Differential compaction
Term used to describe the difference in thickness of an HMA mat before and after compaction. As a general rule-of-thumb HMA compacts about ¼ to ⅜-inch for every inch of mat thickness. Therefore, if the final mat thickness of 1 inch is desired, place about 1¼ – 1⅜ inches of loose HMA. Sometimes this is referred to as “fluffing up” the mix.