A Review of Aggregate and Asphalt Mixture Specific Gravity ...

aggregate with less than 1.0% water absorption. The precision estimates from D 2726 areshown in Table 13 and indicate that the method is less repeatable (i.e. higher within-labprecision) for the coarse-graded specimens compared to fine-graded specimens. Potentialsources of variation for the SSD method discussed by AMRL include differences in thedampness of the towel used to blot the surface of the specimen, temperature of the waterbath, and differences in the interpretations for achieving the SSD condition as quickly aspossible.TABLE 13Single Operator Precision:12.5-mm NMAS (fine-graded)19.0-mm NMAS (coarse-graded)Multilaboratory Precision:12.5-mm NMAS (fine-graded)19.0-mm NMAS (coarse-graded)ASTM D 2726 Precision Estimates for G mbStandard Deviation(1s)0.0080.0130.0150.015Acceptable Range ofTwo Results(d2s)0.0230.0370.0420.042NCHRP Project 9-26 (11) recently completed a significant study that evaluatedthe precision estimates for G mb . The study involved more than 22 laboratories thatcompacted specimens to 100 gyrations in the Superpave Gyratory Compactor inaccordance with AASHTO T 312 then tested the compacted specimens in accordancewith AASHTO T 166 and ASTM D 6752 (the vacuum sealing method). Materialsvariables included two aggregate types (low and high absorption) and two NMASmixtures. The findings of this study were that mixtures with different NMAS and thosecontaining high and low absorptive aggregate yielded similar precision estimates for G mb .This study recommended the precision estimates shown in Table 14 for AASHTO T 166.TABLE 14 NCHRP 9-26 Recommended Precision Estimates for AASHTO T 166Acceptable Range ofStandard Deviation(1s)Two Results(d2s)Single Operator Precision 0.012 0.033Multilaboratory Precision: 0.016 0.0445.3. Alternatives to Standard Test MethodsSeveral alternative methods available for determining G mb are listed in Table 14 withtheir associated advantages and disadvantages.20