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

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

105 IntroductIon This final chapter provides information on expected future developments in full-scale Accelerated Pavement Test- ing (f-sAPT) with emphasis on current plans, anticipated important issues for the future, and an analysis of how the outcome of f-sAPT could guide and influence pavement engineering. Information is discussed around current gaps identified in specific projects, future research needs identi- fied by respondents, and the level to which gaps and needs identified in the previous syntheses have been bridged over the last decade. It further provides conclusions regarding the key findings of the synthesis project, as well as rec- ommendations for f-sAPT research in the near future. It focuses on significant findings that are deemed to influence pavement engineering. contInued trends The following four items for future research were identified by Hugo and Epps Martin in 2004: 1. In situ field performance of pavements tested in f-sAPT for future comparative studies. 2. Closer association between f-sAPT and in-service pave- ment evaluations from SHRP Long-term Pavement Per- formance (LTPP) studies and pavement management systems to validate and evaluate f-sAPT results. 3. Exploration of vehicle–pavement–environment inter- action to enhance the ability to do quantitative perfor- mance prediction of different pavement structures under specific conditions. 4. Prudent use of available information and collaborative research efforts to improve the reliability of findings and establish confidence limits. Evaluating these four items in light of the information cov- ering the period from 2000 to 2011 contained in this synthesis it is evident that the f-sAPT community is moving in the right direction, although it may be slow, with detours in some areas. The importance of in situ sections for performance evaluation and validation of f-sAPT data (long-term monitoring section) is appreciated by most, and there are a number of cases where such validations have been attempted. The problem currently is in the linkage, with good information regarding the perfor- mance trend of the field sections. Primarily, these sections are not as well monitored as the original f-sAPT sections, and it often becomes difficult to obtain reliable traffic and environmental data for a field section. The costs of a long- term field evaluation are often a deterrent to ensuring that long-term data are collected regularly. Vehicle–pavement–environment interaction is receiving ample attention in current programs, and most of the reports at least indicated the conditions under which specific tests were conducted. This includes tire types and conditions, tempera- ture and moisture content at which a test was conducted, and the speed at which the test was conducted. Although not all of these parameters are necessarily controlled during the test, that the researchers appreciate their importance is already a step forward. The importance of incorporating these aspects into the modern pavement design methods also forces an improved evaluation of the methods to control and measure these parameters during f-sAPT, as well as during normal trafficking of pavements. Collaborative research has increased significantly dur- ing the last decade. The development of associations has led to improved communications around common interests and facilities, and different facilities and programs are also using data collectively to evaluate models developed on specific facilities and validate the applicability of specific models and design methods. This allows for data origi- nating from other programs and countries to be analyzed using a model developed under different conditions, and the robustness of models is thus evaluated. The regular inter- national accelerated pavement testing APT conferences and the work at TRB’s Full-Scale Accelerated Pavement Testing AFD40 committee also ensure constant commu- nication between role players. Prudent use of technology such as videoconferencing allows meetings to be organized with international colleagues, fostering the relationships and the multiple uses of data. Thirty-two respondents to the questionnaire indicated that they own an f-sAPT device, whereas 38 had access to a device. Fourteen owners of f-sAPT devices noted that they make data available to non-APT owners, whereas 11 allow APT owners access to their data. Of those respondents who have access to an f-sAPT device, 23 own the device, 2 rent the device, and the remainder access the device through other means (i.e., sharing, part of a consortium etc.). chapter seven trends and conclusIons

106 new trends Many of the trends identified in the synthesis are known issues, but did not receive the required attention in the research and testing environment as would be expected. In this regard, issues such as the stress rotation within pavement layers that affects the performance as indicated in the synthesis, non- linear, elasto-plastic, elasto-visco-plastic, and other complex models are being incorporated into research and analysis procedures more often. The active use of these models and techniques would gain further improvements in the area of pavement behavior based on f-sAPT. Economic evaluation of f-sAPT programs is receiving more attention, and it is not clear whether all programs would pre- fer to enhance the profile of benefit–cost ration (BCR) analy- ses. It does appear that the majority of programs are satisfied that they do get good data from the programs, and that more detailed analyses are not required; rather, more attention and time are focused on the technical analyses and issues around pavement provision and maintenance. Continuous improvements in computer technology allow for the use of more complex models that are providing data much closer to real pavement’s response to loads. These more complex models also incorporate the details of tire– pavement contact stress area and magnitudes. Respondents to the questionnaire indicated that the fol- lowing future research from f-sAPT could aid in the evalua- tion of the following broad topics: • 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 Mechanistic Empirical Pavement Design Guide (MEPDG) validation, • Evaluation of sustainable pavement solutions (energy efficient technologies and re-use of available infrastruc- ture), and • Improved reliability in pavement design. The increased use of dedicated airfield facilities is visible in the academic literature. The special load magnitudes and bogeys used by this traffic require more specific instrument and monitoring equipment than for some of the traditional f-sAPT programs. conclusIons These conclusions are based on the information gained from the questionnaire as well as the published literature. This synthesis on f-sAPT contributes to the body of knowledge by evaluating developments and advances around f-sAPT between 2000 and 2011. The specific objective of this syn- thesis is to expand on the foundation provided in NCHRP Syn- theses 325 and 235 on f-sAPT by adding information generated between 2000 and 2011, and identify gaps in knowledge and future research needs for f-sAPT. To address this objective, the study scope incorporates information on all aspects of f-sAPT since 2000 with a focus on: • 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 (M-E) pavement design methods, and • Identification of the future needs and focus of f-sAPT. The overall finding from this synthesis is that the judi- cious 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 environmen- tal 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 sustain- able and efficient supply of cost-effective pavement-related infrastructure. General perceptions regarding f-sAPT indicate that it is perceived as important, with a major role to be played in pave- ment structure and basic materials research. The future use of f-sAPT is mainly perceived as growing and being a normal part of pavement research operations, benefiting improved structural and material design methods, performance model- ing, and evaluation of novel materials and structures. It was evident from both the questionnaire and the pub- lished literature that many programs share their facilities and data in order to expand their database. 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. It also supports cross-fertilization of ideas and solutions to pave- ment engineering problems. A wide scope of topics is addressed in the research con- ducted by the various programs, with the major focus on hot mix asphalt (HMA) materials. The questionnaire trend indi- cating that f-sAPT focuses on materials used closer to the sur- face were confirmed through the evaluation of the published literature. Apart from the major emphasis on HMA rutting, a developing trend is the evaluation of environmentally sensi- tive materials such as warm mix asphalt and recycled asphalt

107 pavement (RAP), as well as the focus on pavement life exten- sion through application of HMA overlays and ultra-thin whitetopping. Aging effects of HMA are addressed, although accelerated artificial aging of HMA is not necessarily pro- viding similar results to real-time-aged HMA. A number of miscellaneous unique applications of f-sAPT have also been discussed, including the testing of pipes in trenches, snow melting systems, and the use of f-sAPT data to assist in cal- culating road user charges and pay factors. Respondents view temperature as the most important envi- ronmental parameter to relate f-sAPT data with and to control during tests. This is probably related to the high percentage of HMA-type f-sAPT evaluations conducted. 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. The major effects of tire contact stresses and loading conditions on pavement response were highlighted by many researchers and are shown to be incorporated as a factor in many of the test programs evaluated for this synthesis The improved characterization of loading conditions is mirrored by the use of more complicated materials mod- els that can react to these input conditions and model the response of the various materials more realistically. Many programs actively focus on the validation of the models incor- porated in the MEPDG and California Mechanistic Empiri- cal 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 the finite element method 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 lin- ear elastic, but which incorporate the effects on nonlinearity, viscosity, and environmental sensitivity (i.e., moisture and temperature); 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 standing out in terms of f-sAPT modeling. It appears that a virtual process is driven on several fronts, where improved computing technology allows the com- plexity of calculations to increase without becoming too time- and resource consuming, while the understanding of materials properties is improving with the parallel devel- opment of appropriate laboratory and field instruments and tests to obtain these parameters for different materials. The subsequent modeling is improved through the combination of these factors. It does, unfortunately, appear that a lot 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 effects may sometimes affect the surfacing and other upper layers significantly. It is specif- ically 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 per- formance 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 and materials and development of a fundamen- tal understanding of pavement structures. The most signifi- cant strategic level findings from f-sAPT in the last decade focus on issues around materials characterization, pavement modeling, pavement behavior and performance, pavement design method development and calibration, benefits of spe- cific materials and technologies, economic impacts of f-sAPT programs, calibration of pavement design methods, devel- opment of databases of information on pavement perfor- mance 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 var- ious materials’ laboratory and field behavior, and the effect of various real environmental conditions and traffic on pave- ment behavior and performance are seen as major interna- tional findings. The evaluation of economic benefits of f-sAPT has come to the forefront during the past decade with more programs reporting attempts at performing BCR-type evaluations of their research programs. It appears that the general inter- national economic conditions forces researchers to prove the benefit of their research much more and identify, ana- lyze, and quantify the direct and indirect benefits obtained from f-sAPT. Most programs are still only conducting BCR analyses after the research has been completed (43.5%), whereas 17.4% of respondents indicated that they perform BCRs as an input in the research planning. Estimates of BCR from respondents ranged broadly between 1.4 and 11.6, although some respondents to the questionnaire indi- cated that their perception of the BCR for their programs is over 30. It is evident that the f-sAPT community is moving toward the future with a focus 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. Many of the trends iden- tified in the synthesis are old issues that were known to the pavement engineering community for a long time, but

108 chapter summary This chapter focused on an analysis of continued trends and future trends identified through the responses to the questionnaire as well as the literature. The conclusions of the synthesis are also incorporated into the chapter. Over- all it is concluded that the f-sAPT community is actively engaging with real-world pavement performance and behav- ior issues and real attempts are being made to ensure that appropriate f-sAPT research supports the development of improved material response models and pavement design methods. 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 (includ- ing 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.