What is the purpose of tack coat application before overlaying asphalt pavements?

The purpose of tack coat application before overlaying asphalt pavements is to provide a bond between the existing pavement surface and the new asphalt overlay. The tack coat, typically a diluted asphalt emulsion, creates a thin, sticky layer that promotes adhesion between the two layers. This bond is essential for preventing slippage and shear stresses from developing at the interface, which can lead to premature cracking and delamination of the overlay. The tack coat also helps to seal the existing pavement surface, preventing water from infiltrating and weakening the bond. A properly applied tack coat ensures that the overlay acts as a composite layer with the existing pavement, maximizing its structural capacity and extending its service life. Without a tack coat, the overlay may not bond properly to the existing pavement, resulting in reduced performance and premature failure. The application rate and type of tack coat should be carefully selected based on the condition of the existing pavement and the type of asphalt mixture used in the overlay. Too much tack coat can lead to bleeding, while too little tack coat can result in poor bonding.