PSI - Issue 64

Aeneas Paul et al. / Procedia Structural Integrity 64 (2024) 1287–1294 Aeneas Paul et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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2.2. Configuration of the fiber-optical measurement system The altered strain field can be detected even on the concrete surface using DFOS. DFOS are linear, high-resolution sensors that utilize the backscatter of emitted light to measure strain changes (Konertz et al. (2019)). Such sensors are able to detect strains in a resolution of 0.65 mm with an accuracy of up to 1 µm/m (Polytec GmbH (2024)) and sensor lengths of more than 500 m are possible (Kindler et al. (2023)). Depending on the measuring task, a suitable configuration must be selected. The fiber design, the adhesive and the measuring method must be specified (Herbers et al. (2023)). For this project, a sensor with a polyimide coating is chosen. These sensors provide high accuracy due to a chemical bond between the different layers and a stiff coating, but they are vulnerable to sensor loss in the form of fiber breaks (Herbers et al. (2023)). However, since tendon breaks seldom induce cracks (Schacht et al. (2019)), this disadvantage is disregarded here. The adhesive to glue the DFOS on the concrete surface is the epoxy-based Polytec AC2411, which was successfully used elsewhere (Clauß et al. (2021)). To measure strains in two directions, the fiber was applied in a grid running parallel and perpendicular to the tendon as shown in Fig. 1b. That way, strains can be measured in multiple layers with a single fiber. This is necessary since the available interrogator is limited to eight channels simultaneously (Polytec GmbH (2024)). If the fiber is applied in loops, a minimum radius of curvature (≈10 mm) must be adhered, to ensure the integrity of the sensor (Konertz et al. (2019)). Loops were left unglued since detection only takes place in the longitudinal direction of the fiber. The measuring field is characterized by its length l 1 , height l 2 and its density, which is defined by the distances between the layers ( d 1 , d 2 ). A schematic configuration of the grid with all relevant parameters is provided in Fig. 1b, along with installation on a real concrete surface in Fig. 1c.

Fig. 1. a) Qualitative pattern of the longitudinal stresses as iso-lines (red and dark blue) at tendon break based on a linear-elastic finite element simulation along with the proposed measuring grid on the surface b) schematic configuration of the DFOS on the concrete surface and c) application of DFOS on a concrete surface with distances d 2 = 40 mm and 50 mm ≤ d 1 ≤ 75 mm. 3. Fiber-optical measurement system to detect tendon breaks 3.1. Experimental setup Two pre-tensioned concrete beams of the same dimensions (Fig. 2a-c) and three internal tendons were prepared in the laboratory of Ruhr University Bochum. Throughout a unique tendon type with diameter d p = 10.5 mm and ribs of depth 0.19 ± 0.03 mm, made of St 1375/1570 was used to apply the pressure to the concrete in a casting bed. The