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29 CHAPTER 3 Interpretation, Appraisal, and Applications The purpose of this chapter is to discuss and interpret the a rutting/resistivity model (Equations 1 and 2). This model findings presented in Chapter 2, with special emphasis on the allows some quantitative (but approximate) estimates to be practical application of these findings. This chapter is pre- made regarding how changes in the composition require- sented in four sections: ments of HMA might affect rut resistance. The following factors tend to improve the fatigue resistance 1. A summary of the relationships among HMA mixture of Superpave and other HMA mix types: characteristics and performance; 2. A discussion of how HMA mix design specifications have Increasing effective asphalt binder content, at given levels evolved over the past 30 years, and how the resulting of Ndesign, design air voids, and in-place air voids; changes have affected potential pavement performance; Increasing Ndesign, at given levels of VBE, design air voids, 3. A discussion of potential revisions in Superpave require- and in-place air voids; and ments for HMA composition and compaction, and how Decreasing in-place air voids, at given levels of design these revisions might affect various aspects of perform- voids, VBE, and Ndesign. ance; and 4. A discussion of the implementation of the results of this Asphalt binder rheologic type, as reflected in the rheo- research, which includes an Extended Work and Validation logical index R, also affects fatigue resistance; in the labora- Plan. tory, increasing values for R tend to improve fatigue resistance. However, significant experience with actual pave- ments suggests that HMA made using binders with high R Summary of Relationships Among values often exhibit extensive premature surface cracking, HMA Mixture Characteristics contradicting the results of most laboratory fatigue tests. and Performance Therefore, it is not recommended that mix designers The findings presented in Chapter 2 dealt primarily with attempt to improve the fatigue resistance of HMA mixes by relationships among mixture characteristics and various selecting binders with high R values. These findings on aspects of performance. Before discussing the practical impli- fatigue resistance are largely based on continuum damage cations of these findings, a summary of these relationships is theory, which was used to analyze a large amount of labora- useful. The following factors tend to improve the rut resist- tory fatigue data, including data gathered during NCHRP ance of Superpave and other HMA mix types: Projects 9-25 and 9-31 and flexural fatigue data collected during SHRP. This analysis resulted in a fatigue model Increasing binder viscosity; (Equation 9) for predicting the number of cycles required to Decreasing VMA; reach a given damage level for an HMA with specified char- Increasing aggregate specific surface; acteristics (VBE, Ndesign, etc.). Increasing design compaction (Ndesign); and The relationships between mixture composition and age Increasing field compaction (decreasing in-place air voids). hardening are not as easily quantified as other aspects of performance. A large amount of the age hardening observed The relationship among these factors and observed rut in a given HMA in laboratory tests appears to be a function resistance for a wide range of field data has been quantified in of the specific asphalt-aggregate composition and cannot be