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