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54 chapter five ConClusions summary The results of the extensive review of published literature and the survey responses delivered by 45 state departments of transportation (DOTs), the District of Columbia DOT, 10 Canadian provincial ministries of transportation (MOTs), and three local agencies provided valuable insight into how various agencies are approaching the use of performance specifications for both traditional and nontraditional asphalt mixtures. Case examples were developed with state DOTs in six states and one city in Canada. The information obtained from interviews was used to acquire a more precise idea of the concerns and implementation practices regarding perfor- mance testing for the development and use of performance specifications for asphalt mixtures. ConClusions Based on the work carried out in this synthesis the following conclusions can be made: ⢠The literature review, survey responses, and interviews all indicated that a significant amount of research is underway to generate the data and establish the criteria needed to develop performance-based specifications (PBS) for asphalt mixtures, but that very few agencies are actively using performance tests as part of mixture acceptance at this time. The survey data revealed that the performance-based properties most heavily researched included measurement of the stiffness modulus, ther- mal cracking and moisture resistance, and fatigue and durability characteristics. ⢠The current state of the practice reported for asphalt pavement mixture design and acceptance is using vol- umetric 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, includ- ing production acceptance testing at the option of the engineer. Further research is underway to address pre- mature 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. Nineteen DOTs and three agencies in Canada reported that they have the necessary equipment for the labora- tory testing that supports their PBS. ⢠The majority of states and Canadian provinces are build- ing flexible pavements from asphalt mixtures produced with reclaimed asphalt pavement (RAP), recycled asphalt shingle (RAS), or warm mix technologies, and many of the agencies require different test approaches than those used for traditional hot-mix asphalt (HMA) mixtures as a result. ⢠Approximately 80% of the state DOTs (37/45) require the use of performance tests for predicting moisture damage for initial mixture qualification before produc- tion and seven agencies that require APA testing. ⢠Approximately 27% of the state DOTs (12/45) reported using performance specifications as the basis for mixture acceptance and pay adjustments. However, it can be noted that it is not clear whether all of the agencies are using a PBS. ⢠The most frequently reported reasons for the use of per- formance specifications for asphalt mixtures were to achieve longer pavement service lives, in terms of fatigue cracking and other distresses, and to quantify the quality and encourage better construction of flexible pavements. ⢠The survey data revealed that the HWTD test and the APA test were the most commonly used tests for PBS. Nineteen DOTs and three agencies in Canada reported that they have the necessary equipment required for the laboratory testing that supports their PBS. ⢠Approximately one-third of the states reported test time and cost as a deciding factor in implementing PBS. The feedback provided through both the detailed sur- vey responses and interviews were that tests need to be straightforward, relatively quick, and easy to perform. Other reasons reported included the need for the tests to be accepted by industry, as well as the affordability of the test equipment both to purchase and to run (i.e., minimizing the amount of staff and staff time). ⢠From the survey data, it was noted that very few agencies (two state DOTs, two MOTs, and one city in Canada) are currently assessing the costs and benefits of using PBS. However, there appears to be some momentum in mov- ing forward with performance specifications, because 15 state DOTs and one MOT reported that they are plan- ning to assess these factors in the future.
55 suggested Future researCh The results of the synthesis indicate that the primary need for research is to identify more practical and cost-effective performance tests that truly reflect field performance of asphalt mixtures. The challenges most frequently reported to the advancement of performance specifications cited the cost of equipment or testing by private laboratories, insuffi- cient training on performance tests and PBS, and the lack of confidence in test procedures and results. It was repeatedly noted that efforts for technology transfer might be made to assist the industry and agencies in understanding the benefits of added testing, as well as in facilitating agreement between the agencies and industry on which performance tests could be used for mixture acceptance as part of a PBS. Other items related to suggested future research include: ⢠Guidance to agencies and contractors on how to success- fully implement the use of PBS for asphalt mixtures. Of particular interest will be the presentation of this infor- mation to both garner buy-in from industry partners and to train the contractors, consultants, and agency staff who will be involved with performance testing. There is a need for research to facilitate the process of the imple- mentation of PBS to identify approaches to the type of training that will be needed and the types of collabora- tive initiatives for agencies and industry. ⢠There is a need for more research on the quantification of costs and benefits related to the use of performance specifications. The opportunity for studying this element could be accomplished through the use of PBS as part of projects that use publicâprivate partnerships, war- ranty, or design-build hybrid contracts. These types of projects would be ideal in that the responsibility for per- formance is placed on the contractor and this approach could help industry to better engage in the process of implementing PBS. ⢠The development and use of more practical and eco- nomic performance tests that can be applied for field quality control, acceptance, and pay factor adjustment (similar to the initiative underway that was reported by New Jersey DOT) would be an avenue for future research. In this approach, performance tests would be focused on nondestructive testing tailored for produc- tion quality control and acceptance, and could be run by industry, that measure performance properties (e.g., intelligent compaction for stiffness or fatigue or con- tinuous deflection devices). ⢠There are some agencies that reported benefits in researching the use of predictive models [e.g., Simplified Viscoelastic Continuum Design (S-VECD) or other predictive models that target specific distress types, and Quality-Related Specification Software (QRSS)] for the advancement and eventual adoption of PBS. A few states reported exploring the optimization of pavement designs based on life-cycle cost analysis as another area of research to support the future implementation of a PBS for asphalt mixtures. ⢠Significant advancements in pavement design and pave- ment distress predictive models have been made in recent years. It is suggested that research be conducted on integrating aspects of the mechanistic empirical pave- ment models with current and future PBS approaches. ⢠In addition to the affordability of test equipment and staffing, the amount of mixture sampling, specimen preparation, testing time, and test complexity (in terms of interpreting and reporting results) were mentioned numerous times as being major barriers to the imple- mentation of PBS. Therefore, a suggested area for future efforts is to research the streamlining of the entire per- formance testing process. This might be accomplished through efforts that include statistical analyses to refine and streamline the required sampling and testing pro- cedures; field studies to validate the results from both laboratory data and predictive models; collaboration with test equipment manufacturers to improve the effi- ciency, reliability, and affordability of test devices; and coordination with asphalt mixture producers to better tailor the mix approval process to complement a PBS system. Likewise, a national effort to develop consistent and detailed guidance on how to use PBS and the time and effort required to run the supporting performance tests such as the APA, HWTD, and Semi-Circular Bend (in terms of the details of the testing protocols and the various agency or national methods) is needed. Knowledge gaps The following section outlines some knowledge gaps that stemmed from the agency survey and interviews. Based on this work, the following activities are suggested to address knowledge gaps identified as part of this study: ⢠There is little guidance that exists on how to proceed with PBS, both in terms of any quantitative data being collected (or statistical analyses being conducted) and in terms of the availability of tests that address the pri- mary pavement distresses being observed by a particular agency. The agencies observed that the primary benefits of using performance specifications are typically noted on roadways with higher impacts, and that the cost of implementing PBS may not justify the means (benefits) in places that consist primarily of lower volume roads. It appears that this is an area of opportunity for future data collection and should consider the existence of adequate staff resources and available data, as well as define the applicability of PBS for each locale or roadway net- work. In addition, the use of available simple screen- ing test information (e.g., data from Cantabro testing) and from results of full-scale accelerated pavement test facilities could be considered for integration with PBS.
56 ⢠Some agencies reported that they are initiating the pro- cess of having contractors purchase the equipment and run the tests as part of their quality control program. This process would benefit by agencies working with industry to incorporate some of the most promising performance-based tests. It was noted in multiple survey responses and interviews that any future development of PBS mixture design specifications should consider the cost and availability of the performance tests, and that the relationship to actual field performance must be determined, prior to full implementation. ⢠There appears to be confusion with the interpretation of terms commonly associated with performance specifi- cations and the true practical differences between PBS, performance-related specifications, and other types of specifications. It is suggested that more widespread efforts be made to familiarize both the agencies and industry with the terms and associated components of the various specification types. A 2014 workshop con- ducted by FHWA on performance-related specifications would be a suitable model for future dissemination of this type of information on performance specifications. ⢠One gap in knowledge is that various agencies define asphalt mixtures such as warm mix asphalt and those designated as having a high RAP component differently. In addition, the variability during the production and construction of asphalt mixtures also varies from agency to agency. For the implementation of performance spec- ifications to be more comprehensive, a national effort to consistently define these parameters and terms would be beneficial. ⢠Another need is to more fully understand the extent of international experience with performance specifica- tions, and primarily PBS, outside of North America. It is suggested that such efforts seek to more fully research the use of PBS internationally and to better engage prac- titioners in other parts of the world to identify benefits, costs, and experiential information on this topic.
