bitumen in place. Such solutions are termed cutbacks and the process of evap-
oration of the volatile solvents is called curing. The speed at which it occurs will
depend on the nature of the solvent.
The classification of cutbacks is based on the following two characteristics:
The viscosity of the cutback itself
The penetration of the non-volatile residue.The cutback’s viscosity is measured using a standard tar viscometer (STV) which
computes the time in seconds for a given volume of binder to flow through a
standard orifice at a temperature of 40°C. Three common grades for cutback
have viscosities of 50, 100 and 200 seconds. Cutback bitumen is used in surface
dressing. In this process, it is sprayed onto a weakened road surface and chip-
pings are placed on it and then rolled. It serves to provide a non-skid wearing
surface to the pavement, makes the surface resistant to water and prevents its
disintegration.Bituminous emulsions
Bitumen can be made easier to handle by forming it into an emulsion where
particles of it become suspended in water. In most cases, their manufacture
involves heating the bitumen and then shredding it in a colloidal mill with a
solution of hot water and an emulsifier. The particles are imparted with an ionic
charge which makes them repel each other. Within cationic emulsions the
imparted charge is positive, while the charge is negative in anionic emulsions.
When the emulsion is sprayed onto the road surface, the charged ions are
attracted to opposite charges on the surface, causing the emulsion to begin
‘breaking’ with the bitumen particles starting to coalesce together. The break-
ing process is complete when the film of bitumen is continuous.
Bitumen emulsions are graded in terms of their stability or rate of break on
a scale of 1 to 4, with 1 signifying the greatest stability (stable =rapid acting).
Rate of break depends on the composition of the emulsion and the rate at which
the emulsion evaporates. The grading of the aggregate onto which the emulsion
is applied is also important to the rate of break. Dirty aggregates accelerate it,
as will porous or dry road surfaces. Cationic emulsions tend to break more
rapidly than ionic ones. The UK code, BS 434 (BSI, 1984), also designates
cationic emulsions as K and ionic as A. Therefore, K3 denotes a slow acting
cationic emulsion, K2 a medium acting one and K1 a rapid acting one.Chippings
The chippings used are central to the success of the surface dressing process as
they provide essential skidding resistance. The correct rate of spread depends
mainly on the nominal size of chippings used, varying from 7 kg per m^2 for
6 mm nominal size to 17 kg per m^2 for 20 mm. The chippings themselves may be212 Highway Engineering