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Performance SPecificationS for aSPhalt mixtureS Asphalt mixtures are typically designed within a given gradation band to meet a set of volumetric properties at a given laboratory compaction effort. This project will document the state of the practice in performance specifications for asphalt mixtures and their use with traditional volu- metric properties. Performance specifications can be performance-related and/or performance- based. Asphalt mixtures include both hot mix asphalt and warm mix asphalt, and are produced at a plant and may include mixtures that have been modified with recycled materials. Volumetric properties may not provide optimum performance for mixtures that include recycled materials and/or certain types of modifiers. Some agencies have investigated per- formance specifications for asphalt mixtures. These specifications typically include tests for rutting and cracking. Other agencies are investigating mechanistic properties (i.e., dynamic modulus) in conjunction with implementing mechanistic-empirical pavement design. In addition, certain areas may require performance tests to address specific requirements such as studded tire wear and moisture susceptibility tests. This synthesis documents the performance tests used in conjunction with volumetric properties for mixtures. Performance tests are intended to extend service life by guiding material selection (i.e., asphalt binder and aggregate) and proportions (i.e., asphalt content and gradations). The results will benefit government agencies, researchers, and the road building industry by providing guidance on making better use of recycled materials, while also providing better performing and more cost-effective asphalt mixtures for specific applications. The synthesis provides examples of engineering tools used in the develop- ment and implementation of performance specifications for asphalt mixtures, examples of the contents of performance-based specifications (PBS) currently used or in development, information on test program implementation and research related to PBS for asphalt mix- tures, and the reported benefits and challenges with implementing PBS. The findings of this study are intended to provide state departments of transportation (DOTs), Canadian provincial ministries of transportation (MOTs), and local public agencies with useful infor- mation on how to more effectively implement performance testing required as part of PBS for asphalt mixtures and on how to better communicate any implementation benefits and challenges to paving industry partners. The information for this synthesis was gathered through a comprehensive literature review, a survey of DOTs and other transportation agencies, and detailed interviews with multiple organizations in the United States and Canada selected for further study. Forty-six DOTs (45 states and the District of Columbia) responded to the survey, a response rate of 90% (45/50). In addition, responses were received from 10 Canadian provinces and three local agencies. After reviewing documentation in the literature and the detailed survey responses, the city of Edmonton (in Alberta, Canada) along with the states of Florida, Georgia, Louisiana, Maryland, Minnesota, New Jersey, Ohio, Texas, Utah, Virginia, and Wisconsin were selected for further review through interviews. Based on the content analysis of the interviews, six states and Edmonton were fully developed into case examples to display details related to the development and use of PBS, evaluation of performance testing, and implementation efforts Summary
2 related to PBS conducted by these agencies. The following observations were made based on the agency survey data, detailed interviews, and the literature review. ⢠The literature review, survey responses, and interviews all indicated that a significant amount of research is underway, both in the United States and abroad, to generate the data and establish the criteria necessary to develop PBS for asphalt mixtures. The results indicated that a small number of DOTs and municipal agencies are currently using per- formance tests as part of standard mixture acceptance. The survey data indicated that the performance-based properties most commonly used and researched include the measure- ment of stiffness, thermal cracking, moisture resistance, and fatigue cracking. ⢠The current state of the practice reported for asphalt pavement mixture design and acceptance is using volumetric properties in conjunction with performance properties. In a few cases, performance tests such as the Asphalt Pavement Analyzer (APA) and Hamburg Wheel Tracking Device (HWTD), which both measure the rutting resistance and resistance to moisture damage, have been incorporated into standard practice, including production acceptance testing at the option of the engineer. It was reported that further research is also underway to address premature fatigue cracking. ⢠The survey data revealed that the HWTD test, APA test, bending beam rheometer, and flexural beam fatigue test were the most commonly used tests in support of PBS. There were 19 DOTs and three agencies in Canada that reported having the necessary equip- ment required for the laboratory testing that supports their PBS. ⢠The most frequently reported reasons for the use of performance specifications for asphalt mixtures were to achieve longer pavement service lives (in terms of resistance to rutting and cracking and other distresses) and to quantify the quality and encourage better construction of flexible pavements. ⢠The majority of states and Canadian provinces are building flexible pavements from asphalt mixtures produced with recycled materials such as reclaimed asphalt pavement, recycled asphalt shingles, crumb rubber or ground tire rubber, or warm mix additives. Many of these agencies reported that they require different test approaches than those used for traditional hot mix asphalt mixtures as a result of incorporating nontraditional mixture designs. ⢠A number of agencies have observed that Superpave mix designs may have issues when using recycled materials (e.g., recycled asphalt shingles and crumb rubber). Some of the issues noted by the agencies interviewed included overly dry mixes, increased stiffness, and development of premature cracking. For this reason, some agencies are working toward implementing a balanced mix design process that incorporates performance tests to achieve an optimal balance between rutting resistance and fatigue and thermal crack- ing resistance. ⢠Approximately one-third of the state DOTs reported both test time and cost as deciding factors in implementing PBS. The feedback provided through both the detailed survey responses and interviews was that tests need to be straightforward, relatively quick, and easy to perform. Other reasons included for the tests to be accepted by industry and the affordability of the test equipment both to purchase and to run (i.e., minimizing the number of staff and amount of staff time). ⢠The survey data indicated that few agencies (two state DOTs, two MOTs, and one Canadian city) are currently assessing the costs and benefits of using PBS. However, the data collected indicates momentum in moving forward with performance specifi- cations because 15 state DOTs and one MOT reported that they are planning to assess these factors in the future. ⢠Substantial research is underway by a number of agencies focused on developing PBS, in conjunction with mechanistic properties, for mix acceptance and production acceptance.
