A tack coat is a very light application of asphaltic emulsion or paving asphalt on an existing pavement surface. A tack coat is used to ensure a good bond between the existing pavement surface and the new asphalt concrete overlay and between the layers of each lift of asphalt concrete as well as vertical surfaces that the new pavement will be placed against.
Asphaltic emulsion consists of three basic ingredients: paving asphalt, water, and emulsifying agent. The most commonly used emulsion is SS1H. Paving asphalt is the byproduct of refining crude petroleum. Any grade of paving asphalt is acceptable as a tack coat material, however the most commonly used is PG64-10. Local experience and engineering judgment may dictate which type of material is ultimately used. For consideration, the following information and experience is offered for each material type. Emulsions are most commonly used. However, they take longer to break then paving asphalts. For this reason, they are not recommended for use as a tack coat in relatively cool weather, at night, or when there is a short construction window. Paving asphalt is used for night work or work in cool weather because paving asphalt does not require any time to break before it can be overlaid. Paving asphalt is commonly used as a tack coat material for rubberized asphalt concrete (RAC) overlays.