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4 87). Similarly, survey respondents noted its usefulness as searched for FWD usage information. The proceedings of a structural section design aid; FWD data was cited as a the FWDUG meetings provided supplementary informa- pavement rehabilitation strategy decision criterion in five tion to the synthesis. Published research articles, such as states (Indiana, Louisiana, Montana, Nevada, and Oregon) a pooled-fund study related to FWD calibration (Orr et (Appendix B, Question 88). al. 2007), were used as resources. Established guidebooks for FWD usage, such as the Long-Term Pavement Perfor- Calibration protocols suitable for all FWDs currently sold mance Program Manual for Falling Weight Deflectometer in the United States (other than lightweight FWDs) were Measurements (Schmalzer 2006) and the Florida DOT's developed as part of the Long-Term Pavement Performance Falling Weight Deflectometer Handbook (Holzschuher (LTPP) program and adopted by AASHTO. FWD calibration and Lee 2006) provided sensor spacings, load levels, and centers were established to provide service across the con- other useful data. In addition, the standards published in tinental United States. These centers are currently located the Annual Book of ASTM Standards, procedures published in Colorado, Minnesota, Pennsylvania, and Texas. Calibra- by AASHTO, and articles in the Transportation Research tion center records suggest that many of the FWDs currently Record provided valuable procedural descriptions. being used are not calibrated on a regular basis. Absent cali- bration, agencies have no way to be sure that their substan- The bulk of synthesis information was gathered by means tial investments are yielding meaningful results. Similarly, of a survey. Invitations to take the survey were sent to FWD the knowledge and information exchange that takes place at administrators in each of the 50 SHAs in the United States. annual meetings of the FWD User's Group (FWDUG) sug- Continuous communication with SHA representatives gests that many aspects of FWD use and data application are resulted in 45 of those 50 invitees responding; a response inconsistent among owners and operators. rate of 90%. The purposes for collecting FWD data have a major Administrators of the four LTPP FWD calibration cen- influence on the highway agency practices. This synthesis of ters (see Table 2) were asked about their FWD practices. highway practice for FWD use provides information needed Each calibration center provided logs of FWDs calibrated at to support guidelines for advancing the state of the practice. their respective centers. Additionally, the calibration centers described their pricing, durations of calibration sessions, and training protocols. Research Methodology Four manufacturers of FWDs, Carl Bro, Dynatest, Information for this synthesis was acquired by the following JILS, and KUAB, were also contacted. All four provided means: detailed maintenance recommendations, product descrip- tions, descriptions of training services, and data collection Literature search and review and processing software information. The manufacturers Survey of SHA representatives described their sales in the United States, broken down by Communication with calibration center operators agency use. Communication with FWD manufacturers Several sources were explored for the literature review Scope including the Transportation Research Information Services (TRIS) database, which contains bibliographical informa- This synthesis study was limited to FWD usage by SHAs tion from transportation-related research in the United within the United States. Although current practice was States; the International Transport Research Documentation limited to the United States, research published internation- (ITRD) database; And individual SHA websites that were ally was considered for its historical context and potential TABLE 2 SURVEYED LTPP FWD CALIBRATION CENTERS Calibration Center Location Administering State Highway Agency Denver, Colorado Colorado Department of Transportation Maplewood, Minnesota Minnesota Department of Transportation Harrisburg, Pennsylvania Pennsylvania Department of Transportation College Station, Texas Texas Department of Transportation Note: Additional calibration centers are operated by the Indiana Department of Transportation (West Lafayette), Dynatest, Inc. (Starke, Florida), and Foundation Mechanics, Inc. (El Segundo, California).