What is required between each layer of asphalt to ensure proper bonding?

The requirement for a tack coat between each layer of asphalt is crucial for ensuring proper bonding. A tack coat is a thin layer of asphalt emulsion that is applied to the surface of the existing layer before the new asphalt is laid down. This emulsion creates a bond between the layers, allowing them to adhere to each other effectively. Without this bond, the layers may separate over time due to traffic loads, temperature changes, or moisture infiltration, which can lead to premature pavement failure.

Using a tack coat minimizes the risk of delamination, which is the peeling away of asphalt layers, and helps to distribute loads more evenly across the pavement surface. This bonding layer significantly enhances the longevity and performance of the overall asphalt structure. The other options presented do not fulfill the same purpose; for example, a gravel base supports the pavement structure but does not aid in layer bonding, and sealant layers and adhesive primers have different applications unrelated to layer-to-layer adhesion in asphalt paving.