Development of non-contact SIBIE procedure for identifying ungrouted tendon duct

For non-destructive evaluation (NDE) of concrete structures, the impact-echo method has been successfully applied to locate defects and voids in concrete. To compensate the difficulty in selecting the resonant frequency and to visually locate a defect, an imaging procedure named stack imaging of spectral amplitude based on impact-echo (SIBIE) has been developed. Combining the SIBIE analysis with the detection by a laser vibrometer, a non-contact procedure is studied to develop an automated system.Since waveforms detected by the laser vibrometer are undistorted, elastic waves generated due to a steel-ball drop were investigated theoretically as Lamb's problem in elastodynamics. It is confirmed that the first motion of detected waves by the impact-echo method is identical to Lamb's solution for a surface pulse, when reflected waves arrive later than the Rayleigh wave.An applicability of a non-contact SIBIE procedure to identify an ungrouted tendon duct of plastic sheath was examined in the impact-echo tests, as well as the duct of metal sheath. In order to study theoretically the SIBIE procedure for identifying ungrouted tendon ducts, the three-dimensional boundary element method (3D-BEM) was applied to synthesize frequency spectra. It is confirmed that frequency spectra detected in the tests are in good agreement with those synthesized. An applicability of the SIBIE procedure is also confirmed by the synthesized spectra. Results of the experiments show that the presence of the ungrouted duct can be visually identified by the non-contact SIBIE procedure in both the cases of plastic sheath and metal sheath.