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14 CHAPTER 4. CONCLUSIONS AND RECOMMENDATIONS 4.1 CONCLUSIONS This study was conducted to prepare precision estimates for AASHTO Standard Test Method T269, âPercent air Voids in Compacted Dense and Open Asphalt Mixturesâ. The AMRL proficiency data analyzed in this study included the percent air voids and the maximum and bulk specific gravities of the asphalt mixtures. The AMRL data are more up to date than the data used for the current estimate of precision and reflects the various compaction test methods and recent changes to those methods. In most cases the data sets used to derive the precision estimate included well over 60 laboratories. The precision and bias statement recommended for T 269 is provided in Appendix E. The statement has been prepared based on the pooled precisions analyzed from the percent air void data. For majority of cases, following the procedure in section 8.1 of AASHTO T269 resulted in larger precision limits than when percent air void data were used. The reason is because some of the Gmm and Gmb data that are within the tolerance of the specific gravity would result in air void values that are outside of the tolerance of air voids. Since for design and quality control of asphalt mixtures the % air voids is the controlling property, it is therefore more reasonable to set the precisions of the % air voids based on analysis of air void data. The consistency of the computed precision estimates with the precision estimates in Section 8.3 of T 269 was investigated. The comparison indicated that there is a close agreement between the single operator precision estimates in Section 8.3 and the single operator precision estimates computed in this study. There is also a close agreement between the multilaboratory precision estimates in Section 8.3 and the multilaboratory precision estimates computed in this study using Marshall Apparatus and Superpave Gyratory Compactor data. However, the multilaboratory precision estimates computed using California Kneading and Shear Gyratory Compactor data are significantly larger than the current multilaboratory precision estimates in Section 8.3. This brings up the question of whether the data from California Kneading and Shear Gyratory Compactors have been included in the preparation of the Current T 269 precisions. The agreement of the computed precision estimates with the precision estimates computed in Phase 1 of the NCHRP 9-26 study was also investigated. The comparison indicated that the precision estimates for % air voids of specimens compacted according to AASHTO 312 in the Proficiency Sample Program (PSP) were significantly larger than the precision estimates for relative density obtained as part of the interlaboratory study (ILS) in Phase 1 of the project. The reason for this difference is speculated to be the difference in the preparation of the samples in the two studies. The PSP samples were mixed and compacted by the participating laboratories where the ILS samples were mixed in the AMRL laboratory and compacted in various laboratories. This would result in greater variability in the % air voids of the PSP specimens than in those of ILS specimens.
15 4.2 RECOMMENDATIONS The following recommendations are expected to improve the precision estimates of AASHTO T 269: The current precision of T 269 test method depends on the precision of test methods for bulk specific and maximum specific gravities. It was shown in this study that the specific gravities that are measured to be within acceptable range of two results might result in air void values that fall outside of the acceptable range. Therefore, there is a probability of error in accepting an air void that is in fact outside of the acceptable range. Therefore, it is recommended that the precision limits for T269 to be determined based on precision of % air voids. The precision estimates computed in this study are limited to the four compaction methods used. The field compaction was not included in the study. It is recommended to conduct an interlaboratory study to determine the precision estimates of the percent air voids of the field compacted cores. The precision statement recommended in this study only updates the precision estimates in Section 8.3 of AASHTO T269. It is recommended to conduct an interlaboratory study to include AASHTO T275 Test Method to update the precision estimates in Section 8.2 of AASHTO T269. AASHTO T331 âBulk Specific Gravity and Density of Compacted Bituminous Mixtures Using Automatic Vacuum Sealing Methodâ [13] has been added to AASHTO T269 for measuring the bulk specific gravity of compacted asphalt mixtures. It is recommended to conduct an interlaboratory study to determine precision estimates for T269 based on the AASHTO T331 test data. It is recommended that the precision and bias statement in Section 4.4 to be adopted for T269. The current precision statement in T269 is blind to various compaction methods while as observed in this study the precisions resulting from various compaction methods could be quite different. Therefore, the precision estimates should be specific to each method of compaction.
