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OCR for page 42
42 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 25C. 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: 5C, 25C, and 40C. 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

OCR for page 42
43 350 300 y = 0.047x Endurance Limit, Micro-Strain R = 0.650 250 200 150 100 y = 0.031x R = 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.