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OCR for page 41
41
Estimate of Precision optimum, PG 67-22 at optimum plus, and PG 76-22 at opti-
of Beam Fatigue Tests mum asphalt content mixes. The strain levels used for the
beams tested to evaluate the low strain extrapolations were
In Phase II, a small-scale round-robin was conducted to 130 and 220 ms, respectively, for the PG 67-22 and PG 76-22
develop precision estimates for the beam fatigue test and the mixes at optimum asphalt content. The differences arose
extrapolation procedures. ASTM C802 recommends a mini- since not all labs had reported their data when the confirma-
mum of 10 labs for a round-robin study. Due to the testing tion strain levels were selected.
time commitments involved with beam fatigue testing, only The round-robin data were analyzed according to ASTM
five laboratories, three represented by the research team, and E691. When analyzing round-robin data, it is desirable to have
two volunteers, participated. The volunteer labs only tested a constant standard deviation or constant coefficient of varia-
one mix at three strain levels. The experimental matrix for the
tion at the various test (strain) levels. To achieve this, the pre-
mini round-robin is shown in Table 4.10.
cision estimates are based on the log base 10 of the actual test
Variability is expected to consist of four components:
results. This is reasonable since fatigue transfer functions uti-
materials, sample preparation, beam testing, and analysis. To
lized in pavement design are logarithmic.
minimize materials variability, NCAT prepared and batched
Three potential outliers were identified based on the h and k
all of the aggregates. SEM Materials and the University of
statistics utilized in ASTM E691. Only one outlier was removed
California participated on a voluntary basis.
from the data set, Lab 2's results for the PG 67-22 at optimum
The directions provided for preparation of the test samples
asphalt content samples tested at 120 ms and extrapolated using
are presented in Appendix F. Each lab mixed and compacted
the logarithmic model.
their own beams for testing. The aggregate batches were ran-
domized prior to shipping. In some cases, however, additional The precision estimates, without the outlier, are presented
batches were required in order to obtain beams with the appro- in Tables 4.11 and 4.12 for the normal strain and extrapo-
priate air void levels. The samples were short-term oven aged lated data, respectively. The complete data are presented in
for 4 h at 275°F (135°C) according to AASHTO R30 prior to Appendix D. An examination of Table 4.11 suggests that the
compaction. The range of equipment used to compact the repeatability and reproducibility standard deviations for the
beams included linear kneading compactors, vibratory com- normal strain tests are consistent for the different mixes and
pactors, and rolling wheel compactors. strain levels. Based on the data in Table 4.11, the log of two
Testing was conducted in accordance with the draft prac- properly conducted tests by the same operator should not dif-
tice for the determination of the endurance limit, which has fer by more than 0.69 with 95% confidence. Similarly, the log
since been modified (Appendix A). Fatigue lives were also of two properly conducted normal strain tests by two differ-
extrapolated using the logarithmic and RDEC described in ent laboratories should not differ by more than 0.89.
Appendix C. Each lab first tested three beams each at 800 and For the extrapolation methods shown in Table 4.12, the
400 ms. Extrapolations were conducted using each lab's data single-stage Weibull function produces the least variable
to estimate the endurance limit. The average estimated en- results, followed by the logarithmic model. The RDEC pro-
durance limit for all of the participating labs was determined. cedure produces the most variable results. The use of the
Using a single strain level for the endurance limit allowed re- unique plateau value versus cycles to failure line proposed
peatability and reproducibility calculations to be performed in Appendix C appeared to create erroneously low fatigue
on the fatigue life extrapolations. Each lab then tested three lives with one of the data sets and exceptionally long lives
beams at the average estimated endurance limit to 12 million with one of the other lab's data. Based on the potential for
cycles. Low strain beams were not tested for the PG 67-22 mix overestimating fatigue life and the associated variability in
at optimum plus. The average estimated endurance limits calculation, the RDEC model is not recommended for extrap-
were 151, 175, and 188 ms, respectively, for the PG 67-22 at olating endurance limit data.
Table 4.10. Mini round-robin testing matrix.
Lab/Mix PG 67-22 at PG 67-22 at PG 76-22 at
Optimum Optimum Optimum
Plus
NCAT X X X
Asphalt Institute X X X
University of Illinois X X X
SEM Materials X
University of California X
