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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By considering the bond length l b = 5 d p and the circumference U = π ∙ d p , F can be transformed into a mean value of τ b over the bond length by eq. (2). = 51 ⋅ 2 (2) 3.3. Experimental procedure The prestress was transferred to the concrete beams after seven days of hardening. Then the DFOS were installed on the concrete surface. A hard metal driller was used to cut the central tendons through the cladding tubes. The cuts were performed with a rotation of 110 RPM and temporarily stopped at every mm of depth to remove swarf. The experiment was terminated after complete separation of the tendon, which was confirmed by visual inspection through the cladding tube. For the pull-out tests, a vertical load was applied at a rate of 1 mm/min, until a displacement of 5 mm was registered at the free end, to ensure failure (Croppi et al. (2023)). 4. Results and discussion 4.1. Tendon break: DFOS on tendon The longitudinal tendon strains ε p ( x ) after complete separation are plotted in Fig. 3 over the distance from the break point. The strains are normalized by the associated initial prestrain ε p,0 according to Table 1 so that the ordinate runs from zero (no prestress) to one (full re-anchoring). Data recording starts at a distance of about 0.06 m from the break point, as at that point the fiber was guided away from the tendon for protection.

Fig. 3. Normalized longitudinal strains of the tendons after cutting, fitted to exponential functions. An exponential function in the form of = ⋅ exp ( ⋅ ) was used to fit the normalized strain curves. As a constraint, the tendon strain at the break point is known as ε p ( x =0) = 0, too. The parameters a and b of the functions were determined by minimization of the squared errors. From these functions, transfer lengths l pt ≈39 cm for beam A (NSC) and ≈22 cm for beam B (HPC) are estimated. In HPC the transfer length is shortened by ≈44 %.

4.2. Tendon break: DFOS on concrete

For both beams, the concrete strains ε xx and ε yy were measured with DFOS on both sides. In the remainder however, the evaluation focuses on the longitudinal strain ε xx and the cut of the central tendons only. It is located at y = z = 0 m and the cut is made at x = 1.20 m. The concrete strains after cutting the tendon are visualized in Fig. 4. Fig. 4a – b show the left face ( y = -0.15 m) and Fig. 4c – d the right face ( y = +0.15 m) results. The height of the DFOS