Stack imaging of spectral amplitudes based on impact-echo for flaw detection

The impact-echo method recently has drawn a remarkable amount of attention for nondestructive evaluation of defects in concrete structures. Extracting resonance frequencies responsible for the locations of reflectors, the depth and the presence of defects are estimated. So far, however, the technique has some limitations for practical applications. This is because there exist unresolved problems in the application of the impact-echo to concrete structures in service. Consequently, the method is studied theoretically on the basis of the elastodynamics and the signal analysis. In order to circumvent the difficulty to identify peak frequencies in the conventional procedure, a new procedure to evaluate defects in concrete is investigated, applying an imaging procedure. Thus, stack imaging of spectral amplitudes based on the impact-echo is developed. This procedure is applied to a prestressed concrete beam to classify a grouted duct and an ungrouted duct. The location and the presence of the ungrouted duct can be visually identified.