Page 410 - Mechanics of Asphalt Microstructure and Micromechanics

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402 Ch a p t e r w e l v e

frame to accommodate the membranes and to form the pressure cavities. Connection
bolts were provided on each face of the frame to fix the wall assemblies. The function of
the frame is twofold: (1) It forms the top, bottom, and lateral sides of the six pressure
chambers (wall assemblies) that apply the external multiaxial load to the cubical AC
specimen; and (2) It serves as the reaction structure for the application of the total nor-
mal stresses to the top, bottom, and lateral faces of the cubical AC specimen. The frame
was machined from a solid billet of AISI 4140 heat treated, re-sulfurized forged steel.
The outside of the frame was machined to a dimension of 23.0 cm (9.05 in), while the
inner square cavities have a dimension of 10.35 cm (4.07 in). A 0.13 cm (0.05 in) radius
is provided at the inner corners of the square cavities to reduce stress concentrations.
The working pressure limits are 132.50 MPa (19200 psi) for uniaxial pressures, and 64.52
MPa (9350 psi) for a hydrostatic loading condition (Atkinson, 1972; NeSmith, 1997).

12.6.2.2 Top, Bottom, and Lateral Wall Assemblies
Figures 12.6b and c show a photograph of a wall assembly. The walls were machined
from a 7075-T6 aluminum plate (Atkinson, 1972), and are fastened to the frame by steel
studs mounted in tapped holes on the exterior of the frame. Each wall assembly consists
mainly of three components: (1) a cover plate, which provides the wall seal for the inte-
rior pressure cavity, (2) two threaded fluid pressure inlet/outlet connections, and (3)
three threaded holes to receive the stainless steel housing of the LVDTs (linear variable
differential transducers). When the membranes are mounted on the frame, each assem-
bly provides an effective seal against the leaking of the pressurized fluid into the atmo-
sphere. An O-ring groove on the wall forms the pressure seal between the wall assem-
bly and the reaction frame.

12.6.2.3 Deformation Measuring System
The deformation of the cubical asphalt concrete specimen is measured at three points
on the top, bottom, and each of the four lateral faces, using 18 LVDTs (3 LVDTs/face).
The core of each LVDT and its extension rod are thrust into contact with the rigid mem-
branes by a low-stiffness spring (Figure 12.6b, c). Two threaded locknuts were used as
the tip of each LVDT’s extension rod. A zinc-plated steel flat washer allows for complete
extension or compression of the spring. The output from the LVDTs is recorded by
means of a DA/PCS. The 499 XS-C’s Schaevitz series, high-pressure-sealed LVDT (from
Lucas Control Systems Products, Inc.), is suitable for operating pressures up to 210 bars
(3000 psi) in pressure-sealed chambers. The LVDTs have a nominal linear range of +/–
12.50 mm (+/–0.50 in). The EB-welded stainless steel housing is highly resistant to cor-
rosive environments, and its internal magnetic and electrostatic shielding render the
XS-C’s LVDT insensitive to external magnetic influences. The sampling rate during a
test can be easily selected from the menu of a data acquisition and process control com-
puter software (LABTECH-NOTEBOOK from Laboratory Technologies Corp.). The
computer software allows for the real-time display of the 18 LVDT channels. The analog
input (volt) signal delivered by the lead wires of the LVDTs is converted into a digital
output signal by an analog-to-digital converter (RTI-815 board from Analog Devices,
Inc.) plugged into the CPU of the PC-based computer. The deformation of each side of
the specimen along a particular direction is estimated by averaging the three LVDTs’
outputs corresponding to the specimen’s face, X(+/–), Y(+/–), or Z(+/–), perpendicular
to that particular direction. The LVDTs are located at 120 degree spacing on a 3.18 cm
(1.25 in) radius on each of the top, bottom, and lateral wall assemblies.