Using 3-D velocity contour plots to detect voids in grouted tendon ducts in post-tensioned concrete construction

Abstract

3-D velocity contour plots are constructed to detect voids in grouted tendon ducts by a stress-wave test method requiring a receiver and a small instrumented hammer. Each time-domain waveform due to hammer impact is processed using the short-time Fourier transform to obtain the spectrogram (frequency-time relationship), which is enhanced using the reassignment method. The reassigned spectrogram is used to obtain the dispersion curve (velocity-wavelength relationship). The results from all test points are assembled to develop wave-velocity contour plots at different wavelengths. A wall specimen containing ducts with five different grouted conditions was constructed. Two types of test grids were used, namely latticed grids covering the entire wall surface and vertical grids directly above the ducts. For the former method, 3D contour plots of a large area can be constructed with few testing lines. For the latter method, the wave velocity contour plots can distinguish different grouting conditions. Finally, the wave velocity reduction to be expected for different grouting conditions and cover depth is provided.