Dynamic Analysis of Falling-Weight Deflectometer Data

Falling-Weight Deflectometer data from three in-service pavement sections has been used to analyze dynamic response characteristics. The analyzed data will be used later in the project along with computer predictions of pavement response to backcalculate engineering properties of pavements. The pavemeets had thicknesses of 1.5 in., 9 in., and 12 in., and two load levels were tested. The full pulse data is prespoted for the dropweight force and all seven surface displacement sensors. The ana - vsis was performed in the frequency domain using frequency response functions computed from the pulse data. The frequency response functions were computed by dividing the Fast Fourier Transform (FFT) of the displacements by the FFT of the force. Magnitude and phase angles of the frequency response functions are presented. An undesirable oscillation was present in the frequency response functions. This was attributed to a discontinuity in the displacement pulses resulting from zero packing in the FFT Program. The pulses have a non-zero "tail" value at the end of the 60 msec sample period which causes the discontinuity. The pulse "tail II may be due to drift, permanent deformation of the pavement, or premature truncation before the transient response has died out. A linear correction to the pulses that eliminated the discontinuity was applied to one test case. This eliminated the undesirable oscillation, giving the responses a more regular behavior similar to predicted theoretical responses. This correction should facilitate comparison studies of computed versus measured FWD responses.