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13 Reference TxDOT's FWD fleet. Because TxDOT's FWD units are of varying ages, one FWD may produce differing results than Reference calibrations, per LTPP, must be performed at a another. To provide better reproducibility, a three-phase cali- specialized facility. A pooled fund study was commissioned bration plan was created. These phases are as follows (Rocha by the FHWA in 2004 to improve the reference calibration et al. 2003): process. Improvement to the original 1994 SHRP reference calibration procedure was needed for the following reasons Physical inspection and component replacement (Orr et al. 2007): Preliminary calibration--a relative calibration is performed and sensors not passing calibration are The 1994 procedure was designed around Dynatest and identified KUAB FWDs, the only commercially available FWDs Comprehensive calibration--sensors not passing in the United States at the time. Because of differences calibration are calibrated more thoroughly and data- between FWD manufacturers, the original procedure gathering issues are troubleshot was not completely compatible with equipment from other manufacturers. These new protocols greatly improved consistency from The 1994 procedure required individual sensor calibra- one FWD to another, and the researchers recommended that tion, and took six hours to complete as a result. "TxDOT implement the new protocol as soon as possible." The 1994 procedure used a linear variable displace- Section 7 of ASTM D4694-96 acknowledges the UTEP ment transducers (LVDT) for reference deflections, method, which is "more complementary than interchange- the accuracy of which was occasionally compromised able" with the SHRP/LTPP method ("Standard Test Method by movement of the mass and beam to which it was for Deflections . . ." 2005). mounted. Accelerometers were viable replacements for LVDTs, because they are self-referencing. Portable Falling Weight Deflectometer Calibration The 1994 procedure used DOS-based software. DOS is no longer the state-of-the-art PC operating system. Carl Bro's PRIMA 100 PFWD was designed to mimic their existing PRI 2100 trailer-mounted FWD. The design The new procedure addresses each of the pooled fund employs three geophones, compared with PRI 2100's nine study's points. Universal compatibility is achieved through geophones. Because both models use the same geophones, modified triggering mechanisms. Time is saved by placing calibrating the PFWD's geophones uses an identical proce- all FWD sensors into a single support stand and calibrat- dure to the PRI 2100. A time-history system serves as the ing them simultaneously. New accelerometer-based control backbone of Carl Bro's calibration software, which uses a board and data acquisition systems were designed. A new fast Fourier transformation algorithm. Carl Bro calibration program, WinFWDCal, was written in Microsoft Visual equipment employs a test cell connected to an LVDT, and the Basic to provide a graphical user interface (GUI) for calibra- procedure is verified by means of the SHRP 1994 protocol tion. Calibration is recommended once per year, but it takes (Clemen 2003). only about two hours to complete (Orr et al. 2007). By 2006, the updated FWD calibration procedure included Calibration Requirements an FWD calibration results database, conversions were made Calibration Frequencies to the DOS-based FWDCAL software to work with Micro- soft Windows, and software was adapted to work with accel- erometers and modern data acquisition boards. Additionally, For calibration, ASTM D4695-03 recommends that impulse- WinFWDCal was augmented with a utility to convert FWD loading type devices be calibrated "at least once per year file formats from the different equipment types to the PDDX using the procedure in Appendix A of SHRP Report SHRP- format adopted by AASHTO. For sensor calibration, a single P-661" for reference calibration and "relative calibration once support stand was designed so that sensor position was not a month during operation" ("Standard Guide for General significant. With such a support stand, all of an FWD's sen- Pavement Deflection Measurements" 2005). Additionally, sors may be tested simultaneously, as opposed to the one- Section 7 of ASTM D4694-96 recommends that deflection sensor-at-a-time calibration method put forth by the 1994 sensors be calibrated "at least once a month or in accordance procedure. The finalized calibration procedure is discussed with the manufacturer's recommendations" ("Standard Test in a draft final report (Irwin 2006). Method for Deflections . . ." 2005). Independent of the FHWA-pooled fund study, another According to the LTPP manual, reference calibration is calibration procedure was developed at the University of required once per year, unless the FWD is based in Alaska, Texas at El Paso (UTEP). With the support of the Texas DOT Hawaii, or Puerto Rico. Similar requirements are detailed (TxDOT), UTEP developed a new calibration protocol for for Georgia (Pavement Design Manual 2005) and Florida,