About the research
Optimizing the asphalt mixture design process to produce mixes that balance excellent performance with economical materials, requires addressing the current Ndesign levels. Ndesign is where differences between the laboratory mix design compaction effort and the air voids that are ultimately achieved in the field can be improved. Validating this relationship for Iowa asphalt mix designs will lead to better correlations between the mix design target voids, field voids, and performance. A comprehensive analysis of current N-design levels will investigate the current levels with existing mixes and pavements as well as develop asphalt mix designs that identify an optimum N-design through the use of performance data tests.
As a result of the Phase I study, the Ndesign specifications were changed in October 2016. In addition to the mix design changes, the Iowa DOT has implemented new asphalt binder grading criteria.
Phase II is primarily a validation study of the new mix design specifications. The first objective is to evaluate performance of the field mixes produced with the 2016 specifications to ensure performance expectations are being met for rutting, moisture susceptibility and low temperature cracking; both loose mix and cores will be sampled for subsequent testing. The second objective is to evaluate the new mix design specifications in terms of performance and constructability. The third objective entails performance testing of each mix at the optimum binder content in a given Ndesign level using such tests as the dynamic modulus, Hamburg wheel track device, and disk-shaped compact tension to evaluate the stiffness, rutting moisture susceptibility, and resistance to low temperature cracking.The last objective is to take results from the dynamic modulus, and site location information, to use in AASHTOWare Pavement-ME to forecast long-term pavement performance impacts from changing the asphalt content or Ndesign.