Relating N_design to Field Compaction: A Case Study in Minnesota

High field density helps in increasing the durability of asphalt pavements. In a current research effort, the University of Minnesota and the Minnesota Department of Transportation (MnDOT) have been working on designing asphalt mixtures with higher field densities. One critical issue is the determination of the N_design values for these mixtures. The physical meaning of N_design is discussed first. Instead of the traditional approach, in which N_design represents a measure of rutting resistance, N_design is interpreted as an indication of the compactability of mixtures. The field density data from some recent Minnesota pavement projects are analyzed. A clear negative correlation between N_design and field density level is identified, which confirms the significant effect of N_design on the compactability and consequently on the field density of mixtures. To achieve consistency between the laboratory and field compaction, it is proposed that N_design should be determined to reflect the real field compaction effort. A parameter called the equivalent number of gyrations to field compaction effort (N_equ) is proposed to quantify the field compaction effort, and the N_equ values for some recent Minnesota pavement projects are calculated. The results indicate that the field compaction effort for the current Minnesota projects evaluated corresponds to about 30 gyrations of gyratory compaction. The computed N_equ is then used as the N_design for a Superpave 5 mixture placed in a paving project, for which field density data and laboratory performance test results are obtained. The data analysis shows that both the field density and pavement performance of the Superpave 5 mixture are significantly improved compared with the traditional mixtures. The results indicate that N_equ provides a reasonable estimation of field compaction effort, and that N_equ can be used as the N_design for achieving higher field densities.