that employed the lower density subgrade
(f4w6ld) or the "1232" subgrade (f4w6ss) pre-
dicted the pavement to fail much more quickly
than when the higher (optimum) density 1206
subgrade was used. This is due to the fact that
the higher density "1206" subgrade used in
most simulations maintained a resilient modu-
lus during the summer months in excess of
276,000 kPa (40,000 lb/in.2), but the modulus
values for the same period were about 96,000
kPa (14,000 lb/in.2) for f4w6ld and about 6900
kPa (1000 lb/in.2) for f4w6ss. The "low den-
sity" predicted moduli are the closest to the
values measured during the summer months
with an FWD.
2. Variations in water table position in the
Figure 11. Cumulative damage for case f4w6 with opti-
same test section result in different predicted
mum density 1206 subgrade.
lifespans, especially if the water table is less
than 3 m (10 ft) deep at one location and more
than 6 m (20 ft) deep at another. Both the
ML5-F-2 and ML5-F-3 test section simula-
tions illustrate a shorter predicted life with a
shallow water table depth. For the shallow
water table conditions, greater availability of
water results in higher thaw weakening (re-
duced modulus values) in the spring.
3. Most of the sections are predicted to fail
due to cracking of the pavement caused by
tensile stresses at the bottom of the pavement
much before they will fail due to rutting caused
Figure 12. Cumulative damage for case f2w9.
by excessive deformation of the subgrade. It
is only the simulations of the full-depth sec-
tions (ML5-F-4, case f4w6, and ML10-F-14,
f14w50) that indicate rutting failures may oc-
cur before excessive fatigue cracking.
Figures 11 to 13 illustrate examples of the
relation between accumulating damage and
time. Additional plots of heave, frost penetra-
tion, and cumulative damage for all the flex-
ible sections are compiled in Appendix A.
Damage related to the horizontal strain cri-
teria has different patterns with time for dif-
Figure 13. Cumulative damage for case f4w6 with 1232
ferent models. Damage predicted by the MS-1
subgrade.
(Asphalt Institute) model has some increase
during winter thaw events and during the main
spring thaw, but a majority of its increase is in the
mate substantially different pavement lives for the
summer period. Damage from the MS-11 (As-
same strain conditions.
phalt Institute) model has a consistent pattern of
A close review of the values in Table 13 illus-
rising during winter and spring thaw periods, and
trates a variety of interesting occurrences:
remaining constant during the summer. The pat-
1. Cases with Mn/ROAD test section ML5-F-4
23
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