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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 Severe pavement surface texture is another factor that influ- ice control materials treatment are pavement conditions, ences the choice of chemical application rates. It is widely weather conditions, and the performance characteristics of known from operational experiences that chemical applica- the materials. 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. DILUTION POTENTIAL How much of an increase in chemical application rate is required for these surfaces compared to smoother surfaces Dilution potential is a term that relates precipitation, pave- for equivalent chemical performance for a range of operating ment conditions, pavement surface conditions, and operational conditions is a subject for further research. conditions to the choice of snow and ice control material and Pavement type can influence solar heat absorption and application rate that will generally produce a "successful" ultimately pavement surface temperature at the time of treat- result. For simplicity, dilution potential is divided into three ment. Unpaved or gravel roads are not suitable for chemical categories: low, medium, and high. treatment. Precipitation Dilution Potential Pavement Surface Conditions Precipitation dilution potential is the contribution to over- Pavement surface conditions describe any accumulations of all dilution potential caused by the type and rate of precipita- snow and ice that may remain on the pavement at the time of tion of a winter weather event in progress. The higher the treatment (after plowing). These include loose snow, packed moisture content of the event per unit or time, the higher the snow, and ice. A critical surface condition is whether or not dilution potential. 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 Pavement Conditions (in addition to the dilution caused by precipitation). If the snow Pavement conditions are the properties of the pavement or ice is bonded to the pavement, considerably more chemi- itself that influence snow and ice control operations. The most cal will have to be applied in order to achieve an unbonded important of those is pavement surface temperature. Other condition. 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 Operational Conditions concrete surfaces. Pavement surface temperature has a major effect on how The most important operational conditions influencing dilu- ice control chemicals perform and ultimately, on the treat- tion potential are treatment cycle time and traffic. Longer ment decision itself. As pavement temperatures decline below cycle times allow more precipitation to accumulate on the about 12F, most ice control chemicals become very ineffi- roadway between treatments. For equivalent effectiveness, cient because of the slow melting rate and the amount of ice more chemical must be applied for longer cycle times. melted per unit of chemical applied. Pavement temperature The two traffic characteristics thought to influence dilution therefore drives the decision to plow only, plow and apply potential are traffic volume and traffic speed. Higher speeds chemicals, or plow and apply abrasives (depending also on and higher volume will displace ice control chemicals from LOS goals). the roadway.