Tack Coat
From RitchieWiki
A tack coat is thin bituminous liquid asphalt used to provide bonding between an existing pavement surface and hot-mix asphalt overlay.
The tack coat is essential because if the two layers do not bond together they will act as multiple independent layers, none of which are capable of dealing with the anticipated traffic. Cracking will result from an inadequate bond, which will decrease the life of the pavement.
In order for the tack coat to be successful, it must be uniformly applied across the entire surface. Insufficient tack coat can result in inadequate bonding, whereas too much can create lubricated slippage between layers. Therefore, the machine applying the tack coat must be in perfect working condition. Nozzle patterns should be identical, the height of the spray bar must remain constant, and pressure in the distributor must be uniform throughout the entire application.[1]
Equally important for a successful tack coat is the cleanliness of the surface to be paved. If debris exists, the tack coat will bond to it instead of the desired surface. If this occurs, the bond will not be uniform.
It is also important to allow the surface enough time to properly set or cure before driving on it. Otherwise, vehicles can pick up the material on their tires; this is called tracking. In extreme cases, tracking can distort pavement surfaces or hinder a driver’s ability to navigate.[2]
