor as a treatment when pavement tem- peratures are expected to fall below about 20°F during the period of treatment effectiveness. Prewetting with Liquid Ice Control Chemicals Most commercially available liquid ice control chemicals can be used for prewetting solid ice control chemicals, abra- sives, and abrasive/solid chemical mixtures. The primary function of the liquid in prewetting is to provide the water necessary to start the brine generation process for the solid chemicals. When used on abrasives, they help them adhere to the ice surface and provide some ice control chemical to

the roadway that may at some point improve LOS. Organic based chemicals provide some corrosion protection proper- ties and environmental friendliness. Pretreating for and Treating Frost, Black Ice, and Icing with Liquid Chemicals This tactic provides arguably the best use of liquid ice con- trol chemicals. A 23-percent solution of liquid NaCl applied at 40 to 60 gal/LM (or equivalent effective amount of other chemical) has proven to provide protection from these con- ditions that are nonprecipitation events. Table 9 provides the multipliers based on a liquid NaCl application rate to achieve equivalent results with other chemicals. In the absence of 21 precipitation, these treatments are effective for at least 3 days and possibly up to 5 days depending on traffic volume. If the liquid treatment is allowed to dry before the event, it will be slightly more effective. To use the equivalency table shown in Table 9, simply mul- tiply the rate of a 23-percent solution of NaCl by the appro- priate multiplier corresponding to the temperature range and specified chemical. For example, if the treatment were to require 100 lb/LM of dry NaCl in a 23-percent solution and assuming a temperature in the range of 20° to 18°F, then it would only take 85 lb/LM of a 32-percent solution of CaCl2. However, the same temperature condition would require a rate of 194 lb/LM of a 25-percent CMA solution. Treating frost/black ice/icing that has already occurred with liquid chemicals is an excellent tactic. Using application TABLE 8 Correspondence between discharge rate and application rate Application rate in lb/LM (kg/Lkm) Number of lanes being treated Discharge rate in lb/mi (kg/km) 1 2 3 100 (28) 100 (28) 50 (14) 33 (9) 200 (56) 200 (56) 100 (28) 67 (19) 300 (84) 300 (84) 150 (42) 100 (28) 400 (112) 400 (112) 200 (56) 133 (37) 500 (140) 500 (140) 250 (70) 167 (47) 600 (168) 600 (168) 300 (84) 200 (56) 700 (196) 700 (196) 350 (98) 233 (65) 800 (224) 800 (224) 400 (112) 267 (75) TABLE 9 Multipliers for liquid chemical application rates, normalized to 100 lb/LM of dry NaCl in a 23-percent solution Temperature range (°F) 23% NaCl 32% CaCl2 27% MgCl2 50% KAc 25% CMA 32-30 1.00 1.11 0.94 1.58 1.64 30-28 1.00 1.06 0.90 1.50 1.69 28-26 1.00 1.02 0.86 1.42 1.74 26-24 1.00 0.98 0.82 1.34 1.79 24-22 1.00 0.94 0.78 1.25 1.84 22-20 1.00 0.89 0.74 1.17 1.89 20-18 1.00 0.85 0.70 1.09 1.94 18-16 1.00 0.81 0.66 1.01 1.99 16-14 1.00 0.76 0.62 0.92 2.04 14-12 1.00 0.72 0.59 0.84 2.09 12-10 1.00 0.68 0.55 0.76 – 10-8 1.00 0.63 0.51 0.67 – 8-6 1.00 0.59 0.47 0.59 – 6-4 1.00 0.55 0.43 0.51 –

rates for sodium chloride found in Attachment 1 for a low adjusted dilution potential and bonded condition will provide almost immediate results. Pretreating for and Treating General Snow and Ice Events with Liquid Chemicals The use of liquid chemicals during general snow and ice events requires more caution and information in order to achieve satisfactory results. Liquid chemicals are more sen- sitive to pavement temperature, dilution, and ice/pavement bond than solid chemicals. Analytical results were generated during the study to define the time to freeze of chemical brines as a function of application rate, pavement temperature, and rate and moisture content of precipitation. A discussion of time to freeze for chemical brines follows. Relationships Between Time to Freeze of a Chemical Brine and Controlling Variables The nature of the relationships between the time to freeze of a chemical brine and the controlling variables can be sum- marized as follows: • The time to freeze increases proportionally with chem- ical application rate for a given pavement temperature 22 and rate of precipitation. This relationship is a straight- line relationship. • The time to freeze decreases with increasing rate of pre- cipitation for a given chemical application rate and pavement temperature. However, this relationship is not a straight-line relationship. It is of the type shown in Figures 1 through 4, where the rate of decrease is high at low precipitation rates and tapers off as the rate of snowfall increases. • The time to freeze decreases nonlinearly with decreas- ing pavement temperature for a given chemical applica- tion rate and rate of precipitation. This relationship is similar to the one described in the second point above. Sample plots of the time to freeze of liquid NaCl versus snowfall precipitation rates in terms of meltwater equivalent (WE) in inches per hour and snowfall rate in inches per hour were generated to illustrate the second point above. The times to freeze for a 23-percent concentration of NaCl versus snowfall rate are presented in Figures 1 and 2. An applica- tion rate of 100 lb/LM equivalent dry NaCl was used in both figures. Figure 1 applies to a pavement temperature range of 28°F to 31.5°F. Figure 2 applies to a pavement temperature range of 20°F to 27°F. The times to freeze for the dried case of NaCl versus snow- fall rate are presented in Figures 3 and 4. Again, an applica- tion rate of 100 lb/LM equivalent dry NaCl was used in both figures. Figure 3 applies to a pavement temperature range of Figure 1. Time to freezing vs. WE/snowfall rate for a pavement temperature range of 28°F to 31.5°F using 23-percent concentration liquid NaCl.

23 Figure 2. Time to freezing vs. WE/snowfall rate for a pavement temperature range of 20°F to 27°F using 23-percent concentration liquid NaCl. Figure 3. Time to freezing vs. WE/snowfall rate for a pavement temperature range of 28°F to 31.5°F using dry NaCl.

28°F to 31.5°F. Figure 4 applies to a pavement temperature range of 20°F to 27°F. Figures 1 through 4 clearly show the limiting role that liq- uid chemicals play in snow and ice control operations as the pavement temperatures drop and application rates associated with anti-icing are used. The role of liquid chemicals for a given pavement temperature also diminishes as the snowfall rate increases. The times to freezing for the “dried” state of NaCl are longer than those for the “liquid” state, all conditions being equal. The time differences between the two chemical states do not appear to be significant from an operational consideration at the upper temperature range of 28°F to 31.5°F. The time differences increase as the pavement temperature decreases. How signif- icant the time differences are in the 20°F to 27°F temperature range is uncertain because of the small magnitude of the freez- ing times. Conversion of NaCl Application Rates to Application Rates of Four Other Snow and Ice Control Chemicals Calculations were performed to develop application rate data for calcium chloride (CaCl2), magnesium chloride (MgCl2), potassium acetate (KAc), and calcium magnesium acetate (CMA), that were normalized with respect to the 24 application rate data for dry solid NaCl. The ice melting characteristics of each chemical were used in the computa- tions. The equivalent application rates for each of the five ice control chemicals are given in Table A-6 of Attach- ment 1 for a range of pavement temperatures. The applica- tion rates are normalized to 100 lb/LM of dry solid NaCl. The application rates corresponding to a dry solid NaCl rate other than 100 lb/LM are determined by multiplying the equivalent chemical application rates for a given tempera- ture by the ratio of the desired dry solid NaCl rate to 100 lb/LM. For example, if a 200 lb/LM of dry solid NaCl appli- cation rate were recommended at a temperature of 20°F, then switching to a 90- to 92-percent concentration of solid CaCl2 would require a slightly higher application rate of 216 lb/LM. With the previous discussion in mind, liquid ice control chemicals can be effectively used in the treatment of general snow and ice events if the methodology given in Attachment 1 is utilized. Applying Liquid Chemicals to Roadway Surfaces Liquid chemicals are usually applied to the highway with spray bars or spinners. Spray bars may simply have holes in them or nozzles having various spray patterns. When using chemicals other than liquid NaCl, it is recommended that Figure 4. Time to freezing vs. WE/snowfall rate for a pavement temperature range of 20°F to 27°F using dry NaCl.

“streamer” or “pencil” nozzles or just holes in the spray bar be used to apply “strips” of chemical to the surface. The spacing of nozzles or holes should be in the range of 8 in. There have been rare circumstances when using a liquid chemical has resulted in slippery conditions in the absence of precipitation or freezing pavement temperature. This phe- nomenon seems to be related in many instances to the com- bined effect of relative humidity, pavement temperature, and chemical type. The untreated areas between strips should help minimize the potential for this type of slipperiness. 25 ABRASIVES The primary function of abrasives is to provide temporary traction (friction) improvement on snow/ice surfaces. It should be realized that snow/ice covered roadways that have been treated with abrasives provide friction values that are far less than “bare” or “wet” pavement. The application rate for abra- sives varies considerably among maintenance agencies. Appli- cation rates for most agencies fall within the 500 lb/LM to 1,500 lb/LM range with the overall average centering around 800 lb/LM.