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20 structure temperatures are likely to be warmer than the sur- a treatment with an ice control chemical before or early in the rounding pavement. Higher application rates on these struc- winter weather event followed by plowingonly throughout tures are not necessary in these situations. the winter weather event, will make deicing at the end of the winter weather event more efficient and cleanup will be accomplished much quicker. Banked or Superelevated Curves Rapidly accumulating freezing rain is another maintenance nightmare. The best strategy is to apply solid ice control chem- The spread pattern should be kept on the high side of icals, at a high rate, in very narrow bands in the high-side superelevated curves. As the chemical goes into solution, the wheel path of each lane. This approach should produce a loca- brine will migrate over the remainder of the roadway. tion in each lane that will provide enough traction to allow vehicles to stop and steer. Strong Crosswinds Getting the Application Right When spreading in strong crosswinds situations, the spreader should be kept upwind of the intended spread location. Application rates for ice control chemicals are usually Spreading may not be appropriate on downwind lanes when specified in pound-per-lane-mile (lb/LM) or kilogram-per- crosswind speeds are in excess of about 25 mph. 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. Parking Areas The application rate is the number of pounds or kilograms dis- Spreading ice control chemicals as evenly as possible over pensed per mile or kilometer (the discharge rate) divided by the entire paved area is recommended for parking areas. These the number of lanes being treated. Table 8 demonstrates the areas present a unique opportunity for anti-icing with solid relationship between discharge rates and application rates. chemicals because traffic generally will not displace them from the surface. LIQUID ICE CONTROL CHEMICALS Changes in Maintenance Jurisdiction or Level of Liquid chemicals serve a number of functions in snow and Service ice control operations. They are used to prewet solid ice con- trol chemicals, abrasives, and abrasive/solid chemical mix- Sometimes, where maintenance jurisdiction or mandated tures to make those applications more effective. Liquid chem- LOS changes, there will be a dramatic change in pavement icals are used to pretreat and treat "colder highway spots" for conditions, including slipperiness. This is a dangerous con- frost, black ice, and localized icing. They are used as a pre- dition as it is usually unexpected. Appropriate signing should treatment for general storms to facilitate higher LOS in the be used to alert motorists of the situation, or more correctly, initial storm phase and to "buy time" until treatments with transitioning of the LOS treatment should be used by solid chemicals can be made. They may be used also as a maintenance. 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- Worst-Case Scenarios peratures are expected to fall below about 20F during the period of treatment effectiveness. 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, Prewetting with Liquid Ice Control Chemicals very cold temperatures) quickly dilute ice control chemicals and render them virtually useless. If the pavement tempera- Most commercially available liquid ice control chemicals ture going into and coming out of a blizzard is expected to be can be used for prewetting solid ice control chemicals, abra- below about 12F, then plowingonly is probably the best sives, and abrasive/solid chemical mixtures. The primary strategy. If it is still very cold after the blizzard, abrasives function of the liquid in prewetting is to provide the water should be used as necessary until warmer temperatures will necessary to start the brine generation process for the solid allow chemical deicing to work. If the pavement temperature chemicals. When used on abrasives, they help them adhere throughout and after the blizzard is likely to be above 12F, to the ice surface and provide some ice control chemical to

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21 TABLE 8 Correspondence between discharge rate and application rate Application rate in lb/LM (kg/Lkm) Discharge rate Number of lanes being treated 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) the roadway that may at some point improve LOS. Organic precipitation, these treatments are effective for at least 3 days based chemicals provide some corrosion protection proper- and possibly up to 5 days depending on traffic volume. If the ties and environmental friendliness. 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- Pretreating for and Treating Frost, Black Ice, and Icing with Liquid Chemicals priate multiplier corresponding to the temperature range and specified chemical. For example, if the treatment were to This tactic provides arguably the best use of liquid ice con- require 100 lb/LM of dry NaCl in a 23-percent solution and trol chemicals. A 23-percent solution of liquid NaCl applied assuming a temperature in the range of 20 to 18F, then it at 40 to 60 gal/LM (or equivalent effective amount of other would only take 85 lb/LM of a 32-percent solution of CaCl2. chemical) has proven to provide protection from these con- However, the same temperature condition would require a ditions that are nonprecipitation events. Table 9 provides the rate of 194 lb/LM of a 25-percent CMA solution. multipliers based on a liquid NaCl application rate to achieve Treating frost/black ice/icing that has already occurred equivalent results with other chemicals. In the absence of with liquid chemicals is an excellent tactic. Using application 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

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22 rates for sodium chloride found in Attachment 1 for a low and rate of precipitation. This relationship is a straight- adjusted dilution potential and bonded condition will provide line relationship. almost immediate results. 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 Pretreating for and Treating General Snow and Ice Events with Liquid Chemicals Figures 1 through 4, where the rate of decrease is high at low precipitation rates and tapers off as the rate of The use of liquid chemicals during general snow and ice snowfall increases. events requires more caution and information in order to The time to freeze decreases nonlinearly with decreas- achieve satisfactory results. Liquid chemicals are more sen- ing pavement temperature for a given chemical applica- sitive to pavement temperature, dilution, and ice/pavement tion rate and rate of precipitation. This relationship is bond than solid chemicals. Analytical results were generated similar to the one described in the second point above. during the study to define the time to freeze of chemical brines as a function of application rate, pavement temperature, and Sample plots of the time to freeze of liquid NaCl versus rate and moisture content of precipitation. A discussion of snowfall precipitation rates in terms of meltwater equivalent time to freeze for chemical brines follows. (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 Relationships Between Time to Freeze of a snowfall rate are presented in Figures 1 and 2. An applica- Chemical Brine and Controlling Variables tion rate of 100 lb/LM equivalent dry NaCl was used in both figures. Figure 1 applies to a pavement temperature range of The nature of the relationships between the time to freeze 28F to 31.5F. Figure 2 applies to a pavement temperature of a chemical brine and the controlling variables can be sum- range of 20F to 27F. marized as follows: The times to freeze for the dried case of NaCl versus snow- fall rate are presented in Figures 3 and 4. Again, an applica- The time to freeze increases proportionally with chem- tion rate of 100 lb/LM equivalent dry NaCl was used in both ical application rate for a given pavement temperature 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 28F to 31.5F using 23-percent concentration liquid NaCl.

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

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24 Figure 4. Time to freezing vs. WE/snowfall rate for a pavement temperature range of 20F to 27F using dry NaCl. 28F to 31.5F. Figure 4 applies to a pavement temperature application rate data for dry solid NaCl. The ice melting range of 20F to 27F. characteristics of each chemical were used in the computa- Figures 1 through 4 clearly show the limiting role that liq- tions. The equivalent application rates for each of the five uid chemicals play in snow and ice control operations as the ice control chemicals are given in Table A-6 of Attach- pavement temperatures drop and application rates associated ment 1 for a range of pavement temperatures. The applica- with anti-icing are used. The role of liquid chemicals for a tion rates are normalized to 100 lb/LM of dry solid NaCl. given pavement temperature also diminishes as the snowfall The application rates corresponding to a dry solid NaCl rate rate increases. other than 100 lb/LM are determined by multiplying the The times to freezing for the "dried" state of NaCl are longer equivalent chemical application rates for a given tempera- than those for the "liquid" state, all conditions being equal. The ture by the ratio of the desired dry solid NaCl rate to 100 time differences between the two chemical states do not appear lb/LM. For example, if a 200 lb/LM of dry solid NaCl appli- to be significant from an operational consideration at the upper cation rate were recommended at a temperature of 20F, temperature range of 28F to 31.5F. The time differences then switching to a 90- to 92-percent concentration of solid increase as the pavement temperature decreases. How signif- CaCl 2 would require a slightly higher application rate of icant the time differences are in the 20F to 27F temperature 216 lb/LM. range is uncertain because of the small magnitude of the freez- With the previous discussion in mind, liquid ice control ing times. chemicals can be effectively used in the treatment of general snow and ice events if the methodology given in Attachment 1 is utilized. Conversion of NaCl Application Rates to Application Rates of Four Other Snow and Ice Applying Liquid Chemicals to Roadway Control Chemicals Surfaces Calculations were performed to develop application rate Liquid chemicals are usually applied to the highway with data for calcium chloride (CaCl2), magnesium chloride spray bars or spinners. Spray bars may simply have holes in (MgCl2), potassium acetate (KAc), and calcium magnesium them or nozzles having various spray patterns. When using acetate (CMA), that were normalized with respect to the chemicals other than liquid NaCl, it is recommended that