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Publication Number:  FHWA-HRT-07-040    Date:  October 2011
Publication Number: FHWA-HRT-07-040
Date: October 2011


Falling Weight Deflectometer Calibration Center and Operational Improvements: Redevelopment of The Calibration Protocol and Equipment



The primary objectives of the pooled fund study have been achieved because the new protocol greatly increased the speed of FWD calibration without reducing the accuracy or ease of calibration. WinFWDCal is a Microsoft Windows®-based software program, and it guides all aspects of the new calibration procedure.

The overall philosophy for calibration of the load cell and deflection sensors has not changed from the old SHRP procedure. Reference calibration and relative calibration are still the foundation on which the new protocol was developed. The major changes include the use of the self-referencing accelerometer, the updated data acquisition system, the WinFWDCal computer program, and the new multisensor deflection sensor calibration stands.

A small but statistically significant shift in final calibration (gain) factors is evident when comparing results from the old SHRP procedure to the new AASHTO R32-09 procedure.(1) The average gain factor for the deflection sensors increased by less than 0.3 percent, while the average gain factor for the reference load cell increased by 0.4 percent.

It is not possible to determine which procedure is correct, but these results are close to the statistical repeatability of the procedure, and they should not be a cause for concern. The new AASHTO procedure eliminates the beam movement problem that was inherent in the SHRP procedure. As a result, it is likely to be more accurate.

The new procedure works well with all brands of FWDs. However, it is expected that there will continue to be a need for improvements as changes occur in FWDs, computer technology, and Microsoft Windows®. For example, the new Dynatest® truck-mounted FWD did not exist when the new AASHTO protocol was initially developed. In addition, access to the Carl Bro FWD was very limited during the development stage.


  1. Review and update the calibration acceptance criteria.

    There are still a few instances in WinFWDCal where the internal QC criteria are not decisively defined. Using available data along with best judgment, initial tolerances were chosen to get the procedure into use. Over time, use of the new AASHTO R32-09 protocol has provided additional data to help refine the critical criteria. Release of WinFWDCal Version 2.0 includes changes that were created due to experience.

    The following criteria should be reexamined as additional data become available:

    • The allowable change for daily accelerometer calibration.

    • The allowable change in gain factors from one calibration year to the next.

    • The allowable rate of change per year in gain factors for FWDs with at least 4 years of calibration history. (It will be mid2012 before sufficient data are available to allow reconsideration of this criterion.)

    • The allowable noise in the accelerometer signal in the interval before the release of the FWD mass.

    • The allowable noise in the accelerometer signal in the interval just preceding the impact of the falling mass.

    Additional criteria may require review as researchers gain experience applying the WinFWDCal program. Feedback from the calibration center operators has been a great source of information.

    1. Continue to provide support, QA, and operator certifications for the calibration centers.

      The calibration operators need ongoing support for several reasons. If they are experiencing issues calibrating an FWD, they need to communicate with people who can help them solve those problems. They need to know whether their equipment is working properly and if their results are believable. They also need a contact to offer suggestions for procedural improvements and help implement them.

      The annual QA reviews and operator certification are an important part of the center support. They provide a mechanism for two-way communication about changes in the protocol, equipment, and software. They also help identify problems that the centers are encountering.

      Since 2004, the pooled fund study has provided this support. Additionally, the SHRP calibration procedure was first introduced in 1992, and the Cornell Local Roads Program (CLRP) has provided general support to the centers ever since.

      In fall 2010, AMRL provided QA reviews and operator certifications. As a result, the basic support was assured. Careful coordination and communication between AMRL and CLRP is necessary to maintain the other aspects of general support.

    2. Develop a means for interim certification of new calibration operators.

      The retirement and promotion of certified calibration operators seldom occurs just before a QA visit is due. Since the calibration operator must be certified in order to calibrate FWDs, this loss of personnel often requires a temporary shutdown of the center until the new operator can be trained and certified. It is costly and seldom possible to immediately arrange for new operator training and certification, which is a 3-day process.

      To avoid a prolonged closure period, a process whereby a certified operator can train and attest to the proficiency of the new operator should be developed. When the operator certifying agency (i.e., AMRL) receives notice, a temporary certificate should be issued covering the reminder of the pervious operator’s certification period.

      This process avoids a situation where the QA schedule gets out of sync. When the regularly scheduled time for the QA visit arrives, the temporary certificate can be replaced with a regular certificate, and the normal schedule is maintained. A similar procedure should be defined for new calibration operators in overseas centers.

    3. Continue to upgrade WinFWDCal and PDDXconvert and release revised versions.

      Since the first public release of WinFWDCal in November 2006, several major releases of the software have gone out, and numerous small “bug fixes” have been made. While the rate of problem identification from the users has slowed down, programming support will be needed in the future.

      With any software package, there is a continual need for updates. Recommendations from the calibration centers and FWD owners need to be incorporated into the software as soon as possible to reduce the frustration of working with or around a bug.

    4. Provide an electronic means to transfer the results (gain factors) of the calibration into the FWD computer.

      The new calibration procedure provides the means for electronic transfer of data from FWD to the calibration computer via a PDDX file format. The new procedure also outputs the calibration results electronically in the same file format. Electronic data transfer avoids the errors that come from manual entry. Currently, it is not possible to read the electronic output file and update the FWD operating system (field program) with the new calibration factors, calibration date, etc. This will require the assistance and cooperation of the FWD manufacturers.

    5. Improve the electronic data transfer procedures.

      PDDXconvert only partially meets the needs for FWD calibration. Some of the necessary input data from FWD must still be entered manually or left out entirely. Also, several types of FWDs have their input data scattered among several different files.

      The manufacturers need to agree on and provide a standard output file format from the FWD operating system that would overcome the input/output problem. It would put all needed data in a single file and in a harmonized format. This will require the assistance and cooperation of FWD manufacturers.