Technical information

3-4
Depending on the desired mode of application, the wheel assembly can be raised
from the ground by reversing the action of the four pneumatic cylinders and returned to
the startup position for another loading cycle; this will result in a unidirectional mode of
loading. Alternatively, the pavement can also be loaded while the carriage travels back
to the startup position; this will result in a bidirectional mode of loading. Moreover,
trafficking in the APT can be applied repeatedly in the same wheel path or with wander.
In the latter case, the wheel path is randomly selected by a computer to within ±5 in
(±125 mm) from the centerline. The transverse movement for producing wander is
governed by an electric servo-motor also attached to the steel frame. Under this study
APT passes were applied in unidirectional mode only. As will be discussed later (see
Section 3.4), except for the initial part of the experiment passes were applied with
wander.
The utility room houses a boiler, transformer, heating controls and a water
circulating system. The latter can be used to introduce hot water into pipes embedded in
one of the pavement layers. This feature is mostly used when testing rigid pavements or
composite pavements (i.e., HMA overlaying concrete) to heat up the concrete slabs. The
operator control room houses computers and an interface to fully interact with and
control the APT operation. Currently this room is equipped with three personal
computers networked together; one computer is used for operational control of the APT;
the second computer is used for data collection and reduction; the third computer is set
up to monitor the APT functions. The control room also houses two scanners for
collecting data from sensors embedded in the pavement structure. At this time the
system used for the data acquisition is the Vishay Measurements Group System 6100.
Each scanner unit accepts up to 20 input cards and has the ability to scan at a very high
sampling rate, up to 10,000 samples per second per channel.
In this study a gauge array similar to the NCAT study was installed (see Section
3.5). As the APT carriage traversed the pavement, gauges were sampled at a rate of 100
scans per second. However, not all load passes were captured; usually every tenth cycle
was recorded. Also no data was collected in between load passes during which the
pavement recovered as the APT carriage was lifted in the air and returned to the startup
position. This shutdown of the data recording was done in order to save storage space. It