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

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Recent studies have focused on the evaluation of alternative methods but not onthe improvement of the accuracy or precision of the current standard test methods.The CoreLok device shows promise. The CoreLok and AASHTO T 209 producedsimilar results for non-porous aggregate mixtures. For porous aggregate mixtures,the CoreLok produced higher G mm values.For the bulk specific gravity of compacted HMA specimens, the review can besummarized as: AASHTO T 166 and D 2726 procedures are not accurate for determining bulkspecific gravity of many coarse-graded and SMA compacted specimens due to theloss of water from specimen pores during the SSD determination. Precision statements for AASHTO T 166 are not complete. However, research byAMRL provides recommendations for new precision statements. Alternate methods for determining G mb include paraffin coating, parafilm, vacuumsealing, gamma ray, and dimensional measurement. Several recent studies have focused on the comparison of the parafilm, vacuumsealing,gamma radiation, and dimensional measurement to AASHTO T 166. TheG mb values determined using these methods were different. The differencesbetween G mb results from AASHTO T 166 and the vacuum-sealing or gamma raydevices were greater for coarse-graded and SMA specimens. Several studies haverecommended reducing the absorption limit for T 166 to 1.0% or less in order toimprove the accuracy of the G mb determination for coarse-graded and SMAmixtures.The impacts of specific gravity measurements on mix design properties were alsoperformed and are summarized: Based on the current precision indices for G mb and G sb , the acceptable differencefor VMA results performed in two labs on a split sample is 3.7%. This differenceis greater than most VMA quality assurance specifications (typically in the rangeof only 2.0 or 2.5%). This indicates that such specification limits are not valid. For G mm , the ASTM multilaboratory precision for mixtures with non-porousaggregate and the AASHTO/ASTM multilaboratory precision for porousaggregate can result in a difference of 2% in between-laboratory air void values. When the vacuum-sealing method (AASHTO T 331) is used instead of the T 166for determining G mb of coarse-graded compacted HMA specimens, air voids, andVMA will increase approximately 0.5%. In effect, this could result in a slightincrease in asphalt content for coarse-graded mixtures, thereby making suchmixtures more durable and easier to compact. Replacing T 166 with the vacuum-sealing method for roadway cores will decreasefield-relative densities by approximately 1% for coarse-graded mixtures andapproximately 1.75% for SMA mixtures.Based on the review, the automated test methods offer time savings. In addition, thedifferences in specific gravity results between the automated test methods and thestandard test methods significantly impact the mix design properties for some aggregateor mixture types.30
8 RECOMMENDATIONSBased on the review, the following recommendations are offered for improving specificgravity determinations:1. The current standard test methods for determining G sb of coarse aggregate areconsidered satisfactory with respect to accuracy and precision. No change iswarranted in these methods at this time. Research should explore reducing thesoak time.2. The determination of G sb for fine aggregate suffers from poor reproducibility dueto the subjective determination of the SSD condition. The accuracy of the fineaggregate G sb is also questionable for some absorptive materials and those thatcontain highly angular and/or textured particles or that have high dust contents.Further research is needed to improve the reproducibility and accuracy of the fineaggregate G sb determination. Alternate methods of determining the SSDcondition of fine aggregate appear to be promising.3. For agencies that use VMA or VFA in mix design approval or HMA acceptancetesting, the limits for these criteria should be based on well-documented precisioninformation for G sb determinations.4. The current standard test methods for determination of G mm for HMA mixturescontaining aggregate with low absorption are satisfactory. However, themultilaboratory precision estimate for mixtures containing moderately to highlyabsorptive aggregate is so large that it is not valid to distinguish air voids resultsfor split specimens conducted in two laboratories that differ by as much as 2.0percent. Clearly, further work needs to be conducted to improve thereproducibility of the G mm determination for such aggregate. Another importantobjective for further research should be to reduce the time to complete the test formixes containing absorptive aggregate.5. In order to improve the accuracy of the G mb determination, T 166 (and thecorresponding ASTM method D 2726) should be limited to specimens with awater absorption less than or equal to 1.0%. In practice, this will limit the T 166to use with well-compacted, fine-graded mixtures. For specimens with greaterthan 1.0% water absorption, only the vacuum-sealing method (AASHTO T 331,ASTM D 6752) should be used since this method has similar precision estimatesto D 2726 for these mixtures.Note: Agencies should be aware that changing to the vacuum-sealing method willhave substantial consequences with regard to mix designs for coarse-graded andSMA mixtures, and measurement of in-place densities of these mixtures whenmeasurements are based on cores:• For coarse-graded and SMA mixtures, the vacuum-sealing method will yieldhigher air voids and VMA than for the same mixtures tested by T 166. Basedon available data, the average shifts are about 0.5% for both air voids andVMA for coarse-graded mixtures using mix design compactive efforts. ForSMA mixtures, the average shifts in air voids and VMA are 0.9% at a normalmix design compactive effort. These changes will have an effect on futuremix designs. Agencies may want to consider adjusting their mix design VMA31
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 and 32: inaccurate sample volumes caused by
Page 33 and 34: For G sb , the specific gravity val
Page 35 and 36: line). Since it may be more desirab
Page 37: Both standard test procedures, incl
Page 41 and 42: REFERENCES1. The Proficiency Sample

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