A Practical Investigation into Acoustic Wave Propagation in Concrete Structures

Acoustic Emission (AE) testing in concrete structures shows great potential for monitoring and assessing the health condition of structures. Source location is normally based on the arrival times of transient signals, the simplest method is known as the Time of Arrival (TOA) method, where the location of the damage can be determined from the arrival time of the event at two or more sensors. When using this method, the wave velocity of the signals that propagate through the material needs to be determined. Homogenous materials, such as steel, have welldefined velocities, but in non-homogeneous materials such as concrete the wave velocity is more difficult to predict. This makes the use of a single wave velocity as required in the TOA method very difficult due to the variety of wave velocities obtained, especially for large structures. This paper explores wave propagation in concrete structures over a variety of source to sensor distances. Experiments were performed on a reinforced concrete beam and a reinforced concrete slab, using an Hsu-Nelsen (H-N) Source. It is found that, in general, as the source to sensor distance increases, the wave velocity decreases. The presence of longitudinal and transverse waves is demonstrated and the influence of the part of the waveform used for temporal measurement is explored. In order to provide a practical approach to velocity determination, different thresholds are investigated and the results are discussed in relation to the wave modes present.