Issue 64

A. Abdo et alii, Frattura ed Integrità Strutturale, 64 (2023) 11-30; DOI: 10.3221/IGF-ESIS.64.02

C ONCLUSIONS

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his study studied the effect of (UHPFC) with end-hooked steel fiber of different volume fractions (0, 1, and 2%) on joints under repeated loads. UHPFC was poured at the whole sample in some samples and the joint region only in others. Eliminating stirrups of (UHPFC2) joint was studied as a solution for congestion of steel reinforcement. A comparison between the effect of (UHPFC) samples (UHPC, UHPFC with 1% steel fiber, and UHPFC with 2% steel fiber) and normal concrete samples with seismic design recommendations and without was carried out. Eight exterior (BCJ) samples were tested under repeated loading to reveal failure modes. Crack pattern, hysteresis curve, envelope curve, ductility factor, stiffness degradation, and energy dissipation were calculated. The following conclusions are derived obtained from the experimental study results: 1. The flexure failure and the plastic hinge occurred at the beam in all UHPFRC samples and the control sample because UHPFC could strengthen the joint zone well against shear failure. The control sample satisfied the seismic reinforcing specifications. In a normal concrete sample without seismic reinforcing specifications, shear failure occurred in the joint zone. 2. UHPFC samples with 2% steel fibers were the best in the crack pattern then UHPFC samples with 1% were better than the other samples by decreasing the cracks' number, lengths, and widths, increasing load-carrying capacity, and preventing the formation and propagation of cracks. Steel fiber bridging, which enhances the adhesion capacity of embedded reinforcing bars and UHPFRC materials, might be responsible for this favorable shift. 3. UHPFC with 1% and 2% steel fiber increased load-carrying capacity by (28.8 and 22.2%) and (44.4) compared to UHPC without fiber, respectively, and increased initial stiffness with proportions ranging (from 7.5 to 12.5%) and (from 18.3 to 22.1%) compared to UHPC without fiber respectively, ductility factor with (16.1 to 19.5%) and (31.2 to 36.6) compared to UHPC without fiber respectively and dissipated energy with (38.1 to 41.4%) and (69.1 to 80.1) compared to UHPC without fiber respectively. Increased fiber ratio improved the previous properties. UHPC samples without steel fiber improved the previous properties more than the normal concrete sample (J1-NC). 4. The samples poured with UHPFC at the whole sample were extremely identical to the samples poured with UHPFC at the joint only in the crack pattern and load-carrying capacity but slightly more in initial stiffness with an increase (1.9, 2.6, and 1.2%) in the different steel fiber ratios and total energy dissipation with increase (2, 0.7 and 4.3%) for UHPFC samples and ductility factor with increase (1, 1.9 and 2.9%) in the different steel fiber ratios (0,1, and 2%). Limited improvement occurred when pouring the whole sample, so pouring the joint only, which is an economical solution, can be considered. 5. Eliminating stirrups from (J2-UHPFC2-J) didn't affect the behavior of the joint. (J1-UHPFC2) sample with minimum stirrups in the joint was extremely identical to the UHPFC sample without stirrups in the crack pattern, load-carrying capacity but slightly more in initial stiffness with increase (1.2%) and total energy dissipation with increase (4.3%) and ductility factor with increase (2.9%). The specimen from UHPFC without shear reinforcement in the joint zone is suggested as the ideal design owing to the intrinsic confinement of UHPFC materials, ease of construction, and low cost. 6. The testing results of UHPFC specimens show that this material can ensure excellent joint behavior even pouring UHPFC in the joint only or without using shear ties in the joint core. It is possible to achieve much more desired behavior than concrete joints that fulfill the criteria of seismic reinforcing details. Better tensile and shear strength, UHPFC bonding with embedded reinforcements, reinforcement slide avoidance, joint zone damage prevention, and the creation of plastic hinges in the beam are all factors contributing to this. In future studies, the effect of using percentages of fibers not mentioned in the research, such as 0.5% and 1.5%, can be studied. The effect of the repeated load, which is reversed in the other direction, on the behavior of the samples can be studied. The effect of deleting the stirrups from the critical joint zone can be studied on the different ratios of the steel fibers. Studying the possibility of achieving the same improvement effect for the samples in which longitudinal beam bars were anchored in the joint using UHPFRC. The author declares that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors extend their thanks to the journal and the reviewers for their assistance in improving the document level and to the Faculty of Engineering, Zagazig University's laboratory, for their cooperation in pouring samples and conducting tests.

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