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