Section 2: Weather
Introduction
Weather plays an extremely important role in seal coat operations.
There are many things that can result from a sudden change in weather,
most of which are undesirable in seal coat work.
The best conditions for applying a seal coat or surface treatment
are when temperatures are high, humidity is low, and there is little
or no wind. In most parts of Texas, these conditions are most likely
to occur from June through September. Too early in the spring or
too late in the fall brings temperature and wind problems. The extended
forecast should always be checked.
Temperature
of the Standard Specifications requires that seal coats
and surface treatments be placed when the air temperature is above
50°F and rising. Seal coats and surface treatments may not be applied
when the air temperature is below 60°F and falling. In all cases,
no seal coat or surface treatment may be applied when the surface
temperature is below 60F.�
If a polymer-modified asphalt cement is used, it shall be
applied when the air temperature is above 70°F and rising and not
when the air temperature is below 80°F and falling. Surface temperature must
not be below 70°F. When wintertime work is allowed, the Engineer
will approve the air and surface temperature for the asphalt application.�
If asphalt-rubber is used, it shall be applied when the air
temperature is 80°F and above, or above 70°F and rising. In all
cases, do not apply seal coat when surface temperature is below
70°F.�
During the summer months, roadway temperatures are commonly
100°F or higher by 9 a.m. So in most cases, temperature
is not a problem. An inspector must record the surface temperature
every morning before any asphalt is shot. This can be done by placing
a surface contact type of dial thermometer on the roadway. The temperature
reading should be taken under conditions typical of those in which
the asphalt will be shot.
Humidity
If possible, it is best if the asphalt is applied when the
humidity is 50 percent or lower, especially when emulsions are used.
With any type of asphalt a lower humidity is better. High humidity
can cause an invisible film of moisture to collect on the roadway
surface, which may interfere with the asphalt sticking properly
to the surface. With emulsified asphalts, the emulsion will be slower
to break in high humidity. With asphalt cement, which is shot at
much higher temperatures than emulsions, steam can be seen rising
as the hot asphalt hits the moisture on the roadway surface. As steam
is trapped under the asphalt, small bubbles form and break as the
air and moisture work their way to the asphalt surface.
Wind
Wind can be both a disadvantage and an advantage. When an
emulsion is used, a gentle wind of constant velocity can accelerate
the breaking of the emulsion and allow traffic on the roadway sooner.
If the wind varies or is too strong, it can distort the fan pattern
as the asphalt is applied. This may cause streaking and uneven distribution.
Contractors may install a shield in front of the spray bar to minimize
wind effects on the spray pattern.
Wind also tends to blow asphalt onto passing vehicles. It
is particularly important to be careful of wind direction when applying
modified asphalt cements. Small “cobwebs” of asphalt are blown around
and are almost invisible until they land on a light-colored vehicle.
To minimize the effects of blowing asphalt, the sequence of work
should be considered.
Rain
No asphalt binders should be applied during rain. If rain
is in the vicinity and predicted for the area, suspension of operations
should be considered.
If an unexpected shower arises during operations, shut off
the asphalt distributor immediately and continue placement of aggregate
until all asphalt has been covered. This area should be rolled well and
watched carefully after opening to traffic. After a rain, always
suspend operations until the pavement has completely dried.