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1 Performance testing of asphalt mixtures is becoming more widespread as agen- cies move toward performance-related specifications for asphalt-mixture design and construction and evaluate the potential benefits of new materials and products on asphalt-mixture performance. Standard specifications or guidance documents exist for conducting performance tests, but there is little to no guidance on the appropriate meth- odologies for fabricating the test specimens. Various research studies have shown that different specimen fabrication methods can significantly affect measured performance properties. This has substantial impact if performance properties are used as criteria in the design, evaluation, and acceptance of asphalt mixtures. However, general guidance on the best approaches to fabricating specimens that includes the collection of component materials, mixing and reheating protocols, laboratory conditioning, compaction, and coring/cutting have not been developed or documented in a manner that agencies can use to identify appropriate practices. As performance testing transitions to common practice in the design and evaluation of asphalt mixtures, guidelines on the specimen fabrication process will be needed. This synthesis documents state highway agency procedures with respect to laboratory fabrication of asphalt performance test specimens. The synthesis provides examples of sampling procedures used for materials and specific procedures and practices that states and partner laboratories have developed for improved consistency in fabricating performance test specimens, as well as common challenges faced and knowledge gaps that have been identified. The information for this synthesis was gathered through a comprehensive literature review, a survey of state departments of transportation (DOTs), a survey of partner labora- tories that complete asphalt performance testing for state DOTs, and subsequent interviews of personnel from five states selected for further study. Forty-one state DOTs (40 states and the District of Columbia) responded to the survey, attaining a survey response rate of 80%. After reviewing documentation in the literature and the detailed survey responses, the states of Arizona, Illinois, Maine, Montana, and Texas were selected for further review through interviews. Twenty-nine partner laboratories responded to the survey. The following observations were made based on the survey data, detailed interviews, and literature review. ⢠Most states are either fabricating asphalt specimens for performance testing in house or having the work done at partner laboratories. However, states use a range of mixture sampling and splitting procedures for collecting material for specimen fabrication. ⢠Existing ASTM or AASHTO standards are followed as written or with some modification for the performance testing but usually do not provide appropriately detailed guidance with respect to specimen fabrication. S U M M A R Y Practices for Fabricating Asphalt Specimens for Performance Testing in Laboratories
2 Practices for Fabricating Asphalt Specimens for Performance Testing in Laboratories ⢠Loose-mix conditioning is commonly adopted, but various protocols are used for short- and long-term conditioning between agency and partner laboratories. ⢠Most performance test specimens are compacted to a target air void content of 7% using gyratory compactors. The allowable tolerance is typically higher for agencies than partner laboratories, and meeting air void requirements is reported as the most common challenge in specimen fabrication, with average rejection rates of 10% to 15%. ⢠There are minimal restrictions in place at DOT laboratories with respect to maximum allowable storage times during the performance-test specimen-fabrication process. Partner laboratories more commonly use maximum storage time limits; however, the reason for choice of specific maximum allowable storage times is typically not docu- mented. A majority of entities do not have specific protocols to prepare specimens for storage. ⢠Partner laboratories and agencies with more experience with performance testing have established protocols and specific guidance for fabricating specimens to maintain consistency, but these protocols and guidance vary widely.
