Spectral imaging of impact echo tecnique for grouted duct in post-tensioning prestressed concrete beam

An impact echo technique recently draws an attention as nondestructive evaluation of defects in concrete structures. Since it is found that the technique is available to identify embedded defects, an application to the evaluation of grouted ducts in post-tensioning prestressed concrete structures is promising. Although a variety of attempts are reported, it is realized that identification of resonance frequencies due to ungrouted ducts is not easy. To improve the impact echo, a specimen containing an ungrouted duct is tested. Theoretically, frequency responses of the specimen depend on the size of the member, and the location of the void and P-wave velocity, because wave motions in concrete structures are characterized by material properties, incident waves, and the representative length. The frequency response is investigated in respect to the relationship between the wavelength and the depth of the duct. Then, applying scanning procedure, spectral imaging is conducted. Results show that the procedure is quite effective to identify the ungrouted ducts.