# Acoustic Emission Monitoring in Prestressed Concrete:
###### Abstract
The determination of the source characteristics of a wire break is usually based on experience, as comprehensive studies on this topic are rare. A contribution to this topic is the database mentioned in [13], which analyses a variety of events and serves also as a basis for this study. The determination of attenuation and background noise is ideally carried out directly on the structure through object-related measurements (ORMs). For this, several measurement series are recorded with an artificial, reproducible signal source to obtain attenuation profiles and determine attenuation coefficients. The rebound hammer is often used as a reference source for this purpose [13; 14], which generates a strong impact on the concrete surface since its impulse on the structure is necessarily very energetic. This standardised instrument enables reliable reproduction at almost any location on the structure. Despite the successful application of this method in determining the required input parameters for designing a sensor layout, there are significant differences among the source mechanisms of a wire break and the excitation by the rebound hammer to be considered. While a wire break causes signal generation longitudinally to the axis of the tendon inside the construction, the excitation by the rebound hammer occurs on the concrete surface and, as such, orthogonally to the axis of the tendon; see Figure 1. This difference must be considered when transferring the measurement results to the specific issue of wire break detection using AEM. Measurements conducted with the rebound hammer on bridges or other large structures are documented in [14; 15; 16; 17; 18]. In some of these studies, wire breaks were intentionally generated to compare signal characteristics. Similarities were observed between the signals from rebound hammer impacts and those generated by wire breaks. The corresponding attenuation coefficients, determined based on the peak amplitudes of the measured signals, ranged from 2.6 to 7.0 dB/m. These studies primarily used sensors operating in the frequency spectrum below 200 kHz; a tabular overview of the results can be found in [18]. However, these studies focused mostly on one specific test object, with attenuation not being the central focus of investigation. A comparison between the signal sources, considering a variety of specimens and a comprehensive dataset of individual events, both from rebound hammer impacts and wire breaks has, thus far, not been feasible due to these limitations. This research specifically addresses the need for detailed comparative analyses between rebound hammer impacts and wire breaks. By generating both types of events on various girders and structures, a comprehensive real-world database was created. The aim of this analysis was to identify the similarities and differences in the attenuation of the two signal sources and to evaluate their transferability. Hence, not only typical time-domain features such as peak amplitude and signal energy were considered, but frequency domains were analysed as well to cover frequency-dependent effects. Additionally, the propagation behaviour of the signals in both the longitudinal and transverse directions was investigated. Special attention was paid to structural transitions and joints in prefabricated constructions and their influence on wave propagation. The results were processed in a way that statistically derived and validated design parameters for AEM. These are particularly relevant when ORMs, as recommended in [12], are not possible before implementation of the monitoring devices. This might be the case due to lack of expertise on the contractor side or due to time constraints, e.g., in case of particular urgency or imminent danger. In such instances, the insights from this study provide a solid foundation to rely on qualified empirical knowledge and ensure the reliability of the monitoring as well.