PSI - Issue 64

Bertram Richter et al. / Procedia Structural Integrity 64 (2024) 1208–1215 Richter et al. / Structural Integrity Procedia 00 (2019) 000–000

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The strain profile reported by the smart_strand is in agreement with the concrete and reinforcement DFOS. Due to both the integration of the FIMT inside the strand and the measurement method with a spatial sampling of 5cm, individual cracks cannot be identified. The strain profiles of the two FIMTs in the smart_tendon are closely related. As expected, the smart_tendon’s strain profiles are generally lower than the smart_strand’s, especially in the shear zones due to the smart_tendon’s parabolic path and the varying position in the cross-section. Fig. 5 compares the strain profiles of the concrete DFOS and the smart_strand before and after cracking at a load of F = 40kN, which is slightly above the decompression state. In the uncracked state at t 1 , the strain profiles agree well with the theoretical linear-elastic strain profile according to the Bernoulli hypothesis. At t 4 , the strain profiles clearly indicate that the girder transitioned to the cracked state. The smart_strand shows an increased strain level in the center of the span, the concrete DFOS shows strain peaks where cracks occured. The general strain increase compared to load step t 1 can be attributed to concrete (1) creeping during the test experiment and (2) the stiffness reduction due to the loss of the concrete’s tensile contribution (tension stiffening). The strain peaks on the left side of the girder are more pronounced. The asymmetry in the strain profiles is due to the asymmetric course of the tendon, which has its lowest point approximately 1m off the girders’ middle towards axis 30 (at x = +1m) and immediate prestressing losses due to seating of the anchorage on the left side. In addition to the cracks, a further anomaly was detected with the concrete DFOS at x = − 4.1 m, which is characterized by a local strain increase indicating a stiffness reduction in the non-cracked area. The intentionally built-in anomaly is a cavity in the web, modeled with a styrofoam cube with an edge length of 0.1 m.

Fig. 5. Comparison of strain profiles at F = 40 kN before and after the girder’s transition into the cracked state

Fig. 6 shows strain readings in the unloaded state ( F = 0.6 kN) directly before and after the third loading phase P 3 . After unloading, residual strains and also remaining strain peaks can be observed. In the strain profile of the concrete DFOS, remaining strain peaks indicate the presence of cracks, even though the cross-section is completely under compression again and the cracks are almost closed, invisible to the human eye. The differential strain profile (Becks et al., 2021) of t 2 and t 5 reveals new cracks at x = −2.45 m, −2.0 m and −0.34 m. The new cracks indicate that between t 2 and t 5 a load situation occurred to which the girder was not subjected to before. After unloading, the strain profiles

Fig. 6. Strain profiles at t 2 and t 5 (unloaded state, F = 0.6 kN) before and after the P 3 loading phase