DIGEST 2006 - Sabita

Adequate layer bonding critical:
Bond and tack coats to improve
total structural integrity
Kobus Louw
Research and Development Manager
Colas SA (Pty) Ltd
Worldwide, the design
and construction of
long lasting asphalt
pavements is becoming
increasingly important as
greater demands are placed on
the performance of these
asphalt layers.
Adequate bonding between
asphalt layers is critical if the
completed structure is to behave
as a single structural unit and
provide adequate strength. If the
layers are not properly bonded
together, they will behave as
independent thin layers.
Generally, such individual layers
are not designed to accommodate
the stresses imposed by current
traffic patterns. Damage due to
fatigue and deformation proceeds
at a faster rate when a proper
bond has not been created,
resulting in rapid deterioration of
the pavement structure.
Inadequate bonding can result in
delamination, cracking, potholes
and ingress of water into the
pavement structure.
Over-application of the tack coat
can result in slippage of the
asphalt layers due to the
lubricating effect of the excess
binder.
Tack coats
Tack coats have traditionally been
used in South Africa for improving
the adhesion of hot mix asphalt
layers to the substrates on which
they are placed. These mixes are
usually placed at a thickness of 15
– 60mm. Anionic stable mix
emulsion diluted with equal
quantities of water is commonly
used for this purpose. The
application rate specified in COLTO
is 0,55 l/m 2 for a 30% binder
content emulsion, leaving a net
residual binder application of
approximately 0,165 l/m 2 . The
method of application is often with
a hand sprayer, and the resulting
spread rate and evenness of
application usually leaves much to
be desired and may be referred to
as “a lick and promise”. Paving
can only commence after the
emulsion has “broken”.
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