Wave Propagation Based Assessment of Cylindrical Concrete Structural Elements

In structural engineering, the risk assessment of existing structures subjected to seismic action or other severe load conditions requires a complete knowledge of their health state and possible presence of damage and/or defects. Surveys based on low frequency ultrasonic waves are currently carried out for assessing concrete structural elements, aimed both at estimating the material mechanical properties and detecting defects such as voids, in-homogeneity and cracks. The transmission technique is usually adopted in ultrasonics. The evaluation of the recorded data is based on the analysis of important variations both of apparent wave propagation velocity and signal attenuation. Nonetheless, the ultrasonic pulse method presents some limitations in dealing with in-homogeneous materials and in adopting the reflection technique because of signal scattering and attenuation. In order to overcome these application difficulties, the sonic techniques can be adopted, such as the pulse-echo and the impact-echo methods. The latter adopts a high-energy signal and operates by multiple reflection of the propagating wave, analysing then the spectrum of the recorded signal. An experimental research program has been implemented, based on both the ultrasonic pulse and the impact-echo techniques, aimed at evaluating structural elements and at comparing the ability of both techniques in detecting and sizing defects. The testing has included laboratory concrete and masonry specimens containing artificial defects. Nowadays, the two techniques can relay on a long experience with a variety of test cases. Nevertheless, few are the applications on structural elements with circular section. The on-going laboratory work has shown that, especially with impact-echo, the response from cylindrical elements may vary considerably from that obtained on slab-type elements. The research results presented in this paper compare the testing procedure and outcome from ultrasonics and impact-echo on a concrete cylindrical specimen.