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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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· 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.
