# Acoustic Emission Monitoring in Prestressed Concrete:
## 3 Materials and Methods
### Measurements on Bridges In Situ  
The investigations conducted on the girders were enhanced with measurements on a variety of existing structures using the rebound hammer to ensure the transferability of the wire break results from the girders to real structures and to obtain additional reference values for designing the sensor layout. These structures differed notably in terms of their construction and geometrical dimensions, as shown in Figure 4 and elaborated on in the following descriptions:  
* Stennert Bridge, Hagen: The Stennert Bridge in Hagen-Hohenlimburg crosses the federal road B7 over the Lenne River and has a total length of 102.70 m. The load is supported by three hollow boxes connected through the roadway slab. The bridge was constructed in 1959, \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Girder/Bridge** & **Construction** & **Signal Source** & **Direction** & \multicolumn{2}{c}{**No. of Signals**} & \multicolumn{1}{c}{**Attenuation**} \\  & & & & **WB \({}^{*}\)** & **RH \({}^{*}\)** & **Profiles** \\ \hline \multirow{4}{*}{B7} & Roding & Web from a box girder & WB, RH & Long. & 388 & 124 & 6 WB, 1 RH \\  & Stolpe (BT500) & Prefabricated girder & WB, RH & Long. & 454 & 867 & 6 WB, 2 RH \\  & Witzenhausen & Prefabricated girder & WB, RH & Long. & 250 & 1105 & 4 WB, 2 RH \\ \hline \multirow{4}{*}{B7} & Stennert Bridge & Box girder & RH & Long. & - & 237 & 1 Long. \\  & Werra Bridge & T-beam & RH & Long. & - & 356 & 2 Long. \\  & South Ring Bridge & T-beam, multiple webs & RH & Long., Trans. & - & 1327 & 5 Long., 1 Trans. \\  & High Bridge & Slab with hollow bodies & RH & Long., Trans. & - & 1031 & 5 Long., 2 Trans. \\  & Ruhrwald Bridge & Slab with prefab. girders & RH & Trans. & - & 2313 & 5 Trans. \\  & BT500 Bridge & Slab with prefab. girders & RH & Trans. & - & 915 & 2 Trans. \\ \hline \hline \end{tabular}  
* WB: wire break, RH: rebound hammer.  
\end{table} 
Table 1: Overview of the data collected from tests on the girders under laboratory conditions and the in situ measurements on the bridges. Specification of the construction, signal sources, and directions of propagation per girder and bridge. Indication of the number of captured signals per signal source and determined attenuation profiles. Figure 3: General test setup for a girder and fundamental methods for wire break emission and rebound hammer signals. Cross-sections of the tested girders. and the prestressing steel used is classified as particularly susceptible to stress corrosion cracking. Despite its age the structure is generally in good condition, free from cracks, and suitable for targeted monitoring. In 2018, an AEM system was installed, with sensors placed laterally on the 50 cm thick webs. * Werra Bridge, Walldorf: The Werra Bridge was built in the 1980s as a three-span, prestressed concrete structure with a total length of 64.90 m along state road L2624. The structure exhibits damage in various areas in the form of cracking, caused by an alkali-silica reaction (ASR) leading to expansive mineral formation. Material investigations revealed that the superstructure was constructed using prestressing steel susceptible to stress corrosion cracking. To ensure the ongoing operation of the structure, an AEM system was installed in 2021. Additionally, regular visual inspections are conducted. * South Ring Bridge, Leer: This bridge carries a local road over the railway line Rheine-Emden (track 2831). The superstructure was constructed in 1969 as a single-span beam with a span of 31.70 m. The cross-section is a four-web T-beam with a construction height of approximately 1.45 m. The four main girders of the superstructure are prestressed longitudinally. Due to material-specific issues with the prestressing steel and structural shortcomings, the structure was equipped with a monitoring system in 2020. * High Bridge, Wismar: The High Bridge, part of federal road B105, was constructed in 1970 as a multi-span reinforced concrete slab bridge with hollow bodies.