Figure 8-20.—Tars obtained from distillation of bituminous coal.The viscosity grades range from 1 (leastfluid) to 3 (most fluid).Use of Asphalt EmulsionsEmulsions are used for surfacetreatment, road and plant mixes, and crackand joint filling. The mixing grades canbe mixed unheated with damp aggregate.They are preferred over asphalt cutbackwhen the aggregate is very damp. Cationicemulsions coat damp aggregate better thananionic emulsion.Recommended use of emulsionsdepends on setting rate and mixingability. At water-freezingtemperatures, asphalt emulsions donot mix well since the emulsion willseparate from the water. Also,emulsions have a relatively shortshelf life and tend to break whilestill in their unopen drums.TarsTars are obtained from the distillation ofbituminous coal (fig. 8-20) and are seldom used inthe NCF. A road tar is designated by the symbol RTand is manufactured in 12 grades of viscosity(table 8-5). RT-1, RT-2, and RT-3 arePRIMINGOIL. RT-4 through RT-7 are calledCOLD TARSbecause they are fluid enough to be mixed andapplied at relatively low temperatures. RT-8through RT-12 are called HOT TARS because theyare solid enough to require high temperatures formixing and applying.The symbol RTCB refers toROAD TARCUTBACK.RTCBs are manufactured only inviscosity grades 5 and 6. Coal distillate,such as benzene or a solution ofnaphthalene in benzol, may be used tocutback the heaviest grades of road tar toproduce both grades of road tar cutbacks.The viscosity grades of road tars and roadtar cutbackscan be compared to the8-22
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