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A Review of Aggregate and Asphalt Mixture Specific Gravity Measurements and Their Impacts on Asphalt Mix Design Properties and Mix Acceptance

Summary

This report separately examined three specific gravity determinations, the bulk specific gravity of aggregate (Gsb), the maximum specific gravity of HMA mixtures (Gmm), and the bulk specific gravity of compacted HMA specimens (Gmb). Each specific gravity determination was reviewed in terms of: (1) problems and issues with current standard test methods; (2) modifications and/or alternate methods; and (3) areas that need further research and development. In addition, the impacts of specific gravity measurements on mix design properties and mix acceptance were also investigated. The review draws upon information from current AASHTO and ASTM standards, published research studies; state DOTs, equipment manufacturers, and the AMRL website.

With respect to the bulk specific gravity of coarse aggregate, the review can be summarized as:

  • In the AASHTO T 85 and ASTM C127 procedures, the visual method of determining when aggregates reach a SSD condition is highly operator dependent.
  • Both standard test procedures, including aggregate soaking time, cannot be completed in one work day.
  • All of the precision estimates for AASHTO T 85 and ASTM C127 from 1998 through 2005 by the AMRL are much greater than those cited in the standards, and they vary significantly from year to year which is presumed to be due to the use of different aggregate sources in the proficiency sample program.
  • The AggPlus system using the CoreLok device is commercially available as an alternative method for determining Gsb of coarse aggregate. Another device, the Gilson Rapid Water Displacement, is being developed.
  • Recent studies have evaluated the AggPlus system using the vacuum-seal device against the AASHTO T 85 procedure. The AggPlus system does not require the determination of SSD condition and soaking time. The AggPlus produced higher specific gravity values that were significantly different (both statistically and practically) from those produced by AASHTO T 85. The difference was greater for highly absorptive coarse aggregate. Both methods had similar reproducibility.

For the bulk specific gravity of fine aggregate, the review can be summarized below:

  • In AASHTO T 84 and ASTM C128, the SSD condition of various fine aggregates is not consistently determined using the cone and tamp technique.
  • Both standard test methods, including aggregate soaking time, cannot be completed in one work day.
  • As with the standard test methods for bulk specific gravity of coarse aggregate, most of the precision estimates for AASHTO T 84 and ASTM C128 published annually on the AMRL website are greater than those cited in the standards, and they vary significantly from year to year.
  • Several modifications have been made by states to improve the process of determining the SSD condition. However, all modifications still require technician judgment, and the reproducibility improvement is not found in the literature.
  • Alternate methods for determining Gsb of fine aggregate include the CoreLok, SSDetect, and Phunque. In addition, the Langley de-airing device can be used with AASHTO T 84.
  • Most recent studies published have focused on the CoreLok and SSDetect devices. Both devices do not require the determination of SSD weight and soaking time. However, the Gsb values determined using either procedure were significantly different from those produced using AASHTO T 84. Differences were greater for more angular fine aggregate with aggregate having higher dust contents. The SSDetect had the best precision indices, then AASHTO T 84 and the CoreLok.

The review of the maximum specific gravity of HMA mixtures can be summarized as:

  • The ASTM multilaboratory precision for non-porous aggregate and the AASHTO/ASTM multilaboratory precision for porous aggregate appeared very high, resulting in an allowable difference of up to two percent in between-laboratory air void values.
  • The CoreLok and Pressure Meter procedures are alternatives for determining Gmm.
  • Most recent studies have focused on the evaluation of alternative methods but not on the improvement of the accuracy or precision of the current standard test methods. The CoreLok device shows promise. The CoreLok and AASHTO T 209 produced similar results for non-porous aggregate mixtures. For porous aggregate mixtures, the CoreLok produced higher Gmm values.

For the bulk specific gravity of compacted HMA specimens, the review can be summarized as:

  • AASHTO T 166 and D 2726 procedures are not accurate for determining bulk specific gravity of many coarse-graded and SMA compacted specimens due to the loss of water from specimen pores during the SSD determination.
  • Precision statements for AASHTO T 166 are not complete. However, research by AMRL provides recommendations for new precision statements.
  • Alternate methods for determining Gmb include paraffin coating, parafilm, vacuum sealing, gamma ray, and dimensional measurement.
  • Several recent studies have focused on the comparison of the parafilm, vacuum sealing, gamma radiation, and dimensional measurement to AASHTO T 166. The Gmb values determined using these methods were different. The differences between Gmb results from AASHTO T 166 and the vacuum sealing or gamma ray devices were greater for coarse-graded and SMA specimens. Several studies have recommended reducing the absorption limit for T 166 to 1.0 percent or less in order to improve the accuracy of the Gmb determination for coarse-graded and SMA mixtures.

The impacts of specific gravity measurements on mix design properties were also performed and are summarized:

  • Based on the current precision indices for Gmb and Gsb, the acceptable difference for VMA results performed in two labs on a split sample is 3.7 percent. This difference is greater than most VMA quality assurance specifications (typically in the range of only 2.0 or 2.5 percent). This indicates that such specification limits are not valid.
  • For Gmm, the ASTM multilaboratory precision for mixtures with non-porous aggregate and the AASHTO/ASTM multilaboratory precision for porous aggregate can result in a difference of two percent in between-laboratory air void values.
  • When the vacuum sealing method (AASHTO T 331) is used instead of the T 166 for determining Gmb of coarse-graded compacted HMA specimens, air voids and VMA will increase approximately 0.5 percent. In effect, this could result in a slight increase in asphalt content for coarse-graded mixtures, thereby making such mixtures more durable and easier to compact.
  • Replacing T 166 with the vacuum sealing method for roadway cores will decrease field relative densities by approximately one percent for coarse-graded mixtures and approximately 1.7 percent for SMA mixtures.

Based on the review, the automated test methods offer time savings. In addition, the differences in specific gravity results between the automated test methods and the standard test methods significantly impact the mix design properties for some aggregate or mixture types.

Recommendations

Based on the review, the following recommendations are offered for improving specific gravity determinations:

  1. The current standard test methods for determining Gsb of coarse aggregate are considered satisfactory with respect to accuracy and precision. No change is warranted in these methods at this time. Research should explore reducing the soak time.
  2. The determination of Gsb for fine aggregate suffers from poor reproducibility due to the subjective determination of the SSD condition. The accuracy of the fine aggregate Gsb is also questionable for some absorptive materials and those that contain highly angular and/or textured particles, or which have high dust contents. Further research is needed to improve the reproducibility and accuracy of the fine aggregate Gsb determination. Alternate methods of determining the SSD condition of fine aggregate appear to be promising.
  3. For agencies that use VMA or VFA in mix design approval or HMA acceptance testing, the limits for these criteria should be based on well documented precision information for Gsb determinations.
  4. The current standard test methods for determination of Gmm for HMA mixtures containing aggregate with low absorption are satisfactory. However, the multilaboratory precision estimate for mixtures containing moderately to highly absorptive aggregate is so large that it is not valid to distinguish air voids results for split specimens conducted in two laboratories that differ by as much as 2.0 percent. Clearly, further work needs to be conducted to improve the reproducibility of the Gmm determination for such aggregate. Another important objective for further research should be to reduce the time to complete the test for mixes containing absorptive aggregate.
  5. In order to improve the accuracy of the Gmb determination, T 166 (and the corresponding ASTM method D 2726) should be limited to specimens with a water absorption of less than or equal to 1.0 percent. In practice, this will limit the T 166 to use with well-compacted, fine-graded mixtures. For specimens with greater than 1.0 percent water absorption, only the vacuum sealing method (AASHTO T 331, ASTM D 6752) should be used since this method has similar precision estimates to D 2726 for these mixtures.

Note: Agencies should be aware that changing to the vacuum sealing method will have substantial consequences with regard to mix designs for coarse-graded and SMA mixtures, and measurement of in-place densities of these mixtures when measurements are based on cores:

For coarse-graded and SMA mixtures, the vacuum sealing method will yield higher air voids and VMA than for the same mixtures tested by T 166. Based on available data, the average shifts are about 0.5 percent for both air voids and VMA for coarse-graded mixtures using mix design compactive efforts. For SMA mixtures, the average shifts in air voids and VMA are 0.9 percent at a normal mix design compactive effort. These changes will have an effect on future mix designs. Agencies may want to consider adjusting their mix design VMA criteria so that the resulting mixtures can be expected to perform as well or better than those in current use. Reasoning was provided in this report to support an increase in VMA by 0.5 percent for coarse-graded Superpave and SMA mixtures.

Using the vacuum sealing method in lieu of T 166 for measurement of core densities will shift the results more dramatically than for mix designs. Available data shows that in-place air voids are approximately 1.0 percent and 1.7 percent higher on average for coarse-graded mixtures and SMA mixtures, respectively, when using the vacuum sealing method in place of T 166. Therefore changing to the vacuum sealing method for acceptance testing of in-place density will result one of two scenarios for agencies: either leave in-place density criteria as-is and expect contractors to improve their compaction processes to meet the criteria; or adjust the specification criteria for in-place densities to be consistent with the new measurement method so that densities levels are achievable with the current practices for asphalt pavement construction.

References

1. The Proficiency Sample Programs Homepage. AASHTO Materials Reference Laboratory. http://www.amrl.net/Portal/DesktopDefault.aspx?tabindex=3&tabid=12. Accessed July 6, 2007.

2. Spellerberg, P., D. Savage, J. Pielert. Precision Estimates of Selected Volumetric Properties of HMA Using Non-Absorptive Aggregate. Publication NCHRP Web Document 54. NCHRP, TRB, National Academies, Washington, D.C., 2003.

3. Lee, D. Y., J. A. Guinn, P. S. Kandhal and R. L. Dunning.Absorption of Asphalt into Porous Aggregates. Publication SHRP-A/UIR-90-009. Strategic Highway Research Program, National Research Council, Washington, D.C., 1990.

4. Hall, K. D. Using a Single Test to Determine Specific Gravity and Absorption of Aggregate Blends. In Transportation Research Record: Journal of the Transportation Research Board, No. 1874, TRB, National Research Council, Washington, D.C., 2004, pp. 1-10.

5. Mgonella, M. K. Evaluation of the AggPlusTM System and the SSDetect System Against The Current AASHTO T-84 and T85. Master's Thesis. Oklahoma State University, 2005.

6. Sholar, G. A, G. C. Page, J. A. Musselman, P. B. Upshaw, and H. L. Moseley. Investigation of the CoreLok for Maximum, Aggregate, and Bulk Specific Gravity Tests. Transportation Research Record: Journal of the Transportation Research Board, No. 1907, TRB, National Research Council, Washington, D.C., 2005, pp. 135-144.

7. Kandhal, P. S., R. B. Mallick, and M. Huner. Measuring Bulk Specific Gravity of Fine Aggregates: Development of New Test Method. In Transportation Research Record: Journal of the Transportation Research Board, No. 1721, TRB, National Research Council, Washington, D.C., 2000, pp. 81-90.

8. Dana, J. S., and R. J. Peters. Experimental Moisture Determination for Defining Saturated Surface Dry State of Highway Aggregates. Arizona Highway Department, Report No. 6, HPR 1-11, 1974.

9. Prowell, B. D. and N. V. Baker. Evaluation of New Test Procedures for Determining Bulk Specific Gravity of Fine Aggregate by Automated Methods. In Transportation Research Record: Journal of the Transportation Research Board, No. 1874, TRB, National Research Council, Washington, D.C., 2004, pp. 11-18.

10. Cross, S. A., M. K. Mgonella, and Y. Jakatimath.Evaluation of Test Equipment for Determination of Fine Aggregate Specific Gravity and Absorption. In Transportation Research Record: Journal of the Transportation Research Board, No. 1952, TRB, National Research Council, Washington, D.C., 2006, pp. 3-11.

11. Azari, H., B. Lutz, P. Spellerberg.Precision Estimates of Selected Volumetric Properties of HMA Using Absorptive Aggregate. Preliminary Draft Final Report for NCHRP Project 9-26, Phase 4. NCHRP, TRB, National Academies, Washington, D.C., 2006.

12. Franco, C. A. and K. W. Lee. Development of the Pressure Method for Determining Maximum Theoretical Specific Gravity of Bituminous Paving Mixtures. In Transportation Research Record: Journal of the Transportation Research Board, No. 1269, TRB, National Research Council, Washington, D.C., 1990, pp. 101-115.

13. Buchanan, M. S. An Evaluation of Selected Methods for Measuring the Bulk Specific Gravity of Compacted Hot Mix Asphalt (HMA) Mixtures. Journal of Association of Asphalt Paving Technologists, Vol. 69, 2000, pp. 608-634.

14. Hall, K. D., F. T. Griffith, and S. G. Williams. Examination of Operator Variability for Selected Methods for Measuring Bulk Specific Gravity of Hot Mix Asphalt Concrete. In Transportation Research Record: Journal of the Transportation Research Board, No. 1761, TRB, National Research Council, Washington, D.C., 2001, pp. 81-85.

15. Malpass, G., and N. P. Khosla. Evaluation of Gamma Ray Technology for the Direct Measurement of Bulk Specific Gravity of Compacted Asphalt Concrete Mixtures. Journal of Association of Asphalt Paving Technologists, Vol. 70, 2001, pp. 352-367.

16. Cooley, L. A., B. D. Prowell, M. R. Hainin. Comparison of the Saturated Surface-Dry and Vacuum Sealing Methods for Determining the Bulk Specific Gravity of Compacted HMA.Journal of Association of Asphalt Paving Technologists, Vol. 72, 2003, pp. 56-96.

17. Brown, E. R., M. Hainin, A. Cooley, and G. Hurley. Relationships of HMA In-Place Air Voids, Lift Thickness, and Permeability. NCHRP Project 9-27, Vol. II, Part 3 - Task 3, 2003.

18. Williams, S. G. Bulk Specific Gravity of Measurements of 25.0 mm and 37.5 mm Coarse-Graded Superpave Mixes. Proceedings of the 86th Annual Meeting of the Transportation Research Board, Washington, D.C., 2007.

Acknowledgements

ThisTech Briefis the result of an FHWA Mixtures Expert Task Group activity to provide the latest information on the use of effects of specific gravity measurements on asphalt mixtures. Members included Randy West (National Center for Asphalt Technology), Erv Dukatz (Mathy Construction Company), John Haddock (Purdue University), Kevin Hall (University of Arkansas), Julie Kliewer (Arizona Department of Transportation), Chuck Marek (Vulcan Materials Company), Jim Musselman (Florida Department of Transportation), and Ali Regimand (Instrotek, Incorporated).

Further Information

Contact-for information related to the impacts of aggregate and asphalt mixture specific gravity measurements contact the following:

Federal HighwayAdministration Asphalt Pavement Technology Team

John Bukowski - John.Bukowski@dot.gov (Office of Pavement Technology)
Jack Youtcheff - Jack.Youtcheff@dot.gov (Office of Infrastructure R&D)
Tom Harman - Tom.Harman@dot.gov (Pavement & Materials Technical Service Team)

This Tech Brief was developed by the Office of Pavement Technology as part of the Federal Highway Administration's (FHWA's) Asphalt Pavement Technology Program.

Distribution-This Tech Brief is being distributed according to a standard distribution. Direct distribution is being made to the Resource Centers and Divisions.

Notice-This Tech Brief is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The Tech Brief does not establish policies or regulations, nor does it imply FHWA endorsement of the conclusions or recommendations. The U.S. Government assumes no liability for the contents or their use.

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Updated: 11/17/2011
  
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