end. At OCA the subsurface temperature measure-
area concept and the Impulse Stiffness Modulus
ments indicated that the pavement structures were
(ISM) to characterize the changes in pavement perfor-
partially thawed at the beginning of FWD testing and
mance. The basin area method was a good indicator of
completely thawed prior to the end. It should be noted
AC pavement response during thaw periods under
that no FWD data were obtained during the 6-day pe-
controlled conditions (Janoo and Berg 1990, 1991).
riod from 29 March through 4 April, which was un-
The deflection basin obtained from the seven-deflec-
doubtedly the critical thaw-weakening period. This
tion sensor system is used to calculate the basin area
was unfortunate; however, we will work with the data
by the following equation
that we have.
6
∑ [(δ i + δ i+1)(ri+1 - ri )]
1
Basin Area =
2
Bearing capacity analysis
i=0
Any change in the structural capacity of the pave-
where δi is deflection at sensor (i), and ri is sensor (i)
ment during the thaw-weakening period was inferred
from the FWD data. We used both the deflection basin
distance from the center of the loading plate.
30
1000
FWD 3
FWD 7
FWD 22
800
FWD 23
FWD 24
20
600
400
10
200
Outagamie County Airport
0
0
10 Mar
15
20
25
30
4 Apr
9
14
19
24
29
a. Change in basin area.
1000
30
FWD 3
FWD 7
FWD 22
800
FWD 23
FWD 24
20
600
400
10
200
Outagamie County Airport
0
0
10 Mar
15
20
25
30
4 Apr
9
14
19
24
29
b. Change in ISM.
Figure 9. Changes in basin area and impulse stiffness modulus during spring thaw at Outagamie County Airport.
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