Falling-Weight Deflectometers Calibration

Falling Weight Deflectometer (FWD) calibrations are critical for highway monitoring activities. Falling Weight Deflectometers must be properly calibrated and used; otherwise, measurements will be inaccurate. A first generation FWD calibration system was developed in the early 1990s for the Strategic Highway Research Program's (SHRP) LTPP program. Four State-run FWD calibration centers were subsequently established.

In order to keep up with the technology change and make FWD calibration sustainable for the next decade without a loss of precision, the FHWA initiated a Transportation Pooled Fund Study TPF-5(039): "Falling Weight Deflectometer (FWD) Calibration Center and Operational Improvements" (http://www.pooledfund.org). A much improved FWD calibration system was developed under this study. Subsequently, American Association of State Highway and Transportation Officials (AASHTO) Standard Practice R 32: Calibrating the Load Cell and Deflection Sensors for a Falling Weight Deflectometer has been updated, reviewed, balloted and published in 2009 as AASHTO R 32-09. The new practice R 32-09 has been successfully deployed at all seven pooled fund-supported, State department of transportation- (DOT-) run FWD calibration centers. The new practice has also been deployed at three manufacturer-run centers in the United States and Europe.

To familiarize FWD owners with the new calibration procedure, and also to encourage the State DOTs to utilize the calibration facilities, a brief 11-minute video, "Calibrating the Falling Weight Deflectometer", has been produced. It is available at the LTPP Web site and hard copies in CD format can be obtained at no cost from the FHWA LTPP Customer Service Support at LTPPINFO@dot.gov or 202–493–3035.

This video addresses the question "Why should you calibrate your FWD?" It is intended to educate FWD managers, engineers and operators on the need for calibrating FWD equipment in order to collect high quality data for durable pavement designs. The following topics are covered in the video:

• Illustration of the new calibration procedure
• Explanation of the preparation needed to get a successful calibration
• Description of how calibration improves the quality of backcalculated data and its impact on overlay design.