PSI - Issue 54

Anna Karolak et al. / Procedia Structural Integrity 54 (2024) 460–467 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Experimental tests were performed in the Building Construction Laboratory of the Faculty of Civil Engineering of the Wroclaw University of Science and Technology. Beam models were subjected to the 4-point bending tests according to the standard procedure. Tested beams were freely supported with the fork support (in order to avoid lateral buckling). The span between the beam supports was 3.24 m. The beams were loaded with a force applied at 2 points (in order to obtain pure bending in the central parts with the joints). The loading force was applied with a recommended speed of 5 mm/min according to the standard procedure. In the tests an electronically controlled hydraulic jack Instron 500 was used, and the results were recorded by the Hottinger Baldwin Messtechnik MGC plus measurement system. The moisture content was controlled using a resistance hygrometer (FMW moisture metre) in several locations in each test beam and kept close to the level required by the standard. The test procedure and a view of the laboratory test set are shown in Fig. 3.

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Figure 3 a) View of the testing set in the laboratory, b) Scheme of the 4-point bending test according to the standard procedure

3. Results and discussion During the tests each beam was loaded up to failure. The destructive force – ultimate, maximum loading was registered and on that basis the load-carrying capacity of each beam was calculated. The failure modes for all of the beams: solid and beams with the joints were presented below. Failure of the solid beams of A-series was in the result of the breakage of the lower fibres in the central part, without or with occurring cracks, depending on the element being tested (A01, A02 or A03). The destructive force for the beams in this series varied depending on the beam and its values were accordingly: 44 kN, 37 kN and 55 kN. As evident, the variability of results in this series was quite significant. In case of the beams in series B, a visible joints loosening occurred, along with cracking at the height of upper fasteners in the upper area of the beam by the force of dozen kN. Failure was the result of fibres breakage – delamination in the lower area (tension zone), at the height of the lower fasteners, by the force of dozen kN (Fig. 4 a). The destructive force values for this series ranged from 12 kN to almost 18 kN. The beams of series C were damaged in the result of breakage the fibres – delamination that occurred at the level of the lower fasteners and in the middle height of the beam by the force of 18-20 kN. Cracks were observed in the joint part with cutout as a result of compression against the other part, which may be apparent in Fig. 4 b. The beams of D-series obtained the destructive force values from 15 kN to 20 kN. Failure was a result of breakage the fibres – delamination at the level of the fasteners (fasteners in one row) in the middle of the height and also below the middle. In this case also a significant loosening of the joint was observed and cracks occurring due to compression of the one part against the other part of the joint, what can be observed in Fig. 4 c. In turn, failure of the beams of E-series was in the result of breakage of the middle and upper fibres and the complete loosening of the joint in the lower area of the joint. Destructive force values for this series were 11-13 kN. The loosening of the joint could be observed already by a force of several kN and then cracking mainly at the edges of the joint, in one part of it – the upper part occurred, that was growing, leading to destruction (Fig. 4 d). Failure of the beams of F-series was similar. First, cracking at the edges, in the upper part of the joint occurred and was growing, leading to destruction of the joint (Fig. 4 e). The destructive force values noted for this series were: 7 kN, 14 kN and 17 kN, evidently these are widely divergent values. Also G-series was characterised by a high variability of the results, considering the obtained values of the destructive force – 9 kN, 16 kN and 17 kN. Also in this case, a significant loosening of the joint in the lower area and occurring od the cracking on the edge of the joint were observed, leading to the beam failure (Fig. 4 f). A significant loosening of the joint was also observed in the case of H-series beams. Failure was due to cracking at the height of the lower connectors in the lower area of the beam, also at the height of the upper connectors in the upper area of the beam, and in the middle of the height between the connectors, what can be observed in Fig. 4 g. The values