Determining minimum lift thickness for hot mix asphalt (HMA) mixtures

One of the main issues that have been discussed among asphalt technologists is the minimum lift thickness necessary to provide a high probability of adequate compaction under a reasonable compactive effort. In many cases, pavements constructed with thin lifts have been difficult to compact to the desired density level. Thinner lifts tend to cool faster and prevent particle orientation during compaction. To date, little research has been performed to determine the minimum desired lift thickness needed to facilitate optimum compaction. This research project was undertaken to determine the recommended minimum lift thickness. A total of 36 mixes were designed comprising various gradations and aggregate types. Laboratory samples were prepared using both a Superpave gyratory compactor (SGC) and an automated vibratory compactor (AVC). Seven field sections were also constructed to evaluate the relationship between air voids and thickness to nominal maximum aggregate size (t/NMAS). The results of the study indicated that t/NMAS has a significant impact on the compactibility of HMA mixtures. For improved compactibility, it is recommended that the t/NMAS be at least 3.0 for fine-graded mixes and at least 4.0 for coarse-graded mixes. Layers thinner than that recommended can be constructed but will likely require more compactive effort to achieve adequate density. The cooling rate for mixes less than 38 mm is very high making compaction difficult. Hence for best results, the thickness should be at least 38 mm even during warm weather. Paving in cold weather may require an even greater layer thickness. The work reported in this report was a part of NCHRP 9-27.