Final answer:
A tack coat is used as a bonding agent between new pavement layers and existing surfaces to ensure a strong, lasting bond and to prevent structural failures.
Step-by-step explanation:
The purpose of a tack coat is to ensure a strong bond between the new pavement layer and the existing surface. Without a tack coat, the new and old layers might not bind properly, leading to premature wear and structural failures. It can be described as a glue that unites layers, preventing slippage and providing a better long-term pavement performance.
The tack coat is typically a liquid asphalt emulsion that is applied evenly across the existing surface. It must be allowed to break, which means the water in the emulsion must evaporate before the new asphalt layer is placed. This ensures the proper adhesion of the pavement layers.