# Acoustic Emission Monitoring in Prestressed Concrete:
## 5 Aggregation of Results and Discussion  
The attenuation profiles shown in the previous sections, along with the detailed data documentation, already outline initial patterns. To underscore the central tendencies further, a statistical representation of the data through boxplots, as illustrated in Figure 13, is employed, focusing on the evaluation of the medians and RSDs of these distributions. This approach considers the results of wire break and rebound hammer impulses on the girders' concrete surfaces in addition to all measurements from the bridges. The analysis is focused on the time-domain features peak amplitude and energy given their practical importance. The medians of the rebound hammer impulses across both girders and bridges range between approximately \(-2.8\) and \(-3.6\,\mathrm{dB/m}\), with RSDs in the range of about \(2.3\) to \(2.5\,\mathrm{dB}\). Girders show slightly higher RSDs when compared to bridges. This indicates that the outcomes from both girders and bridges share a fundamental comparability and can be mutually transferred. However, the boxplots reveal a tendency for rebound hammer results from bridges to underestimate the attenuation coefficients observed from wire breaks on girders. Specifically, the median attenuation coefficients from wire breaks, considering both peak amplitude and energy, are consistently lower, despite the bridge coefficient ranges' covering those from wire breaks. This indicates that attenuation coefficients derived from rebound hammer tests on structures do not lead  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**Attenuation Coefficient (dB/m)**} & \multicolumn{6}{c}{**Residual Standard Deviation (dB)**} \\ \cline{2-13} \multicolumn{1}{c}{**Partial**} & \multicolumn{3}{c}{**Longitudinal**} & \multicolumn{3}{c}{**Transverse**} & \multicolumn{3}{c}{**Longitudinal**} & \multicolumn{3}{c}{**Transverse**} & \multicolumn{3}{c}{**Longitudinal**} & \multicolumn{3}{c}{**Transverse**} & \multicolumn{3}{c}{**Transverse**} \\
**Spectra** & **(kHz)** & **(kHz)** & **(kHz)** & **(kHz)** & **(kHz)** & **(kHz)** & **(kHz)** & **(kHz)** & **(kHz)** & **(kHz)** & **(kHz)** & **(kHz)** & **(kHz)** & **(kHz)** & **(kHz)** \\
**Features** & **(kHz)**  
implicitly to a conservative assumption, especially since one of the sampled structures has pre-existing damage, thus unfavourably affecting the distributions. Furthermore, wire breaks typically show RSDs 1.0 to 1.5 dB higher than those from rebound hammer impulses, a pattern replicated across frequency bands. This difference is attributed to wire breaks being subject to complex conditions, leading to less reproducible signals and, quite naturally, to greater variance. These findings confirm the rebound hammer's suitability as a reliable test and reference source, capable of yielding consistent outcomes for broad application. Nevertheless, they also underscore the need for an extra safety margin when transferring rebound hammer test results to scenarios involving wire breaks. Based on the data at hand, a transfer factor is proposed herewith to adjust the results of ORMs to account adequately for wire break events. For this purpose, the attenuation coefficient of the ORM \(\beta_{\text{orm}}\) is scaled by a factor \(F\). To account for the variance in the  
Figure 13: Statistical distributions of the attenuation coefficients and residual standard deviation of the regressions in their respective frequency bands and for the time-domain features for wire break and rebound hammer signals at the girders in (**a**,**c**) and for rebound hammer signals at bridges in longitudinal and transverse directions in (**b**,**d**).