DIGEST 2006 - Sabita

Properties Tar Prime Cutback bitumen
prime
Invert bitumen
emulsion
Water content 0% 0% < 20%
Solvent type Polar Non-polar Non-polar
Solvent Creosote-like Paraffin-like Paraffin-like
Viscosity Low Highest Lower than
cutback bitumen
Table 1: Tar vs bitumen primes
construction. Placing hot mix
asphalt or a seal on a prime that
has not penetrated into the base
substrate raises the probability of
problems appearing later during
the life of the surfacing.
Penetration
The penetration and drying
problems with primes are worse in
winter and this led the technical
team at Tosas to investigate
various ways to overcome these
problems. The fact that the
viscosity of bituminous products
increases at lower temperatures
prompted the Tosas team to
investigate ways of decreasing
viscosity. The alternatives were:
• Increase the percentage of
cutter to reduce the viscosity:
This is standard practice and
all suppliers can make the
alterations, formulating a
non-SABS 308 product,
(unfortunately at a cost to
the client). For example: MC
30 + 10% additional paraffin;
• Change the solvent type:
(This was also investigated
and may prove to be a
longer-term solution if the
specifications for cutback
products are amended);
• Emulsify the cutback bitumen
in water: i.e. normal
‘oil-in-water’ as opposed to
invert emulsion where the
water is the discontinuous
phase. (Although the
viscosity is reduced, one can
be fooled by false penetration
– although the water and
some of the solvent
penetrate, the bitumen
droplets are unable to do so,
and coagulate on top to form
a skin. A high concentration
of soap is also required to
emulsify the product, which
has a water content of
between 40 and 50%. This
large proportion of free water
reduces the net binder
content dramatically, and the
contractor would be better off
spraying bitumen primes at
30-40% below specified
application for the same
effect.);
• Look at the effect of
temperature.
Temperature
Everyone knows that it is colder in
winter and warmer in summer, air
temperatures are lower than the
road temperatures as it is warmer
in the direct sun than in the
shade.
The clever philosopher who once
said: “You do not know that which
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