# Acoustic Emission Monitoring in Prestressed Concrete:
## 3 Materials and Methods  
### Experiments on Bridge Girders under Laboratory Conditions  
Wire breaks are rarely measured under real-world conditions and are at best artificially provoked in a limited number in investigations on operating structures. However, for this research a total of six girders were sourced from three bridges at risk from stress corrosion cracking during their dismantling and then destructively tested. These girders were predominantly in an undamaged state. Detailed descriptions of the girders and documentation of these experiments are available in [13,18]. Briefly:  
* Girders of a box girder bridge near Roding: The bridge was built in 1965 near the city of Roding. Two web segments, each with a length of 7 m, were obtained; see Figure 2a. The girders examined contained 21 tendons, each with 42 wires. * Typified prefabricated post-tensioned girders (BT500) of a bridge near Stolpe: The bridge near Stolpe supported an agricultural road over the A24 motorway and was constructed in 1981 from prefabricated elements of the BT500 series with BSG-50 tendons made in the former GDR; see Figure 2b. The lengths of the girders were uniformly 11 m. Each tendon contained 16 wires. The oval prestressing wires are known to be very susceptible to stress corrosion cracking. * Prefabricated, prestressed girders from a bridge near Witzenhausen: The third pair of girders was obtained during the dismantling of a side road over a railway track; see Figure 2c. The prefabricated elements were produced in 1962, each with a length of 9 m. Each girder contained 12 ribbed, oval wires. Compared to the specimens from the other bridges, the girders of the Witzenhausen bridge were less massive, with a profiled cross-section. Measurements on each girder followed a consistent methodology, as depicted in Figure 3. Initially, the tendons or wires were exposed in specific areas, where individual wires were then cut. Sensors were positioned along the longitudinal axis of each tendon, aligned with the site of the wire break location. Subsequently, depending on accessibility, either in the vicinity of this opening or at the ends of the girders, rebound hammer impulses were generated directly on the prestressed steel; see also Figure 3. It was ensured that the integrity of the tendon was preserved to avoid affecting signal propagation by having introduced separations. These measurements allowed for the direct comparison of signals from both sources given their identical direction of action. However, the rebound hammer impulses were also generated on the concrete surface to ensure the measurements' applicability and comparability with ORMs conducted on actual structures. The number of measured signals is detailed in Table 1.  
For the experiments, an AMSY-6 measurement system and VS30-V sensors with a flat frequency response ranging from 25 to 80 kHz from Vallen Systeme GmbH (Wolfrathhausen,  
Figure 2: Bridge girders tested under laboratory conditions: (**a**) in situ-cast, post-tensioned element of the web of a hollow box bridge near Roding; (**b**) prefabricated post-tensioned element (BT500) from a bridge near Stolpe; and (**c**) prefabricated, prestressed element with single wires from a bridge near Witzenhausen. Germany) were used [38; 39]. The AMSY-6 system's analogue, high-pass filter was set to a cutoff frequency of 0.5 kHz. No further analogue or digital filtering was applied. Capturing wire break and rebound hammer signals poses a challenge due to the high energy of the source signal. When too close to the source, the input level on the measuring device could be surpassed, necessitating the hardware configuration of the measurement chain to attenuate the signals purposefully for recording. To this end, special attenuators were employed, achieving effective attenuation levels of \(-20\) and \(-40\) dB. The sampling rate was 1 MHz. The time-amplitude signal is recorded on the basis of a fixed threshold.