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Section 3: Traffic Volume Considerations for Seal Coat Treatment

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Although a seal coat can be applied to high-traffic volume roadways, it is generally limited to low-traffic volume roadways. Facilities with average daily traffic in excess of 10,000 vehicles per day will be considered as high traffic for purposes of this manual. Problems that can occur on high-traffic volume roadways are sometimes related to the following:

Scott Shuler1 (1990) offers several solutions to alleviate the impediments to using chip seals (seal coats) on high-volume facilities. Some of these solutions are discussed below.

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Short-Term Aggregate Loss

Short-term aggregate loss refers to aggregate loss within hours or days after construction. If loss occurs within a few days, causes may be related to the following:

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Vehicular Damage

The potential liability due to vehicle damage from dislodged aggregate may be the primary reason for not using seal coats on high-traffic volume roadways. Damage can occur to windshields, headlights, radiators, and vehicle paint.

While there is sometimes a tendency to apply excess aggregate to avoid tracking by rollers, this excess aggregate can cause damage to vehicles. In addition, when more than one aggregate thickness is present, additional aggregate particles on the surface are pushed into those below. This action dislodges aggregate in the first layer causing loss of aggregate and changes in grading (due to aggregate crushing). The correct aggregate quantity should produce a layer, which is one-stone thick.

Allowing slow-moving traffic on a new seal coat after final rolling and sweeping is one of the best means to reduce premature aggregate loss. Slowly moving vehicles also seem to provide a level of aggregate orientation not achievable by conventional pneumatic rollers. One method to assure the traffic will move slowly is to use pilot vehicles. This practice is often not followed because of the inconvenience to motorists on high-volume facilities. To alleviate this problem, seal coat operations can be performed when traffic is at reduced levels.

The potential for vehicle damage can also be reduced by using a small aggregate size (such as a Grade 4) or by using lightweight aggregates which have a much lower specific gravity than conventional mineral aggregates.

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Tire Noise

The best seal coats are those that are effective sealing mechanisms and those that provide a long-lasting, high-friction riding surface. One of the ways to achieve these two objectives is by using large, one-sized aggregates (1/2 inch or greater). The larger aggregates require greater asphalt quantities to bind the aggregate that provides greater sealing capabilities while providing necessary friction. These larger aggregates often generate complaints by motorists because the tire noise level is greater. While the larger aggregates can increase the surface texture and improve friction, the key to friction is in the skid properties of the aggregate.

Double application seal coats (two-course surface treatment) using a smaller aggregate for the top layer will provide for less tire noise. This results in a first layer with more voids in the surface, while the second application of aggregate fills in the voids.

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Prolonged Traffic Control

Prolonged traffic control is primarily associated with emulsified asphalt binders. Increased traffic control is often necessary until the emulsion has had time to break and develop tensile strength to hold the aggregate.

Modified binders may offer a higher level of adhesion than corresponding conventional binders. Therefore, aggregate retention is better during the early life of the seal coat, and often the rigid levels of traffic control required for emulsions are not as significant when polymer-modified binders are used.

1. Shuler, T.S., 1990. “Chip Seals for High Traffic Pavements,” Transportation Research Record No. 1259, Transportation Research Board, National Research Council, Washington D.C.

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