# Acoustic Emission Monitoring in Prestressed Concrete:
## 6 Conclusions  
Acoustic emission monitoring has become a standard practice for monitoring pre-stressed concrete bridges, particularly for the detection of wire breaks caused by stress corrosion cracking. The success of such monitoring measures crucially depends on a properly designed sensor layout tailored to the specific structure to ensure the detection of all relevant damage signals with high probability. The understanding of the propagation of elastic waves within the structure, especially their attenuation characteristics, is fundamental in this context. Typically, the essential knowledge required for effective monitoring are derived from in situ, object-related measurements (ORMs) on the bridge under investigation using a reproducible reference source, such as the rebound hammer. This approach assumes that the signal characteristics generated by the rebound hammer are analogous to those of wire breaks, allowing for the direct application of the findings from ORMs to the issue of wire break detection without additional modifications. This study challenged this assumption by providing a comprehensive analysis of the attenuation behaviour of impulsive, energetic source events within large prestressed concrete structures, enabling a critical evaluation and comparison based on an extensive dataset. The research involved conducting experiments on various test objects to generate the data. First, six girders from three different bridges were subjected to controlled destruction in a laboratory setting. These girders underwent artificial wire breaks followed by comparative rebound hammer tests. The rebound hammer introduced impulses both longitudinally into the tendons' cross-section and orthogonally onto the concrete surface, evaluating the directionality of signal propagation. To complement these laboratory experiments, additional measurements were carried out on six in-service bridge structures, each varying in geometric dimensions and the extent of pre-existing damage. Notably, one of these structures exhibited signs of damage due to alkali-silica reaction, a condition that affects the integrity of concrete due to cracks, whereas the others did not display any significant or otherwise known damage. The combination of these experiments aimed at establishing a transfer factor that could reliably map the rebound hammer impulses to actual wire break events, ensuring the applicability of test results to real-world scenarios. The analysis covered generating attenuation profiles from the captured signals. This process involved evaluating both classical time-domain features--such as peak-amplitude and energy--and conducting a frequency analysis across ten different frequency bands spanning 0 to 100 kHz. For the time-domain features and the initial 0 to 50 kHz frequency bands, linear regression analysis was applied to interpret the data, which was then statistically parameterized to describe the attenuation characteristics observed. In conclusion, the following aspects can be noted:  
* The results of the girder experiments indicate that attenuation coefficients derived from rebound hammer impulses on the concrete surface generally tended to be lower than those of wire breaks. The observed attenuation coefficients and their RSDs for wire breaks are greater. This is particularly pronounced in the relevant time-domain features, suggesting that the variations are attributable to source-specific causes. Within the frequency bands, a comparable level of attenuation is noted. * To adapt the findings from rebound hammer measurements to address wire break scenarios, a transfer factor was established using the data collected from both bridges and girders. This involves a scaling factor of 1.2 to modify the average attenuation coefficient, complemented by a variance of \(\pm 1.0\) dB/m to incorporate a 95 % confidence interval accounting for sample scattering. It is crucial to account for sensor-specific characteristics when implementing this factor.