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Section 2: Weather

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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.

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Item 316 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.

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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.

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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.

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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.

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