PSI - Issue 64

Christoph M. Monsberger et al. / Procedia Structural Integrity 64 (2024) 1665–1672 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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BRUsens DTS (Solifos AG 2024b), where the fiber is loosely and thus, strain-free embedded inside the cable, is installed to provide a reliable temperature compensation of the measured strain sensing profiles as well as the cross sectional temperature distribution. Both cables are mounted in well-defined arrangement along the reinforcement using cable ties to ensure a fixed position inside the segment before concreting. The reliable bond between the concrete and the strain sensing cable (and therefore, the stress transfer to the glass fiber core) is given once the concrete is hardened, as demonstrated in loading tests of full-scale segments by Monsberger et al. (2018). Inside the segment, two connecting boxes are placed at each side, in which the optical connectors are stored during concreting.

Fig. 2. Schematic representation of fiber optic sensing cable installation along reinforcement (left) and practical realization (right)

As an alternative monitoring solution, individual segment rings at the BBT have also been equipped with conventional vibrating wire sensors (VWS) at the initial stage. These electrical sensors deliver strain and temperature information at discrete points, but no complete distribution along the entire segment. Every VWS additionally requires its own lead-in cable and therefore, the number of sensors, which can be installed inside one segment, is limited due to practical reasons. As an example, Fig. 3 depicts the installation of both technologies along the same segment (manufactured at another construction site, not at BBT). Whereas massive six connecting cables (red) are required to realize six individual VWS along the segment, the fiber optic sensing cable (thin blue, diameter of 3.2 mm) can not only significantly reduce disturbing influences on the segment itself, but also provide a full coverage with approx. 1,200 sensing points per segment.

Fig. 3. Sensor installation along reinforced tunnel segments: Conventional strain gages (left, red) and distributed fiber optic sensors (right, blue)