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Publication Number: FHWA-RD-95-202
Date: June 1996

 

Manual of Practice for An Effective Anti-Icing Program

APPENDIX C. OPERATIONS GUIDE FOR MAINTENANCE FIELD PERSONNEL

C.1 INTRODUCTION

This appendix is a guide to highway anti-icing operations for maintenance field personnel. Its purpose is to suggest maintenance actions for preventing the formation or development of packed and bonded snow or bonded ice during a variety of winter weather events. It is intended to complement the decision-making and management practices of a systematic anti-icing program so that roads can be efficiently maintained in the best possible condition.

The guidance is based upon the results of four years of anti-icing field testing conducted by 15 State highway agencies and supported by the Strategic Highway Research Program (SHRP) and the Federal Highway Administration (FHWA). It has been augmented with practices developed outside the U.S., where necessary, for completeness. The recommendations are subject to refinement as U.S. highway agencies gain additional experience with anti-icing operations. Final decisions for their implementation rests with management personnel.

C.2 GUIDANCE FOR ANTI-ICING OPERATIONS

Guidance for anti-icing operations is presented in Tables 8 to 13 for six distinctive winter weather events. The six events are:

The tables suggest the appropriate maintenance action to take during an initial or subsequent (follow-up) anti-icing operation for a given precipitation or icing event. Each action is defined for a range of pavement temperatures and an associated temperature trend. For some events the operation is dependent not only on the pavement temperature and trend, but also upon the pavement surface or the traffic condition at the time of the action. Most of the maintenance actions involve the application of a chemical in either a dry solid, liquid, or prewetted solid form. Application rates ("spread rates") are given for each chemical form where appropriate. These are suggested values and should be adjusted, if necessary to achieve increased effectiveness or efficiency, for local conditions. The rates given for liquid chemicals are the equivalent dry chemical rates. Application rates in volumetric units such as L/lane-km (or gal/lane-mi) must be calculated from these dry chemical rates for each chemical and concentration.

Comments and notes are given in each table where appropriate to further guide the maintenance field personnel in their anti-icing operations.

C.3 GLOSSARY OF TERMS

Black ice. Popular term for a very thin coating of clear, bubble-free, homogeneous ice which forms on a pavement with a temperature at or slightly above 0°C (32°F) when the temperature of the air in contact with the ground is below the freezing-point of water and small slightly supercooled water droplets deposit on the surface and coalesce (flow together) before freezing.

Dry chemical spread rate. The chemical application rate. For solid applications it is simply the weight of the chemical applied per lane kilometer (or mile). For liquid applications it is the weight of the dry chemical in solution applied per lane kilometer (or mile).

Freezing rain. Supercooled droplets of liquid precipitation falling on a surface whose temperature is below or slightly above freezing, resulting in a hard, slick, generally thick coating of ice commonly called glaze or clear ice. Non-supercooled raindrops falling on a surface whose temperature is well below freezing will also result in glaze.

Frost. Also called hoarfrost. Ice crystals in the form of scales, needles, feathers or fans deposited on surfaces cooled by radiation or by other processes. The deposit may be composed of drops of dew frozen after deposition and of ice formed directly from water vapor at a temperature below 0°C (32°F) (sublimation).

Light snow. Snow falling at the rate of less than 12 mm (1/2 in) per hour; visibility is not affected adversely.

Liquid chemical. A chemical solution; the weight of the dry chemical in solution applied per lane kilometer (or mile) is the chemical application rate – the "dry chemical spread rate" – used in this appendix.

Moderate or heavy snow. Snow falling at a rate of 12 mm (1/2 in) per hour or greater; visibility may be reduced.

Sleet. A mixture of rain and of snow which has been partially melted by falling through an atmosphere with a temperature slightly above freezing.

Slush. Accumulation of snow which lies on an impervious base and is saturated with water in excess of its freely drained capacity. It will not support any weight when stepped or driven on but will "squish" until the base support is reached.

Table 8. Weather event: light snow storm.
PAVEMENT TEMPERATURE RANGE,
AND TREND
INITIAL OPERATION SUBSEQUENT OPERATIONS COMMENTS
pavement surface at time of initial operation maintenance action dry chemical spread rate, kg/lane-km (lb/lane-mi) maintenance action dry chemical spread rate, kg/lane-km (lb/lane-mi)
liquid solid or prewetted solid liquid solid or prewetted solid
Above 0°C (32°F),
steady or rising
Dry, wet, slush, or light snow cover None, see comments     None, see comments     1) Monitor pavement temperature closely for drops toward 0°C (32°F) and below
2) Treat icy patches if needed with chemical at
28 kg/lane-km (100 lb/lane-mi); plow if needed

Above 0°C (32°F),
0oC (32°F) or below is imminent;

ALSO
-7 to 0°C
(20 to 32°F),

remaining in range

Dry Apply liquid or prewetted solid chemical 28
(100)
28
(100)
Plow as needed; reapply liquid or solid chemical when needed 28
(100)
28
(100)
1) Applications will need to be more frequent at lower temperatures and higher snowfall rates
2) It is not advisable to apply a liquid chemical at the indicated spread rate when the pavementtemperature drops below -5°C (23°F)
3) Do not apply liquid chemical onto heavy snow accumulation or packed snow
Wet, slush, or light snow cover Apply liquid or solid chemical 28
(100)
28
(100)
28
(100)
28
(100)
-10 to -7°C
(15 to 20°F),

remaining in range
Dry, wet, slush, or light snow cover Apply prewetted solid chemical   55
(200)
Plow as needed; reapply prewetted solid chemical when needed   55
(200)
If sufficient moisture is present, solid chemical without prewetting can be applied
Below -10°C (15°F),
steady or falling
Dry or light snow cover Plow as needed     Plow as needed     1) It is not recommended that chemicals be applied in this temperature range
2) Abrasives can be applied to enhance traction

Notes

CHEMICAL APPLICATIONS. (1) Time initial and subsequent chemical applications to prevent deteriorating conditions or development of packed and bonded snow. (2) Apply chemical ahead of traffic rush periods occurring during storm.

PLOWING. If needed, plow before chemical applications so that excess snow, slush, or ice is removed and pavement is wet, slushy, or lightly snow covered when treated.

Table 9. Weather event: light snow storm with period(s) of moderate or heavy snow.
PAVEMENT TEMPERATURE RANGE, AND TREND INITIAL OPERATION SUBSEQUENT OPERATIONS COMMENTS
pavement surface at time of initial operation maintenance action dry chemical spread rate, kg/lane-km (lb/lane-mi) maintenance action dry chemical spread
rate, kg/lane-km
(lb/lane-mi)
liquid solid or prewetted solid liquid solid or prewetted solid
light snow heavier snow light snow heavier snow
Above 0°C (32°F),
steady or rising
Dry, wet, slush, or light snow cover None, see comments     None, see comments         1) Monitor pavement temperature closely for drops toward 0°C (32°F) and below

2) Treat icy patches if needed with chemical at 28 kg/lane-km
(100 lb/lane-mi); plow if needed
Above 0 °C (32°F), 0°C (32oF) or below is imminent;

ALSO
-4 to 0oC
(25 to 32°F),
remaining in range
Dry Apply liquid or prewetted solid chemical 28
(100)
28
(100)
Plow as needed; reapply liquid or solid chemical when needed 28
(100)
55
(200)
28
(100)
55
(200)
1) Applications will need to be more frequent at lower temperatures and higher snowfall rates 2) Do not apply liquid chemical onto heavy snow accumulation or packed snow 3) After heavier snow periods and during light snow fall, reduce chemical rate to
28 kg/lane-km (100 lb/lane-mi); continue to plow and apply chemicals as needed
Wet, slush, or light snow cover Apply liquid or solid chemical 28
(100)
28
(100)
-10 to -4°C
(15 to 25°F),
remaining in range
Dry, wet, slush, or light snow cover Apply prewetted solid chemical   55
(200)
Plow as needed; reapply prewetted solid chemical when needed     55
(200)
70
(250)
1) If sufficient moisture is present, solid chemical without prewetting can be applied

2) Reduce chemical rate to 55 kg/lane-km (200 lb/lane-mi) after heavier snow periods and during light snow fall; continue to plow and apply chemicals as needed
Below -10°C (15°F),
steady or falling
Dry or light snow cover Plow as needed     Plow as needed         1) It is not recommended that chemicals be applied in this temperature range

2) Abrasives can be applied to enhance traction

Notes

CHEMICAL APPLICATIONS. (1) Time initial and subsequent chemical applications to prevent deteriorating conditions or development of packed and bonded snow. (2) Anticipate increases in snowfall intensity. Apply higher rate treatments prior to or at the beginning of heavier snowfall periods to prevent development of packed and bonded snow. (3) Apply chemical ahead of traffic rush periods occurring during storm.
PLOWING. If needed, plow before chemical applications so that excess snow, slush, or ice is removed and pavement is wet, slushy, or lightly snow covered when treated.

Table 10. Weather event: moderate or heavy snow storm.
PAVEMENT TEMPERATURE RANGE,
AND TREND
INITIAL OPERATION SUBSEQUENT OPERATIONS COMMENTS
pavement surface at time of maintenance action
initial operation
maintenance action dry chemical spread rate, kg/lane-km (lb/lane-mi) maintenance action dry chemical spread rate, kg/lane-km (lb/lane-mi)
liquid solid or prewetted solid liquid solid or prewetted solid
Above 0°C (32°F),
steady or rising
Dry, wet, slush, or light snow cover None, see comments     None, see comments     1) Monitor pavement temperature closely for drops toward 0°C (32°F) and below
2) Treat icy patches if needed with chemical at
28 kg/lane-km (100 lb/lane-mi); plow if needed

Above 0°C (32°F),
0°C (32°F) or below is imminent;

ALSO
-1 to 0°C
(30 to 32°F),
remaining in range

Dry Apply liquid or prewetted solid chemical 28
(100)
28
(100)
Plow accumulation and reapply liquid or solid chemical as needed 28
(100)
28
(100)

1) If the desired plowing/treatment frequency cannot be maintained, the spread rate can be increased to
55 kg/lane-km (200 lb/lane-mi) to accommodate longer operational cycles

2) Do not apply liquid chemical onto heavy snow accumulation or packed snow

Wet, slush, or light snow cover Apply liquid or solid chemical 28
(100)
28
(100)
-4 to -1°C
(25 to 30°F),
remaining in range
Dry Apply liquid or prewetted solid chemical 55
(200)
42-55
(150-200)

Plow accumulation and reapply liquid or solid chemical as needed

55
(200)
55
(200)

1) If the desired plowing/treatment frequency cannot be maintained, the spread rate can be increased to 110 kg/lane-km (400 lb/lane-mi) to accommodate longer operational cycles
2) Do not apply liquid chemical onto heavy snow accumulation or packed snow

Wet, slush, or light snow cover Apply liquid or solid chemical 55
(200)
42-55
(150-200)
-10 to -4°C
(15 to 25°F),
remaining in range
Dry, wet, slush, or light snow cover Apply prewetted solid chemical   55
(200)
Plow accumulation and reapply prewetted solid chemical as needed   70
(250)
1) If the desired plowing/treatment frequency cannot be maintained, the spread rate can be increased to
140 kg/lane-km (500 lb/lane-mi) to accommodate longer operational cycles
2) If sufficient moisture is present, solid chemical without prewetting can be applied
Below -10°C (15°F),
steady or falling
Dry or light snow cover Plow as needed     Plow accumulation as needed     1) It is not recommended that chemicals be applied in this temperature range
2) Abrasives can be applied to enhance traction

Notes

CHEMICAL APPLICATIONS. (1) Time initial and subsequent chemical applications to prevent deteriorating conditions or development of packed and bonded snow -- timing and frequency of subsequent applications will be determined primarily by plowing requirements. (2) Apply chemical ahead of traffic rush periods occurring during storm.
PLOWING. Plow before chemical applications so that excess snow, slush, or ice is removed and pavement is wet, slushy, or lightly snow covered when treated.

Table 11. Weather event: frost or black ice.

PAVEMENT
TEMPERATURE RANGE,
TREND, AND RELATION TO DEW POINT

TRAFFIC
CONDITION
INITIAL OPERATION SUBSEQUENT OPERATIONS COMMENTS
maintenance action dry chemical spread rate, kg/lane-km (lb/lane-mi) maintenance action dry chemical spread rate, kg/lane-km (lb/lane-mi)
liquid solid or prewetted solid liquid solid or prewetted solid
Above 0°C (32°F),
steady or rising
Any level None, see comments     None, see comments     Monitor pavement temperature closely; begin treatment if temperature starts to fall to 0°C (32°F) or below and is at or below dew point
-2 to 2°C
(28 to 35°F),
remaining in range or falling to 0oC (32°F) or below, and equal to or below dew point
Traffic rate less than 100 vehicles per h Apply prewetted solid chemical   7-18
(25-65)
Reapply prewetted solid chemical as needed   7-18
(25-65)
1) Monitor pavement closely; if pavement becomes wet or if thin ice forms, reapply chemical at higher indicated rate
2) Do not apply liquid chemical on ice so thickthat the pavement can not be seen
Traffic rate greater than 100 vehicles per h Apply liquid or prewetted solid chemical 7-18
(25-65)
7-18
(25-65)
Reapply liquid or prewetted solid chemical as needed 11-32
(40-115)
7-18
(25-65)
-7 to -2°C
(20 to 28°F),
remaining in range, and equal to or below dew point
Any level Apply liquid or prewetted solid chemical 18-36
(65-130)
18-36
(65-130)
Reapply liquid or prewetted solid chemical when needed 18-36
(65-130)
18-36
(65-130)
1) Monitor pavement closely; if thin ice forms, reapply chemical at higher indicated rate
2) Applications will need to be more frequent at higher levels of condensation; if traffic volumes are not enough to disperse condensation, it may be necessary to increase frequency
3) It is not advisable to apply a liquid chemical at the indicated spread rate when the pavement temperature drops below -5°C (23°F)
-10 to -7°C
(15 to 20°F),
remaining in range, and equal to or below dew point
Any level Apply prewetted solid chemical   36-55
(130-200)
Reapply prewetted solid chemical when needed   36-55
(130-200)
1) Monitor pavement closely; if thin ice forms, reapply chemical at higher indicated rate
2) Applications will need to be more frequent at higher levels of condensation; if traffic volumes are not enough to disperse condensation, it may be necessary to increase frequency
Below -10°C (15°F),
steady or falling
Any level Apply abrasives     Apply abrasives as needed     It is not recommended that chemicals be applied in this temperature range

Notes
TIMING. (1) Conduct initial operation in advance of freezing. Apply liquid chemical up to 3 h in advance. Use longer advance times in this range to effect drying when traffic volume is low. Apply prewetted solid 1 to 2 h in advance. (2) In the absence of precipitation, liquid chemical at 21 kg/lane-km (75 lb/lane-mi) has been successful in preventing bridge deck icing when placed up to 4 days before freezing on higher volume roads and 7 days before on lower volume roads.

Table 12. Weather event: freezing rain storm.
PAVEMENT
TEMPERATURE
RANGE,
AND TREND
INITIAL OPERATION SUBSEQUENT OPERATIONS COMMENTS
maintenance action chemical spread rate, kg/lane-km (lb/lane-mi) maintenance action chemical spread rate,
kg/lane-km (lb/lane-mi)
Above 0°C (32°F),
steady or rising
None, see comments   None, see comments   1) Monitor pavement temperature closely for drops toward 0°C (32°F) and below
2) Treat icy patches if needed with prewetted solid chemical at 21-28 kg/lane-km (75-100 lb/lane-mi)
Above 0°C (32°F),
0oC (32°F) or below is imminent
Apply prewetted solid chemical 21-28
(75-100)
Reapply prewetted solid chemical as needed 21-28
(75-100)
Monitor pavement temperature and precipitation closely
-7 to 0°C
(20 to 32°F),

remaining in range
Apply prewetted solid chemical 21-70
(75-250)
Reapply prewetted solid chemical as needed 21-70
(75-250)
1) Monitor pavement temperature and precipitation closely
2) Increase spread rate toward higher indicated rate with decrease in pavement temperature or increase in intensity of freezing rainfall
3) Decrease spread rate toward lower indicated rate with increase in pavement temperature or decrease in intensity of freezing rainfall
-10 to -7°C
(15 to 20°F),

remaining in range
Apply prewetted solid chemical 70-110
(250-400)
Reapply prewetted solid chemical as needed 70-110
(250-400)
1) Monitor precipitation closely
2) Increase spread rate toward higher indicated rate with increase in intensity of freezing rainfall
3) Decrease spread rate toward lower indicated rate with decrease in intensity of freezing rainfall
Below -10oC (15°F),
steady or falling
Apply abrasives Apply abrasives as needed It is not recommended that chemicals be applied in this temperature range

Notes
CHEMICAL APPLICATIONS. (1) Time initial and subsequent chemical applications to prevent glaze ice conditions. (2) Apply chemical ahead of traffic rush periods occurring during storm.

Table 13. Weather event: sleet storm.

PAVEMENT TEMPERATURE RANGE,
AND TREND
INITIAL OPERATION SUBSEQUENT OPERATIONS COMMENTS
maintenance action chemical spread rate, kg/lane-km (lb/lane-mi) maintenance action chemical spread rate,
kg/lane-km (lb/lane-mi)
Above 0°C (32°F),
steady or rising
None, see comments   None, see comments   1) Monitor pavement temperature closely for drops toward 0°C (32°F) and below
2) Treat icy patches if needed with prewetted solid chemical at 35 kg/lane-km (125 lb/lane-mi)
Above 0°C (32°F),
0oC (32°F) or below is imminent
Apply prewetted solid chemical 35
(125);
Plow as needed, reapply prewetted solid chemical when needed 35
(125)
Monitor pavement temperature and precipitation closely
-2 to 0°C
(28 to 32°F),

remaining in range
Apply prewetted solid chemical 35-90
(125-325)
Plow as needed, reapply prewetted solid chemical when needed 35-90
(125-325)
1) Monitor pavement temperature and precipitation closely
2) Increase spread rate toward higher indicated rate with increase in sleet intensity
3) Decrease spread rate toward lower indicated rate with decrease in sleet intensity
-10 to -2°C
(15 to 28°F),
remaining in range
Apply prewetted solid chemical 70-110
(250-400)
Plow as needed, reapply prewetted solid chemical when needed 70-110
(250-400)
1) Monitor precipitation closely
2) Increase spread rate toward higher indicated rate with decrease in pavement temperature or increase in sleet intensity
3) Decrease spread rate toward lower indicated rate with increase in pavement temperature or decrease in sleet intensity
Below -10°C (15°F),
steady or falling
Plow as needed   Plow as needed   1) It is not recommended that chemicals be applied in this temperature range
2) Abrasives can be applied to enhance traction

Notes
CHEMICAL APPLICATIONS. (1) Time initial and subsequent chemical applications to prevent the sleet from bonding to the pavement. (2) Apply chemical ahead of traffic rush periods occurring during storm.

 

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