Tack Coat Emulsion
A tack coat is a very light spray application of diluted asphalt emulsion. It is used to ensure a bond between a surface being paved and the new course. For most overlays, a tack coat is advisable. Perhaps the only exception is when an additional course is placed within two or three days on a freshly-laid asphalt surface. In this case, ample bond between the two courses should develop without the use of a tack coat. In all cases, however, the surface must be clean and free of loose material.
The more common emulsion types for tack coats are diluted SS-1, SS-1h, CSS-1, and CSS-1h. The emulsion is diluted by adding an equal amount of water. To prevent premature breaking, the water is always added to the emulsion, not the emulsion to the water. Warm water is used, if practical, and added slowly. First, a test dilution is made to be certain that the water to be used is compatible with the emulsion. The diluted material is then applied at a rate of 0.25 to 0.70 liter/m² (0.05 to 0.15 gal/yd²). No more tack coat should be applied to an area than can be covered by the same day’s paving operations.
Tack coats should not be applied during period of cold or wet weather. Best results are obtained if the road surface is dry, has a surface temperature above 27°C (80°F), and there is no threat of rain. If rain occurs, care should be taken to prevent the tack coat from entering storm drains, rivers, creeks, and other bodies of water.
The goal is a very thin but uniform coating of asphalt left on the surface when the emulsion has broken. Too much tack coat may create a plane of slippage between the two pavement courses as the asphalt acts as a lubricant rather than an adhesive. It may even create “fat spots” or bleeding on the surface of the new pavement, a condition that is not only unsightly, but produces a dangerously slick pavement. Pneumatic-tired rolling of spotty tack coats will help spread the asphalt for better coverage. It will also help to lessen the probability of fat spots.
After spraying the tack coat, enough time must be allowed for complete breaking to occur before the overlay is placed. Traffic should be kept off the tacked area. If that is not possible, vehicle speeds should be kept below 32 km/hr (20 mph). The freshly-tacked pavement may be too slick for safe driving if excessive speeds are permitted, especially before the emulsion breaks.
A tack coat is also an essential part of a good patching operation. First, the area to be patched must be thoroughly cleaned and all loose material removed. Then, a fairly heavy tack coat of asphalt emulsion is sprayed, or painted, over the entire area, including the vertical sides. The tack helps hold the patch in place and imparts a watertight seal between the patch and the surrounding pavement.