An investigation of the impact-echo technique /

Nondestructive testing of concrete for maintenance purposes is the focus of this thesis. The application of a new nondestructive testing technique known as impact-echo was examined. The technique is based on transient stress wave propagation for the detection of defects in concrete, as well as measuring the setting time, early age strength, and elastic properties. Impact-echo functions by a mechanical impact, where stress pulses are generated in the test subject. The stress pulses undergo multiple reflections between the top and bottom of concrete layers. The surface displacements are recorded and the frequency of the successive arrivals of the reflected pulses is determined. Thus, knowing the thickness of a given layer, together with the measured frequencies, P- and S-wave velocities can be calculated. If on the other hand, the thickness is unknown, the time-distance graph of the primary surface wave arrivals could be used to calculate the thickness. The position of the impact source relative to the receiver must be selected in such a way as to detect P- and S-waves at their maximum reflection amplitude. In this study, defects were detected using the reflected P-waves from their top surfaces. In addition, the change in elastic moduli of various types of concrete mixes was monitored for a 30-day period by measuring P- and S-wave velocities. The impact-echo elastic moduli were then compared with static and dynamic values obtained by standard methods in order to assess their accuracy