Validation of Ndesign in Iowa

Abstract

<p>The design number of gyrations, or Ndesign introduced by Strategic Highway Research Program (SHRP) used in the Superpave mix Design method has been commonly used in flexible pavement design throughout the United States. The Ndesign, also known as the compaction effort used to simulate field compaction during construction has been reported to produce air voids that are unable to reach ultimate pavement density after some period of time. Other states had conducted studies validating the Ndesign for their specific region. This study will focus on the validation of Ndesign in the State of Iowa. Pavement sections constructed in 2011 were randomly selected to determine if 4% target air voids was being achieved four years post-construction. The objective of the study is to determine if the current mix design gyratory levels are creating mixes that will reach target densification under traffic. The quality control and quality assurance (QC/QA) information at construction was matched with four year post-construction densities from field cores to determine if traffic loading is adequately compacting the surface mix. The compaction effort is critical in design. Over-compaction during design may lead to under-compaction in the field as well as reduce asphalt content and affect overall durability. Findings of the study suggest majority of mixes were not achieving 4% air voids four years post-construction and mixes with lower design gyrations compact more readily post-construction even though traffic levels are lower. Majority of the projects in the highest traffic volume were unable to reach ultimate pavement density with the current design gyrations.</p>