Page 117 - MODELING OF ASPHALT CONCRETE

Complex Modulus Characterization of Asphalt Concr ete 95

FIGURE 4-6 General schematic of gauge points (not to scale). (Witczak et al. 2000, with
permission from Association of Asphalt Paving Technologists.)

wheel compaction method. In the Superpave volumetric mix design method, the
specimens are compacted using Superpave Gyratory Compactor (SGC), which is also
used for compacting test specimens for the simple performance testing. In addition, the
SGC will be used to compact test specimens for the new pavement design guide material
characterization testing.
The standard protocol for laboratory determination of asphalt concrete stiffness
dictates that the specimen size and boundary conditions produce a homogeneous stress
distribution in the test specimen. A study by Witczak et al. (2000) determined the minimum
test specimen dimensions that provide measured responses and material properties that
are independent of the test specimen size (end effects) and aggregate size using gyratory
compacted laboratory test specimens. The experiment included four height-to-diameter
ratios being 1, 1.5, 2, and 3, and three specimen diameters being 70, 100, and 150 mm. The
specimen diameter was also used as gauge length for the LVDTs. Testing was conducted
with three mixtures with nominal aggregate sizes of 12.5, 19.0, and 37.5 mm. For the
dynamic modulus test, the 70-mm-diameter specimens with height-to-diameter ratio of
1.5 or greater gave acceptable test results when tested at 4 and 40°C. The specimen size of
100 mm in diameter and 150 mm in height has been selected for the new dynamic modulus
test protocol development at ASU. The gauge length used is 100 mm.
After compaction, the 100-mm-diameter cylindrical test specimens are cored from
the center of the 150-mm-diameter gyratory specimens. If the desired specimen height
is 150 mm, the gyratory specimen has to be compacted to a height of at least 170 mm in
the gyratory mold. This gives 10 mm of each specimen end that can be sawed to obtain
smooth ends perpendicular to the axis of the specimen. A recommended tolerance for
perpendicular ends is 0.05 mm across any diameter, which can be checked using a
straight edge and feeler gauges. The specimen end should not depart from perpendicular
to the axis of the specimen by more than 0.5°.
The ASTM protocol required capping specimens with sulfur mortar to ensure
parallel specimen ends to prevent any misalignment and rocking during testing.
Witczak et al. (2000) have recommended that specimens should not be capped and
friction-reducing treatments should be placed between the specimen ends and the
platens (hardened steel disks). This treatment is needed to have homogenous stress
distribution in the specimens by avoiding any restraints capping may introduce. The
end treatment consists of two 0.5-mm-thick latex sheets separated with silicone or
vacuum grease.