# Acoustic Emission Monitoring in Prestressed Concrete:
## 3 Materials and Methods
### Measurements on Bridges In Situ
The bridge consists of 15 spans with lengths of 22.0 m and 28.0 m (total length 396.0 m). It was prestressed using BSG 25 and BSG 100 tendons made of Hennigsdorfer prestressing steel, which is known to be susceptible to stress corrosion cracking. Static deficits were only identified in two fields, which have been monitored since 2023. * Ruhrwald Bridge, Dortmund: The Ruhrwald Bridge crosses federal road B54 over a double-track railway line in the city of Dortmund. The structure consists of several substructures of different construction years and types. The substructure examined was built in 1957 using post-tensioned, prefabricated concrete elements, which were connected to form a slab with a reinforced concrete topping in situ. The span and width are 9.15 m and 19.45 m, respectively. The prestressing steel used is classified as susceptible to stress corrosion cracking. Computational analysis revealed deficiencies, leading to the installation of an AEM system in 2022. * BT500 Bridge, Coswig: The bridge carries a state road near an urban area over a railway track and was constructed in 1985 as a prefabricated bridge. It consists of two spans, each of 11.27 m. The superstructure contains 8 post-tensioned prefabricated beams of type BT500N, supplemented with cast-in-place concrete to form a slab. The beam elements were made with Hennigsdorfer prestressing steel. Corrosion pits were found on the steel during material investigations, attributed to stress corrosion cracking. Structural deficits prompted an ORM to assess the applicability of an AEM system on the bridge. The in situ measurements on the bridges were executed either through distinct campaigns or during the installation of a permanent monitoring system. Contrary to the girder measurements, where the signal emission originated from a stationary point, the in situ measurements featured variable signal emission locations. The structure was gradually \"tapped\" at predetermined intervals. The source of the signal was a rebound hammer, identical to the one employed in the destructive tests detailed in Section 3.1. To achieve adequate statistical reliability, multiple impacts from the rebound hammer were applied at each signal emission location. Figure 4 depicts one example sensor position and the directions of investigation for each structure. This information was subsequently utilized to generate the attenuation profiles. For some structures the measurements were conducted throughout the entire  
structure, involving multiple sensor positions. While multiple sensors were analysed, not all are depicted in Figure 4 to maintain clarity within the figure. Nonetheless, the quantity of measured signals and attenuation profiles is documented in Table 1.  
Figure 4: Views and cross-sections of the bridges on which the in situ measurements were conducted. Qualitative marking (in red) of an example sensor position, as well as the direction of the series of measurements with the rebound hammer longitudinal and transverse to the axis of the structure. In every instance, the AMSY-6 measurement system from Vallen Systeme GmbH, paired with VS30-V sensors, was employed [38; 39]. The setup required no hardware configuration beyond a preamplification setting of 0 dB. The measurement system's analogue, high-pass filter was adjusted to a cutoff frequency of 0.5 kHz, with no further analogue or digital filtering applied. This consistent use of measurement technology components and sensors across different tests ensures the reliability and comparability of the measurements conducted on both bridges and beams. The sampling rate was 1 MHz. The time-amplitude signals were recorded using the same procedure as described in Section 3.1.