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U of MD asphalt mix design
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In order to select target "design" asphalt contents for each of the six mixtures investigated (mixture evaluation with dynamic mechanical tests); asphalt mix designs were developed for all mixes. In this study, two separate mix design procedures were used to estimate the "design" asphalt content. These approaches were the: (1) Marshall Mix design and (2) SHRP (SUPERPAVE) Gyratory Level I Mix design. For each mix, the design asphalt contents were determined by both procedures and a final "design" value was subjectively selected for further sample preparations in the dynamic material characterization tests described in the following [text] of this report.
Marshall Mix Design The Marshall test was used to obtain the optimum asphalt content based upon ASTM D 1559, "Resistance to Plastic Flow of Bituminous Mixtures Using Marshall Apparatus." For this study, the mix design criteria of the Asphalt Institute (as defined in MS-2) was used to estimate the "design" asphalt content for each of the six mixtures investigated. The test criterion selected was for a 75 blow Marshall compaction.Selection of Design Asphalt Content In order to determine the optimum asphalt content, three levels of asphalt content (4, 5 and 6%) were used for each mix. At each asphalt content, three replicates were prepared and the average of the three were then used to develop the typical Marshall test result plots. . . . Using the TAI - MS-2 design criterion, the final "Marshall Design" asphalt contents and appropriate volumetric/stability results (at the design asphalt content) are shown [here]:Marshall Design Summary
The table indicates that the design binder content, for each of the two virgin mixtures, is increased by 0.5% binder when 2.0% Elvaloy® polymer is added. The Marshall Stability results also show that a slight improvement in stability occurs as the Elvaloy® percentage is increased. Another Marshall parameter of interest is the Stability/Flow ratio (sometimes referred to as the Marshall Quotient). This parameter is thought to be an empirical indicator of the mix stiffness. As noted in the table, the Marshall Quotient appears to slightly decrease with increased percentage of the Elvaloy® polymer. The slight decrease of the ratio values with the modified asphalts is the result of the increase in the flow values for these mixtures. For both studies (DuPont No. 1 and No. 2), the virgin mix exhibited the largest Marshall Quotient. SUPERPAVE Gyratory Level I Mix Design The second mix design approach used to determine the "design" asphalt content for each of the six mixtures evaluated was the SUPERPAVE Level I (volumetric) mix design approach. This approach requires specimen compaction with a gyratory compactor in accordance with SHRP Standard Method of Test M-002. A Rainhart gyratory compactor was used at the University of Maryland lab. This compactor provides a consolidation pressure of 0.60 MPa with an angle of gyration of 1.25 degrees and speed of gyration of 30 rpm. The gyratory device continuously monitors the increase in specimen density (expressed as a percent of its theoretical maximum specific gravity). For all mixes, specimens were compacted in a 100-mm diameter mold for up to 200 gyrations.Similar to the Marshall test, three levels of asphalt content, each with two replicate samples were used for each mixture evaluated. The gyratory test results and graphical plots of percent theoretical maximum specific gravity versus number of gyrations for each asphalt content are shown in Appendix C (omitted*). The "design" asphalt content was based on number of gyration criteria specified by the SUPERPAVE Level I procedure. A design gyration number of 96 was used, along with a 4% air void criterion to select the design asphalt content. This gyration number corresponds to an average design air temperature between 37-38 degrees C and traffic level of less then 10 million ESAL, in order to be as comparable as possible to the Marshall 75 blow compactive effort. Based upon the procedure previously presented, the "design" asphalt contents were found to be:
Selection of Final Design Asphalt Contents The following table is a summary of the "design" asphalt contents determined for each of the six mixtures evaluated, by both the Marshall and SUPERPAVE Level I approaches. Also shown in this table, are the final recommended design asphalt contents, for each mix, that were subsequently used to prepare all remaining specimens to be evaluated for the: dynamic (complex) modulus; static creep testing and repeated load permanent deformation tests.Summary of Design Asphalt Contents
It is quite obvious from the results shown that little, if any, correlation is noted between "design" asphalt contents found by the Marshall and Gyratory Level I results. For the DuPont Study No. 1 mixtures, the two Elvaloy® modified mixtures yielded lower gyratory based design asphalt contents compared to the Marshall design. However, the opposite trend was found for the polymer modified mixtures used in DuPont Study No. 2. Because the SUPERPAVE Level I approach to mix design is a recent technical innovation, there is a paucity of historic data in the literature to either agree or disagree with the differences in asphalt content found from each "mix design procedure." As a result, the principal author developed the final recommended values for the design based upon an evaluation of well established volumetric mix principles relating to air voids; voids in the mineral aggregate and voids filled with bitumen. As can be observed, these "final" values are generally "compromises" between the Marshall and Gyratory approaches. With each study mix, asphalt content increments of 0.2% were used for each level of Elvaloy® percentage compared to the virgin (0% Elvaloy®) mixture.
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