A tack coat is a thin bituminous liquid bitumen emulsion applied between HMA pavement lifts to promote bonding. Adequate bonding between construction lifts and especially between the existing road surface and an overlay is critical in order for the completed pavement structure to behave as a single unit and provide adequate strength. If adjacent layers do not bond to one another they essentially behave as multiple independent thin layers – none of which are designed to accommodate the anticipated traffic-imposed bending stresses. Inadequate bonding between layers can result in delamination (debonding) followed by longitudinal wheel path cracking, fatigue cracking, potholes, and other distresses such as rutting that greatly reduce pavement life.
Tack coats should be applied uniformly across the entire pavement surface and result in about 90 percent surface coverage. In order for this uniformity to be consistently achieved, all aspects of the application must be considered and carefully controlled. Specific aspects are:
The pavement surface receiving the tack coat should be clean and dry to promote maximum bonding. Emulsified tack coat materials may be applied to cool and/or damp pavement, however, the length of time needed for the set to occur may increase. Since existing and milled pavements can be quite dirty and dusty, their surfaces should be cleaned off by sweeping or washing before any tack coat is placed, otherwise the tack coat material may bond to the dirt and dust rather than the adjacent pavement layers. This can result in excessive tracking of the tack coat material. Construction vehicles and equipment pick up the tack-dirt mixture on their tires and leave the existing roadway with little or no tack coat in the wheel paths. Slippage cracking and delamination are distresses typically seen when cleanliness is lacking.