# Acoustic Emission Monitoring in Prestressed Concrete:
## 4 Results
### Rebound Hammer Impacts on Bridges  
The girders discussed in Sections 4.1 and 4.2 may yield signal propagation influenced by effects such as reflections due to their smaller dimensions compared to actual bridges, potentially skewing the assessed attenuation coefficients. To evaluate the findings from these girders further, measurements from six bridges were included. The attenuation was evaluated in both the longitudinal and transverse directions for each of these structures. Figure 10 depicts the frequency-dependent attenuation profiles for the structures in the longitudinal direction, while Figure 11 shows them in the transverse direction. Each structure is represented by a sensor's characteristic profile, even when multiple datasets were available; see Table 1. Figure 12 presents the time-domain features for both directions of analysis, with coefficients and RSDs detailed in Table 4.  
Compared to the girders' profiles, it is notable that in the bridge measurements the initial amplitudes across different frequency bands resemble each other more than those observed in the girder measurements; see Figure 8, concrete surface. This distribution pattern is somewhat mirrored in the case of girder \(B\) from the Roding bridge; see Figure 8d. Given that the Roding bridge's girders were more heavily reinforced than the others, it suggests that the increased reinforcement and the proximity of tendons to the surface contribute to the reduced attenuation of high-frequency components. Along the bridges' longitudinal measurements, such as on the web, reinforcement such as tendons was also present. Similarly, the transverse profiles of the BT500 Bridge and Ruhrwald Bridge, measured in the load-bearing areas, which are characterized by significant reinforcement  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Partial Spectra**} & \multicolumn{4}{c}{**Attenuation Coefficient (dB/m)**} & \multicolumn{4}{c}{**Residual Standard Deviation (dB)**} \\ \cline{2-13}  & **Stolpe (BT500)** & **Roding** & **Witzenhausen** & **Stolpe (BT500)** & **Roding** & **Witzenhausen** \\ \cline{2-13}
**and Features** & **Tendon** & **Concrete** & **Tendon** & **Concrete** & **Tendon** & **Concrete** & **Tendon** & **Concrete** & **Tendon** & **Concrete** \\ \hline
0–10 & \(-\)4.8 & \(-\)4.6 & \(-\)5.5 & \(-\)7.4 & \(-\)5.2 & \(-\)5.5 & 6.4 & 4.5 & 3.5 & 6.1 & 4.1 & 3.7 \\
10–20 & \(-\)5.  
due to shear loads and the anchorage of the tendons, exhibited comparable patterns, as depicted in Figure 10.  
Moreover, the attenuation profiles in Figure 10 align with prior observations, showing significant attenuation of high-frequency components up to 50 kHz within a spread of approximately 3 m from the source to the sensor, in contrast to a much lesser decrease observed for low frequencies. Notably, at distances of around 5 to 6 m from the source, the amplitudes for frequencies below 30 to 40 kHz begin to increase again in the Werra Bridge and in the High Bridge. This phenomenon may be attributed to the structural design and variations in reinforcement configuration along the structure's length, both of which influence signal pathways. Additionally, it is remarkable that the pre-existing damage to the Werra Bridge's concrete due to ASR manifests only minor differences in the frequency range analysis when compared to the Stennert Bridge; yet a more pronounced correlation is observed in the time-domain features. This highlights the challenges associated with individual assessments and underscores the necessity of performing detailed statistical analysis with particular emphasis on time-domain features, as outlined in Section 5. The attenuation anisotropy is examined by means of three examples, each containing results in both the longitudinal and transverse directions.