Lesson 1/Learning Event 3
Learning Event 3
TYPES AND GRADES OF TARS
TYPES OF TARS
Tars consist of road tars and road tar cutbacks.
Road Tar (RT)
Road tar is manufactured in twelve grades based on viscosity or hardness. Grades 1 through 7
are liquid at room temperature, while grades 8 through 12 are semi-solid to solid at room
temperature. The difference in the liquid and solid grades of road tars is that the liquid road tars
contain more of the of the liquid coal distillates than do the solid grades.
Road Tar Cutback (RTCB)
In the same manner that asphalt can be cut back with a petroleum distillate, so can road tars be
cut back with a coal tar distillate. Road tar cutbacks are manufactured in two viscosity grades
only, grades 5 and 6. The more volatile coal distillates such as benzene or a solution of
naphthalene in benzol may be used to cut back the heavier grades of road tar to produce the
RTCB 5 and 6. These RTCBs react much like the RC asphaltic cutbacks in that they cure
rapidly.
ADVANTAGES/DISADVANTAGES OF ROAD TAR
Advantages. Not only can tars do almost any job that asphalt can, but they even have some
advantages over them. Probably the biggest advantage of tars is that they do not dissolve in
petroleum distillates as do asphalts. This is an advantage where there is danger of petroleum
distillates being spilled on the surface, such as jet fuel and gasoline on such areas as refueling
and parking aprons and gasoline and oil dispensing services. The asphalts in an asphalt surface
could, and have been, dissolved by spilled petroleum distillate but since tars are products of coal
they would not be dissolved. Tars also seem to have better penetrating qualities than asphalts. If
a tar and an asphalt of equal viscosities and curing rates were applied to a soil base, a tar would
penetrate deeper into the base than the asphalt.
Disadvantages. Tars, however, have a disadvantage that stems from the fact that they are
subject to extreme consistency variations with only average temperature changes. In the summer
when it is hot, a tar binder may get so soft that the tar mix pavement may become so brittle that it
will crack and cause the pavement to disintegrate.
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