Click for next page ( 28


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 27
Attachment 1 Using Road and Weather Information to Make Chemical Ice Control Treatment Decisions

OCR for page 27
A-2 ATTACHMENT 1 USING ROAD AND WEATHER INFORMATION TO MAKE CHEMICAL ICE CONTROL TREATMENT DECISIONS This Attachment contains recommended steps for using road and weather information to make snow and ice control treatment decisions. Its purpose is to define a step-by-step procedure that winter maintenance field personnel can follow in determining an appropriate treatment action to take in response to a variety of conditions. Snow and ice control material rate guidelines are presented. These application rates are based upon results of three winters of field testing various strategy/tactic combinations by 24 highway agencies. The recommended rates apply to both state and local highway agencies engaged in snow and ice control operations on highways, roads, and streets. Appropriate application rates for solid, prewetted solid, and liquid sodium chloride are given as a function of pavement temperature range, adjusted dilution potential level, and the presence or absence of ice/pavement bond. The adjusted dilution potential level accounts for precipitation type and rate, snow and ice conditions on the road, and treatment cycle time and traffic volume conditions. The recommended snow and ice control material application rates depend on atmospheric and pavement conditions at the time of treatment and on how these conditions are expected to change over the time period (window) prior to the next anticipated treatment. Implicit in the recommended treatment steps is the requirement that plowing, if needed, should be performed before chemical applications are made. This is necessary so that any excess snow, slush, or ice is removed and the pavement surface is wet, slushy, or lightly snow covered when treated. When applying solid, prewetted solid, or liquid snow and ice control chemicals, the usual intent is to achieve or maintain an unbonded, bare, or wet pavement condition. The following procedure will provide a generally successful result. STEP 1 The first step in the procedure is to determine the pavement temperature at the time of treatment and the temperature trend after treatment. A judgment, either estimated or predicted by modeling techniques, will need to be made of what the pavement temperature will be in the near term, 1 to 2 hours after treatment. This is one aspect of what is commonly called "nowcasting." Nowcasting refers to the use of real-time data, or best information available, for very short-term forecasting. It relies on the rapid transmittal of data from RWIS installations, weather radar, patrols, and other information sources for making a judgment of the probable weather and pavement condition/temperature over the next hour or two. Nowcasts can be provided by a private weather service or performed within the maintenance agency. The end result of this step in the procedure will be the determination of the "pavement temperature and trend."

OCR for page 27
A-3 STEP 2 The second step in the procedure is to establish the dilution potential that a chemical treatment must: (1) endure before another treatment is made during a winter weather event, or (2) produce a satisfactory result in the absence of precipitation at the end of an event. The establishment of the dilution potential for each treatment includes consideration of precipitation type and rate (including none), precipitation trend, the presence of various wheel path area conditions, treatment cycle time, and traffic speed and volume. The dilution potential for the precipitation at the time of treatment and its anticipated trend in the short-term is determined from Table A-1. The level of precipitation dilution potential will be either low, medium, or high. In the absence of precipitation, the dilution potential is determined from the wheel path area condition and is shown in Table A-2. STEP 3 In the third step, an adjustment to the precipitation dilution potential shown in Table A-1 may have to be made for various wheelpath area conditions. These adjustments are given in Table A-3. STEP 4 In the fourth step, an additional adjustment to the precipitation dilution potential may have to be made for treatment cycle time. This is the time between anticipated successive treatment passes. In the case of pretreating, it is the time between the onset of precipitation and the next anticipated treatment. These adjustments are given in Table A-4. STEP 5 In the fifth step, an extra adjustment to the precipitation dilution potential may have to be made for traffic speeds greater than 35 mph and traffic volume greater than 125 vehicles per hour. These adjustments are also given in Table A-4. No adjustment is made for traffic volume when traffic speeds are 35 mph or below. When making additional level adjustments to the precipitation dilution potential, an adjustment level of 1 would change a low level to a medium level or a medium level to a high level. An adjustment level of 2 would change a low level to a high level. The end result of adding various factor adjustment levels to the precipitation dilution potential is termed "adjusted dilution potential." The final adjusted dilution potential level cannot exceed "high." STEP 6 The sixth and final step in the procedure is to make a judgment of whether an ice/pavement bond condition exists. This determination (yes or no) is made based on field observations or sensor data.

OCR for page 27
A-4 TABLE A-1 Precipitation dilution potential in the presence of precipitation Precipitation rate Precipitation type Light Moderate Heavy Unknown 1. Snow 1 (powder) Low Low Medium Low 2. Snow 2 (ordinary) Low Medium High Medium 3. Snow 3 (wet/heavy) Medium High High High 4. Snow U (unknown) Medium 5. Rain Low Medium High Medium 6. Freezing rain Low Medium High Medium 7. Sleet Low Medium High Medium 8. Blowing snow Medium 9. Snow with blowing snow (Same as type of snow) 10. Freezing rain with sleet Low Medium High Medium TABLE A-2 Precipitation dilution potential in the absence of precipitation for various wheel path area conditions Precipitation Wheel path area condition Precipitation dilution potential Dry or damp Not applicable ("NA") Wet Low None Frost or black ice (thin ice) Low Slush or loose snow Medium Packed snow or thick ice High TABLE A-3 Adjustment table to precipitation dilution potential for the presence of various wheel path area conditions Increase precipitation dilution Precipitation Wheel path condition potential by number of levels Bare 0 Frost or thin ice 0 Yes Slush, loose snow, packed snow, or 1 thick ice TABLE A-4 Cycle time and traffic volume adjustments to precipitation dilution potential (final level not to exceed "high") Increase precipitation dilution Cycle time, hours potential by number of levels: 0 1.5 0 1.6 3.0 1 More than 3.0 2 For traffic speeds > 35 mph Traffic volume (vehicles per hour) Less than 125 0 More than 125 1

OCR for page 27
A-5 The appropriate application rates for solid, prewetted solid, and liquid sodium chloride can then be determined from Table A-5 using the results from the previously described steps. Calculations were performed to develop application rate data for calcium chloride (CaCl 2), magnesium chloride (MgCl2), potassium acetate (KAc), and calcium magnesium acetate (CMA), that were normalized with respect to the application rate data for dry solid NaCl. The ice melting characteristics of each chemical were used in the computations. The equivalent application rates for each of the five ice control chemicals are given in Table A-6 for a range of pavement tempera- tures. The application rates are normalized to 100 lb/LM of dry solid NaCl. A word of caution is in order concerning the use of the application rates in Table A-6. The equivalent application rates for a 23-percent concentration solution of NaCl determined from the use of Table A-6 are more conservative (larger) than those in Table A-5 for unbonded ice- pavement conditions. The liquid application rate data in Table A-6 were derived from freezing point (ice melting) data of the five chemical solutions. The liquid application rate data in Table A-5 for unbonded ice-pavement conditions were derived from field test data and include the influ- ence of such variables as precipitation type and rate, pavement wheel path conditions, maintenance treatment cycle time, and traffic volume. As such, the equivalent application rates for the five ice control chemicals in Table A-6 should be considered as starting points in determining the appro- priate rates for snow and ice control operations. Local experience should refine these values. Two forms were developed to assist in the process of selecting an appropriate treatment chemical application rate. Form 1 shown in Figure A-1 is a weather and pavement condition sheet. Here, all relevant weather and pavement data are arrayed for various points in time of interest. These time points may be: shortly before a winter weather event begins at the onset of precipitation at the beginning of each treatment cycle at the end of an event at various points in time after the winter weather event The data may come from a variety of sources. The form is intended to display all relevant weather and pavement condition data in one convenient location and format. The form could be used as a format for private sector weather forecasters to deliver their products. Form 2 shown in Figure A-2 is a snow and ice control treatment design worksheet. It was developed to assist in determining an appropriate treatment and application rate by arraying the necessary data in a logical order. Both forms could be easily computerized to assist in the treatment decision-making process in support of level of service requirements. An example of how to select a treatment using the treatment design procedure is given in Attachment 2.

OCR for page 27
A-6 TABLE A-5 Application rates for solid, prewetted solid, and liquid sodium chloride Application rate Pavement Adjusted Solid (1) Liquid (2) Temperature (F) dilution potential Ice pavement bond lb/LM gal/LM No 90 (3) 40 (3) Low Yes 200 NR (4) No 100 (3) 44 (3) Over 32 F Medium Yes 225 NR (4) No 110 (3) 48 (3) High Yes 250 NR (4) No 130 57 Low Yes 275 NR (4) No 150 66 32 to 30 Medium Yes 300 NR (4) No 160 70 High Yes 325 NR (4) No 170 74 Low Yes 350 NR (4) No 180 79 30 to 25 Medium Yes 375 NR (4) No 190 83 High Yes 400 NR (4) No 200 87 Low Yes 425 NR (4) No 210 92 25 to 20 Medium Yes 450 NR (4) No 220 96 High Yes 475 NR No 230 NR Low Yes 500 NR No 240 NR 20 to 15 Medium Yes 525 NR No 250 NR High Yes 550 NR No 260 NR Low Yes 575 NR No 270 NR 15 to 10 Medium Yes 600 NR No 280 NR High Yes 625 NR A. If unbonded, try mechanical removal without chemical. Below 10F B. If bonded, apply chemical at 700 lb /LM. Plow when slushy. Repeat as necessary. C. Apply abrasives as necessary. NR = Not recommended. Specific Notes: 1. Values for "solid" also apply to prewet solid and include the equivalent dry chemical weight in prewetting solutions. 2. Liquid values are shown for the 23-percent concentration solution. 3. In unbonded, try mechanical removal without applying chemicals. If pretreating, use this application rate. 4. If very thin ice, liquids may be applied at the unbonded rates. General Notes: 5. These application rates are starting points. Local experience should refine these recommendations. 6. Prewetting chemicals should allow application rates to be reduced by up to about 20% depending on such primary factors as spread pattern and spreading speed. 7. Application rates for chemicals other than sodium chloride will need to be adjusted using the equivalent application rates shown in Table A-6. 8. Before applying any ice control chemical, the surface should be cleared of as much snow and ice as possible.

OCR for page 27
A-7 TABLE A-6 Equivalent application rates for five ice control chemicals NaCl CaCl2 MgCl2 KAc CMA 90- 100%* 23%* 92%* 32%* 50%* 27%* 100%* 50%* 100%* 25%* Temperature Solid Liquid Solid Liquid Solid Liquid Solid Liquid Solid Liquid (F) lb/LM gal/LM lb/LM gal/LM lb/LM gal/LM lb/LM gal/LM lb/LM gal/LM 31.5 100 45 109 32 90 31 159 30 159 69 31 100 46 111 32 91 32 161 31 161 72 30.5 100 47 111 33 91 32 155 30 155 71 30 100 48 107 33 94 33 158 31 158 74 29 100 49 109 34 91 33 155 31 155 79 28 100 52 109 34 91 33 152 31 152 81 27 100 54 109 35 90 34 153 31 153 86 26 100 56 104 34 96 36 161 33 161 95 25 100 57 102 34 99 35 167 35 167 108 24 100 61 108 38 102 41 167 35 167 114 23 100 62 112 41 102 41 164 35 164 117 22 100 65 110 41 102 42 160 35 160 121 21 100 68 107 40 101 42 155 35 155 125 20 100 70 108 42 98 42 150 34 150 129 15 100 90 103 44 96 44 142 34 142 170 10 100 120 101 49 95 47 138 35 138 265 5 100 165 104 57 96 51 139 37 139 630 NaCl: Sodium chloride. CaCl2: Calcium chloride. MgCl2: Magnesium chloride. KAc: Potassium acetate. CMA: Calcium magnesium acetate. * Typical percent concentrations of the solid and liquid forms with the balance being largely water. General Notes: 1. The above application 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 temperature 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 application rate were recommended at a temperature of 20F, then switching to a 90 to 92 percent concentration of solid CaCl2 would require a slightly higher application rate of 216 lb/LM. 2. The above application rate data were derived from the freezing point (ice melting) data of the five chemical solutions. As such, the data are more conservative (larger) than field data would suggest for anti-icing operations.

OCR for page 27
A-8 Weather and Pavement Condition Sheet Date Time Forecast (F) or Actual (A) Precipitation Type Precipitation Intensity (H, M, or L) Percent Clouds Weather Data Cloud Density (H, M, or L) Radiational Effects (0, + or ) Air Temperature (F) Air Temperature Trend (0, + or ) Wind Velocity (mph) Wind Direction Relative Humidity (%) Dewpoint (F) Pavement Temperature (F) Pavement Temperature Pavement Condition Data Trend (0, + or ) Treatment Cycle Time (hr) Traffic Speed (mph) Traffic Volume (vph) Slush, Loose Snow, or Packed Snow in Wheelpath (Yes or No) Ice Pavement Bond (Yes or No) Text Forecast and Other Operational Data Figure A-1. Form 1--Example weather and pavement condition worksheet.

OCR for page 27
Agency________________________ Date__________________ Route__________________________________ Operator_________________________________ Date Time Dilution Potential Precipitation and Trend (L, M, or H) Cycle Time (0, +1, or +2) Wheel Path Condition (0 or +1) Traffic (0 or +1) Final (do not exceed H) Pavement Temperature and Trend Ice/Pavement Bond (Yes or No) Recommended Treatment Figure A-2. Form 2--Example snow and ice control treatment design worksheet.