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13 CHAPTER 3. COMPARISON OF AVAILABLE PRECISION ESTIMATES To examine the reliability of the precision limits obtained in this study, two sets of comparisons were made. The first comparison is between the computed precision limits and the precision limits that are currently available in Section 8.3 of AASHTO T 269. The second comparison is between the precision limits from Superpave Gyratory Compactor data (AASHTO T312) as part of this study and the precision limits for AASHTO T312 determined in Phase 1 of the NCHRP 9-26 project [1]. Table 3-1 shows the proposed precision limits for AASHTO T 269 from this study using the Professional Sample Program (PSP) Data for various methods of compaction. The current precisions in Section 8.3 of T 269 are also presented in the table. The comparison of the proposed and current precision limits indicates that the single operator precisions from the two sources are reasonably consistent. Also, the proposed multilaboratory precisions derived from Marshall Apparatus and Superpave Gyratory Compactor data are consistent with the multilaboratory precision limit in Section 8.3 of T 269. However, the multilaboratory precisions derived from California Kneading and Shear Gyratory Compactor data are significantly larger than the current precision in T269. It is speculated that the reason for this significant difference is because these two compaction methods were not included in the development of the current T269 precisions. Table 3-1 also provides the precision estimates for the air voids of the specimens compacted according to AASHTO T 312 as part of the interlaboratory study (ILS) in Phase 1 of the NCHRP 9-26 project [1]. As indicated from the table, the multilaboratory precision obtained from Superpave Gyratory Compactor data in this study is significantly larger than the multilaboratory precision from Phase 1. The reason for this could be the difference in the preparation of the specimens in each study. The PSP specimens for this study were mixed and compacted at the participating laboratories; however, the specimens for the ILS in Phase 1 were mixed in AMRL laboratory and compacted at different laboratories. Therefore, it seems reasonable that the multilaboratory precision from PSP to be larger than that from ILS. Table 3-1- Summary of the Available Precision Estimates for T269 Data Type Single operator Precision 2ds Multilaboratory Precision 2ds Marshall (Phase 5) 0.48 1.35 1.08 3.06 California Kneading (Phase 5) 0.52 1.46 1.39 3.94 Shear Gyratory (Phase 5) 0.50 1.41 1.49 4.22 Superpave Gyratory (Phase 5) 0.47 1.33 1.01 2.87 AASHTO T269 (Sec. 8.3) 0.51 1.44 1.09 3.08 ILS (Phase 1 NCHRP 9-26) 0.301- 0.502 0.901- 0.1402 0.60 1.70 1 12.5-mm nominal maximum aggregates 2 19.0-mm nominal maximum aggregates
