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

1290

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tendons were pre-tensioned with an initial strain ε p,0 of approximately 4000 µm/m ( σ p,0 = 784 N/mm²). Beam A was made of normal strength concrete (NSC) while beam B was made of a high-performance concrete (HPC) to purposely increase bond stiffness and strength. Table 1 summarizes the parameters and properties of the beams from accompanying material and pull-out tests.

Table 1. Overview of the prestressing parameters, material parameters and geometric properties of the tested beams. Label P 0 [kN] ε p,0 [µm/m] f ' cm,cube [N/mm²] d p [mm] Standard Pull-out test Pull-out test w/ prestress

✓ ✓

✓

Beam A Beam B

65.34 68.40

3940 4030

55.8 71.8

10.5 10.5

✗ Cladding tubes were installed from the tendon to the upper surface to ensure accessibility after casting. The tubes were offset 200 mm from the center to one side. A length of 1200 mm was calculated to be sufficient for a complete load introduction from both sides. Besides the main experiments, three series of pull-out tests (Fig. 2d) in four repetitions (cf. Table 1) according to RILEM (RILEM TC (1994)) were conducted to quantify the bond-stress slip relation of both concretes. One series was devoted to the Hoyer effect in prestressed members. Pre-stress was applied to those NSC specimens in a casting bed (Hegger et al. (2007)). 3.2. Measurement equipment The measurement system consisted of DFOS and strain gauges (SG). The DFOS were arranged in loops according to Fig. 1b. Longitudinal strains were measured at five different heights with a distance of d 2 = 40 mm between the horizontal sensors ( l 2 = 160 mm) and over the length of l 1 = 1400 mm for beam A. For beam B, l 1 was reduced to 1000 mm, because the results of beam A showed this length to be sufficient to detect relevant strain changes. Measurements were taken in the highest possible spatial resolution (0.65 mm pitch) at a frequency of 1 Hz. Supplementary strain measurements were taken over the entire length of the tendon with DFOS as well. A notch of 1 mm depth was cut into each tendon and a DFOS was glued inside, circumventing the cladding tube in a capillary to maintain the sensors integrity during tendon cutting. Additional SGs on the tendons served for reference. The pull-out tests were accompanied by measurements of the vertical tendon displacement at the free end (equivalent to the slip s ) with a displacement transducer and the corresponding vertical force F at the fixed end (see Fig. 2d).

Fig. 2. Sketches of a) the concrete beams along with its dimensions b) the section where the tendon is cut c) a general section d) the setup of the pull-out tests according to RILEM.