AASHTO R 32-20

AASHTO R 32-20 covers the annual calibration of the load cell and the deflection sensors and monthly relative calibration of the deflection sensors of a falling weight deflectometer (FWD) or a heavy weight deflectometer (HWD). It is used to establish calibration factors for correcting FWD and HWD measurements.

This procedure is not applicable to the calibration of light weight deflectometers or of cyclic loading and other types of pavement deflection testing equipment.

A reference calibration is performed once per year or after the load cell has been replaced on the FWD. It shall be performed by a certified technician.

Monthly relative calibration is performed on the deflection sensors at least once per month and immediately after a deflection sensor is replaced. A certified technician is not required.

The procedure results in calibration factors are entered into the FWD software as multipliers. When the FWD measurements are multiplied by the calibration factors the result is a set of measurements that have been corrected to agree with the calibration instrumentation.

Calibration procedures may vary slightly among FWD types. This procedure can be used for all types of FWDs with minor modifications within the limits of the reference calibration equipment.

This procedure has been automated in the software package WinFWDCal. It is required that the computer program be used to carry out the procedure.

To use this procedure, the FWD computer must be capable of electronic data transfer.

Data files for all types of FWDs can be read by the WinFWDCal software. The PDDX file format is required for data input. FWD data files that are in native format can be converted to PDDX format using PDDXconvert, which is an integral component in WinFWDCal.