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Section 2: Functions of Surface Treatments and Seal Coats

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A surface treatment is placed on a crushed stone base to provide a roadway with the least expensive permanent type of bituminous surface. It seals and protects the base and provides strength at the road surface so that the base can resist the abrasive and disruptive forces of traffic. It also provides many of the functions that a seal coat provides.

When applied to a bituminous pavement surface, a seal coat provides a durable all-weather surfacing that:

The functions of the asphalt binder are to bind the aggregate particles to the underlying surface and to provide a waterproof seal. The functions of the aggregate are to resist traffic abrasion, to transmit wheel loads and to provide skid-resistance and the desired surface texture.

A seal coat or surface treatment has little or no structural strength itself but by preventing the ingress of water it enables the inherent strength of the pavement and the subgrade to be preserved. If a pavement shows evidence of traffic load associated cracking (alligator, longitudinal, transverse), a seal coat is only a temporary solution. Areas that show load-associated cracking may require base repair prior to a seal coat or overlay. A thick asphalt concrete overlay or reconstruction is normally required to correct these problems. Seal coats applied to pavements showing signs of non-traffic load associated longitudinal and transverse cracks have proved somewhat effective. Seal coats usually bridge these cracks in a more satisfactory manner than thin asphalt concrete overlays.

Ride quality of a pavement cannot be improved significantly by the application of a seal coat. Overlays of various thickness, spot level-up maintenance patches, or reconstruction are normally required to restore pavement ride quality.

Pavements demonstrating flushing or bleeding are difficult to repair with seal coats. The binder normally migrates through an added seal coat unless the asphalt quantity applied to the roadway can be altered at these spot locations. Seal coats utilizing a large maximum size aggregate are suggested if seal coats are used on flushed surfaces.

Seal coats have been used successfully on both low- and high-traffic volume roadways, but tend to be more successful on low-volume roadways, especially low-volume truck traffic. The use of seal coats in urban areas where accelerating/decelerating traffic and turning movements frequently occur should be approached with caution and is addressed more in Chapter 2.

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