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Figure 3. Proper nozzle angle setting (14).
will result in some areas of the pavement having thicker or
thinner coverage and possible interference between noz-
zles. Differing coverage will result in streaks and gaps in
the tack coat (see Figure 4).
· The size of the nozzles needed to apply an asphalt emul-
sion material for a surface treatment, chip seal, or seal
Figure 5. Small nozzle opening (19).
coat is significantly larger than the size of the nozzles
needed to apply a tack coat. Using a nozzle that is too small
with too much pressure results in a surface that has a and 54°C. Excessive heating may cause the emulsion to
spider web coating of tack coat material (see Figure 5). break while still in the distributor.
· Spray bar height should remain constant. As tack coat is
applied, the vehicle will become lighter, causing the spray
bar to rise. The tack coat application vehicle should be able 2.4.2 Proper Tack Coat Application
to compensate for this. Excessively low spray bars result Proper application of tack coat is a key component in high-
in streaks (see Figure 4), while excessively high spray bars quality asphalt pavement rehabilitation. Proper tack coat appli-
cause non-uniform transverse coverage. cation begins with properly calibrated application equipment.
· Pressure within the distributor must be capable of forc- If the distributor has not been used for some period of time,
ing the tack coat material out of the spray nozzles at a con- the operator should place a trial tack coat application over
stant rate. Inconsistent pressure will result in non-uniform some convenient, unused area to ensure that all of the nozzles
application rates. are open and operating properly. In addition, the distributor
· Tack distributors must be capable of maintaining tem- application rate needs to be calibrated, both in the transverse
perature of the asphalt cement material to ensure the direction and in the longitudinal direction, using the proce-
material will adequately flow. For slow-setting asphalt dure described in ASTM Method D 2995 (19). Spray bar height
emulsions such as SS-1, the spraying temperature within depends on truck speed, nozzle configuration, and application
the distributor should be maintained between about 24°C pressure. Operators should adjust the spray bar height through-
out the day depending on the amount of emulsion in the tank.
As a summary, the literature suggests the fundamental aspects
of achieving tack coat success are
· Having a thoroughly clean roadway surface,
· Ensuring all the equipment functions properly and is set
up correctly,
· Choosing the proper application rate for the tack material
used and the existing surface conditions,
· Applying the materials uniformly, and
· Allowing the tack to set prior to paving to ensure the best
possible bond between layers.
One perpetual problem often associated with tack coat
application using distributor trucks is that haul trucks
normally drive on the applied tack coat, thus tracking the
Figure 4. Non-uniform tack coat: streaks. tack coat material and removing it from the pavement, as
