What is the recommended minimum lift thickness for placing HMA?

Minimum lift thickness should be at least 3 times the nominal maximum aggregate size to ensure aggregates can align themselves during compaction to achieve required density and also to ensure mix is impermeable. The maximum lift thickness is also dependent on the type of compaction equipment being used. When static steel-wheeled rollers are used, the maximum lift thickness that can be properly compacted is 3 inches. When pneumatic or vibratory roller is used, the maximum lift thickness that can be compacted is almost unlimited. Generally, lift thicknesses are limited to 6 or 8 inches. Proper placement becomes a problem in lifts thicker than 6 or 8 inches. For open-graded mixes, compaction is not an issue since it is intended that these types of mixes remain very open. Therefore, the maximum size aggregate can be as much as 80 percent of the lift thickness.