U.S. Department of Transportation
Federal Highway Administration
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Washington, DC 20590
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
A winter maintenance program consists of several elements with varying degrees of importance depending on the size of the operational jurisdiction it covers and the complexity of its road network. One element, level of service (LOS), is important for all jurisdictions. It must be considered, along with the climatic conditions, in the design of any snow and ice control program. This Section describes how the level of service and climatic conditions influence an anti-icing program, and how anti-icing fits into the context of an overall winter maintenance program and level of service.
The extent to which maintenance services will be provided to a road section is determined by management by the assignment of a level of service. In the case of winter maintenance, this will require establishing a prescribed end-of-storm road condition, what intermediate conditions will be acceptable while obtaining that condition, or the frequency of snow and ice control maintenance operations. Some examples of LOS are: maintain 24-h coverage until near normal surface conditions are restored with coverage at a rate of 2 to 4 cycles (passes one vehicle would make over a given point on the road) per shift (however defined) as conditions and resources allow; bare pavement during daylight hours; center bare at all times; or snow-covered to a maximum of 12 mm (1/2 in) during daylight hours. The LOS will largely be determined by the importance of the road, and hence the average daily traffic. As a reflection of the desired bare pavement condition, high winter maintenance service levels are often generically called "bare pavement policies."
As defined in the Introduction, anti-icing is the snow and ice control practice of preventing the formation or development of bonded snow and ice by timely applications of a chemical freezing-point depressant. A winter maintenance crew that is persistent in this practice is best able to support demanding road condition requirements set forth by a higher LOS. If the LOS requirements are in terms of operational frequency rather than road condition, a crew operating at higher frequency would find anti-icing practices to provide the best road conditions possible within a given set of operational constraints. Because of the proven compatibility between anti-icing and higher LOS, this manual presents anti-icing as a strategy for support of high service levels.
Figure 1 depicts the components of an anti-icing program in the context of a winter maintenance program and the LOS assignment. It shows anti-icing as a support strategy for "bare pavement" service levels.
In addition to the service level, maintenance effort will vary with climatic conditions. A factor of great importance is pavement temperature. Pavement temperature directly influences the formation, development, and breaking of a bond between fallen or compacted precipitation and the road surface as well as the effectiveness of chemical treatments. It is also important when high humidity levels are accompanied by low dew point temperatures. Under these conditions there will be a greater potential for formation of frost and black ice. Unless some external source of heat is provided, the pavement temperature will generally track air temperature with a time delay. For road sections without obstructions to a clear sky view, solar radiation during the day and exposure to the clear night sky will affect the road surface temperature to a greater extent than on sections influenced by air contact only.
Other important climatic factors are type and rate of precipitation. Together with pavement temperature, they are the most important variables to consider when performing anti-icing operations. The operational guidance described later in this manual is presented in terms of these variables.
Figure 1. Outline of the components of an anti-icing program in the context of a winter maintenance program.
There are two distinct snow and ice control strategies that make use of chemical freezing-point depressants: deicing and anti-icing. They differ in their fundamental objective. Whereas anti-icing operations are conducted to prevent the formation or development of bonded snow and ice for easy removal, deicing operations are performed to break the bond of already-bonded snow and ice. Deicing is familiar to most agencies since it has been the most widely used strategy in the past. The design of deicing operations as a bond-breaking operation stems from its timing: it is commonly initiated only after 25 mm (1 in) or more of snow has accumulated and bonded to the road.
A distinction between anti-icing and deicing has not always been clear. This is because anti-icing is a relatively new term. Some agencies that have instinctively adopted anti-icing practices over the years have used the term deicing to describe their operations, even when they were intended to prevent a strong ice-pavement bond. In this manual a distinction between a primarily-preventive and a primarily-reactive strategy is intended when referring to anti-icing and deicing, respectively.
There are other strategies and techniques that can and have been used in snow and ice control operations. These include such approaches to a bare pavement policy as use of heated pavements, structural covering of a road to protect it from precipitation (e.g., covered bridges or avalanche sheds), and the use of a modified pavement surface in conjunction with plowing, to name a few. These are not covered in this manual.
As illustrated in Figure 1, both deicing and anti-icing can be used to support higher service level objectives. However, because deicing is reactionary, it cannot support strict requirements for safe road conditions during a winter storm. Anti-icing can meet such requirements, but to successfully and efficiently do so the maintenance manager must ensure that the timing of the operations is consistent with the objective of preventing the formation or development of bonded snow and ice. This is not an easy task. It requires use of much more judgment in making decisions, requires that available information sources be utilized methodically, and requires that the operations be anticipatory or prompt in nature. In short it requires a systematic approach.
The elements of a systematic anti-icing program are shown in Figure 1. As indicated, support of an anti-icing strategy is divided into tools and operations. The supporting tools can be organized according to operations, decision-making, and personnel "toolboxes," which are further broken down according to capabilities, information sources, and procedures that may be available for a given operation. A toolbox analogy is followed to suggest that managers should use their available resources systematically as they would use mechanical tools in the course of a methodical repair job. Operations are broken down into initial and subsequent operations in order to convey the importance of the initial chemical treatment in anti-icing operations, and to signal that subsequent operations throughout a storm or weather event should follow the anti-icing strategy as well.
While the outline of Figure 1 indicates the underlying complexity of anti-icing practices, it also reveals that what may initially be thought of as an overly demanding practice is actually an organized set of preparations, decisions, and operations. Such a methodical program can be designed for an agency’s unique conditions using tools that are generally available to agencies today and using operational guidance provided later in this manual. The tools are described in Section 3, and the operational guidance is presented in Section 4 and Appendix C.
Common to many snow and ice control operations is the use of abrasives. It is recognized that abrasives may be necessary when a rapid increase in friction coefficient is required, particularly at temperatures so low that chemical action is slow, and in conditions where snow or ice is strongly bonded to the pavement and cannot easily be removed. As these latter conditions are more likely to occur in the course of deicing, abrasives treatments can be an important tool for deicing operations. Abrasives are not ice-control chemicals, however, and will not support the fundamental objective of either anti-icing or deicing. Their sole function is to increase the coefficient of friction. This increase may be short lived, because traffic will rapidly disperse abrasives.
There is generally no advantage gained from the routine use of abrasives in an anti-icing program. When anti-icing operations have successfully prevented or mitigated the hazards of packed snow, for example, straight abrasives applications will provide no significant increase in friction or improvement in pavement condition. Further, a mix of abrasives and chemical will usually be no more effective as an anti-icing treatment during snowstorms than the same amount of chemical placed alone. It even appears that the use of abrasives in the mix can be detrimental to the effectiveness of the chemical. Because of the cost associated with both application and clean-up of roads and drainage facilities, and because of the potential airborne dust problem accompanying their use, abrasives applications should not be a routine operation of an anti-icing program.
Although this manual does not present abrasives application capability as a tool for anti-icing, Section 4 and Appendix C do provide guidance for conditions under which abrasives use may be appropriate during operations. Recommended practice for abrasives applications, or for deicing, is not provided in this manual.