An overlay is any operation that consists of laying either Portland Cement Concrete (PCC) or Hot Mix Asphalt (HMA) over an existing pavement structure. This is different than a total replacement of the structure, and is typically done when there is only minor to modest damage to the existing pavement structure.
When constructing an overlay, the old surface is typically milled or ground off. Any minor structural deficiencies are then repaired. Finally, a new surface is applied.