The objectives of this project were to (1) determine the relationship between mix properties measurable during Superpave gyratory compaction and permanent deformation of pavements in service and (2) recommend any practical modifications to existing Superpave gyratory compactors, test methods, or both, to measure the identified properties.

Of the candidate SGC parameters identified though the literature review and survey of research results, the one that offered the greatest early promise was k x AV, the product of the compaction slope and the percent air voids. However, the laboratory testing program failed to validate the general utility of this parameter for mixes from several field projects, including NCHRP Project 9-7, the 1992 LTPP SPS-9 pilot projects, the WesTrack experiment, and several LTPP GPS projects used in the original development of the SGC N_design levels in the SHRP Asphalt Research Program.

The research team then tested a second good candidate, N-SRmax, the number of gyrations at maximum stress ratio. It found that N-SRmax was capable of correctly grouping laboratory mixes with good, fair, and poor expected rutting resistance and that this finding was reasonably validated by the data from the field projects described above. The parameter is directly obtainable from any commercial SGC capable of measuring shear stress during compaction; for compactors not meeting this criterion, the use of an auxiliary external device, such as the Gyratory Load Cell Plate Assembly developed by the University of Wisconsin, may produce acceptable results.

The research results suggest that N-SRmax provides a rapid means of identifying gross HMA mix instability or rutting potential during volumetric mix design. Thus, it can serve as a useful screening or indicator parameter; it is not, however, a fundamental material property for predicting the development of permanent deformation. Moreover, although N-SRmax is reasonably sensitive to aggregate structure and binder volume, it cannot detect the effect of changes in asphalt binder stiffness (i.e., the performance grade of the asphalt binder) on mix stability. The research team has also prepared a draft test method in AASHTO standard format for screening HMA mix designs for gross mix instability by measurement of N-SRmax during compaction with the SGC.

The final report has been published as NCHRP Report 478.