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Development of mix design procedures for gap-graded asphalt-rubber asphalt concrete
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    FHWA-AZ-06-524 ; Report No. 4975-03-3008 ; Final Report 524
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  • Abstract:
    A research project was conducted to identify and document current modifications to ARIZONA 815c (75-blow Marshall method) used to develop gap-graded asphalt rubber asphalt concrete (GG AR AC) mix designs, and to develop and test improvements to provide a standard mix design method for use by contractors and consultants. Based on field performance data provided by the Arizona Department of Transportation (ADOT), the existing mix design method was successful and should serve as the standard for comparison of proposed improvements. Best practices were synthesized to develop proposed improvements. Three aggregate sources and two AR binders were used for initial testing of the existing (control) mix design method and of the proposed changes. Rebound of compacted AR AC specimens was evaluated, as were Rice test results at 6% and 7% AR binder by weight of mix. The composition of the AR binders (rubber gradation and content) had more effect on the results than which mix design method was used. Additional replicate testing was performed by MACTEC and ADOT to confirm these findings. Changes to the AR AC mix design method consist primarily of making and curing Rice specimens in the same manner as Marshall specimens, tighter temperature ranges for mixing and compaction, incorporating Asphalt Institute calculations in a "User's Guide," and improving presentation. An ADOT construction project was used as an "acid test" to pilot the proposed mix design method and provide materials for a four-laboratory round robin to evaluate the precision of testing AR AC materials. The precision of round robin testing appears very similar to that of conventional AC mixtures based on data from Proficiency Sample Programs of the AASHTO Materials Reference Laboratory and ADOT. The results indicate that the mix design method developed can be used by qualified laboratories to provide suitable AR AC mix designs.

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