Falling Weight Deflectometer Usage (2008)

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3 Ninety percent of all SHAs responded to this synthesis sur- vey. Responding agencies are listed in Table 1. The number of FWDs in use and the importance of their role in pavement engineering practice are expected to increase 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. The importance of FWDs among SHAs was reflected in survey results. SHAs conduct FWD tests on up to 24,100 lane-km (15,000 lane-mi) annually (Appendix B, question CHAPTER ONE inTroducTion State Responding Agency Alabama Alabama Department of Transportation Alaska Alaska Department of Transportation and Public Facilities Arizona Arizona Department of Transportation Arkansas Arkansas Highway and Transportation Department California California Department of Transportation Colorado Colorado Department of Transportation Connecticut* Connecticut Department of Transportation Florida Florida Department of Transportation Hawaii Hawaii Department of Transportation Idaho Idaho Transportation Department Illinois Illinois Department of Transportation Indiana Indiana Department of Transportation Iowa Iowa Department of Transportation Kansas Kansas Department of Transportation Kentucky* Kentucky Transportation Cabinet Louisiana Louisiana Department of Transportation Maine Maine Department of Transportation Maryland Maryland State Highway Administration Michigan Michigan Department of Transportation Minnesota Minnesota Department of Transportation Mississippi Mississippi Department of Transportation Missouri Missouri Department of Transportation State Responding Agency Montana Montana Department of Transportation Nebraska Nebraska Department of Roads New Hampshire* New Hampshire Department of Transportation New Jersey New Jersey Department of Transportation New Mexico New Mexico Department Of Transportation New york New york State Department of Transportation North Carolina North Carolina Department of Transportation North Dakota North Dakota Department of Transportation Ohio Ohio Department of Transportation Oregon Oregon Department of Transportation Pennsylvania Pennsylvania Department of Transportation Rhode Island Rhode Island Department of Transportation South Carolina South Carolina Department of Transportation South Dakota South Dakota Department of Transportation Tennessee Tennessee Department of Transportation Texas Texas Department of Transportation Utah Utah Department of Transportation Vermont Vermont Department of Transportation Virginia Virginia Department of Transportation Washington Washington State Department of Transportation West Virginia West Virginia Department of Transportation Wisconsin Wisconsin Department of Transportation TABLE 1 RESPONDING STATE HIGHWAy AGENCIES *Responded by stating that the agency does not have an FWD program. purpose Falling weight deflectometers (FWDs) have been in use since the 1980s. These devices are used to measure pave- ment 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. Based on a survey conducted for this synthesis, 45 state high- way agencies (SHAs) reported using 82 FWDs produced by three different manufacturers (Appendix B, questions 2–6).

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

5 Chapter eight shares lessons learned from a series of case studies using FWDs. Chapter nine discusses FWD-related research projects, which were either recently concluded or ongoing at the time of the preparation of this report. Chapter ten concludes the synthesis with a summary of findings and suggestions for further study. These chapters are followed by References, a bibliogra- phy, a list of abbreviations, and two appendices. Appendix A includes a copy of the print version of the survey question- naire. Appendix B describes the survey results in tabular and graphical form. deFiniTions This section defines several key terms that pertain to FWD use and data analysis. These definitions are largely based on ASTM standards (“Standard Guide for General Pavement Deflection Measurements” 2005). Variations of these defi- nitions may be found in literature published by AASHTO, FWD manufacturers, and researchers. Additional terms are defined within the context of their relevant sections. Back-calculation: An iterative process by which pave- ment layer moduli, or other stiffness properties, are esti- mated from FWD deflection data. The process begins with a hypothesis of a given layer’s modulus, which is repeatedly compared with the FWD’s output using an iterative math- ematical model. The iteration stops once a predetermined level of tolerance has been reached between subsequent cal- culated estimates. Geophone: An electrical sensor that translates dynamic velocity into electrical voltage. Based on the principle of magnetic induction, these devices translate vibration infor- mation into an analog electrical signal. Because of their prevalence with FWDs, the terms “geophone” and “deflec- tion sensor” are used interchangeably. For the sake of brev- ity, this report refers to the device as a “sensor.” Forward calculation: A noniterative process in which stresses, strains, and displacements are calculated from layer data and applied load. Deflection basin: The bowl shape of the deformed pave- ment surface caused by a specialized load as depicted from the peak measurements of a series of deflection sensors placed at radial offsets from the center of the load plate (“Standard Guide for General Pavement Deflection Mea- surements” 2005). future research topics. Because synthesis studies summarize current practices, most information reviewed was published after 1999; exceptions were made if more current informa- tion was not available. While searching for case studies among the research articles, the focus was on projects that used the FWD for a specific application. orGanizaTion oF reporT This synthesis report is organized into ten chapters. The bal- ance of chapter one reviews the report’s structure and defines key terms and phrases. The report structure is summarized with brief explanations of chapter content. Key terms are provided within the Definitions section. This chapter con- cludes by describing the survey that was completed by SHA representatives. Chapter two describes FWD equipment. Although not intended to be a comprehensive, technical description, the general mechanism is explained. Additionally, this chapter briefly lists FWD manufacturers, models, and maintenance practices. The physical setup, including sensor spacings and nominal loads practiced by SHAs, is discussed. Chapter three reviews calibration practices. Manufactur- ers’ recommended calibration schedules, as well as other calibration schedules, are provided. Locations of calibration centers, calibration frequency, and related costs of calibra- tion center operation are provided. This chapter relates costs incurred by SHAs related to FWD calibration. Chapter four examines the collection, management, and storage of FWD data. Titles and vendors of FWD software are listed, along with the file formats they support. Field data quality control and quality assurance measures are described, along with each method’s popularity. Test site protocols are also reviewed, including SHA operator safety and traffic control methods. Chapter five describes analysis of FWD data by SHAs. The principles of back-calculation and forward calculation are briefly reviewed as are software packages for FWD data analysis. Chapter six focuses on personnel training methods. qualifications and certifications for new FWD operators and data analysts, as described by SHA survey respondents, are included. Additionally, training opportunities outside one’s SHA, are described, such as the FWDUG and the National Highway Institute, are examined. Chapter seven discusses FWD program administration, including the topics of budgeting, allocation, and staffing. This chapter briefly describes outsourcing requirements.

6 testing or traveling (“Standard Guide for General Pavement Deflection Measurements” 2005). Load plates: Capable of an even distribution of the load over the pavement surface for measurements on conventional roads and airfields or similar stiff pavements. The plate shall be suitably constructed to allow pavement surface deflection measurements at the center of the plate (“Standard Guide for General Pavement Deflection Measurements” 2005). Load transfer test: A test, usually on portland cement concrete (PCC) pavement, with deflection sensors on both sides of a crack or joint in the pavement. The test is used to determine the ability of the pavement to transfer load from one side of the break to the other. Also, the load deflection data can be used to predict the existence of voids under the pavement (“Standard Guide for General Pavement Deflec- tion Measurements” 2005). Test location: “The point at which the center of the applied load or loads are located” (“Standard Guide for General Pavement Deflection Measurements” 2005). Deflection basin test: A test with deflection sensors placed at various radial offsets from the center of the load plate. The test is used to record the shape of the deflection basin resulting from an applied pulse load. Information from this test can be used to estimate material properties for a given pavement structure (“Standard Guide for General Pavement Deflection Measurements” 2005). Deflection sensors: An electronic device(s) capable of measuring the relative vertical movement of a pavement sur- face and mounted to reduce angular rotation with respect to its measuring axis at the expected movement. Such devices may include seismometers, velocity transducers (geo- phones), or accelerometers (“Standard Guide for General Pavement Deflection Measurements” 2005). Load cells: Capable of accurately measuring the load that is applied to load plate and placed in a position to minimize the mass between the load cell and the pavement. The load cell shall be positioned in such a way that it does not restrict the ability to obtain deflection measurements under the cen- ter of the load plate. The load cell shall be water resistant and resistant to mechanical shocks from road impacts during