PSI - Issue 70

Abutu Simon John Smith et al. / Procedia Structural Integrity 70 (2025) 11–18

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1. Introduction Researchers in recent times have intensified their efforts on investigating the structural behaviour of ultra-high performance fibre reinforced concrete (UHPFRC) due to its high strength, ductility, excellent durability (Azmee and Shafiq, 2018) and capability of improving the sustainability of buildings and other infrastructure components (Schmidt and Fehling, 2005). The structural behaviour of UHPFRC is different from ordinary concrete, and they cannot be designed using standards and codes for ordinary concrete because steel fibre’s presence in UHPFRC beam contributes immensely to its structural capacity. Considering shear capacity for instance, fibre topology and fibre UHPFRC matrix bond play vital roles in resisting applied shear load before crack appearance and after crack appearance. So using ordinary concrete beam ’s shear capacity equation to e valuate UHPFRC beam ’s shear capacity will lead to serious underestimation. This is why researchers, through series of studies have developed many techniques and provided a lot of data to help simplify the analysis and design of the structural integrity of UHPFRC members. One of such techniques is the development of equations for predicting the shear capacity of UHPFRC beams through the modification of either normal concrete beam’s shear capacity equation or steel fibre reinforced concrete (SFRC) beam’s shear capacity equation. Some of which include: The modification of ACI 318 -14 shear capacity equation for normal concrete by Ahmad et al. (2019) to develop UHPFRC’s shear capacity equation; which was reported to be reasonably accurate in predicting the shear capacity of UHPFRC beam. The development of UHPFRC’s shear capacity equation from ACI 318 -14 shear capacity equation, whose results after comparison with the shear capacity obtained from experiment showed reasonable correlation (Hussein, 2015). Jia-nan et al. (2020) through experimental and theoretical analysis of UHPFRC beams’ shear strength using research variables like concrete contribution, fibres and stirrups, proposed a model for estimating the shear strength of UHPFRC beams. The procedures for evaluating the shear capacity of UHPFRC beams have also been included in different standards like the Chinese technical specification for UHPFRC structures (CECS2020, 2020 ); Swiss standard for UHPFRC’s materials, design and execution (SIA 2052, 2016); Japanese recommendations for design and construction of UHPFRC structures (JSCE, 2006); French rules for UHPFRC design (NF P 18-710, 2016); and test and design methods for SFRC (RILEM TC 162-TDF, 2003). In recent times, beside the modification of existing normal concrete/SFRC beam’s shear capacity to develop UHPFRC beam’s shear capacity; and the release of technical guide and specification for UHPFRC structures, there have been serious advancements in the field of shear capacity prediction for UHPFRC beams as researchers are developing new shear capacity equations for UHPFRC beams and standards are being updated to include shear design of UHPFRC beams. For instance, Smith and Xu (2023) developed a new shear capacity equation that incorporates both direct and indirect shear influencing factors; and which is capable of correctly estimating the shear capacity of both UHPFRC and UHPFRC-CA beams. Also, AASHTO (2024) released the firstedition of the Guide Specifications for Structural Design with ultra-high performance concrete, which greatly improved the design procedures of UHPFRC beams against shear loading. Despite the advancements, there are still challenges in the field of shear capacity prediction of UHPFRC as there are still a lot of data needed in the area of expanding the different shear influencing parameters into many levels, so that the effects of changes in those levels can be ascertained. This challenge is a serious one because conducting experiment to cater for different levels of shear parameters for UHPFRC is highly expensive. However, this challenge may be overcome by conducting parametric study of the shear behaviour of UHPFRC beams using finite element numerical modelling and simulation. The structural failure of concrete beams in shear is a complex mechanism as explained by Smith and Xu (2023) in their study that UHPFRC beam’s shear resistance is provided by three different mechanisms: (1) UHPFRC -steel fiber bond (2) Stirrup strength (3) UHPFRC strength (i. e. UHPFRC compression zone, aggregate interlock and dowel action). These shear mechanisms occur in sequential stages starting with dowel action resistance, then aggregate interlock after dowel action reaches its optimum capacity. The next shear resistance is provided by the UHPFRC’s compression zone, after which stirrups in the UHPFRC beam contribute their resistance to the applied load when diagonal cracks appear in the beam. The stirrups continue to provide the shear resistance until they yield, leading to the widening of the diagonal crack. Once the crack widens, the steel fibre resisting pull out in the cracked UHPFRC is the last to provide shear resistance until the UHPFRC-steel fibre bond is broken. The loss of UHPFRC steel fibre bond leads to the attainment of its pull-out force capacity resulting in their pull out from the beam and eventual failure.