F-days), and mean 3 coldest out of 30--1964-65
4. The 10-year conventional section with class
(1391 C-days, 2503 F-days). Results from these
3 special subbase (F19) may fail due to asphalt
cracking, but not from subgrade rutting.
years were compared with F3W8 predictions for
5. Predictions for the 10-year full-depth sec-
the mean year that had been run in the "Schmidt"
tion indicate that it will not fail from asphalt crack-
series, since this Phase 2B series also used the
ing, but two of the three criteria for subgrade rut-
Schmidt model for asphalt modulus calculations
ting indicate early failure.
(eq 8) and the high density 1206 form for the
subgrade modulus calculations.
Figure 15 shows the freeze/thaw penetration in
Phase 2B
To investigate the influence of the applied
the F3 section predicted by FROST for all four
freeze/thaw season on the predicted performance
freeze seasons. In the mean year, multiple small
of the sections, an environmental effects sensitiv-
freeze/thaw events were followed by a long freeze
ity series was conducted using the single conven-
event that penetrated the subgrade and a spring
tional flexible test section, F3, which included
warming that remained above freezing. The mini-
class 5 special and class 3 special materials as its
mum year had several freeze/thaw events, but they
base and subbase courses (Fig. 6) and a water
were less severe--none of them penetrated be-
table at a depth of 2.4 m (8 ft). Prior simulations
yond the base course materials. The maximum
had applied upper boundary temperatures from
year began with a very severe freeze event that
the season with a freezing index near to the 30-
penetrated to the subgrade and lasted all winter. In
year mean, 195960 (freezing index 1003 C-days,
the spring, thawing of this freeze bulb was inter-
1806 F-days). This new series applied tempera-
rupted by two smaller freeze events, such that
tures from three other freeze/thaw seasons, in-
there existed a thawed layer in the subbase be-
cluding those with freezing indices equal to the
tween two frozen zones. The year with a freezing
mean of the three coldest seasons during the pe-
index equal to the mean of the three coldest in 30
riod 1959 to 1987 and the maximum and mini-
(3/30 year) had a freeze season consisting of a
mum for the 19591987 period. These seasons
single severe freeze event that penetrated into the
were maximum--197879 (1477 C-days, 2658
subgrade, which then thawed with no small freeze
F-days), minimum--198687 (467 C-days, 841
events.
(cm)
(in.)
0
0
AC
CL5
Frozen
50
20
CL3
Unfrozen
100
40
195960
Mean
198687
Min.
150
60
0
0
50
20
100
40
197879
Max.
196465
3/30
150
60
0
60
120
180
240
300 0
60
120
180
240
300
Elapsed Time (days)
Figure 15. Frost and thaw penetrations predicted by FROST for freeze
seasons in Phase 2B environmental sensitivity study. Simulations start on
1 October.
27
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