DIGEST 2006 - Sabita

promote the breaking of the
emulsion. It also enhances early
mosaic development of the stone
matrix in a seal.
The quantity of flux in cationic
spray grade is varied on a
seasonal basis, with the higher
hydrocarbon levels being used
during winter months. SABS 548
specifies a maximum allowable
flux level of 5% m/m of residual
binder. Should cationic spray
grade emulsion be used as a tack
coat, the flux component will be
trapped under the overlaid
material. Although generally not a
problem in most paving
applications, the use of cationic
spray grade emulsions should be
avoided on high stress areas, due
to possible problems with slippage
or creeping of the asphalt layer.
Anionic stable grade emulsions
contain no hydrocarbon solvents
and are thus preferable for tack
coat applications.
Typical properties
No national specifications exist for
bond coat emulsions. These
emulsions are supplied by the
manufacturers, who provide their
own specifications. Typical
properties used to classify a bond
coat emulsion are: binder content,
viscosity and residue on sieve
values. Minimum softening point
and elastic recovery values are
specified on the residual binder
after evaporation of the water
phase. These properties should
comply with the requirements of
TG1’s SC-E1 grade modified
emulsion for seals, except that the
product must not contain any
hydrocarbon fluxes. It is possible
that the specifications for these
emulsions will be included in the
revised TG1 guideline to be
published later this year.
European developments
A common problem associated
with the use of tack coats is “pick
up” of the broken emulsion
residue under the wheels of the
tipper trucks supplying the hot mix
asphalt to the pavers. This
effectively results in areas where
little or no binder remains on the
surface, which can result in
slippage cracking or delamination
of the asphalt layer.
A new type of tack coat emulsion,
such as the Colnet system
registered in the name of Colas SA
in France, has been developed in
Europe, and is known as “clean
emulsion”. Essentially a hard
grade of bitumen is emulsified
using specialised equipment, and
the emulsion is applied with a
specially adapted binder
distributor using three spray bars.
The first spray bar applies an
adhesion agent to the existing
surface, while the second, or main
bar, applies the emulsion. The
third spray bar applies a chemical
“breaking agent” on the emulsion
to destabilise the emulsion and
induce chemical breaking. Paving
can commence immediately after
application of the emulsion and
“pick-up” on the truck tyres does
not occur (see Figure 2). This
system allows paving work to
proceed at night, with no delays
being caused by slow breaking of
the tack coat.
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