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Table 4.11. Summary of normal strain round-robin results.
Code No. Log of Standard Repeatability Reproducibility Between- Within-Lab Between-
of Average Deviation Standard Standard Lab Coefficient Lab
Labs of All between Deviation (Sr) Deviation (SR) Standard of Coefficient
Labs Log of Deviation Variation, of
Cell of Log of % Variation,
Averages Lab %
(Sx) Means
(SL)
PG 67-22 Opt. at 800 ms 7 3.876 0.220 0.249 0.300 0.167 6.4 7.7
PG 67-22 Opt. at 400 ms 7 5.370 0.365 0.240 0.414 0.338 4.5 7.7
PG 76-22 Opt. at 800 ms 3 3.932 0.127 0.261 0.261 0.000 6.6 6.6
PG 76-22 Opt. at 400 ms 3 5.624 0.299 0.295 0.384 0.246 5.2 6.8
PG 67-22 Opt.+ at 800 ms 3 4.203 0.107 0.207 0.207 0.000 4.9 4.9
PG 67-22 Opt.+ at 400 ms 3 5.717 0.134 0.243 0.243 0.000 4.2 4.2
Pooled 0.234 0.248 0.318 0.164 5.4 6.8
Indirect Tensile Strength as Samples of the PG 67-22 mixtures at optimum and opti-
a Surrogate for Endurance mum plus asphalt content and PG 76-22 mixtures at optimum
Limit Determination and optimum plus asphalt content were compacted in the
Superpave Gyratory Compactor (SGC) for IDT testing to
Indirect Tensile Strength (IDT)--or, more correctly, the the same air void levels used for the beam fatigue tests. The
tensile strain at failure from the IDT test--was examined samples were tested in the IDT at 25°C. Testing was conducted
as a surrogate for beam fatigue tests to identify the fatigue according to AASHTO T322. Tensile strain was calculated as
endurance limit. The Asphalt-Aggregate Mixture Analysis described by Kim and Wen (60). Strain was calculated at 98%
System (AAMAS) used a mixture's resilient modulus and ten- of the peak stress, and the results are shown in Figure 4.24
sile strain at failure from the IDT test to assess fatigue resist- versus the predicted and 95% lower confidence limits for the
ance (59). In the AAMAS system, testing was conducted at the endurance limit determined from the beam fatigue tests.
following three temperatures: 5°C, 25°C, and 40°C. The mea- From Figure 4.24, it is apparent the predicted and 95% lower
sured resilient modulus and tensile strain at failure were com- confident interval for the endurance limit are approximately
pared to the properties of a "standard" mix, the dense-graded 5 and 3%, respectively, of the indirect tensile failure strain.
mix used at the AASHO Road Test. Von Quintus (personal However, although the indirect tensile test appears to be sen-
communication) suggested that long-life pavements be designed sitive to the two different binders, it does not appear to be
with tensile strains at the bottom of the asphalt layer that sensitive to binder content. This procedure appears to have
were < 1% of the tensile strain at failure. Maupin and Freeman some potential to predict the magnitude of the endurance
(9) demonstrated satisfactory correlations between constant limit. Additional work is necessary with a broader range of
strain fatigue-life curves and indirect tensile test results. materials.
Table 4.12. Summary of round-robin extrapolations at the estimated endurance limit.
Code Number Log of Standard Repeatability Reproducibility Between- Within- Between-
of Labs Average Deviation Standard Standard Lab Lab Lab
of All between Deviation Deviation (SR) Standard Coefficient Coefficient
Labs Log of Cell (Sr) Deviation of of
Averages of Log of Variation, Variation,
(Sx) Lab % %
Means
(SL)
Logarithmic
PG 67-22 Opt. at 130 ms 5 10.609 1.642 1.322 1.965 1.454 12.5 18.5
PG 76-22 Opt. at 220 ms 3 8.209 0.441 1.207 1.207 0.000 14.7 14.7
Pooled 1.192 1.279 1.681 0.909 13.3 17.1
Weibull
PG 67-22 Opt. at 130 ms 5 8.794 0.915 0.586 1.032 0.850 6.7 11.7
PG 76-22 Opt. at 220 ms 3 7.563 0.309 0.719 0.719 0.000 9.5 9.5
Pooled 0.688 0.636 0.915 0.531 7.7 10.9
RDEC
PG 67-22 Opt. at 130 ms 4 10.275 3.421 2.341 3.919 3.143 22.8 38.1
PG 76-22 Opt. at 220 ms 3 9.607 0.967 2.151 2.151 0.000 22.4 22.4
Pooled 2.369 2.259 3.161 1.796 22.6 31.4
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350
300
y = 0.047x
Endurance Limit, Micro-Strain
R² = 0.650
250
200
150
100
y = 0.031x
R² = 0.400
50
0
0 1000 2000 3000 4000 5000 6000
Tensile Strain at Failure, Micro-Strain
Predicted Endurance Limit 95% Lower Confidence Endurance Limit
Linear (Predicted Endurance Limit) Linear (95% Lower Confidence Endurance Limit)
Figure 4.24. IDT tensile strain at failure versus beam fatigue endurance limit.
