Iowa Department of Transportation
Iowa Highway Research Board
Chris Williamsrwilliam@iastate.edu email >
About the research
Optimizing the asphalt mixture design process to produce mixes that balance excellent performance with economical materials, requires addressing the current Ndesign values. 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 mix design target voids, field voids, and performance.
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 Department of Transportation (DOT) implemented new asphalt binder grading criteria. Phase II is primarily a validation study of the new mix design specifications.
The main purpose of this study was to evaluate and compare differences in performance between mix designs made using old and new Ndesign values (different optimal binder contents) for low, medium, and high traffic. Performance comparisons were made for three mix designs with one for each traffic level (low, medium, and high) using tests such as Hamburg wheel tracking for rutting and moisture susceptibility, flow number for rutting, disk-shaped compact tension for low-temperature cracking, -point beam fatigue for fatigue cracking, and dynamic modulus for characterizing asphalt mix stiffness with changing temperatures. Lastly this study used results from the dynamic modulus and dynamic shear rheometer, along with site location information, for use in AASHTOWare Pavement ME Design, to forecast long-term pavement performance impacts by changing the asphalt content Ndesign value and comparing performance to field distress surveyed performance.
When produced with new Ndesign values (higher optimal binder content), the three mix designs were observed to perform better against rutting, fatigue cracking, and low-temperature cracking than that of mixtures made using old Ndesign values. Significant differences were shown between old and new Ndesign for IA 4 using flow number results and IA 330 and I-235 for Hamburg wheel tracking test results in terms of resistance against rutting. Laboratory results appear to validate that changing to new Ndesign value specifications does improve mix performance.
When comparing predicted performance from AASHTOWare Pavement ME Design against field performance in the Iowa DOT’s Pavement Management Information System, there were significant differences for both old and new Ndesign produced mixtures. A possible reason for this is a need for more level 1 input data in ME Design such that predicted performance can be better correlated to field performance.