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127 APPENDIX F NCHRP 9-38 Beam Fatigue Round Robin The beam fatigue testing conducted to date indicates that 3. After mixing, the batch should be split to the size required there is an endurance limit for hot mix asphalt (HMA). for your compaction device. We use the following formula Techniques have been developed to identify the endurance for estimating the target sample weight for compaction: limit using beam fatigue testing. One concern is that there is no precision statement for AASHTO T321, the beam fatigue Target Weight = (Target Density - Correction Factor) test. A full round robin is beyond the scope of this study. ÷ 100 × Gmm × Compacted Sample Volume However, a mini-round robin should provide an indication of the variability of beam fatigue testing and of the determi- where, nation of the endurance limit. The round robin will encom- Target Density = 93%, pass: sample preparation, beam fatigue testing, calculations Correction Factor = 2.5 accounts for surface voids and to assess the endurance limit. Three techniques will be used the fact that the center of a com- to analyze the beam fatigue results: single-stage Weibull pacted sample tends to be denser, function, logarithmic extrapolation, and ratio of dissipated Gmm = 2.586, energy. The following describes the sample preparation and Compacted Sample = length x width x height in cm3 = testing. A second document will be sent at a later date, which Volume (for us) 7.78 × 39.8 × 11.25 = will describe the data analysis. 3483 cm3. The mixes included in the study and the labs testing each mix are shown in Table 1. The mixes are the same mixes used previ- Please contact us if we did not supply a large enough sam- ously in the NCHRP 9-38 study and are based on the lower lay- ple for your mold. If the initial sample does not produce 7 ± ers of the structural sections of the 2003 NCAT Test Track. 0.5 percent air voids, this number may need to be adjusted. Optimum asphalt content for both mixes is 4.5 percent by The target density for the optimum plus mix is 96.7 percent. total weight of mix. The optimum plus asphalt content is 5.2 percent by total weight of mix. 4. The mix should then be aged for four hours at 275°F (135 °C) according to AASHTO R30. 5. The sample should then be compacted using your in- Directions for Preparation house procedure. Vary the compaction effort to achieve of Samples the predetermined volume. Normally this means com- 1. Each aggregate "batch" of material consists of two parts pacting to a specified height. "A" and "B." The aggregate batches were randomized 6. After the sample has cooled, it is a good idea to bulk the before shipping to the individual labs. To make one com- sample according to AASHTO T166 before sawing the plete batch, an "A" and "B" can should be dry mixed. The sample to the test dimensions. This will be used, if neces- combined aggregate weight should be 8,776 grams. sary, to adjust the mass of future samples to achieve the 2. Therefore, 413.5 grams of binder should be mixed with correct air voids. one batch for the optimum asphalt content samples and 7. Use a wet saw to cut the compacted beam to 380 ± 6 mm 481.4 grams of asphalt for the optimum plus samples. The in length, 63 ± 6 mm in width, and 50 ± 6 mm in height. mixing temperatures are: 350°F for the PG 64-22 and 8. Determine the mass under water and SSD mass accord- 350°F for the PG 76-22. ing to AASHTO T166. Dry the sample in front of a fan to
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128 Table 1. Testing matrix. Lab/Mix PG 64-22 at PG 64-22 at PG 76-22 at Optimum Optimum Optimum Plus NCAT X X X Asphalt Institute X X X University of Illinois X X X VA Transportation Research Council X SEM Materials X University of California X a constant mass to determine the dry mass. Calculate the You can then solve: Y = 2E + 19(x)-5.3087 for 50,000,000 sample density and air voids using a Gmm = 2.586. If the cycles. air voids for the optimum asphalt content samples are not 7 ± 0.5 percent or the air voids of the optimum plus 2 E + 19 Recall that x5.3087=1/x5.3087, then x = 5.3087 samples are not 3.3 ± 0.5 percent, the sample density is 50, 000, 000 not in tolerance and a new sample must be made. Evalu- = 153 micro-strain ate whether the sample mass should be adjusted. We nor- mally adjusted by multiplying the dry mass in step 6 by You can also avoid the algebra and solve this using a the desired density divided by the measured density. least-squares procedure and solver in Excel. 9. Is the sample is not going to be tested within 5 days, wrap the sample in plastic wrap and store it in a freezer. 13. Test three beams at the strain value predicted to give 10. Condition the sample to 20.0 ± 0.5°C for two hours. 50,000,000 cycles, in this example 153 micro-strain. Ten- Samples that have been frozen should be allowed to thaw tatively, we would like all of the labs to use the same strain at room temperature for 16 hours prior to conditioning. level. Once you determine the strain level for 50,000,000 11. Test the sample according to AASHTO T321 cycles in Step 12, please contact Brian Prowell. These sets a. Test three samples at 800 micro-strain of beams should be tested to a maximum of 12 million b. Test three samples at 400 micro-strain cycles. Later instruction will describe the procedure to 12. Plot the results on a log-log graph and fit a regression extrapolate the data and confirm the identification of the line. This would be a power model in excel (Figure 1). endurance limit. 100000000 10000000 1000000 Cycles to 50% Initial Stiffness 100000 -5.3087 y = 2E+19x 2 R = 0.9987 10000 1000 100 10 1 1 10 100 1000 micro-strain Figure 1. Log-log plot of strain versus cycles to failure.