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future research topics. Because synthesis studies summarize Chapter eight shares lessons learned from a series of case
current practices, most information reviewed was published studies using FWDs.
after 1999; exceptions were made if more current informa-
tion was not available. While searching for case studies Chapter nine discusses FWD-related research projects,
among the research articles, the focus was on projects that which were either recently concluded or ongoing at the time
used the FWD for a specific application. of the preparation of this report.
Chapter ten concludes the synthesis with a summary of
Organization of Report findings and suggestions for further study.
This synthesis report is organized into ten chapters. The bal- These chapters are followed by References, a bibliogra-
ance of chapter one reviews the report's structure and defines phy, a list of abbreviations, and two appendices. Appendix A
key terms and phrases. The report structure is summarized includes a copy of the print version of the survey question-
with brief explanations of chapter content. Key terms are naire. Appendix B describes the survey results in tabular and
provided within the Definitions section. This chapter con- graphical form.
cludes by describing the survey that was completed by SHA
representatives.
Definitions
Chapter two describes FWD equipment. Although not
intended to be a comprehensive, technical description, the This section defines several key terms that pertain to FWD
general mechanism is explained. Additionally, this chapter use and data analysis. These definitions are largely based on
briefly lists FWD manufacturers, models, and maintenance ASTM standards ("Standard Guide for General Pavement
practices. The physical setup, including sensor spacings and Deflection Measurements" 2005). Variations of these defi-
nominal loads practiced by SHAs, is discussed. nitions may be found in literature published by AASHTO,
FWD manufacturers, and researchers. Additional terms are
Chapter three reviews calibration practices. Manufactur- defined within the context of their relevant sections.
ers' recommended calibration schedules, as well as other
calibration schedules, are provided. Locations of calibration Back-calculation: An iterative process by which pave-
centers, calibration frequency, and related costs of calibra- ment layer moduli, or other stiffness properties, are esti-
tion center operation are provided. This chapter relates costs mated from FWD deflection data. The process begins with
incurred by SHAs related to FWD calibration. a hypothesis of a given layer's modulus, which is repeatedly
compared with the FWD's output using an iterative math-
Chapter four examines the collection, management, and ematical model. The iteration stops once a predetermined
storage of FWD data. Titles and vendors of FWD software level of tolerance has been reached between subsequent cal-
are listed, along with the file formats they support. Field culated estimates.
data quality control and quality assurance measures are
described, along with each method's popularity. Test site Geophone: An electrical sensor that translates dynamic
protocols are also reviewed, including SHA operator safety velocity into electrical voltage. Based on the principle of
and traffic control methods. magnetic induction, these devices translate vibration infor-
mation into an analog electrical signal. Because of their
Chapter five describes analysis of FWD data by SHAs. prevalence with FWDs, the terms "geophone" and "deflec-
The principles of back-calculation and forward calculation tion sensor" are used interchangeably. For the sake of brev-
are briefly reviewed as are software packages for FWD data ity, this report refers to the device as a "sensor."
analysis.
Forward calculation: A noniterative process in which
Chapter six focuses on personnel training methods. stresses, strains, and displacements are calculated from layer
Qualifications and certifications for new FWD operators and data and applied load.
data analysts, as described by SHA survey respondents, are
included. Additionally, training opportunities outside one's Deflection basin: The bowl shape of the deformed pave-
SHA, are described, such as the FWDUG and the National ment surface caused by a specialized load as depicted from
Highway Institute, are examined. the peak measurements of a series of deflection sensors
placed at radial offsets from the center of the load plate
Chapter seven discusses FWD program administration, ("Standard Guide for General Pavement Deflection Mea-
including the topics of budgeting, allocation, and staffing. surements" 2005).
This chapter briefly describes outsourcing requirements.
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Deflection basin test: A test with deflection sensors placed testing or traveling ("Standard Guide for General Pavement
at various radial offsets from the center of the load plate. Deflection Measurements" 2005).
The test is used to record the shape of the deflection basin
resulting from an applied pulse load. Information from this Load plates: Capable of an even distribution of the load
test can be used to estimate material properties for a given over the pavement surface for measurements on conventional
pavement structure ("Standard Guide for General Pavement roads and airfields or similar stiff pavements. The plate shall
Deflection Measurements" 2005). be suitably constructed to allow pavement surface deflection
measurements at the center of the plate ("Standard Guide for
Deflection sensors: An electronic device(s) capable of General Pavement Deflection Measurements" 2005).
measuring the relative vertical movement of a pavement sur-
face and mounted to reduce angular rotation with respect to Load transfer test: A test, usually on portland cement
its measuring axis at the expected movement. Such devices concrete (PCC) pavement, with deflection sensors on both
may include seismometers, velocity transducers (geo- sides of a crack or joint in the pavement. The test is used to
phones), or accelerometers ("Standard Guide for General determine the ability of the pavement to transfer load from
Pavement Deflection Measurements" 2005). one side of the break to the other. Also, the load deflection
data can be used to predict the existence of voids under the
Load cells: Capable of accurately measuring the load that pavement ("Standard Guide for General Pavement Deflec-
is applied to load plate and placed in a position to minimize tion Measurements" 2005).
the mass between the load cell and the pavement. The load
cell shall be positioned in such a way that it does not restrict Test location: "The point at which the center of the applied
the ability to obtain deflection measurements under the cen- load or loads are located" ("Standard Guide for General
ter of the load plate. The load cell shall be water resistant Pavement Deflection Measurements" 2005).
and resistant to mechanical shocks from road impacts during
