DIGEST 2006 - Sabita

Bond coats
Bond coats were first used with
the introduction of the integrated
paver/sprayers for application of
Ultra Thin Friction Course (UTFC)
asphalt in South Africa in 1999.
These thin, open graded asphalt
layers are placed at an average
thickness of 18mm. The strains
induced by traffic at the surface/
binder interface are much greater
than the forces acting on the
thicker asphalt layer/binder
interfaces, and undiluted rapid
setting cationic polymer modified
emulsions are thus used for this
purpose to ensure adequate bond
strength at the interface.
The thin asphalt
surfacing layer and
the bond coats are
usually offered as
an integral package
by the applicators.
The paving unit
applies the binder
uniformly using a
calibrated,
computer
controlled, spraying
system. In the case
of simultaneous application, the
free water in the emulsion bond
coat foams when it comes into
contact with the hot mix asphalt.
The reduced viscosity of the
foamed binder and its subsequent
expansion allows the binder to
penetrate more effectively into the
existing substrate and overlying
mix, thus creating an improved
bond.
Application rate of the bond coat
varies typically between 0,4 – 0,6
l/m 2 depending mainly on the
Deformation
proceeds
at a faster
rate when
a proper bond
has not been
created
substrate texture, leaving a
residual modified binder
application of 0,26 to 0,39 l/m 2 ,
which is considerably higher than
the residual binder content used
for the conventional, thicker
asphalt lifts. The thicker residual
binder layers also provide better
waterproofing, as the thinner,
open graded asphalt layers are
more prone to allowing water
infiltration compared to the
conventional, dense layers.
Specific requirements
• High softening point of
residual binder to prevent
excessive migration into the
open UTFC
aggregate
structure under
traffic after
paving;
• Very rapid
setting
characteristics.
The UTFC layer is
very thin and
rapidly cools
down after
paving. During
the time interval
when the mat is at a
temperature in excess of
100 0 C, most of the water in
the emulsion should boil off.
The emulsion should be
sufficiently de-stabilised
during this short period to
prevent re-emulsification,
which could occur if
unexpected rain falls on the
surface within the first few
minutes after paving;
• Despite the rapid setting
characteristics required of a
bond coat, the emulsion
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