Page 150 - MODELING OF ASPHALT CONCRETE

128 Cha pte r F i v e

Specimen Fabrication
Asphalt mixtures were mixed and compacted at temperatures that were in accordance
with the requirements for each binder. All mixtures were aged at 135°C for 4 hours (i.e.,
short-term oven aging) before compaction.
For mixtures tested in the axial compression mode, samples were compacted into
gyratory plugs of 150 mm in diameter by 178 mm in height. Later, they were cut and cored
to cylindrical specimens with dimensions of 100 mm in diameter and 150 mm in height. For
mixtures tested in the IDT mode, samples were compacted into gyratory plugs of 150 mm
in diameter by 60 mm in height, and they were cut to 38 mm height. Both ends were cut to
ensure a more consistent air void distribution along the height of the test specimens.
The target air voids for the final cut and cored specimen were 4% ± 0.5%. In order
to achieve this density, the target air voids for the gyratory plug had to be higher than
that of the cut and cored specimen. The difference between the target air voids for the
gyratory plug and for the cut and cored specimen typically increased as the NMAS
increased with an average being about 1.5% to 2%.
For IDT specimens, a small study was conducted using four mixtures with different
NMASs to estimate the reduction in air voids when the specimen is cut from a 60 mm
height to 38 mm. The study showed that, in general, the reduction in air voids is around
1% for 9.5-mm NMAS and the reduction increases by approximately 0.5% increments
as the NMAS increases to 12.5, 19, and 25 mm.
Air voids were measured using the Corelok vacuum sealing device. Specifications
provided in ASTM D6752-03 were followed in taking these measurements and making
the calculations. Appropriate adjustments were made to account for the density of
water when measurements were taken at temperatures other than 25°C.
For each of the 24 mixtures there were 3 replicates of each tested. During fabrication
or testing, if errors were made or densities were not met, then the specimen was
discarded and an additional specimen was manufactured and tested.

Testing and Analysis

Test Setup
Testing was performed using a closed-loop servo-hydraulic machine, manufactured by
Material Testing System (MTS). A temperature chamber, cooled by liquid nitrogen, was
used to control the test temperature. Dummy specimens with thermocouples embedded
in the middle of the specimen were used to monitor the temperatures to which the
specimens were subjected.
For axial compression testing, 100-mm-diameter metal end plates attached to the top
and bottom rams were used to apply the compressive load to the specimen. Friction-
reducing end treatments were used to lessen the confinement that occurs as a result of
the friction against the specimen and the metal end plates. The end treatments were
made of two latex membranes, each 0.0125 in. thick. A very small amount of silicon
grease was smeared between the latex membranes to allow the specimen to move (dilate)
with respect to the metal end plates. Caution was used to apply only the minimum
amount of grease necessary because excess grease creates a slick interface that causes the
specimen to slip out of the test setup under high load and high frequency combinations.
Also, an approximate 25-mm-diameter hole was introduced in the center of the membrane
to allow a small amount of contact between the metal plate and the specimen, thereby
increasing the friction and reducing the movement of the specimen during testing.