"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
