Models of Asphalt Concrete Field Compactibility with Focus on Lift Thickness

Compaction of asphalt concrete, quantified by air voids, is a significant performance factor. Although compactibility is affected by numerous factors (e.g., mix temperature), this paper focuses on lift thickness. The pavement industry throughout the years has observed a general trend toward thicker lifts that generally maintain temperature longer and provide more room for aggregate particle orientation during compaction. The current ratios of lift thickness (t) to nominal maximum aggregate size (NMAS) typically range from three to five. The objective of this study was to investigate the effects of lift thickness on the compactibility of 22 field paving projects (72 test locations) through the use of multiple regression analysis. Two regression models were developed to this end. The findings supported general, but not well-documented, perceptions that a t/NMAS ratio that was too high could adversely affect air void levels. Furthermore, optimal t/NMAS range with respect to air voids may be higher (i.e., four to six times higher) than the three to five times higher range allowed by many state departments of transportation. Consideration should be given to further exploration of greater t/NMAS ratios.