tions to failure. And finally, in case f4w6ss, with
Results from the Phase 2A series that used the
the 1232 subgrade that produced lower resilient
Ullidtz model for asphalt modulus calculations
modulus values in lab tests, failure is predicted
and a recovered summer subgrade modulus of 1.03
105 kPa (15,000 lb/in.2), a close approximation
after only 72,000 applications, or in less than one
of the value back-calculated from FWD measure-
year.
ments on subgrade during fall 1991, yield perfor-
After the Phase 1 simulations were complete,
mance predictions that are as "fine-tuned" as pos-
the resilient modulus data from the 1206 subgrade
sible, so far (Table 17 "Ull-15K"). When these
were reviewed. A calibration error of unknown
Mechanistic Pavement Design Procedure predic-
magnitude was discovered in the resilient modu-
tions are compared with the designed failure at
lus equipment used to measure the unfrozen modu-
3,300,000 applications for the 5-yr sections and
lus on that material. Modulus testing of the 1206
6,600,000 applications for the 10-yr sections, the
subgrade in the frozen condition and all testing on
following statements can be made:
the other materials were conducted on a different
1. Predictions for the full depth sections, both 5
machine, which passed its calibration checks.
and 10 yr, indicate that they will not fail prema-
Comparing frozen and unfrozen resilient modulus
turely from asphalt cracking, but two of the three
test results from the 1206 subgrade with those
criteria for subgrade rutting indicate early failure.
from other materials tested (Fig. 14) indicates that
the unfrozen Mr values for the 1206 subgrade may
2. Conventional sections are predicted not to
be about an order of magnitude too high. A lower
fail due to subgrade rutting; however, sections with
Mr for the 1206 subgrade would have resulted in
more frost-susceptible bases are predicted to fail
earlier failures than computed in this report where
because of asphalt cracking relatively early in their
the 1206 optimum density subgrade was used.
design life, and sections with non-frost-suscep-
We are currently investigating the unfrozen 1206
tible bases are predicted to fail towards the end of
subgrade results and will include findings and re-
their design life.
vised performance predictions in Berg (in prep).
By modeling 21 years of environmental condi-
It is obviously extremely important to use the
tions in the Phase 3 series, we were hoping to
representative subgrade conditions in the design
identify the characteristics of a winter that could
simulations. Results are consistent with observa-
be used for design purposes. The results were com-
tions from in-service pavements; e.g., weak areas
plex and a relation between predicted failure and
fail much more rapidly than strong ones and high
characteristics of freeze seasons was elusive, at
water tables cause failures before similar pave-
best.
ments constructed over lower water tables. Both
It is recommended that funding be located for
the "1206" and "1232" subgrade samples were
the following studies:
obtained from the test site and were located less
The class 4 special and class 5 special base
than one-half mile apart, yet their tested behavior
materials should receive the full complement of
was quite different. It is hoped that the actual
laboratory tests so that simulations may be run
subgrade will perform more like the "1206"
using properties of actual materials from the site
subgrade than the "1232" material.
rather those of substitute materials.
Another important aspect governing the results
Once performance data are received from Mn/
of the Mechanistic Pavement Design Procedure is
ROAD, predicted and measured values of mois-
the method used to calculate the asphalt concrete
ture, temperature, strains and failure times should
resilient modulus. The use of the Schmidt model,
be compared to complete the verification compo-
which produces extremely low summer season re-
nent of the study.
silient moduli, partially accounts for the short
Use performance data from Mn/ROAD to
lifespans predicted in the Phase 1 modeling se-
revise, refine or develop new damage models for
ries. Replacement with the Ullidtz model at tem-
rutting and fatigue cracking.
peratures above 1C in the Phase 2A modeling
Use performance data from Mn/ROAD to
series increased the predicted lifespans of the sec-
refine or develop a new model for the change of
tions when judged by the horizontal strain criteria
the asphalt modulus with temperature.
(Table 17).
Use data from falling weight deflectometer
43
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