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 asphalt rubber (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 asphalt concrete 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

The objective of this project was to develop an asphalt mix design method incorporating crumb rubber and using the "Wet" or "Dry" method of producing Crumb Rubber Modified Asphalt (CRM). Several resurfacing projects have been constructed using both the "Wet" and "Dry" methods. Based on this study, the Kansas Department of Transportation could use CRM mixes with a binder content between 7.5% and 9.0% depending on the percent air voids, with 19% to 22% rubber content. In this study, it was observed that using 24% rubber produced mixes that were too sticky to manage. With a rubber content of less than 18% combined with AC-5 it was difficult to satisfy the minimum viscosity requirements. Fracture tests can be used as a basis to determine the optimum binder content for any asphalt-rubber mix.

The mixture design and performance characteristics of crumb rubber modified asphalt concretes were investigated in this research project to meet the requirements of the Intermodal Surface Transportation Efficiency Act (ISTEA) of 1991, which has required each State to incorporate scrap tire rubber into its asphalt paving materials. Specifically, the objectives of this research encompass the following: (i) investigation of the rheological properties of asphalt-rubber binder to determine optimum content of crumb rubber; (ii) development of optimum mix design for various applications, including both wet and dry mix processes; (iii) characterization of mechanical properties of recommended paving mixtures, including resilient modulus, fatigue cracking behavior, low-temperature thermal cracking resistance, water sensitivity test, incremental creep test and loaded wheel track test; and (iv) comparison of performance of selected paving mixes.

This synthesis on the use of recycled rubber tires in highways will be of interest to administrators and policy-makers; pavement, materials, geotechnical, environmental, and traffic operations engineers; and research engineers involved with highway design and construction issues. Information is provided on the uses of rubber tires in asphalt paving materials as well as other uses, such as on fills and embankments, for erosion control and on railroad grade crossings. Specifically, information is included which identifies the agencies using or implementing applications for recycled rubber tires and defines the design parameters, technical and construction limitations, performance, costs, benefits, environmental limitations, specifications, and availability. This synthesis of information defines the use of recycled rubber tires in highways and is based on a review of nearly 500 references and on information recorded from state highway agency responses to a 1991 survey of practice. Updates are included for as much of the state practice information possible through 1993. The use of scrap tires for highway applications is dynamic with regard to policy and technical issues. Therefore, the reader should keep in mind that the information presented reflects the best available data at a particular time. The synthesis also identifies current research in the topic area, critical research needs, and legislative issues that affect application and use of recycled rubber tires.

This report outlines both the general development and mix-design of gap-graded asphalt. Although only gap-graded mixes are considered many of the principles discussed can be applied to the design of other types of mixtures. The increase in price of bitumen, the shortage of high quality aggregates and the increases in traffic intensity and loading, are all factors that have influenced the need to understand the effect of the mix constituents on the behaviour of the bituminous mixture. Emphasis is therefore on the development of mix-design procedures that will enable the engineer to optimize the performance of the asphaltic mixture, according to the material characteristics. Among those characteristics considered are the shape, surface texture, grading, and mix combinations of the coarse and fine aggregate, and their associated effect on the binder requirements of the finished mixture.