Highway Pavement Materials and Design 211
Table 7.8 indicates the penetration and softening point valuations for differ-
ent bitumen grades (BS 3690) (BSI, 1990b).
Heated water
bath
25 mm gap
Steel ball
Binder and ring
Thermometer
Figure 7.13Softening
point test.
Grade of bitumen
Property 15 pen 50 pen 100 pen 200 pen
Penetration at 25°C 15 ± 5 50 ± 10 100 ± 20 200 ± 30
Softening point (°C) min. 63 47 41 33
Softening point (°C) max. 76 58 51 42
Table 7.8Properties of penetration grade bitumens (BSI, 1990b)
7.6.2 Surface dressing and modified binders
Surface dressing involves the application of a thin layer of bituminous binder
to the surface of the pavement slab followed by the spreading and rolling into
it of single sized stone chippings. In order to apply the binder effectively, its stiff-
ness must be modified during the construction phase of the pavement. Two such
binder modifications used during surface dressing are cutback bitumen and
bitumen emulsion.
Cutback bitumen
Bitumen obtained from the refining process described briefly above can be
blended with some of the more volatile solvents such as kerosene or creosote to
form a solution that has a viscosity far below that of penetration grade bitumen
and will act as a fluid at much lower temperatures. However, when the solution
is exposed to the atmosphere, the volatile solvents evaporate leaving solely the
