Technical information

3-3
interconnected pneumatic cylinders. A precision air pressure gauge is used to adjust and
control the magnitude of the force throughout the test. The downward force is applied to
the pavement surface through a wheel assembly. Two tire assembly types are available:
dual/conventional and single wide-base. The tire inflation pressures in each case are
adjustable, up to a maximum of 120 psi (0.84 MPa). Either assembly is mounted on a
carriage capable of traversing the test pit by traveling on the steel beams. The carriage is
cable driven by a motor and a control drive. This motor was designed to accelerate the
carriage (with wheel assembly) from a static startup position to a speed of 5 mph (8
km/h or 2.235 m/s) within the first 5 ft (1.52 m) of the test pit. The carriage speed is
then maintained constant, at 5 mph (8 km/h), for the next 12 ft (3.66 m). Finally, the
speed is reduced back to zero within the last 3 ft (0.91 m) of the test pit. For this study
passes were applied via the dual-wheel assembly loaded to 15,000 lb (6,810 kg) with
tires inflated to 100 psi (0.70 MPa). Figure 3.1.2 shows a picture of the APT loading
system (the test pit is empty in this picture).
Figure 3.1.2: Picture of empty test pit and APT loading system.