Technical information

5-31
As before, the elastic properties of the base and subgrade (and rigid bottom)
were assumed to be identical in both experiments, unaffected by the different loading
conditions. The appropriate HMA temperature at NCAT was determined in Subsection
4.3.3 to be 80.6ºF (27.0ºC); based on Figure 4.3.2 this temperature level is associated
with a time temperature shift factor of 0.062 (i.e.,
=
T
a 0.062). Using a loading speed
(
V in Figure 5.2.1) that is nine times higher than the APT, responses were generated by
the calibrated viscoelastic model for the different axle configurations and loads given by
Table 2.1.1. The travel paths of the truck axles shown in Figure 4.3.5 were reused for
performing the forward calculations here. Recall that these were obtained by forcing the
measured and calculated stresses to match in the isotropic LET case (see subsections
4.3.3 and 5.1.3). Hence, the modeling capabilities should only be judged based on the
strain responses. Additionally, in order to position the axles longitudinally (as their
location was not measured in NCAT), the calculated and measured peaks were made
(forced) to coincide with each other.
Figures 5.2.5 to 5.2.9 graphically contrast the computational model and the
measured resilient responses at NCAT (vs. time). Each figure separately presents the
stresses and strains due to a different half-axle. Referring to Figure 2.1.2 and Table
2.1.1, these are respectively: steering wheel (1S), drive axle (1D and 2D), first trailer
axle (1T), third trailer axle (3T), and fifth (last) trailer axle (5T). Each figure is
comprised of nine charts, individually depicting the measured response (circular
markers) and calculated response of the viscoelastic model (solid lines) for the gauges
shown in Figure 4.3.5. The isotropic LET case is also shown for graphical comparison
(dashed lines), reproduced from Figures 4.3.6 to 4.3.10. The abscissa represents time in
seconds, matching the timeline in Figures 2.5.1 to 2.5.4. The ordinate depicts either
vertical stress (in psi) or horizontal strain (in microstrains) depending on the gauge
considered (note that the scale changes from case to case). In addition, each figure also
includes a picture of the NCAT truck with an arrow identifying the half-axle
considered.
As a general observation, these figures show that the model predictions capture
relatively well the magnitudes as well as the trends in the measured responses.