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OCR for page 145
Design of Dense-Graded HMA Mixtures 145 these adjustments, and the one recommended in this manual, is to adjust the test temperature upwards as design traffic speed decreases. The recommended temperature adjustment is +6C for slow traffic and +12C for very slow traffic. However, as with other aspects of performance testing, agencies should use judgment and experience with local conditions and materials when establishing performance test requirements for slow and very slow traffic speeds. For many applications, performance testing is probably only necessary on material placed within 100 mm of the pavement surface. However, for critical projects, material placed 100 to 200 mm within the pavement surface might also be tested. In such cases, the test temperature should be adjusted to reflect the estimated temperature at the surface of the material as placed within the pavement structure, as determined using LTPPBind. For example, a base course placed 100 mm below the pavement surface will have an estimated critical high pavement temperature 7.7C lower than material at the pavement surface; the test temperature for this material would then be reduced by 7.7C. Adjusting Mix Designs to Improve Rut Resistance As mentioned earlier in this section, the rut resistance tests and recommended minimum and maximum values for test results have been selected so that most dense-graded HMA designs developed following the procedures given in this manual will meet the requirements, and no additional laboratory work will be needed. However, some mix designs will fail to meet require- ments for rut resistance. In such cases, the test results should first be checked to make sure there were no errors in either the procedures used or in the calculation of the test results. If no errors are found, and the test results are close to meeting the requirements, the test can be repeated. In this case, the results of both tests should be averaged and compared to the test criteria. If the mix still fails to meet the requirements for rut resistance testing, the mix design will have to be modified. Rut resistance of an HMA mix design can be improved as follows: Increase the binder high-temperature grade. If the binder is not modified, consider using a polymer-modified binder of the same grade or one high-temperature grade lower. If the binder is polymer-modified, try a different type of modified binder. Increase the amount of mineral filler in the mix, adjusting the aggregate gradation if necessary to maintain adequate VMA. Decrease the design VMA value, if possible, by adjusting the aggregate gradation. Replace part or all of the aggregate (fine or coarse or both) with a material or materials having improved angularity. If a different asphalt binder is used in the mix, the volumetric composition should not change. However, if other aspects of the mix design are changed, the volumetric composition might change significantly, which will require further refinement of the mix prior to further rut resistance testing. Step 11. Compile Mix Design Report The final step in preparing an HMA mix design is compiling a report documenting the mix design. In many states, standard forms must be filled out by hot-mix producers and submitted to the appropriate state agency or office for approval. In some cases, engineers or technicians may wish to develop their own mix design reports, for internal purposes or for use on private jobs. In such cases, the following information should be included in the report: The organization that performed the mix design. The name of the technician or engineer responsible for developing the mix design.

OCR for page 145
146 A Manual for Design of Hot Mix Asphalt with Commentary The date the mix design was completed. The name of the client for which the mix design was developed. The name of the project for which the mix design was developed (if applicable). General mix design information, including the type of mix (surface course, intermediate course, base course), the nominal maximum aggregate size, the design traffic level, the Ndesign value, and any special requirements. Complete aggregate information, including for each aggregate the producer, the size designation of the aggregate, gradation, specific gravity, and all applicable specification properties. Binder information, including the binder performance grade and the name of the supplier. Composition of the mixture, including the design air void content, the design VMA, the design VBE, the mineral filler content, the target dust/binder ratio, and the estimated unit weight for the mix Brief comments on the workability of the mix. The results of moisture resistance testing. The results of rut resistance testing, if applicable (generally for mixtures designed for traffic levels of 3 million ESALs and over). The spreadsheet, HMA Tools, can generate a comprehensive mix design report containing all of this information as well as additional information on the results of trial mixtures evaluated during the mix design process. This report might be useful to some engineers and technicians for internal purposes and might also serve as a template for those wishing to develop their own customized mix design report. Bibliography AASHTO Standards M 320, Performance-Graded Asphalt Binder M 323, Standard Specification for Superpave Volumetric Mix Design R 30, Mixture Conditioning of Hot-Mix Asphalt (HMA) R 35, Standard Practice for Superpave Volumetric Design for Hot-Mix Asphalt (HMA) T 166, Bulk Specific Gravity of Compacted Asphalt Mixtures Using Saturated Surface-Dry Specimens T 209, Theoretical Maximum Specification Gravity and Density of Bituminous Paving Mixtures T 269, Percent Air Voids in Compacted Dense and Open Asphalt Mixtures T 275, Bulk Specific Gravity of Compacted Bituminous Mixtures Using Paraffin-Coated Specimens T 283, Resistance of Compacted Asphalt Mixture to Moisture-Induced Damage T 312, Preparing and Determining the Density of Hot-Mix Asphalt Specimens by Means of the Superpave Gyratory Compactor T 320, Determining the Permanent Shear Strain and Stiffness of Asphalt Mixtures Using the Superpave Shear Tester. T 324, Hamburg Wheel-Track Testing of Compacted Hot-Mix Asphalt (HMA) TP 63-09, Determining Rutting Susceptibility of Asphalt Paving Mixtures Using the Asphalt Pavement Analyzer (APA) Other Publications The Asphalt Institute (2001) Superpave Mix Design (SP-2), 128 pp. The Asphalt Institute (1997) Mix Design Methods for Asphalt Concrete and Other Hot-Mix Types (MS-2), 6th Ed., 141 pp. Bonaquist, R. F. (2008) NCHRP Report 629: Ruggedness Testing of the Dynamic Modulus and Flow Number Tests with the Simple Performance Tester, TRB, National Research Council, Washington, DC, 130 pp. Brown, E. R., et al. (2004) NCHRP Report 531: Relationship of Air Voids, Lift Thickness, and Permeability in Hot-Mix Asphalt Pavements, TRB, National Research Council, Washington, DC, 37 pp. Burns, Cooley, Dennis, Inc., AAPTP Project 04-03: Implementation of Superpave Mix Design for Airfield Pavements, Quarterly Progress Report for the Period January 1, 2007 through March 31, 2007, available at www.aaptp.us/ Report.Interim.04-03.pdf.

OCR for page 145
Design of Dense-Graded HMA Mixtures 147 Christensen, D. W., and R. F. Bonaquist (2006) NCHRP Report 567: Volumetric Requirements for Superpave Mix Design, TRB, National Research Council, Washington, DC, 57 pp. Christensen, D. W., et al. (2004) "Indirect Tension Strength as a Simple Performance Test," New Simple Performance Tests for Asphalt Mixes, Transportation Research Circular E-C068, http://gulliver.trb.org/publications/ circulars/ec068.pdf, TRB, National Research Council, Washington, DC, pp. 44-57. Kandhal, P. S., and L. A. Cooley (2003) NCHRP Report 508: Accelerated Laboratory Rutting Tests: Evaluation of the Asphalt Pavement Analyzer, TRB, National Research Council, Washington, DC, 73 pp. Leahy, R. B., and R. B. McGennis (1999) "Asphalt Mixes: Materials, Design and Characterization," Journal of the Association of Asphalt Paving Technologists, Vol. 68A, pp. 70-127. Maupin, G. W., Jr. (2003) Final Report: Additional Asphalt to Increase the Durability of Virginia's Superpave Surface Mixes, Report VTRC 03-R15, Charlottesville, VA: Virginia Transportation Research Council, June, 13 pp. Vavrik, W. R., et al. (2002) Bailey Method for Gradation Selection in HMA Mixture Design, Transportation Research Circular E-C044, http://gulliver.trb.org/publications/circulars/ec044.pdf, TRB, National Research Council, Washington, DC, October, 34 pp. West, R. C., et al. (2010) NCHRP Report 648: Mixing and Compaction Temperatures of Asphalt Binders in Hot-Mix Asphalt, TRB, National Research Council, Washington, DC, 77 pp.