U.S. Department of Transportation
Federal Highway Administration
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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
|This report is an archived publication and may contain dated technical, contact, and link information|
|Publication Number: FHWA-RD-95-202 Date: June 1996|
Publication Number: FHWA-RD-95-202
Date: June 1996
This section presents general recommendations for successful anti-icing practices that can be employed for various combinations of precipitation, pavement temperature, traffic volumes, and mandated levels of service. It presents information that will assist in the development of a systematic anti-icing program, and complements Section 3 on the capabilities, information sources, and procedures that make up the tools of an anti-icing program. It contains four major components: initial operations, subsequent operations, special considerations, and post-storm assessment of operations.
The section is intended as a companion and background document to Appendix C, which presents specific and concise recommendations for anti-icing operations for six weather events. These are:
Guidance on maintenance actions for each event is provided in the Appendix C tables for several pavement temperature ranges and for initial and subsequent operations. Temperature trend, an important factor, is also indicated. Solid, liquid, and prewetted solid chemical application rates are suggested where appropriate. These rates are not to be considered as fixed values but rather the middle of a range to be selected by an agency according to its local conditions and experience. Traffic volumes have not been found to have a consistent or dominant influence on pavement condition or traction to suggest varying chemical application rates except in the case of frost and black ice, and that category is the only one incorporating traffic as an operational consideration. Special considerations associated with maintenance actions are listed in the Comments and Notes given on the tables.
The guidance presented in this section and in Appendix C is based upon the results of 4 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. However, no short discussion or list of recommendations can completely cover the range of conditions facing agencies continent wide. Therefore every agency is encouraged to use the guidance of this document as a starting point for developing its own anti-icing program, and to modify the recommendations when necessary in order to accommodate local experience, specific site concerns, and agency objectives. Careful recording of conditions and the operations made in response will provide the basis for fine-tuning the program for specific service levels and conditions.
The initial anti-icing operation is most often the application of a chemical freezing-point depressant to the pavement before enough snow has accumulated to keep the chemical from reaching the pavement. However, before this action is taken, information about the nature and characteristics of the anticipated storm should be assembled and a decision made concerning the action. Each of these steps is discussed next.
4.1.1 Information assembly
Several pieces of information need to be assembled upon first notice that a winter storm or frost/black ice event may affect the maintenance area. This information includes weather forecasts, weather radar data, satellite data, local road condition and RWIS data, pavement temperature forecasts, and any RWIS data from areas outside the immediate maintenance jurisdiction that might have already have been affected by the approaching storm. The information must be reviewed to estimate when and where the event will begin, its extent, and severity.
4.1.2 Decision point
The decision on whether or not to initiate a treatment, when to start it and what type of treatment to apply can be made after the review is made of the information assembled. The decision is based on when precipitation is expected to start, what form it will be, the probable air and pavement temperatures, the anticipated trend of the temperatures, the expected sky conditions, the wind speed and direction, and the intended timing of the treatment.
22.214.171.124 Apply chemical
Either dry solid chemicals, liquid chemicals, or prewetted solid chemicals can be used as an initial anti-icing treatment. Whichever is used, the timing of the application should be consistent with the underlying objective of preventing the formation or development of bonded snow or ice, and should reflect an underlying readiness consistent with a preventive strategy. That is, it should be made in anticipation of or in prompt response to worsening pavement conditions. Applications in advance of snowfall are not necessary for preventing bonded snowpack, but early applications when the pavement condition is no worse than wet, slushy, or lightly snow covered are for the most part necessary for anti-icing success. As this may not always be possible, for example because of a limited fleet or heavy traffic, pretreating the road before a snowstorm may be the only way to ensure that all areas are treated before conditions deteriorate.
Residual chemical from previous operations has a short-lived effect on highway conditions at the beginning of storms, and should not be relied upon for timing of initial anti-icing operations without independent indications of adequate chemical concentration.
Recommendations for use of liquid chemicals
Guidance for initial liquid treatments is given in Appendix C for four events: light snow storm; light snow storm with period(s) of moderate or heavy snow; moderate or heavy snow storm; and frost or black ice. The use of a liquid is not recommended during either a freezing rain or sleet storm because of the large quantity needed to retain an effective concentration. The suggested application rates are the equivalent dry chemical rates. All are given only for pavement temperatures at -5°C (23°F) and above because of the ineffectiveness of the indicated rates at pavement temperatures below -5°C (23°F).
For snowstorms, initial liquid applications can be made either as a "pretreatment" in advance of the storm or as an "early-storm treatment," i.e., soon after snowfall has begun and/or when the pavement temperature is dropping toward freezing. A pretreatment can be made well ahead of a storm as long as the storm does not start out with above freezing temperatures and rain, washing the chemical away. In the case of early-storm treatment, the application may be made onto dry, wet, light slush, or lightly snow covered pavement. Late applications onto pavements with more than a light covering of slush or snow can result in excessive dilution of the chemical, and risk failure. These should always be coordinated with plowing.
Relative to conventional practices in which surfaces are not treated until later in a storm, benefits from liquid pretreatments can include higher friction and better pavement conditions early in a storm. These benefits are generally short-lived, however, and should not be expected over a long period. Subsequent chemical applications should be made as soon as conditions begin to deteriorate. In essence, pretreatments can be thought of as “buying time” in the earliest stages of a storm until subsequent chemical applications become effective.
For preventing the formation of frost or black ice -- caused by radiational cooling of the pavement in the presence of high humidity -- the chemical should be applied in advance of the expected time of ice formation so that the water component of the brine will evaporate or be removed by traffic action. This will leave only the chemical on the road surface, and thus result in the greatest concentration when frost or black ice conditions occur. Of the recommended frost or black ice treatments given in Appendix C, traffic condition is a dominant factor only in the temperature range of -2 to 2°C (28 to 35°F).
As an example of frost prevention, one agency has found that a solution of 27 percent MgCl2 can be applied to bridge decks in valley areas to prevent preferential icing (frosting) conditions. The liquid is applied at the rate of 100 L (25 gal) per lane-mile at speeds up to 50 km/h (30 mph). Depending on traffic and weather conditions, the residual chemical can prevent frosting conditions for about one week on low-volume roads and 3 to 4 days on higher volume freeways. Appendix C includes this experience as guidance, presenting the application rate in terms of the weight of the dry chemical in the applied solution.
Recommendations for use of solid and prewetted solid chemicals
Guidance for initial prewetted solid chemical applications is given in Appendix C for all of the events. Guidance for initial dry solid chemical treatments is given for three events only: light snow storm; light snow storm with period(s) of moderate or heavy snow; and moderate or heavy snow storm (dry solid chemical is referred to as solid chemical in the Appendix C tables). The differences in the recommendations for solid and prewetted solids are primarily a result of the additional benefits that prewetting provides and the limitations on the use of solid chemicals when insufficient moisture or accumulation is present on the road. These have been discussed in Sections 3.1.1 and 3.1.3. For some conditions identified in the Appendix C tables, prewetting is not listed as a recommended maintenance action even though dry solid chemicals are. These are cases where sufficient moisture is present to trigger the effectiveness of the dry chemical. Under such conditions, however, there would be no loss of effectiveness in applying prewetted solids.
For precipitation events, prewetted solid applications, like liquid treatments, can be made either in advance of the storm or as an early-storm treatment. In the latter case, the application can be made onto dry, wet, slush, or lightly snow covered pavement. It should be completed before accumulation or pack bonds to the pavement. Applications onto dry pavement, either as a pretreatment or early-storm treatment, should be monitored to avoid excessive loss of material. Late applications onto pavements with more than a light covering of slush or snow can result in excessive dilution of the chemical, and should be coordinated with plowing. Where there is sufficient moisture after snowfall has begun, dry solid chemicals can be applied.
Application of dry solid chemical onto dry pavement is not recommended, and therefore should not be used as a pretreatment. Timing of an initial dry solid chemical application for snowstorm events is therefore critical; it should be made as soon as possible after sufficient precipitation has fallen to prevent loss, but before snowpack or ice bonds to the pavement.
If the pavement and snow are cold and dry, and it is apparent that snow in tire tracks is not adhering to the pavement, plowing is all that will be necessary to remove accumulation. If residual chemical or pavement temperature is high enough to form some liquid, wetting the snow or causing slush, then plowing with an appropriate cutting edge or slush blade is recommended (see Section 3.1.4).
126.96.36.199 Do nothing
When the pavement is cold (below -9.5°C (15°F)) and new or blowing snow are light and cold, traffic and wind (speeds of 25 km/h (15 mph) or higher) may be sufficient to prevent accumulation and compaction in tire tracks. In this case, application of any chemical, even that added as freeze-proofing to an abrasive, may create rather than cure a problem. Once a wet surface develops where before it was cold and dry, the dry snow can adhere and begin to build up. Prompt removal with a plow may prevent pack from developing, but this situation may have been avoided by refraining from chemical application. If the weather forecast is for rising temperatures, however, chemical should be applied before snow becomes wet with the potential of forming pack. Application should be made when the temperature rises high enough for the chemical to act rapidly, usually above about -5°C (23°F). Application can be made at temperatures as low as -9.5°C (15°F) if a rapid rise in temperature is forecast.
An initial application of chemical may suffice for some conditions and short duration events, but it is far more likely that further treatments will be required during a storm. In many cases these do not differ from initial treatments, although other considerations become very important such as the coordination of the application with plowing.
4.2.1 Monitoring of conditions
It is important that roadway and weather conditions, weather forecast updates, and RWIS data be closely monitored once the initial anti-icing operation has taken place. Special attention should be paid to pavement temperature and trend and to changes in precipitation type and intensity. This information, plus observations of precipitation, observations of pavement conditions, and evaluations of treatment effectiveness (as discussed in Section 3.2.6) are needed to determine when, or if, additional anti-icing treatments are necessary. Only by the systematic use of available information can the most efficient anti-icing actions be taken over the course of a storm.
188.8.131.52 Apply chemical
As for the initial operation, guidance for solid, liquid, and prewetted solid chemical applications is given in Appendix C for subsequent operations that may be necessary. Again the guidance for liquid applications is limited to four events: light snow storm; light snow storm with period(s) of moderate or heavy snow; moderate or heavy snow storm; and frost or black ice. Also, guidance for prewetted solid chemical applications is given for all of the events, whereas guidance for dry solid chemical treatments is given for three events only: light snow storm; light snow storm with period(s) of moderate or heavy snow; and moderate or heavy snow storm. Much of the discussion above for the initial chemical application is pertinent to the subsequent application.
Whichever treatment is used, the timing of subsequent applications, like the initial application, should be consistent with the underlying objective of preventing the formation or development of bonded snow or ice, and should reflect an operational readiness consistent with a preventive strategy. That is, they should be made in anticipation of or in prompt response to worsening pavement conditions.
For snowfalls, an initial anti-icing treatment may be all that is necessary to cope with a light or short duration event. There will be little dilution of the chemical, so the freezing-point may not reach the pavement temperature. When snowfall continues, and pavement temperature is about -9.5°C (15°F) or higher, subsequent treatments may be required to prevent the formation or development of pack or bonded pack. In such a case, the snow should be plowed off as completely as possible before chemical is reapplied, allowing the necessary small amount of chemical to reach the pavement surface.
In snow storms with generally steady precipitation and pavement temperature conditions, subsequent chemical operations made at regular intervals are generally adequate. However, in storms with significant changes in precipitation and pavement temperature conditions, operations will likely be at irregular intervals. In either case, systematic operations using all available decision-making tools should be conducted. Subsequent chemical applications that are made in prompt response to changing conditions can improve friction and pavement conditions. Subsequent chemical applications that are made in anticipation of changing conditions can prevent deteriorating conditions or mitigate their effects.
A limited period of heavier snow in an otherwise light snow storm should be treated as "a storm within a storm." That is, chemical operations should be conducted just prior to or at the beginning of the intense snow period to reduce the likelihood that snowpack will develop or be sustained by a strong bond, and to increase the likelihood that plowing operations can readily remove any packed snow that may develop. Use of reliable short-term forecasting tools would facilitate the timing of these operations, but they would otherwise be routine extensions of the responses to heavy snow seen in current snow and ice control practice. The result may not only be the prevention of a strong bond between the packed snow and the pavement, but also the prevention of excessive reductions in friction that precede the development of snowpack. Guidance for the weather event "Light Snow Storm with Period(s) of Moderate or Heavy Snow" was developed specifically to underscore the importance of being ready for these heavy snow periods and the poor road conditions that often result.
Although subsequent liquid applications can be successful, operational constraints such as the capacities of truck tanks and storage facilities, as well as operational preferences, may lead to anti-icing chemical operations that include transitions from liquid to solid applications. In such cases, the transition should be anticipated to allow continuous anti-icing operations and to avoid operational delays, and the solid chemical operations should be performed according to anti-icing practices.
Plowing passes should be made as necessary to prevent build-up of a compacted snow layer. Before applying any more chemical, the pavement surface should be cleared of frozen precipitation as best as possible to allow the small amount of chemical to reach the pavement surface. This is very important for the application of liquids.
As described for the initial operation, if the pavement and snow are cold and dry, and it is apparent that snow in tire tracks is not adhering to the pavement, plowing is all that will be necessary to remove accumulation.
184.108.40.206 Do nothing
If the initial or previous anti-icing treatments have done their job, the pavement temperature is around -2°C (28°F) and holding steady or rising, and there is no additional precipitation coming down or forecast, there may be no need for further action. This is especially the case when the pavement temperature is above 0°C (32°F) and steady or rising, whether it is during or after precipitation. Recognition of such conditions, and communication of these conditions to operators, can result in significant material savings. However, it is important to monitor conditions closely when pavement temperature is below or slightly above 0°C (32°F), and to be aware of the potential for surprise freeze-ups.
Several special factors influence choice or timing of an operational treatment. These are introduced or amplified here.
4.3.1 Traffic condition
Other than the influence that traffic volume has on the assignment of level of service, the influence of traffic on anti-icing storm operations comes primarily from the variation of traffic rate throughout the period of a storm or icing event. Although traffic density has an effect on friction, this is less important and not as direct or consistent as the effects of precipitation type and rate and pavement temperature. In fact, anti-icing snowstorm operations have been shown to be successful in high volume rush hour traffic as well as low volume middle-of-night traffic. Because of this, and because traffic can have both positive and detrimental effects on road condition during anti-icing operations, traffic rate has not been included as a variable in the anti-icing guidance provided in Appendix C, except in the table for frost or black ice. The routine use of traffic information should be mostly to ensure that storm operations are completed ahead of rush periods in order to avoid delays which can lead to bonded pack or ice. This is reflected in the Notes provided in the Appendix C tables. Local experience or level of service may warrant further incorporation of traffic information within the anti-icing operational guidance. If so, the Appendix C tables can be appropriately modified.
Experience has shown that crosswind speeds in excess of about 24 km/h (15 mph) may cause local drifting across a pavement and retention of snow if, for example, the pavement is wet. The threshold wind speed at which this becomes a problem will vary widely with road siting and other conditions. Maintenance personnel should be alert to the conditions that may cause this interception of snow and incorporate the information in operating procedures. For liquid applications, when a decision is made to apply chemical during windy conditions, experience has shown that adjusting the sprayer closer to the pavement can be successful in avoiding loss and achieving the desired application.
4.3.3 Hybrid precipitation events
Few storms will fit neatly into specific classifications based on intensity, duration, and type of precipitation. The operational treatment used early in a storm may have to be modified during the course of the event or operation. This is particularly critical when a light snowfall is interspersed with one or more periods of heavy snowfall. "Light Snow Storm with Period(s) of Moderate or Heavy Snow" has been defined for this case in Appendix C. However, other "hybrid" events are not specified in the remaining Appendix C tables, which are organized according to the type of storm or event. In the occurrence of changing precipitation conditions, maintenance managers should be prepared to base operational decisions on more than one of the tables.
4.3.4 Prewetting quantities
In theory, only sufficient liquid to wet every particle of dry solid chemical is required for prewetting. The actual rate to achieve this wetting will vary with the particle size distribution. In practice it has been found, for example, that 37 L (10 gal) of 23 percent concentration sodium chloride solution will be sufficient for a 1000 kg (1 ton) of dry chemical of coarse gradation. 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.
4.3.5 Development of snowpack and bond
Even when anti-icing operations are performed successfully, a snowpack may still develop and a bond may form between the pack and pavement. Technically this generally implies that deicing is necessary. However it is often observed in practice that the previous anti-icing treatments have inhibited the development of a bond, resulting in a weak bond that is easily broken, leading to a rapid return to acceptable pavement conditions. Those charged with developing anti-icing programs should recognize that bonded snowpack can occur even when anti-icing is "successful," but that it will not usually have the strength of bonded pack observed during traditional deicing operations. Indeed, the stated objective of anti-icing is not only to prevent formation of a bond, but to prevent development if one does form.
If during anti-icing operations rapidly changing climatic conditions or delays in operations lead to strongly bonded snow or ice, maintenance managers should be prepared (1) to utilize deicing techniques to break the bond to return the road to acceptable conditions, and (2) to apply abrasives if a rapid increase in friction coefficient is required in the interim. However, once acceptable road conditions are recovered, a return to preventive anti-icing operations would be appropriate.
4.3.6 Use of abrasives
Abrasives may be necessary when a rapid increase in friction coefficient is required, particularly at pavement temperatures so low that chemical action is slow, and, as discussed above, in conditions where snow or ice is strongly bonded to the pavement and cannot be removed rapidly. Several tables in Appendix C contain low pavement temperature conditions that may warrant abrasives use, most directly in the tables for frost/black ice events and freezing rain storms. However, abrasives are not ice-control chemicals, and will not support the fundamental objective of anti-icing when applied either straight or in a mix with chemicals. Because of this, and because of evidence that they are generally ineffective when used routinely in anti-icing operations, they should not be a customary operation of an anti-icing program outside of the low pavement temperature conditions presented in Appendix C. However, even in these conditions, chemical typically added as freeze-proofing to abrasives may create a problem by causing a wet pavement surface. As described previously in Section 220.127.116.11, once a wet surface develops where before it was cold and dry, the dry snow can adhere and begin to build up. Maintenance crews should be aware of this possibility.
Lessons can be learned from both the successes and failures of any winter maintenance operation. Anti-icing operations are no exception. Improvements in operations, and even in equipment, can be identified and implemented through a post-storm assessment of the practices and treatments used. It is important that all levels of maintenance personnel, from district level supervisors to equipment operators, be involved in these evaluations. Many times, what is witnessed during a storm by a supervisor is not readily seen by an operator, and vice versa. As part of the post-storm assessment, it is suggested that the highway agency track the cost and effectiveness of the anti-icing program, and, if possible, do the same for conventional snow and ice control operations in comparable areas. This process should include evaluations of treatment effectiveness, as discussed in Section 3.2.6, and examinations of costs. Techniques for these evaluations can be based upon an agency’s current storm data collection practice, or upon those used for FHWA T&E 28 (1).
The cost data of a storm or weather event can be organized according to the total costs for materials (chemicals and abrasives), labor, and equipment employed in operations. For chemicals, this includes the purchase price, transportation to storage site, storage, truck loading, handling and mixing of solid chemicals, and solution preparation. For abrasives it should include both material costs and any clean-up costs. Labor costs should be fully burdened and include overtime, while equipment costs would be the agency’s hourly rates times the hours used. Other costs can be considered, if appropriate. These include the cost of dispatchers, costs of specialized equipment, cost of patrols, etc.
The costs and effectiveness measures of the anti-icing and conventional operations need to be recorded separately for each highway section or route considered. The cost per lane mile can then be calculated for each type of operation, effectiveness measures can be evaluated, and the relative success of the two operations compared.
While the implementation of systematic anti-icing practices can be thought of as a means for maintaining roads in the best conditions possible during a winter storm, and a way to do so in an efficient manner, it should not be thought of as an improvement that will automatically result in reduced overall costs. Most importantly, anti-icing techniques provide the potential for maintaining roads in the best conditions possible during winter storms. Cost savings will depend on the current practice: for example, what level of service governs, what materials it uses, whether it is more deicing than anti-icing, and what information sources it uses. Examples of success include providing the same level of maintenance effectiveness at less total cost, and providing a higher level of maintenance effectiveness at the same cost. As these examples illustrate, for those examining the costs or success of anti-icing operations, an examination of both costs and level of maintenance effectiveness is important.