Falling Weight Deflectometer Calibrations
Falling-Weight Deflectometer's (FWD's) are critical for highway monitoring activities. FWD's, however, must be properly calibrated and used or else measurements taken will be inaccurate. To address this issue, the Strategic Highway Research Program (SHRP) and the Long Term Pavement Performance Program (LTPP) developed a set of standardized FWD calibration procedures and also set up four regional FWD Calibration Centers in the US. The procedures were subsequently adopted as an AASHTO provisional standard, "Practice for Calibrating the Load Cell and Deflection Sensors for a Falling-Weight Deflectometer."
The Strategic Highway research Program, which was established by Congress in 1987, and is continued by the Federal Highway Administration (FHWA), established the four regional FWD calibration centers. SHRP also developed standardized procedures for FWD calibration, which have since been adopted by the American Association of State Highway and Transportation Officials (AASHTO).
The center for the North Atlantic Region is located at the PennDOT BOMO Annex facility, and has been in operation since 1992. Twenty to thirty calibrations are performed at the center annually, for State and Federal agencies, as well as private vendors, some travel from as far as North Carolina and Canada to have FWD devices calibrated at this facility. Devices are typically calibrated on an annual basis.
One person is needed to perform calibrations; a permanent Roadway Programs Specialist operates PennDOT's center.
The calibration center is a cement concrete pad constructed over sub base and sub grade materials with known and constant properties. Therefore, the deflection of the pad under a given load is constant. The deflection transducers from the FWD are calibrated individually against this reference; this procedure is called "reference calibration." The FWD device's load cell is calibrated through the use of a reference load cell.
The calibration of the FWD deflection sensors is further refined by comparing them to each other in a process referred to as "relative calibration."
The calibration procedures result in calibration factors, which are entered into the FWD software as multipliers. When the FWD measurements are multiplied by the calibration factors, the result is a measurement which has been corrected to agree with the calibration instrumentation. Data from the calibration tests is collected and stored on a computer at the calibration center, and can be compiled and plotted through various types of output programs and graphs.
Historical records of equipment, calibration dates, and results are well maintained at the calibration center.
The standard cost for a calibration is $300. If the customer or problems with the device cause delays which result in the calibration taking more than one business day (7.5 hours), then an additional rate of $40 per hour will be applied for the additional time. The full hourly rate will be charged for partial hours, and will apply whether the additional hours are spent the same day, or subsequent days. Charges are applied whether the device "passes" or "fails".
The FWD North Atlantic Calibration Center is directed and controlled by PennDOT's Bureau of Maintenance and Operations (BOMO), Asset Management Division, Roadway Inventory and Testing Unit (RITU). The following individuals, with RITU, manage the operation:
Chief, Roadway Inventory and Testing Section
Manager, Pavement Evaluation and Testing Unit
PennDOT can schedule FWD calibrations at any time during the year, but it is preferable to have them take place January 2-March 31. It is the responsibility of the FWD device owner to contact PennDOT for scheduling. Please call 717-783-6843 for all scheduling of FWD calibrations.