Using gyratory compactor to measure mechanical stability of asphalt mixtures

Abstract

In this study several asphalt mixtures were produced using four different aggregate sources, different asphalt contents, and different aggregate gradations. Every mixture was compacted using the Superpave gyratory compactor (SGC). To evaluate if the densification results from the SGC can be related to rutting of mixtures, the new axial compression test procedure for rutting measurements recommended by the National Cooperative Highway Research Program project 9-19 and used in the AASHTO 2002 pavement design manual, was also used for evaluating the rutting behavior for laboratory produced samples. Densification curves produced by the SGC were used to determine volumetric properties of the mix as well as for the calculation of the construction and the traffic densification indices. The construction densification index (CDI), which is the value of the area under the densification curve from density at 8 gyrations to density of 92% Gmm, represents the work done during the construction period to achieve 8% air voids. The traffic densification index (TDI), which is the value of the area under the densification curve from 92% density to 98% density, represents the work needed to resist traffic loading during pavement service life. Two more indices are calculated, construction force index (CFI) and traffic force index (TFI). CFI is related to the amount of work done to raise the density of the mix to 92%. The TFI is the amount of work done to increase the density of the mix from 92% to 98%. The results from the mixture rutting tests were used to estimate the rutting rate and the flow number (FN), which is the point at which the mixture starts to exhibit tertiary flow. The FN, which is considered an important mixture property, is shown to have a strong correlation to the TDI derived from the mixture volumetric behavior measured in the SGC. The main finding of the study is that SGC appears to give information that can be used to characterize the stability of the mixtures. Such information could be used as an initial screening criterion to select mixture for various traffic ESAL (equivalent single axle load) levels, in addition to indicating an expected performance level.