Validating the Fatigue Endurance Limit for Hot Mix Asphalt (2010)

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127 The beam fatigue testing conducted to date indicates that there is an endurance limit for hot mix asphalt (HMA). Techniques have been developed to identify the endurance limit using beam fatigue testing. One concern is that there is no precision statement for AASHTO T321, the beam fatigue test. A full round robin is beyond the scope of this study. However, a mini-round robin should provide an indication of the variability of beam fatigue testing and of the determi- nation of the endurance limit. The round robin will encom- pass: sample preparation, beam fatigue testing, calculations to assess the endurance limit. Three techniques will be used to analyze the beam fatigue results: single-stage Weibull function, logarithmic extrapolation, and ratio of dissipated energy. The following describes the sample preparation and testing. A second document will be sent at a later date, which will describe the data analysis. 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- ously in the NCHRP 9-38 study and are based on the lower lay- ers of the structural sections of the 2003 NCAT Test Track. Optimum asphalt content for both mixes is 4.5 percent by total weight of mix. The optimum plus asphalt content is 5.2 percent by total weight of mix. Directions for Preparation of Samples 1. Each aggregate “batch” of material consists of two parts “A” and “B.” The aggregate batches were randomized before shipping to the individual labs. To make one com- plete batch, an “A” and “B” can should be dry mixed. The combined aggregate weight should be 8,776 grams. 2. Therefore, 413.5 grams of binder should be mixed with one batch for the optimum asphalt content samples and 481.4 grams of asphalt for the optimum plus samples. The mixing temperatures are: 350°F for the PG 64-22 and 350°F for the PG 76-22. 3. After mixing, the batch should be split to the size required for your compaction device. We use the following formula for estimating the target sample weight for compaction: Target Weight = (Target Density − Correction Factor) ÷ 100 × Gmm × Compacted Sample Volume where, Target Density = 93%, Correction Factor = 2.5 accounts for surface voids and the fact that the center of a com- pacted sample tends to be denser, Gmm = 2.586, Compacted Sample = length x width x height in cm3 = Volume (for us) 7.78 × 39.8 × 11.25 = 3483 cm3. Please contact us if we did not supply a large enough sam- ple for your mold. If the initial sample does not produce 7 ± 0.5 percent air voids, this number may need to be adjusted. The target density for the optimum plus mix is 96.7 percent. 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- house procedure. Vary the compaction effort to achieve the predetermined volume. Normally this means com- pacting to a specified height. 6. After the sample has cooled, it is a good idea to bulk the sample according to AASHTO T166 before sawing the sample to the test dimensions. This will be used, if neces- sary, to adjust the mass of future samples to achieve the correct air voids. 7. Use a wet saw to cut the compacted beam to 380 ± 6 mm in length, 63 ± 6 mm in width, and 50 ± 6 mm in height. 8. Determine the mass under water and SSD mass accord- ing to AASHTO T166. Dry the sample in front of a fan to A P P E N D I X F NCHRP 9-38 Beam Fatigue Round Robin

a constant mass to determine the dry mass. Calculate the sample density and air voids using a Gmm = 2.586. If the air voids for the optimum asphalt content samples are not 7 ± 0.5 percent or the air voids of the optimum plus samples are not 3.3 ± 0.5 percent, the sample density is not in tolerance and a new sample must be made. Evalu- ate whether the sample mass should be adjusted. We nor- mally adjusted by multiplying the dry mass in step 6 by the desired density divided by the measured density. 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. 10. Condition the sample to 20.0 ± 0.5°C for two hours. Samples that have been frozen should be allowed to thaw at room temperature for 16 hours prior to conditioning. 11. Test the sample according to AASHTO T321 a. Test three samples at 800 micro-strain b. Test three samples at 400 micro-strain 12. Plot the results on a log-log graph and fit a regression line. This would be a power model in excel (Figure 1). You can then solve: Y = 2E + 19(x)−5.3087 for 50,000,000 cycles. Recall that x–5.3087=1/x5.3087, then = 153 micro-strain You can also avoid the algebra and solve this using a least-squares procedure and solver in Excel. 13. Test three beams at the strain value predicted to give 50,000,000 cycles, in this example 153 micro-strain. Ten- tatively, we would like all of the labs to use the same strain level. Once you determine the strain level for 50,000,000 cycles in Step 12, please contact Brian Prowell. These sets of beams should be tested to a maximum of 12 million cycles. Later instruction will describe the procedure to extrapolate the data and confirm the identification of the endurance limit. x E = +2 19 50 000 000 5 3087 , , . 128 y = 2E+19x-5.3087 R2 = 0.9987 1 10 100 1000 10000 100000 1000000 10000000 100000000 1 10 100 1000 micro-strain Cy cle s to 5 0% In itia l S tif fn es s Figure 1. Log-log plot of strain versus cycles to failure. Lab/Mix PG 64-22 at Optimum PG 64-22 at Optimum Plus PG 76-22 at Optimum 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 Table 1. Testing matrix.