Impact-echo

Impact echo is an acoustic NDT method successfully used for the investigation of concrete structures. For the interpretation of the measurements a detailed investigation of the acoustic wave propagation is highly relevant. This paper describes useful techniques for that purpose. By using a scanning laservibrometer it is possible to obtain a visualization of sound waves propagating along the surface of a concrete specimen. The interaction of the sound waves with tendon ducts as well as their behaviour when reaching the backwall of the specimen are investigated. Impact-echo measurements in transmission arrangement, in which the transducer is placed on the opposite side to the excitation, enable further investigation of the so-called geometry effects, which disturb the signals obtained from structures with limited dimensions. In order to visualize the surface wave propagation an arrangement showing the instantaneous wave field on all planes of the specimen in phase with the excitation is used. Time slices then show the instantaneous wave field for all points on the surface of the specimen. The experimental results are compared with numerical simulations.