133.5 Operator’s Guide for Anti-Icing
Anti-icing is the snow and ice control practice of preventing the development of a bond between snow and/or ice and the pavement surface with the timely application of salt. Applying the right amount of salt at the right time will make snow removal operations more efficient and produce safer driving conditions during winter storms.
This article provides guidelines for the application of salt to the roadway for a variety of winter storm conditions. The timing and application rates were developed under the SHRP/FHWA Research Program, in which MoDOT participated. Many Maintenance personnel who have tested these procedures have found them to be valuable in their winter maintenance operations. The statewide implementation of this technology will help produce safer driving conditions on MoDOT’s continuous operations routes during winter months.
133.5.1 Operations Guide for Maintenance Field Personnel
This is a guide to highway anti-icing operations for Maintenance field personnel. Its purpose is to recommend maintenance actions for preventing the formation or development of packed and bonded snow or bonded ice on the continuous operations routes 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 continuous operations routes can be efficiently maintained in the best possible condition.
These guidelines are based on the results of four years of anti-icing field testing conducted by 15 state DOTs, including MoDOT, and is supported by the Strategic Highway Research Program (SHRP) and the Federal Highway Administration (FHWA). Since then, it has been augmented with many additional years of anti-icing experience in different parts of our state.
Guidance for anti-icing operations during five winter weather events is available. The five events are:
- Type 5 Winter Event: Frost, flurries, freezing fog, blowing snow & refreeze
- Type 4 Winter Event: Dusting to 1 in. of snow, sleet or other frozen precipitation
- Type 3 Winter Event: 1 – 6 in. of snow/frozen precipitation in 24 hours OR a trace to ½ in. of ice
- Type 2 Winter Event: 6 – 12 in. of snow in 24 hours OR ½ to ¾ in. of ice
- Type 1 Winter Event: More than 12 in. of snow in 24 hours OR more than ¾ in. of ice
| Related Information |
| MoDOT Snow Academy Official Guide, Participant's Manual and Snow Academy Website |
The tables suggest the appropriate maintenance action to take during initial and follow-up anti-icing operations 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. Many of the maintenance actions involve the application of salt in either a dry solid, pre-wetted or brine (liquid) form. Pre-wetted solid and brine are the two primary forms on which we need to concentrate. Application rates are given for each form where appropriate. These are suggested rates and should be adjusted, if necessary, to achieve the effectiveness for local conditions.
Comments and notes are given in each table where appropriate to further guide field maintenance personnel for their anti-icing operations.
133.5.2 Glossary
Black ice. Popular term for a very thin coating of clear, bubble-free, homogenous ice which forms on a pavement with a temperature at or slightly above 32º 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.
Brine. Water saturated with common salt (NaCl), also liquid salt or liquid chemical.
Continuous Operations Routes. This system includes all major highways, minor highways with traffic volumes of 2,500 ADT or greater and other urban and community routes designated by the district in consultation with the Maintenance Division. This also includes all designated incident bypass routes.
These routes will receive plowing and application of snow and ice control treatments on an as needed basis, throughout the storm until all lanes are restored to a near normal condition.
Dew Point. The temperature that a vapor begins to condense.
Freezing rain. Super-cooled 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-super-cooled 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 0º C (32º F) (sublimation).
Light Snow. Snow falling at the rate of less than 1/2 in. per hour; visibility is not affected adversely.
Moderate or heavy snow. Snow falling at a rate of 1/2 in. per hour or greater; visibility may be reduced.
Pre-treatment. This is the practice of applying salt brine at 44 gallons per lane mile to dry pavement prior to the winter event, or the application of pre-wetted salt to the surface prior to snow and ice bonding to the pavement.
Pre-wetting. Pre-wetting is the practice of applying salt brine to dry salt before it is placed on the pavement, and should be done at 10 to 15 gallons per ton. Liquid calcium chloride may be used for pre-wetting salt at temperatures below 15° F.
Sleet. A mixture of rain and of snow that has been partially melted by falling through an atmosphere with a temperature slightly above freezing.
Slush. Accumulation of snow that 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.
Spread Rate. The salt application rate in either the solid or liquid form. For solid applications it is simply the weight of the salt applied per lane mile. For liquid applications it is the volume (gallons) of brine applied per lane mile.
133.5.3 Tables
Table 133.5.3.1 How to Use Liquid Anti-Icers
| Temperature, ° F | One Pound of Salt (sodium chloride) |
|---|---|
| 30 | 46.3 lbs. of ice |
| 25 | 14.4 lbs. of ice |
| 20 | 8.6 lbs. of ice |
| 15 | 6.3 lbs. of ice |
| 10 | 4.9 lbs. of ice |
| 5 | 4.1 lbs. of ice |
| 0 | 3.7 lbs. of ice |
| Printable pdf of "Equivalent Salt Spread Rates" |
Table 133.5.3.2 Equivalent Salt Spread Rates
| Solid or Pre-wetted Solid (lbs./lane-mile) | Salt Brine, 23% Concentration NaCL (gallon/lane-mile) |
|---|---|
| 25 | 11 |
| 50 | 22 |
| 75 | 33 |
| 100 | 44 |
| 125 | 55 |
| 150 | 65 |
| 200 | 87 |
| Printable pdf of "Pure salt concentration and corresponding specific gravity (measured by a hydrometer) at 59º F" |
Table 133.5.3.3 Pure salt concentration and corresponding specific gravity (measured by a hydrometer) at 59º F
| Percent salt | Specific gravity at 59º F | Percent of saturation | Weight 1 of salt, lb/gal |
|---|---|---|---|
| 0 | 1.000 | 0 | 0 |
| 5 | 1.035 | 20 | 0.43 |
| 6 | 1.043 | 24 | 0.52 |
| 7 | 1.050 | 28 | 0.61 |
| 8 | 1.057 | 32 | 0.71 |
| 9 | 1.065 | 36 | 0.80 |
| 10 | 1.072 | 40 | 0.90 |
| 11 | 1.080 | 44 | 0.99 |
| 12 | 1.087 | 48 | 1.00 |
| 13 | 1.095 | 52 | 1.10 |
| 14 | 1.103 | 56 | 1.29 |
| 15 | 1.111 | 60 | 1.39 |
| 16 | 1.118 | 63 | 1.49 |
| 17 | 1.126 | 67 | 1.60 |
| 18 | 1.134 | 71 | 1.71 |
| 19 | 1.142 | 75 | 1.81 |
| 20 | 1.150 | 79 | 1.92 |
| 21 | 1.158 | 83 | 2.03 |
| 22 | 1.166 | 87 | 2.14 |
| 23 | 1.175 | 91 | 2.26 |
| 24 | 1.183 | 95 | 2.37 |
| 25 | 1.191 | 99 | 2.45 |
| 25.2 | 1.200 | 100 | |
| 1Note: Weight of commercial salt required = (weight of pure NaCl from table) ÷ (purity in percent) | |||
| Printable pdf of "Gradation of salt specified by ASTM D 632 and MoDOT" |
Table 133.5.3.4 Gradation of salt specified by ASTM D 632 & MoDOT
| Sieve size | Weight % passing | ||
|---|---|---|---|
| ASTM Gr. 1 | ASTM Gr. 2 | MoDOT | |
| 3/4 in. | - | 100 | - |
| 1/2 in. | 100 | - | 100 |
| 3/8 in. | 95-100 | - | 95-100 |
| No. 4 | 20-90 | 29-100 | 15-95 |
| No. 8 | 10-60 | 10-60 | 5-65 |
| No. 30 | 0-15 | 0-15 | 0-15 |
| Note: ASTM Gr. 1 is the most commonly used gradation in the U.S. | |||
| Printable pdf of "Proportions for preparing sodium chloride solution from commercial grade salt" |
Table 133.5.3.5 Proportions for preparing sodium chloride solution from commercial grade salt (i.e., up to 5 percent impurities)
| Actual %NaCl | Weight NaCl | Crystalization temperature, º F | Weight per unit volume of solution, lb/gal | |
|---|---|---|---|---|
| per volume solution, lb/gal | per volume water, lb/gal | |||
| 10 | 0.9 | 0.8 | 20 | 8.95 |
| 15 | 1.4 | 1.3 | 12 | 9.28 |
| 20 | 1.9 | 1.7 | 0 | 9.6 |
| 231 | 2.3 | 1.9 | -6 | 9.76 |
| 25 | 2.5 | 2.1 | -16 | 10.3 |
| 1 Note: This is the approximate eutectic composition, i.e., the composition that results in the lowest temperature at which a solution can exist while remaining completely liquid. | ||||
133.5.3.6 Tables for Continuous Operations Routes
| Printable file for "Type 5 Winter Event" |
Table 133.5.3.6.1 Type 5 Winter Event: Frost, Flurries, Freezing Fog, Blowing Snow and Refreeze
| Pavement Temperature Range and Trend | Traffic Condition | Initial Operation | Subsequent Operations | Comments | ||||
|---|---|---|---|---|---|---|---|---|
| Maintenance Action | Spread Rates | Maintenance Action | Spread Rates | |||||
| Pre-wetted solid salt (lb/lane-mile) | Brine (gal/lane-mile) | Pre-wetted solid salt (lb/lane-mile) | Brine (gal/lane-mile) | |||||
| Above 32º F, steady or rising | Any level | None, see comments | - | - | None, see comments | - | - | 1) Monitor pavement temperature closely; begin treatment if starts to fall to 32ºF and below and is at or below dew point. |
| 28º F to 32º F, remaining in range or falling to 32º F or below, and equal to or below dew point | Traffic rate less than 100 vehicles per hr | Apply brine or pre-wetted solid salt | 25-65 | 11-28 | Reapply pre-wetted solid salt as needed | 25-65 | - | 1) Monitor pavement closely; if pavement becomes wet or if thin ice forms, reapply salt at higher indicated rate.
2) Do not apply brine on ice so thick that the pavement cannot be seen. |
| Traffic rate greater than 100 vehicles per hr | Apply brine or pre-wetted solid salt | 25-65 | 11-28 | Reapply brine pre-wetted solid salt as needed | 25-65 | 11-28 | ||
| 20º to 28º F, remaining in range and equal to or below dew point | Any level | Apply brine or pre-wetted solid salt | 65-130 | 28-57 | Reapply brine pre-wetted solid salt as needed | 65-130 | 28-57 | 1) Monitor pavement closely; if thin ice forms, reapply salt 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 brine at the indicated spread rate when the pavement temperature drops below 20ºF. |
| 10º to 20º F, remaining in range and equal to or below dew point | Any level | Apply pre-wetted solid salt | 130-200 | - | Reapply pre-wetted solid salt as needed | 130-200 | - | 1) Monitor pavement closely; if thin ice forms, reapply salt 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 0º F, steady or falling | Any level | Apply abrasives | - | - | Apply abrasives as needed | - | - | 1) Monitor pavement closely, salt will have limited melting power in this temperature range.
2) Liquid calcium chloride may be used for pre-wetting salt/abrasive mix at colder temperatures. |
| Notes: TIMING. 1) Conduct initial operation in advance of freezing. Apply brine up to 3 hr in advance. Use longer advance times in this range to effect drying when traffic volume is low. Apply pre-wetted solid salt 1 to 2 hr in advance. 2) In the absence of precipitation, brine at 33 gal/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. | ||||||||
| Printable file for "Type 4 Winter Event" |
Table 133.5.3.6.2 Type 4 Winter Event: Dusting to 1 in. of snow, sleet, or other frozen precipitation
| Pavement Temperature Range and Trend | Initial Operation | Subsequent Operations | Comments | |||||
|---|---|---|---|---|---|---|---|---|
| Pavement surface at time of initial operation | Maintenance action | Salt spread rates | Maintenance action | Salt spread rates | ||||
| Pre-wetted solid salt (lb/lane-mile) | Brine (gal/lane-mile) | Pre-wetted solid salt (lb/lane-mile) | Brine (gal/lane-mile) | |||||
| Above 32º 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 32º F and below.
2) Treat icy patches if needed with pre-wetted solid salt at 100 lb/lane-mile; plow if needed. |
| Above 32º F, 32º F or below is imminent; ALSO 15º to 32º F, remaining in range | Dry | Apply brine or pre-wetted salt | 100 | 44 | Plow as needed, reapply liquid or solid chemical when needed | 100 | 44 | 1) Applications will need to be more frequent at lower temperatures and higher snowfall rates
2) It is not advisable to apply a straight brine at the indicated spread rate when the pavement temperature drops below 20ºF 3) Do not apply brine onto heavy snow accumulation or packed snow |
| Wet, slush, or light snow cover | Apply liquid or solid salt | 100 | 44 | |||||
| 0º to 15º F, remaining in range | Dry, wet, slush or light snow cover | Apply pre-wetted solid chemical | 200 | - | Plow as needed, reapply pre-wetted solid chemical when needed | 200 | - | 1) Abrasives may be added to the salt to enhance traction at colder temperatures
2) Liquid calcium chloride may be used for pre-wetting solid salt at colder temperatures |
| Below 0º F, steady or falling | Dry or light snow cover | Plow as needed | - | - | Plow and apply salt/abrasive mix as needed | - | - | 1) 1 If pavement becomes slick apply salt/abrasive mix to enhance traction. Salt will have limited melting power in this temperature range.
2) Pre-wet salt/abrasive mix with liquid calcium chloride. |
| Notes: SALT APPLICATIONS. 1) Time initial and subsequent chemical applications to prevent deteriorating conditions or development of packed and bonded snow. 2) Apply salt ahead of traffic rush periods occurring during storm.
PLOWING. If needed, plow before salt applications so that excess snow, slush, or ice is removed and pavement is wet, slushy, or lightly snow covered when treated. | ||||||||
| Printable file for "Type 3 Winter Event" |
Table 133.5.3.6.3 Type 3 Winter Event: 1 – 6 in. of snow/frozen precipitation in 24 hours OR a trace to 1/2 in. of ice
| Pavement Temperature Range and Trend | Initial Operation | Subsequent Operations | Comments | |||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Pavement surface at time of initial operation | Maintenance action | Salt spread rates | Maintenance action | Salt spread rates | ||||||
| Pre-wetted solid salt (lb/lane-mile) | Brine (gal/lane-mile) | Pre-wetted solid salt (lb/lane-mile) | Brine (gal/lane-mile) | |||||||
| Light snow | Heavier snow | Light snow | Heavier snow | |||||||
| Above 32º 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 32º F and below.
2) Treat slick patches if needed with pre-wetted salt at 100 lb/lane-mile or brine 44 gal/lane-mile; plow if needed. |
| Above 32º F, 32º F or below is imminent; ALSO 20º to 32º F, remaining in range | Dry | Apply brine or pre-wetted salt | 100 | 44 | Plow as needed, reapply brine or pre-wetted solid salt when needed | 100 | 200 | 44 | 88 | 1) Applications will need to be more frequent at lower temperatures and higher snowfall rates.
2) Do not apply brine onto heavy snow accumulation or packed snow. 3) After heavier snow periods and during light snowfall, reduce salt rate to 100 lb./lane-mile or 44 gal./lane-mile brine; continue to plow and apply salt as needed |
| Wet, slush, or light snow cover | Apply brine or pre-wetted salt | 100 | 44 | |||||||
| 10º to 20º F, remaining in range | Dry, wet, slush or light snow cover | Apply pre-wetted salt | 200 | - | Plow as needed, reapply pre-wetted solid salt when needed | 200 | 250 | - | - | 1) Reduce salt rate to 200 lb./lane-mile after heavier snow periods and during light snowfall; continue to plow and apply salt as needed.
2) Liquid calcium chloride may be used for pre-wetted salt at colder temperatures |
| Below 10º F, steady or falling | Dry or light snow cover | Plow as needed | - | - | Plow and apply salt/abrasive mix as needed | - | - | - | - | As pavement becomes slick, apply salt/abrasive mix to enhance traction. Salt will have limited melting power at these temperatures. |
| Notes: SALT 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 salt ahead of traffic rush periods occurring during storm.
PLOWING. If needed, plow before salt applications so that excess snow, slush or ice is removed and pavement is wet, slushy or lightly snow-covered when treated. | ||||||||||
| Printable file for "Type 2 Winter Event" |
Table 133.5.3.6.4 Type 2 Winter Event: 6 – 12 in. of snow in 24 hours OR ½ to ¾ in. of ice
| Pavement Temperature Range and Trend | Initial Operation | Subsequent Operations | Comments | |||||
|---|---|---|---|---|---|---|---|---|
| Pavement surface at time of initial operation | Maintenance action | Salt spread rates | Maintenance action | Salt spread rates | ||||
| Pre-wetted solid salt (lb/lane-mail) | Brine (gal/lane-mile) | Pre-wetted solid salt (lb/lane-mile) | Brine (gal/lane-mile) | |||||
| Above 32º 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 32º F and below.
2) Treat slick patches if needed with pre-wetted solid salt at 100 lb/lane-mile or with brine at 44 gal/lane-mile; plow if needed. |
| Above 32º F, 32º F or below is imminent; ALSO 30º to 32º F, remaining in range | Dry | Apply brine or pre-wetted solid salt | 100 | 44 | Plow accumulation and reapply brine or solid salt as needed | 100 | 44 | 1) If the desired plowing/treatment frequency cannot be maintained, the spread rate can be increased to 200 lb/lane-mile to accommodate longer operational cycles.
2) Do not apply brine onto heavy snow accumulation or packed snow. |
| Wet, slush or light snow cover | Apply brine or pre-wetted solid salt | 100 | 44 | |||||
| 20º to 30º F, remaining in range | Dry | Apply brine or pre-wetted solid salt | 150-200 | 65-87 | Plow accumulation and reapply brine or solid salt as needed | 200 | 87 | 1) If the desired plowing/treatment frequency cannot be maintained, the spread rate can be increased to 400 lb/lane-mile to accommodate longer operational cycles.
2) Do not apply brine onto heavy snow accumulation or packed snow. |
| Wet, slush or light snow cover | Apply brine or pre-wetted solid salt | 150-200 | 65-87 | |||||
| 10º to 20º F, remaining in range | Dry, wet, slush or light snow cover | Apply pre-wetted solid salt | 200 | - | Plow accumulation and reapply brine or solid salt as needed | 250 | - | 1) If the desired plowing/treatment frequency cannot be maintained, the spread rate can be increased to 500 lb/lane-mile to accommodate longer operational cycles.
2) Liquid calcium chloride may be used for pre-wetting salt at colder temperatures. |
| Below 10º F, steady or falling | Dry or light snow cover | Plow as needed | - | - | Plow accumulation as needed | 250 | - | As pavement becomes slick, apply salt/abrasive mix to enhance traction. Salt will have limited melting power in this temperature range. |
| Notes. SALT APPLICATIONS. 1) Time initial and subsequent salt applications to prevent deteriorating conditions or development of packed and bonded snow – timing and frequency of applications will be determined primarily by plowing requirements. 2) Apply salt 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. | ||||||||
| Printable file for "Type 1 Winter Event" |
Table 133.5.3.6.5 Type 1 Winter Event: More than 12 inches of snow in 24 hours OR more than ¾ inch of ice
| Pavement Temperature Range and Trend | Initial Operation | Subsequent Operations | Comments | |||||
|---|---|---|---|---|---|---|---|---|
| Pavement surface at time of initial operation | Maintenance action | Salt spread rates | Maintenance action | Salt spread rates | ||||
| Pre-wetted solid salt (lb/lane-mail) | Brine (gal/lane-mile) | Pre-wetted solid salt (lb/lane-mile) | Brine (gal/lane-mile) | |||||
| Above 32º 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 32º F and below
2) Treat slick patches if needed with pre-wetted salt at 100 lb/lane-mi or with brine at 44 gal/ln-mi; plow if needed. |
| Above 32º F, 32º F or below is imminent; ALSO 30º to 32º F, remaining in range | Dry | Apply brine or pre-wetted solid salt | 100 | 44 | Plow accumulation and reapply brine or solid salt as needed | 100 | 44 | 1) If the desired plowing/treatment frequency cannot be maintained, the spread rate can be increased to 200 lb/lane-mi to accommodate longer operational cycles.
2) Do not apply brine onto heavy snow accumulation or packed snow. |
| Wet, slush, or light snow cover | Apply brine or pre-wetted solid salt | 100 | 44 | |||||
| 20º to 30º F, remaining in range | Dry | Apply brine or pre-wetted solid salt | 150-200 | 65-87 | Plow accumulation and reapply brine or solid salt as needed | 200 | 87 | 1) If the desired plowing/treatment frequency cannot be maintained, the spread rate can be increased to 400 lb/lane-mi to accommodate longer operational cycles.
2) Do not apply brine onto heavy snow accumulation or packed snow. |
| Wet, slush, or light snow cover | Apply brine or pre-wetted solid salt | 150-200 | 65-87 | |||||
| 10º to 20º F, remaining in range | Dry, wet, slush, or light snow cover | Apply pre-wetted solid salt | 200 | - | Plow accumulation and reapply brine or solid salt as needed | 250 | - | 1) If the desired plowing/treatment frequency cannot be maintained, the spread rate can be increased to 500 lb/lane-mi to accommodate longer operational cycles.
2) Liquid calcium chloride may be used for pre-wetting salt at colder temperatures |
| Below 10º F, steady or falling | Dry or light snow cover | Plow as needed | - | - | Plow accumulation as needed | 250 | - | As pavement becomes slick apply salt/abrasive mix to enhance traction. Salt will have limited melting power in this temperature range. |
| Notes: SALT APPLICATIONS. 1) Time initial and subsequent salt 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 salt 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. | ||||||||