Technical information

5-26
the responses computed in the last simulated pass. However, due to computational
power limitations, only one movement of the half-axle was simulated. Furthermore, it
should be noted that unlike the previous analyses (refer to Figures 4.2.2 and 5.1.3), the
matching of computed and measured responses was not limited to the approaching
branches and was also performed for the receding branches.
The pavement system was modeled as a four layered half-space. Only the top
layer, representing the HMA, with a thickness of 5 in. (127 mm) was treated as
viscoelastic while the remaining three layers were treated as time-independent (elastic).
A constant Poisson’s ratio of 0.30 was assumed for the HMA with a relaxation modulus
)(tE that follows equation 5.2.12. The extreme values of )(tE at
0→t
and
∞
→t
were prefixed to: 5,920,000 psi (40,825 MPa) and 23,800 psi (164 MPa) respectively.
These values were obtained from the combined dynamic modulus master curve in
Figure 4.3.1 using
*
0
lim
comf
EE
r
∞→
= and
*
0
lim
comf
EE
r
→∞
= . The second layer from
the top represented the aggregate base with a thickness of 6 in. (152 mm), Poisson’s
ratio of 0.35 and modulus
2
E . The third layer from the top represented the subgrade
with a thickness of 61 in. (1,549 mm), Poisson’s ratio of 0.40 and modulus
3
E . The
bottom (fourth) layer with semi-infinite thickness represented the concrete floor of the
test pit, having the following properties:
=
4
ν
0.20 and
=
4
E 4,000,000 psi (27,580
MPa). Consequently, four unknown parameters were determined by the inverse
analysis, namely: the two remaining HMA properties
D
τ
and
D
n (equation 5.2.12), the
base modulus
2
E , and the subgrade modulus
3
E .
Table 5.2.1 presents the calibrated material properties of the layered viscoelastic
model corresponding to APT pass number 5,000. The global error term (equation 4.2.7)
was 1.32% which is much lower than, but not directly comparable to, the time-
independent cases, mainly because matching was performed for both the approaching
and receding branches of the responses. As can be seen in the table, the base modulus
was found to be lower than the subgrade modulus, which contradicts the findings from
the time-independent analyses. With reference to the resilient modulus tests (see Figure
3.3.1), a subgrade modulus of 25,915 psi (180 MPa) seems too high (i.e., exceeding the