Figure 8-20.—Tars obtained from distillation of bituminous coal. The    viscosity    grades    range    from    1    (least fluid)    to    3    (most    fluid). Use of Asphalt Emulsions Emulsions are used for surface treatment,   road   and   plant   mixes,   and   crack and    joint    filling.    The    mixing    grades    can be    mixed    unheated    with    damp    aggregate. They     are     preferred     over     asphalt     cutback when    the    aggregate    is    very    damp.    Cationic emulsions    coat    damp    aggregate    better    than anionic emulsion. Recommended use of emulsions depends on setting rate and mixing ability. At water-freezing temperatures, asphalt emulsions do not     mix     well     since     the     emulsion     will separate from the water. Also, emulsions have a relatively short shelf life and tend to break while still in their unopen drums. Tars Tars  are  obtained  from  the  distillation  of bituminous  coal  (fig.  8-20)  and  are  seldom  used  in the NCF. A road tar is designated by the symbol RT and   is   manufactured   in   12   grades   of   viscosity (table  8-5).  RT-1,  RT-2,  and  RT-3  are  PRIMING OIL.  RT-4  through  RT-7  are  called  COLD  TARS because  they  are  fluid  enough  to  be  mixed  and applied    at    relatively    low    temperatures.    RT-8 through  RT-12  are  called  HOT  TARS  because  they are  solid  enough  to  require  high  temperatures  for mixing and applying. The  symbol  RTCB  refers  to  ROAD  TAR CUTBACK.   RTCBs   are   manufactured   only   in viscosity    grades    5    and    6.    Coal    distillate, such as benzene or a solution of naphthalene     in     benzol,     may     be     used     to cutback   the   heaviest   grades   of   road   tar   to produce    both    grades    of    road    tar    cutbacks. The   viscosity   grades   of   road   tars   and   road tar cutbacks can be compared to the 8-22