Verification of the Superpave Gyratory Ndesign Compaction Levels

Abstract

The original Superpave Ndesign table contained 28 levels based on a limited laboratory experiment. This was later consolidated to 4 levels based on the sensitivity of mixture volumetric properties to Ndesign; however, these data were not verified as being correct for field conditions. An experiment was conducted to verify the Ndesign levels in the field. Samples were collected, tested and analyzed from 40 field projects. The projects were selected in a total of 16 states. The projects represent a wide range of traffic levels, binder grades, aggregate types, and gradations. Each project was visited at the time of construction and at 5 additional times after construction. The 40 pavements studied in this project appeared to reach their ultimate density after two years of traffic. A fair relationship was determined between the as-constructed and the density after two years of traffic. The high temperature PG binder grade was found to significantly affect pavement densification, with stiffer binders resulting in less densification. The ultimate in-place densities of the pavements evaluated in this study were approximately 1.5 percent less than the densities of the laboratory compacted samples at the agency specified Ndesign. The number of gyrations to match the ultimate in-place density was calculated for each project in this study. The calculated values for the two compactors used in this study differed by approximately 20 gyrations. This was attributed to differences in their dynamic internal angle. The predicted gyrations, adjusted to a dynamic internal angle of 1.16 degrees showed good agreement between the two machines. A relationship was developed between predicted Ndesign and design traffic for the projects which were not constructed using PG 76-22. Although there was a great deal of scatter in the data, the scatter was expected. The predicted gyration levels were generally less than those currently specified. All of the projects in this study were very rut resistant. The maximum observed rutting for the field projects was 7.4 mm with an average rut depth for all of the projects of 2.7 mm after 4 years of traffic. Based on the densification and performance data the Ndesign levels can be reduced for higher traffic levels and the Ninitial and Nmaximum criteria can be eliminated.