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FALLING WEIGHT DEFLECTOMETER USAGE Summary Falling weight deflectometers (FWDs) have been in use since the 1980s. These devices are used to measure pavement deflections in response to a stationary dynamic load, similar to a passing wheel load. The data obtained are used to evaluate the structural capacity of pavements for research, design, rehabilitation, and pavement management purposes. The number of FWDs in use and the importance of their role in pavement engineering practice are expected to rise as agencies move toward mechanistically based pavement design. The interpretation of FWD data is a key method for estimating the in situ moduli of pavement layer materials. This synthesis of highway practice for FWD use will provide information needed to support guidelines for advancing the state of the practice. Information for this synthesis was gathered in the following four phases: Literature search and review Survey of state highway agency (SHA) representatives Communication with calibration center operators Communication with FWD manufacturers The literature review was conducted from several sources. TRB maintains the Trans- portation Research Information Services (TRIS) database, which contains bibliographical information from transportation-related research in the United States. Further information was found through the International Transport Research Documentation (ITRD) database. Individual SHA websites were searched for FWD usage information. The proceedings of the FWD User's Group meetings provided supplementary information to the synthe- sis. Published research articles, such as a pooled-fund study related to FWD calibration, were used as resources. Established guidebooks for FWD usage, such as the Long-Term Pavement Performance Program Manual for Falling Weight Deflectometer Measurements and the Florida Department of Transportation's (DOT's) Falling Weight Deflectometer Handbook provided sensor spacings, load levels, and other useful data. In addition, the standards published in the Annual Book of ASTM Standards, procedures published by AASHTO, and articles in the Transportation Research Record provided valuable proce- dural descriptions. The bulk of synthesis information was gathered by means of a survey. Survey invitations were sent to FWD administrators in each of the 50 SHAs in the United States. Forty-five of those 50 invitees responded, for a response rate of 90%. The following observations were made based on survey data and literature research: SHAs are currently using 82 FWDs. Most SHAs are currently following FWD guidelines of their own creation rather than the Long-Term Pavement Performance guidelines. Although most SHAs do not have written FWD maintenance plans, maintenance activities are being performed.

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2 The 1994 Strategic Highway Research Program (SHRP)/LTPP FWD reference calibration procedure has been replaced by a newly developed 2007 FHWA calibration procedure that has been adopted by calibration centers. Of SHAs surveyed, 55% review a written equipment inspection checklist before departing for testing and the same percentage follows a written warm-up procedure. Despite accident prevention measures such as traffic controls, 29% of survey respon- dents reported accidents occurring within the past 5 years. The survey indicated that 89% of survey respondents keep raw FWD field data for more than 5 years and 84% keep these data indefinitely. Among SHAs with an FWD program, an average of 2,194 lane-km (1,363 lane-mi)-- with a median of 644 lane-km (400 lane-mi)--are tested annually. Additionally, 187 full-time employees work for these programs. From the survey results, the responding SHAs' expenditures varied widely (from no program to $850,000 annually) for their FWD programs.