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

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Williams (18) evaluated four methods for measuring G mb , including the T 166,vacuum-sealing, dimensional, and gamma ray method using coarse-graded 25.0- and37.5-mm mixtures compacted to approximately 2, 4, and 7% air voids. The resultsindicated that four methods produced statistically different G mb results. In addition, T166had the lowest levels of variability, followed by the vacuum-sealing method.In NCHRP Project 9-26 (11), a significant part of the study evaluated theprecision estimates for G mb using ASTM D 6752 (the vacuum-sealing method). Thefindings of this study were that mixtures with different NMAS and those containing highand low absorptive aggregate yielded similar precision estimates for G mb . This studyrecommended the precision estimates shown in Table 16 for ASTM D 6752. These aregreater than the recommended precision estimates for AASHTO T 166 (see Table 14).TABLE 16 NCHRP 9-26 Recommended Precision Estimates for ASTM D 6752Acceptable Range ofStandard Deviation(1s)Two Results(d2s)Single Operator Precision 0.013 0.036Multilaboratory Precision: 0.021 0.059In summary, it has been reported in the existing literature that significant differences inmeasured G mb using different test methods exist. These differences are more pronouncedfor coarse-graded HMA mixtures. AASHTO T 166 exhibited the smallest level ofvariability, followed by the CoreLok, then dimensional method, and finally the gammaray device.6 IMPACTS OF SPECIFIC GRAVITY MEASUREMENTS ON MIXTUREPROPERTIESAs stated previously, one motivation for adopting a new test method is reducing thevariability of test results. An analysis was performed to assess the relative effect ofreducing the variability of aggregate specific gravity and compacted HMA test (G mb ) onthe VMA of HMA mixtures. The study involved a Monte-Carlo simulation of VMAresults calculated using Equation 2. Details of the simulation include the following: Values of G mb and G sb were randomly drawn from a population exhibiting anormal probability distribution. Each simulation included 50,000 calculated values for VMA. Baseline “mean” values for the normal distributions were selected to yield a VMAresult of approximately 15.1%, to represent a typical 12.5-mm NMAS hot-mixasphalt mixture. The baseline standard deviation for each of the normal distributions wascalculated as the average value of all multi-lab standard deviations (1s) reportedby the AASHTO Materials Reference Laboratory (AMRL) for the respectivespecific gravity. The standard deviation values reported for the traditional SSDmethod for both G mb and G sb were used.24
For G sb , the specific gravity value used in the simulation was calculated using a50/50 split between coarse (AASHTO T-85) and fine (AASHTO T-84)aggregate. To assess the effect of reducing aggregate and HMA bulk specific gravities onVMA results, the standard deviation of each property was reduced from thebaseline value in steps of 10 percent to a final value of 50 percent of the baseline.Each simulation produced a normal distribution of VMA values. Figure 1 shows theoverall result from the simulation analysis. The y-axis represents the variability of VMA,expressed as the standard deviation of the VMA distribution. The x-axis represents thestepwise reduction of the G mb standard deviation. The discrete points arranged verticallyrepresent the stepwise reduction of G sb at each x-axis (G mb ) reduction step. Thus, the areabounded by the points shown in the figure illustrates the potential reduction in VMAvariability (standard deviation) resulting from reductions in constituent specific gravities.It is possible to compute the percent-reduction in VMA standard deviation as afunction of the reductions in standard deviation of both G mb and G sb , as illustrated inEquation 9:where:VMA red = 0.4894 (G mb ) red + 0.4880 (G sb ) red (9)VMA red = reduction in VMA standard deviation (%),(G mb ) red = reduction in G mb standard deviation (%),(G sb ) red = reduction in G sb standard deviation (%),It is apparent from Equation 9 that, in general, the improvement in VMAvariability is approximately half (in terms of percent from baseline, or original) that ofany improvement in compacted HMA and/or aggregate specific gravity.The focus on variability (standard deviation) is reasonable in the context of theassociated range of two test results. Typically, the acceptable range of two test results iscalculated using Equation 10.where:d2sd2s = 2.83 = acceptable range of two test results= standard deviation of testIn the simulation study, the “baseline” standard deviation values for G mb and G sbyielded a distribution of VMA values with a standard deviation of approximately 1.31percent. Using Equation 10, the acceptable range of two VMA results would be 3.7%.Typical HMA mix design and QA/QC criteria for VMA specifies a total VMA range ofonly 2.0 or 2.5%. Thus, in this example, two VMA results that should be consideredacceptable could in fact fall outside VMA specifications.25
Page 1 and 2: NCAT Report 12-06A REVIEW OF AGGREG
Page 3 and 4: Objective and Process of the Review
Page 5 and 6: warranted in these methods at this
Page 7 and 8: Table of Contents1. Introduction ..
Page 9 and 10: 1 INTRODUCTIONCurrent practices for
Page 11 and 12: 3 BULK SPECIFIC GRAVITY OF AGGREGAT
Page 13 and 14: TABLE 3 Advantages and Disadvantage
Page 15 and 16: TABLE 4 AASHTO T 84 and ASTM C128 P
Page 17 and 18: TABLE 6 Modifications of Standard T
Page 19 and 20: In 2000, Kandhal et al. (7) conduct
Page 21 and 22: angular fine aggregate with higher
Page 23 and 24: ASTM does not specify an absorption
Page 25 and 26: 4.3. Alternatives to Current Standa
Page 27 and 28: 5 BULK SPECIFIC GRAVITY OF COMPACTE
Page 29 and 30: TABLE 15 Advantages and Disadvantag
Page 31: inaccurate sample volumes caused by
Page 35 and 36: line). Since it may be more desirab
Page 37 and 38: Both standard test procedures, incl
Page 39 and 40: 8 RECOMMENDATIONSBased on the revie
Page 41 and 42: REFERENCES1. The Proficiency Sample

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