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"Using the Gyratory Compactor to Measure the Mechanical Stability of Asphalt Mixtures"
Research by
Ahmed Fatin Faheem Mahmoud/ Prof. Hussain Bahia
Dept of Civil & Environmental Engineering

In this study several asphalt mixtures were produced using four different sources, different asphalt contents, and different gradations. Every mixture was compacted using the Superpave Gyratory Compactor(SGC). To evaluate if the 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 behavior in the laboratory for the same mixtures.

The SGC shows the change in height for each gyration, this helps to determine the change in density from one gyration to the other. A plot with the density against the number of gyration is called densification curve. These curves produced by the SGC were used to determine volumetric properties of the mix as well as 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 of 88% of maximum possible density to density of 92%, 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) using the rutting laboratory test recommended by AASHTO 2002. CFI is related to the amount of work done to change the density of the mix to 92%. The TFI is the amount of work done to change 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 failure. The flow number, which is considered an important mixture property, is shown to have a strong correlation to the TDI derived from the mixtures 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 levels. This result helps in developing a mechanical testing protocol for asphalt mixtures.

Fig. 4.16: Relationship between the mixture performance and traffic force index

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