Snow and Ice Control: Guidelines for Materials and Methods (2004)

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13 CHAPTER 6 FACTORS INFLUENCING THE CHOICE OF MATERIALS, THEIR FORM, AND ASSOCIATED APPLICATION RATES The major factors to consider when choosing a snow and ice control materials treatment are pavement conditions, weather conditions, and the performance characteristics of the materials. DILUTION POTENTIAL Dilution potential is a term that relates precipitation, pave- ment conditions, pavement surface conditions, and operational conditions to the choice of snow and ice control material and application rate that will generally produce a “successful” result. For simplicity, dilution potential is divided into three categories: low, medium, and high. Precipitation Dilution Potential Precipitation dilution potential is the contribution to over- all dilution potential caused by the type and rate of precipita- tion of a winter weather event in progress. The higher the moisture content of the event per unit or time, the higher the dilution potential. Pavement Conditions Pavement conditions are the properties of the pavement itself that influence snow and ice control operations. The most important of those is pavement surface temperature. Other factors that sometimes warrant consideration are severely tex- tured pavement surfaces such as open-graded asphalt con- crete surfaces and grooved or heavily textured Portland cement concrete surfaces. Pavement surface temperature has a major effect on how ice control chemicals perform and ultimately, on the treat- ment decision itself. As pavement temperatures decline below about 12°F, most ice control chemicals become very ineffi- cient because of the slow melting rate and the amount of ice melted per unit of chemical applied. Pavement temperature therefore drives the decision to plow only, plow and apply chemicals, or plow and apply abrasives (depending also on LOS goals). Severe pavement surface texture is another factor that influ- ences the choice of chemical application rates. It is widely known from operational experiences that chemical applica- tion rates need to be increased for severely textured pave- ment surfaces such as found with open-graded asphaltic con- crete and newly grooved/tined Portland cement concrete. How much of an increase in chemical application rate is required for these surfaces compared to smoother surfaces for equivalent chemical performance for a range of operating conditions is a subject for further research. Pavement type can influence solar heat absorption and ultimately pavement surface temperature at the time of treat- ment. Unpaved or gravel roads are not suitable for chemical treatment. Pavement Surface Conditions Pavement surface conditions describe any accumulations of snow and ice that may remain on the pavement at the time of treatment (after plowing). These include loose snow, packed snow, and ice. A critical surface condition is whether or not the snow or ice is bonded to the pavement surface. Any remaining snow or ice on the roadway surface after plowing will cause chemical treatments to dilute more quickly (in addition to the dilution caused by precipitation). If the snow or ice is bonded to the pavement, considerably more chemi- cal will have to be applied in order to achieve an unbonded condition. Operational Conditions The most important operational conditions influencing dilu- tion potential are treatment cycle time and traffic. Longer cycle times allow more precipitation to accumulate on the roadway between treatments. For equivalent effectiveness, more chemical must be applied for longer cycle times. The two traffic characteristics thought to influence dilution potential are traffic volume and traffic speed. Higher speeds and higher volume will displace ice control chemicals from the roadway.

PROPERTIES OF ICE CONTROL MATERIALS The four basic types of ice control materials are (1) abra- sives, (2) solid ice control chemicals, (3) prewet solid ice control chemicals, and (4) liquid ice control chemicals. Abrasives Abrasives are a vital part of most snow and ice control programs. They support lower LOS and can provide at least some measure of traction enhancement when it is too cold for chemicals to work effectively. They are suitable for use on unpaved roads and on thick snow pack/ice surfaces that are too thick for chemicals to penetrate. When mixed with enough ice control chemical, abrasives will support anti-icing and deicing strategies; however, this is very inefficient and costly as the abrasives for the most part are “going along for the ride” while the chemical portion of the mix is doing the “work.” Solid Ice Control Chemicals Solid ice control chemicals are a very popular treatment option for most highway maintenance agencies. They support high LOS and both anti-icing and deicing strategies. When anti-icing, they are most effective when applied early in a win- ter weather event, before ice/pavement bond has a chance to develop. Some snow/ice/water on the pavement will minimize bouncing and scattering of the chemicals. Field observations indicate solid chemicals may be used as a pretreatment, espe- cially when applied at traffic speeds under about 30 mph and traffic volumes under 100 vehicles per hour. Solid chemicals, particularly those with a “coarser” gra- dation or particle size distribution, are well suited to deic- ing operations. The larger particles are able to “melt” through snow/ice on the surface and continue to cause melting at the ice/pavement interface until the ice/pavement bond is broken and the snow/ice can be removed mechanically. The use of fine graded salt during anti-icing operations gen- erally is not cost effective compared to the use of coarse graded salt (8). This is true for most forms of frozen precipi- tation, including freezing rain and sleet. Fine graded salt dilutes faster than coarse graded salt and has to be reapplied more often and at greater rates during a winter weather event than does coarse graded salt to achieve a similar chemical effectiveness. The fast brine generation of fine graded salt when applied to a pavement will produce a wet pavement sooner than coarse graded salt, but the condition will not be long lasting. This situation can quickly lead to a refreeze of the brine solution unless additional salt applications are made. The use of fine graded salt is better suited for the treatment of thin ice and, when prewetted, as a pretreatment for frost conditions when applied just prior to daylight. Fine graded salt applications are not well suited for deic- ing operations because of the high dilution potential. 14 Solid ice control chemicals are often mixed in small quan- tity (less than 10 percent) with abrasives to prevent “chunk- ing” and freezing in stockpiles. They are also mixed with abrasives in sufficient quantity (greater than 20 percent) to do some ice melting. Prewet Solid Ice Control Chemicals Prewet solid ice control chemicals are used in the same way as solid chemicals except they are generally not mixed with abrasives. They consist of solid ice control chemicals that have been “coated” with liquid ice control chemicals by a variety of mechanisms. The water in the liquid ice control chemical starts the process of allowing the solid chemical to generate “brine” more quickly than “uncoated” solid chemical the coating also allows the solid chemical to better “stick” to the surface. This reduces bounce and scatter and accelerates deicing. Liquid Ice Control Chemicals Liquid ice control chemicals are generally a solution of solid ice control chemicals with water being the predominant com- ponent. They support high LOS and anti-icing strategy. They are particularly well suited to pretreating for anticipated frost/ icing/black ice situations. Here, the water evaporates and the residual dry chemical is relatively immune to dispersal by traffic. Liquid chemicals are also used to pretreat roadways prior to a general snow or ice event. This is an effective way to initiate the anti-icing strategy. Since liquid ice control chemicals are mostly water, they are already fairly well diluted. They are not well suited to deicing operations as they have little ability to penetrate thick snow ice. However, they may be used in limited situations for deic- ing if the treatment is immediately followed by an application of solid chemicals or the process is reversed. The Illinois DOT has reported success with a deicing strategy that utilizes approximately 250 lb/LM (lane mile) dry salt applied on top of compacted snow followed immediately by 30 to 50 gal/LM of liquid salt or calcium chloride with air temperatures above 10°F and sunny conditions (9). This is a variation of prewetting. Liquid chemicals are probably not a good choice at pave- ment temperatures below about 20°F. Here, the limited ice melting ability of most chemicals would make application rates excessive and potentially cause refreeze if the pavement was not dried by traffic or other atmospheric mechanisms. Liquid chemicals, as a within-winter weather event treat- ment, should be limited to lower moisture content events, pave- ment temperatures above 20°F, and cycle times less than about 1.5 hours. This will minimize the risk of ice/pavement bond formation. It is not advisable, however, to use liquid chemicals during moderate or heavy snow, sleet, and freezing rain events. At pavement temperatures higher than about 28°F, liquid chemicals are a very effective treatment for thin ice in the absence of precipitation. The ice melting process in this sit- uation is almost immediate.