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34 CHAPTER 4 Conclusions and Suggested Research The objective of this research was to use accelerated pave- aggregate UVA of at least 45 percent would be required for ment testing techniques to conduct the rutting, fatigue, and traffic above 100,000 ESAL. moisture susceptibility validation experiments identified in The preceding discussion of coarse aggregate UVA applies NCHRP Project 4-19 by Kandhal and Parker (1) as shown in to the coarse aggregate UVB as well. The UVB test results had Table 1. The validation effort involved subjecting HMA mix- a high descriptive ranking as did the UVA test results. Thus, tures prepared with various aggregates to full-scale accelerated either test can be used to specify coarse aggregates used in pavement testing and measuring their performance according HMA mixtures. to one of three HMA failure modes: (1) rutting; (2) moisture susceptibility; and (3) fatigue. The results are discussed below and are summarized in Table 26. Fatigue The research shows the trend that, as the coarse aggregate UVA increases, HMA mixture resistance to fatigue cracking Coarse Aggregate Uncompacted increases. Although this trend can easily be seen in the data Voids Content and appears logical, it is based on only three data points and Rutting other factors contribute to HMA mixture fatigue perfor- mance, such as initial mixture density, binder type, and pave- The coarse aggregate UVA was found to be the best single ment cross section. Nevertheless, there is no logical reason to predictor of rutting performance of the coarse-graded mix- discard the coarse aggregate UVA test as being unrelated to tures as indicated by the descriptive ranking. The test appears mixture fatigue performance. to capture information related to particle shape and texture and rutting decreases as the coarse aggregate UVA increases. As discussed in Chapter 3, the coarse aggregate UVA appears Flat or Elongated Particles to be less important at lower traffic levels. The number of APT Rutting wheel passes to reach a rut depth of 3.5 mm covers a range of approximately 100. The coarse aggregate UVA is more sensi- The percentage of flat or elongated particles, 2:1 ratio, does tive at higher traffic levels where the number of wheel passes exhibit a predictive relationship with HMA mixture rutting to reach a 7-mm rut depth covers a range of approximately performance. Rutting increases with increasing FOE21. The 14,000. In both cases, the relationship between traffic and FOE21 has a descriptive ranking only slightly lower than coarse aggregate UVA seems less sensitive for UVA values in coarse aggregate UVA. In fact, FOE21 was found to be posi- the range of 40 to 45 percent. The relationship becomes tively correlated with UVA (i.e., as FOE21 increases, so does stronger in the coarse aggregate UVA range of 45 to 50 per- UVA). The two tests seem to predict HMA mixture rutting cent. Previous testing in the APT has indicated that one APT equally well. However, when the two descriptors are com- pass is equivalent to approximately 2,500 ESAL. Applying this bined, they do not improve rutting prediction. If only one of equivalency factor to the coarse aggregate UVA/wheel pass the two tests is to be used, the coarse aggregate UVA seems data, a performance limit occurs at 100,000 ESAL. For preferable because it is typically less time consuming than the expected traffic below 100,000 ESAL, a minimum coarse FOE21 test. However, given that the FOE21 and coarse aggre- aggregate UVA of 40 percent would be required. A coarse gate UVA results are positively correlated, it would be good