Cross-Sectional Modes in Impact-Echo Testing of Concrete Structures

The impact-echo method is a promising technique for the detection of damage in concrete structures, particularly when applied to plate-like structures. However, when the technique is applied to beam-like rather than plate-like structures the resulting amplitude spectra are complex and difficult to interpret. The complications are due to the excitation of cross-sectional modes of vibration in the beam. This paper discusses an investigation into the nature of these modes, shows that their frequency is lowered by the presence of a large void, and discusses the potential of using these modes for nondestructive testing. Plane strain finite-element models have been used to model cross-sectional modes in the presence of both grouted and ungrouted ducts. The results from these analyses are described together with an experimental validation using a beam containing a large duct, some of which is grouted and some of which contains a void. Results are compared with previous analytical findings and found to be in agreement.