Significant Findings from Full-Scale Accelerated Pavement Testing (2012)

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Full-scale accelerated pavement testing (f-sAPT) forms a vital link between the laboratory evaluation of materials used in pavement layers and the field behavior of these materials when combined into pavement structures. For many years f-sAPT provided pavement engineers with knowledge that improved their understanding of pavement materials and structures, as well as their behavior under typical traffic and environmental loading. It formed the basis for developing various theories about pavement behavior and supports most of the current pavement design methods. This third NCHRP synthesis on f-sAPT contributes to the body of knowledge by evalu- ating current developments and advances around f-sAPT. Its objective is to expand on the foundation provided in NCHRP Syntheses 325 and 235 on f-sAPT by adding information generated between 2000 and 2011, and identifying gaps in knowledge and future research needs. To address these objectives, the synthesis covers evaluation of the operational f-sAPT programs; discussion of material-related issues as researched through f-sAPT; discussion on pavement structure-related research using f-sAPT; application of f-sAPT in the evaluation and validation of new Mechanistic–Empirical pavement design methods; and identification of the future needs and focus of f-sAPT. Research and developments conducted outside the focus period of this synthesis (pre-2000) are excluded as this has been covered extensively in the previous syntheses. For the purposes of this synthesis, the definition of f-sAPT covers the controlled applica- tion of full-scale wheel loads to layered, structural pavement systems to determine pavement response and performance under controlled, accelerated, accumulation of damage loading in a compressed time period. This is done while environmental effects on the pavement are typically controlled and measured. This synthesis is based on two major sources of information. A questionnaire covering relevant aspects of f-sAPT was distributed by an online system to 43 known U.S. and international operators and owners of f-sAPT devices, the 50 U.S. state departments of transportation representatives, and a group of specialists active in the field of pavement engineering. The questionnaire focused on the perceptions of the respondents around various f-sAPT issues, as well as information on their own activities in the field. The second source of information is the conference and journal published papers over the last decade. The synthesis specifically covers the period 2000 to 2011, and thus information before this period was not included in the source material, although the work covered in the synthesis is based on the history of f-sAPT that essentially started in the 1960s. Thirty-eight U.S. state departments of transportation, 29 f-sAPT programs and 5 specialists responded to the questionnaire, for a total of 72 unique responses. The overall finding of this synthesis is that the judicious use of f-sAPT contributes to and supports the body of knowledge regarding the way that pavement materials and structures react to controlled traffic and environmental loads. Through well-planned studies the f-sAPT work conducted over the last decade highlighted the following strategic findings that provide important information to the pavement engineering community to ensure the sustainable and efficient supply of cost-effective pavement-related infrastructure. Summary Significant findingS from full-Scale accelerated Pavement teSting

2 The importance of f-sAPT is perceived as high, with a major role to be played in pavement structure and basic materials research. The future of f-sAPT is primarily perceived as growing and being a normal part of pavement research operations, benefiting improved structural and material design methods, performance modeling, and evaluation of novel materials and structures. It was evident from both the questionnaire and the published literature that many programs share their facilities and data in order to expand the database that their specific research is founded upon. In this regard the formation of associations of f-sAPT users with the general objective of improving the cost-effectiveness of the overall programs through cooperative efforts of program planning, data analysis, and device improvements is evident. A wide scope of topics is addressed in the research conducted by the various programs, with traditional focus areas such as hot mix asphalt (HMA) response to applied loads still quite evident. However, the data also show that topics related to environmental issues, such as the increased use of warm mix asphalt and recycled asphalt products are receiving increasing attention. The trend derived from the questionnaire indicating that f-sAPT focuses on materials used closer to the surface was confirmed through the evaluation of the published literature. Another focus is on the extension of pavement life through the application of HMA overlays and ultra-thin whitetopping. Evaluation of materials models for newer pavement design methods [such as the Mechanistic Empirical Pavement Design Guide (MEPDG) and California Mechanistic Empirical design method (CalME)] is also quite common. F-sAPT aims to evaluate pavement sections under a range of loading and environmental conditions to improve the knowledge of the potential performance of the pavement layers and structure under a full range of operational conditions. The majority of respondents relate their f-sAPT data to pavement temperature and ambient air temperature, while also actively managing these parameters during tests. This trend may be related to the majority of tests being conducted on temperature-sensitive HMA materials. New focus was placed by a number of researchers on the major effects that tire contact stresses and loading conditions have on pavement response, and the active management of this parameter is visible in the published literature originating from many programs. The improved characterization of loading conditions is mirrored by the use of more complicated materials models that can react to these input conditions and provide improved models of the materials’ load responses. Many programs actively focus on the validation of the models incorporated in the MEPDG and CalME pavement design processes, thereby reducing the risk involved in pavement design as more appropriate parameters are incorporated into the design and the effect of each of the parameters are better understood. The increased use of finite element modeling for analysis of moving loads (as opposed to static load analysis) where factors such as mass inertia and stress rotation are incorporated into the model; the increased use of materials models that are not simply linear elastic, but that incorporate the effects on nonlinearity and viscosity; the increased use of detailed definition of the applied loads in terms of both load history and contact stress patterns; and the increased cognizance given to the effects of the environment on pavement response are prominent in terms of f-sAPT modeling. It appears that a virtual process is driven on several fronts where improved computing technology allows the complexity of calculations to increase without becoming too time- and resource-consuming, while the understanding of materials properties are improving with the parallel development of appropriate laboratory and field instruments and tests to obtain these parameters for different materials, and the subsequent modeling is improved through the combination of these factors. A major part of the modeling is still focused only on the surfacing layers and that the effect and contribution of lower materials layers are generalized and simplified, although these

3 effects may sometimes significantly affect the surfacing and other upper layers. It is specifically the strength-balance of the pavement that often appears to be ignored in test planning and modeling. Respondents viewed improved structural and material design methods, evaluation of novel materials, improved performance modeling, and the development of performance-related specifications as the major benefits of f-sAPT, while perceptions regarding the way that f-sAPT has changed the pavement engineering world focused on proving new techniques, materials, and development of a fundamental understanding of pavement structures. In the last decade the most significant strategic level findings from f-sAPT have focused on materials characterization, pavement modeling, pavement behavior and performance, pavement design method development and calibration, benefits of specific materials and technologies, economic impacts of f-sAPT programs, calibration of pavement design methods, development of data- bases of information on pavement performance that are shared between different pavement research programs, cost savings through implementing f-sAPT, and the development of improved instrumentation and analysis methods. In terms of more practical examples, issues such as an improved understanding of failure mechanisms of top-down cracking, critical strain limits in HMA, the effect of adequate layer compaction, variability of materials and layer properties, improved understanding of the links between various materials’ laboratory and field behavior, and the effect of various real environmental conditions and traffic on pavement behavior and performance are seen as major international findings. Evaluation of the economic benefits of f-sAPT has come to the forefront during the past decade with more programs reporting attempts at performing benefit–cost ratio (BCR)-type evaluations of their research programs. It appears that the general international economic conditions force researchers to prove the benefit of their research much more and identify, analyze, and quantify the direct and indirect benefits obtained from f-sAPT. The majority of programs are still only conducting BCR analyses after the research has been completed (43.5%), while 17.4% of respondents indicated that they perform BCRs as an input in the research planning. Estimates of BCRs from respondents ranged broadly between 1.4 and 11.6, although some respondents to the questionnaire indicated that their perception of the BCR for their programs is greater than 30. It is evident that the f-sAPT community is moving forward by focusing on calibration of f-sAPT outputs with in situ pavement data, specifically with the view of incorporating environmental and real traffic issues that cannot be modeled using f-sAPT. The potential exists for evaluating the effects of novel research questions around climate change on pavements through the judicious application of artificial temperature and moisture changes (based on expected weather conditions) during f-sAPT. Many of the trends identified in the synthesis are old issues that have been known to the pavement engineering community for a long time, but which did not receive the required attention in the research and testing environment. Questionnaire respondents indicated that issues such as a more detailed focus on vehicle– pavement interaction (including improved load and contact stress models), environment– pavement interaction (including climate change issues), development of and improvements in performance-related specifications, improved MEPDG validation, evaluation of sustainable pavement solutions (energy efficient technologies and re-use of available infrastructure), and improved reliability in pavement design are important future focus areas.