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5 CHAPTER 4 TEST RESULTS AND ANALYSIS 4.1 PART 1--MIX DESIGNS FOR SPECIMENS est air void level. This figure indicates that for ARZ mixes, TO STUDY THE EFFECT OF t/NMAS ON the two methods provide similar results. For the TRZ, BRZ, DENSITY and SMA mixes, Figures 2 through 4 suggest that the bulk specific gravity measurements derived from the two methods Of the 36 mix designs, 27 were Superpave-designed mixes moved farther apart as density decreased. The results also and 9 were SMA mixes. The Superpave mixes were classified indicate that, as the gradation became coarser, the difference according to three gradations: above the restricted zone (ARZ), in the test results for the two test methods increased. This through the restricted zone (TRZ), and below the restricted finding agrees with the research by Cooley et al. (11). zone (BRZ). The optimum asphalt content, the effective The apparent reasons for the different results according to asphalt content (Pbe), voids in mineral aggregate (VMA), the two test methods is loss of water during density measure- voids filled with asphalt (VFA), percent theoretical maxi- ment when using the T-166 method and the effect of surface mum density at Ninitial (% Gmm at Nini), and ratio of dust to texture. The loss of water when blotting in the T-166 method effective asphalt content (P0.075/Pbe) for the Superpave mixes causes a test error resulting in higher measured density. The are summarized in Table 3. Data for SMA mixes are shown in Table 4. The mix design information for both mix types surface texture can result in the vacuum seal device measur- is presented in Appendix A. Optimum asphalt binder con- ing a lower density than the actual density. Because the vac- tent was chosen to provide 4 percent air voids at the design uum seal device is more accurate in measuring the density number of gyrations. However, for the 19-mm NMAS lime- of porous samples, it was used to determine density for this stone SMA mix, 4 percent air voids could be achieved with research project. 5.7 percent asphalt content, which did not meet the mini- The main objective of this part of the study was to deter- mum asphalt content requirement in accordance with the mine the minimum t/NMAS. To achieve this objective, rela- "Standard Practice for Designing SMA," AASHTO PP44-01. tionships of average air voids for the three aggregate types Therefore, the minimum asphalt content of 6.0 percent was versus t/NMAS with respect to NMAS and gradation were chosen, which resulted in 3.7 percent air voids at the design evaluated; the results are illustrated in Figures 5 through 10. number of gyrations. Some designs did not meet the re- Originally it was intended to determine the t/NMAS at which quirements of VMA, VFA, % Gmm at Nini, and/or dust/Pbe. the air voids began to level out and to pick that t/NMAS level Efforts were made to redesign the respective mixes by as the minimum level recommended to achieve optimum changing the gradation until the requirements were met or compaction. However much of the data in Figures 5 through closely approximated. This is important in that the mixes 10 indicate that the air voids continue to drop with increasing used in this project were intended to duplicate mixes uti- t/NMAS past typical t/NMAS values. These data therefore lized in the field. No modification was made for the TRZ did not provide reasonable guidance for selecting a mini- mixes that did not meet the requirements, as little could be mum t/NMAS. Hence an air void content of 7.0 percent was done to modify these gradations and still pass through the selected as the criteria to determine the minimum t/NMAS. restricted zone. This level of air voids was selected because compaction of most pavements in the field is targeted at 92.0 to 94.0 per- cent of theoretical maximum density. Because of the uncer- 4.2 EVALUATION OF EFFECT OF t/NMAS ON tainty in the relationship of average air voids to t/NMAS, as DENSITY USING GYRATORY COMPACTOR indicated by the data, it was determined to compact some laboratory samples with a vibratory compactor and also to Before the evaluation was done, two methods of measuring compact some mixes in the field during reconstruction of density, or bulk specific gravity, were compared: the AASHTO the NCAT test track. These two efforts, which are discussed T166 (SSD) and the vacuum sealing (ASTM D6752-02a) later in the report, should provide sufficient information to methods. All samples were measured using both methods. Fig- make reasonable conclusions concerning desired t/NMAS ures 1 through 4 present these measurements for the three levels. gradations of Superpave mixes and the SMA mixes. One potential problem with the Superpave gyratory com- As shown in Figure 1, the air voids for ARZ mixes as mea- pactor is that it applies a constant strain to the mix during com- sured by the two methods are approximately equal at low air paction and the force required varies as necessary to provide voids and deviate by approximately 0.5 percent at the high- the desired strain. This is not the approach that is observed in