EXPERT ADVICE
chemicals, salts, etc. from penetrating and
damaging the asphalt pavement underneath.
Understandably, any deficiency in the curing
process will not allow the binder to fuse properly
and leave voids in the film, thus resulting
in inferior performance or failure.
During the curing process, sealcoating films
transition through various stages of water
evaporation from the applied film. First it
attains initial drying when the film becomes
“tack free” to the light touch, then it becomes
firmer (about 80-90% cured) to take light
pedestrian traffic, and finally full firmness to
withstand light vehicular traffic, when all the
water is lost through evaporation.
For understanding the drying and cure
process, visualize the wet film not as one solid
entity, but a composite of numerous layers
of molecularly thin films (imagine a sheet of
plywood). Like most water-based coatings,
sealcoating dries, in successive layers, from top
to bottom. As each layer dries, it shrinks in
volume, becomes tight and relatively impervious,therefore,
impeding the evaporation of
water from the bottom layers. It has been established
that the cumulative water loss happens
rapidly in the early stages of film drying
i.e. the first 80-90% of the film dries relatively
fast, but the release of the last 15-20% of water
takes much longer, requiring the right set
of ambient and pavement conditions. For optimum
performance, all water must leave the
film: 80-90% is not good enough. The uncured
bottom layers of the sealcoating will be
torn or dislodged if traffic is allowed on it too
soon. The percentages noted above are strictly
to explain the phenomenon of the cure stages.
The final cure will depend on many factors;
mix design, coverage rate and the ambient
conditions of temperature, humidity and the
wind velocity.
Ambient Cure Conditions
The ambient conditions play the decisive
role in determining the thoroughness
of the overall cure process, therefore, sealcoating
performance. These conditions
are the recommendations of the industry
and its research association, Pavement
Coatings Technology Council (PCTC).
1. Temperatures: Both ambient and pavement
Sealer should not be applied unless the
pavement temperature is at least 50°F and the
air temperature is 50°F and rising. The fusion
of the binder particles (in sealcoatings) to
form a uniform and continuous film depends
on their ability to soften under the ambient
and pavement temperatures. The process
of fusion is greatly enhanced at higher temperatures;
say 75°F to 85°F. Conversely, it is
significantly reduced at temperatures below
50°F. Let us review the extremes; cold and
hot temperature applications.
Cold Temperature Application.
When sealcoating is applied below 50°F, refined
tar (or asphalt) particles do not soften
and form a continuous film, thus leaving clay
and filler particles uncoated. The color of the
sealcoating cured under such conditions usually
turns out grey and blotchy in appearance
and never returns to its characteristic dark
slate/black color, even when the pavement
temperatures rise, later. Simply the temperature
of the pavement normally does not reach
high enough to re-melt the binder particles
and force them to flow and form a continuous
film. Even if the pavement temperatures
reached high enough, still it will not be sufficient
to re-mobilize the binder particles to
flow and envelope the clay and filler particles
to form a continuous film. Needless to say
sealcoating cured under cold weather conditions
lack the integrity and are liable to fail
prematurely.
Hot Temperature Application.
Applications under hot temperatures can
be equally problematic. Sealcoatings should
not be applied under the summer sun (over
90°F,ambient) without first cooling the surface
with a fine mist of water, also called “fogging”.
Water should only dampen the pavement,
without leaving puddles.
If applied to a hot pavement without
“fogging” it, the sealcoating film almost gets
“baked” as soon as it hits the pavement. With
the sudden loss of the film fluidity, the bind-
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