PSI - Issue 64

Giovanni Pietro Terrasi et al. / Procedia Structural Integrity 64 (2024) 1347–1359 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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(passive cable end, with the longitudinal RSG at 40 mm form the anchor front and the transverse RSG at 225 mm from the sleeve end). Moreover, RSG-based displacement measuring transducers were built to measure the LTM draw-in at the end of the passive cable anchor (based on cantilever beam springs with two RSG on its edges). All RSG were of type HBM 1-LY41-6/700 (HBK 2024). All RSG were built as half bridges and were compensated for temperature. After calibration of all sensors in the unloaded cable systems, the cables were installed and anchored in the bridge box girder and the prestress force was build up. The strain logging started at the beginning of the prestressing process and the data has been recorded periodically since then. Measuring amplifiers of the type DL DLR2-012 (for Analog Strain Gauge Sensor) were used as measuring amplifiers. The sensor data is transmitted in real time via LoRaWAN® wireless technology (Decent Lab 2024). Lo-RaWAN® enables encrypted radio transmissions over long distances with very low power consumption. The sensor data is transmitted to Empa via Decent Lab's data storage and visualization system. The strain monitoring rate was set to one measurement cycle every 10 minutes during prestressing and it was increased to one measurement cycle every 3 hours for the long term monitoring. The measurements of the wire elongations show stable values over 7.5 months, this is shown in Figure 11 for the north cable. However, there is a certain variability between the strains of the 4 wires of a single post-tensioning cable, which is influenced by three factors: - Cable production: there is a slightly different alignment of the individual wires when casting the anchor with the LTM (i.e. variation of the base length l 0 ). - Edge distance / position of the wire within the cable (also due to the cable deflection in the middle) - Accuracy of measurement (which is estimated at ±1% according to the strain gauge specification)

Fig. 11: Longitudinal strains of the 4 CFRP wires monitored in the north cable over time (bridge post-tensioning occurred on 23.8.2023). Right y-axis: Average prestress reduction over monitoring time.

There are also certain differences in the degree of prestressing between cable north and cable south (max. 5.7%). This seems to be primarily due to the prestressing process itself. The total clamping force could not be monitored in real time using a load cell as in the laboratory (in the ELS tests), but was calculated using the oil pressure of the hydraulic prestressing system. The target force value could therefore not be approached with high precision. When the cables were prestressed to the planned nominal load of 1 MN, the average CFRP stresses achieved are 1427 MPa (north cable) and 1350 MPa (south cable), which corresponds to an average CFRP wire strain of 8755 and 8281 μm /m, respectively. After 7.5 months of monitoring, the wire elongations have decreased by an average of 1% (Figure 11, right y-axis).