Technical information

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be shown in the following section, large differences were recorded between gauges
installed in Section n1 that were expected to measure identical responses. These
differences are assumed to originate from structural heterogeneity and slight
dissimilarity in gauge installation conditions. Consequently, any inherent dissimilarity
in the response to load of the two sections is masked by these differences.
In summary, and based on the aforementioned sequence of events, the
experimental dataset obtained during loading of Section n1 in the APT between July
and August 2004 is selected for pursual of the study objectives. Although in Chapter 4
this dataset will be analyzed twice, for the pavement in its initial condition and also
after 80,000 passes, the forecasting of NCAT response will focus on the initial loading
phases during which the two experiments were most closely linked.
3.5 STRUCTURAL BEHAVIOR
3.5.1 Instrumentation
Instrumentation types and placement techniques used in the APT study were similar to
these in the NCAT study (see Chapter 2, section 2.4). Pressure cells (Model 3500
manufactured by Geokon) were used as vertical stress gauges; CTL Group gauges
(Model ASG-152) were used for measuring horizontal strains at the bottom of the
HMA. These gauges were checked for functionality before and after embedment. The
achieved level of survivability was 100%. For further details refer to the reports
included in Appendix C.
From a mechanical point of view, the introduction of a gauge in a pavement
system produces changes to the stress and strain fields which influence the response of
the pavement in the vicinity of the gauge and hence influence the recorded values. This
disturbance was mostly investigated for the case of pressure cells (e.g., Tory and
Sparrow, 1967; Brown, 1977; Tabatabaee and Sebaaly, 1990). Numerous factors have
been identified that affect the measurements of pressure cells, including the ratio of cell
thickness to diameter, the ratio of medium stiffness to cell stiffness, cell size, and field
placement effects (Weiler and Kulhawy, 1982; Dunnicliff, 1988).