PSI - Issue 72

Serhii Drobyshynets et al. / Procedia Structural Integrity 72 (2025) 210–215

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step. The force magnitude was measured using a dynamometer. The relative deformations were recorded using strain gauges with a 50 cm base and dial indicators. Deflections were measured using deflectometers. The failure criterion for the beams was defined as the condition in which the reinforcement deformations in the normal cross-section of the beams reached their ultimate values (yield strength limit).

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Fig. 1. (a) diagram of the experimental setup (1 – screw for load application, 2 – test frame, 3 – spring block, 4 – dynamometer, 5 – traverse, 6 – beam, 7 – support, 8 – angle bracket); (b) steel fiber reinforced concrete beam B-2 during testing A total of three steel fiber reinforced concrete beams were tested. The first beam B-1 was tested under a single short-term load. The second beam B-2 was tested under repeated low-cycle loading-unloading with 10 cycles, and on the 11th cycle, the beam was loaded to failure. The lower loading level was η = 0 , and the upper level was η = 0.75 . The third beam B-3 was tested under repeated low-cycle loading-unloading with 20 cycles, and on the 21st cycle, the beam was loaded to failure. The lower loading level was η = 0 , and the upper level was η = 0.9 . 3. Results and discussion After the experimental studies, the load-bearing capacity of beam B-1 was determined as P u = 27.5 ( M u = 19.25 ∙ ). The first cracks appeared at a load P = 5 kN, with a crack width of 0.02 mm. Compared to reinforced concrete beams, the cracks were more frequently interwoven. No inclined cracks were observed. The beam failed gradually, without concrete spalling, due to the reinforcement reaching its ultimate strain. The relative strain of the extreme fiber in the compressed zone of steel fiber reinforced concrete at failure was  sfb = ∙ − , while the relative strain of the reinforcement was  s = 290×10⁻⁵ . The beam deflections were as follows: under concentrated forces, f = 12.5 mm and f = 13.7 mm, and at mid-span, f = 16.0 mm. The deformation diagrams of steel fiber reinforced concrete and reinforcement for beam B-1 are shown in Fig. 2a, and the deflection graph is presented in Fig. 2b. The behavior of beam B-2 was studied under repeated loading at a level of η = 0.75 over 10 loading-unloading cycles, with the beam being loaded to failure on the 11th cycle. The load-bearing capacity of the beam was P u = 30 kN ( М u = 21 kN∙m). Cracks began to appear during the first cycle at a load of P = 5 kN along the entire span of the beam, spreading in height from 2.5 to 7 cm. As the load increased to P = 7.5...20 kN, new cracks formed. No significant changes in crack development were observed in the following cycles, with normal cracks progressing further under increased loading on the 11th cycle. The crack width remained constant throughout the 10 cycles, measuring 0.02 mm at P = 5 kN and 0.2 mm at P = 20 kN. Strains and deflections developed rapidly until the 3rd to 5th cycle, then grew more slowly until the 7th cycle, and stabilized during the 8th to 10th loading-unloading cycles.