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42 Conclusions Data from tests performed during NCHRP Projects 9-25 and 9-31 and data gathered from various previous research projects have been analyzed and effective models have been developed for predicting a range of performance-related properties for HMA. These models were used to analyze the effect of changes in VMA, air voids, aggregate fineness, and related factors on the potential performance of HMA. In gen- eral, rut resistance increases with decreasing VMA and increasing aggregate specific surface. Fatigue resistance increases with increasing effective binder content, which tends to increase with increasing VMA. Both rut resistance and fatigue resistance increase with increasing levels of design compaction and increasing levels of field compaction relative to design compaction, when other factors are held constant. The permeability of HMA increases with increasing air void content and decreasing aggregate specific surface. Recommendations The results of NCHRP Projects 9-25 and 9-31 suggest that current Superpave requirements for volumetric design of HMA do not need major revision. However, there appears to be some need for refinements in the system since many high- way agencies have recently funded research and engineering projects dealing with both top-down cracking and perme- ability of HMA. Current HMA mixtures tend to be somewhat lower in asphalt binder content compared with mixtures designed and placed prior to the implementation of Super- pave; this may be a contributing factor to the observed fre- quency of raveling and surface cracking in Superpave mixtures. Because the Superpave system has encouraged the use of coarse aggregate gradationsâbelow the maximum density gradationâthey also often contain relatively few fines; this lack of aggregate fines, in combination with rela- tively high in-place air voids, can result in mixtures with high permeability and less resistance to age hardening. The poten- tially low fines content, when combined with high VMA val- ues, can also lead to poor rut resistance, although this problem is relatively uncommon in HMA designed using the Superpave system. Many highway agencies have already modified volumetric requirements in the Superpave system, the most common changes being establishing maximum VMA values 1.5% to 2.0% above the minimum values, increasing minimum VMA by 0.5% to 1.0%, and/or a broadening of design air void con- tent from 4.0% to a range of 3.0% to 5.0%. Establishing max- imum VMA values and eliminating VFA requirements make the Superpave system simpler and more direct and reduce the chances of designing HMA with poor rut resistance. Increas- ing VMA while maintaining design air voids at 4.0% will improve fatigue resistance since this will increase VBE. How- ever, unless care is taken to ensure that adequate aggregate specific surface is maintained while increasing VMA, rut resistance will be reduced. Increasing aggregate specific sur- face while increasing minimum VMA will improve both fatigue resistance and rut resistance, and will tend to decrease permeability. Changing design air voids in essence has the effect of changing the design compaction level since this changes the amount of compaction energy that will be required in the field to reach the target air void levels. The effect of changing design air voids depends in part on whether VMA or VBE is held constant. If VBE is held constant, design air void contents below 4.0% reduce the required field com- paction effort, and both fatigue resistance and rut resistance will be decreased; increasing design air voids to levels above 4.0% has the opposite effect. If VMA is held constant, decreas- ing design air voids will still result in a decrease in field com- paction effort, but this will be offset in part by increasing binder content. Decreasing design air voids to from 4.0% to 3.0% while decreasing the target air voids in the field a simi- lar amount will improve both fatigue resistance and rut resist- ance while decreasing permeability. Decreasing Ndesign for a C H A P T E R 4 Conclusions and Recommendations
given aggregate blend will tend to produce a mixture with somewhat higher binder content, which is also easier to com- pact in the field. However, it should be recognized that many materials suppliers will adjust aggregate gradations in such a situation to maintain minimum allowable binder contents, and some contractors might also adjust field compaction practices so that in-place air voids are not reduced. Therefore, agencies that choose to reduce Ndesign in order to obtain higher binder contents and better field compaction should also increase minimum binder content requirements and decrease allowable in-place air voids. A slight increase in dust-to- binder ratio in such cases will help maintain current levels of rut resistance. Other approaches are possible to improving the fatigue resistance of HMA while maintaining or improv- ing rut resistance and decreasing permeability. Agencies contemplating modification in Superpave spec- ifications should first evaluate the level of in-place air voids being achieved during flexible pavement construction and verify that acceptable levels of field compaction are being achievedâpoor field compaction will have a significant negative impact on pavement performance that can only be partially offset by proper mix design. Any changes in current Superpave requirements should be carefully evaluated using performance models tempered with engineering judgment and experience with local conditions and materials. Although performance models are useful tools for evaluat- ing the effects of modifications in HMA specifications, they should be used with cautionâunderstanding that such models provide only approximate results. Care is also needed when instituting multiple changes in Superpave specifications or in specifications for any other HMA mix type: changes in volumetric requirements, compaction lev- els, and materials specifications are additive, and unless such changes are carefully evaluated and implemented, signifi- cant and unanticipated reductions in pavement perform- ance can result. Chapter 3 of this report includes an Extended Work and Validation Plan (see the end of Chapter 3). This plan has been devised to extend the results of this research to mixtures made with larger aggregate sizes (25- and 37.5-mm) and also to val- idate the results of this research using accelerated pavement testing and other field data. 43
