PSI - Issue 66

Sneha et al. / Procedia Structural Integrity 66 (2024) 419–425

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Sneha, S.Ray/ Structural Integrity Procedia 00 (2025) 000–000

1. Introduction Concrete is one of the most versatile materials used in the construction industry. With ongoing advancements, there is a growing need for materials that offer enhanced toughness, durability, and ductility to address the challenges of modern construction. Ensured longer service life and improved resilience under extreme environmental conditions. Consequently, the development of advanced concrete materials is crucial for addressing the increasing demands for performance, durability, sustainability, and innovation in the construction industry. Hence, UHPFRC is emerging as the future of construction materials. UHPFRC is characterized by its ultra-high compressive strength, superior durability, and enhanced resistance to cracking. Steel fibres play a crucial role in improving the tensile strength and ductility of the material. In UHPFRC, the inclusion of steel fibres is essential for bridging cracks and distributing stresses throughout the material. These fibres enhance the post-cracking behaviour of the concrete. Research by Richard and Cheyrezy (1995) on UHPFRC emphasizes the role of steel fibres in improving the tensile and post cracking behaviour of the material [1]. Graybeal (2006) explored the mechanical properties of UHPFRC and highlighted the importance of steel fibres in controlling crack propagation and dissipating energy [2]. To further enhance the mechanical and flexural performance of UHPFRC, researchers have explored the use of hybrid fibre combinations using different types of fibres to benefit from the unique properties of each. Hybridization involves the integration of fibres with varying geometries, materials, and dimensions, which allows for better control over the cracking behaviour and ductility of UHPFRC. There are different ways of hybridization for better performance. Firstly, it can be done by combining macro steel fibre and synthetic fibres, i.e. with different moduli and different shapes or the same shape. Secondly, macro steel fibres (hooked end) and micro steel fibres (straight fibres) having different shapes but the same moduli. Lastly micro steel fibres with the same moduli but different size like 6 mm, 13 mm and 20mm steel fibres. Shah et al. (2010) have shown that the hybridization of steel macro fibres (0.5 mm in diameter) and polyvinyl alcohol (PVA) micro-fibre (diameter less than 0.022 mm) can significantly enhance both the strength and toughness of concrete composites [3]. In another study, Chun and Yoo (2019), reported that the combination of hybrid steel fibres and synthetic fibres (including polyvinyl alcohol and polyethylene fibres) in ultra-high performance concrete (UHPC) resulted in enhanced tensile properties [4]. One major challenge in utilizing long steel fibres is the issue of poor fibre dispersion, which can lead to the phenomenon known as fibre balling. When this occurs, the fibres are not uniformly distributed throughout the concrete matrix, resulting in a reduction of flexural performance. To mitigate this, Yoo et al. (2020) recommend that steel fibres shorter than 30 mm are more suitable for achieving optimal fibre dispersion and, consequently, better mechanical performance [5] The use of shorter straight micro steel fibres, such as those measuring 13 mm and 20 mm, has become increasingly popular in UHPFRC formulations. It has been shown that the use of 13 mm and 20 mm steel fibres in UHPFRC significantly enhances the flexural performance of the composite [6]. The incorporation of these short fibres allows for better fibre dispersion, reduces the likelihood of fibre balling, and ensures a more uniform stress distribution across cracks. The straight geometry of these fibres allows for better mechanical interlocking, providing greater resistance to pullout under stress. Therefore, the incorporation of hybrid fibres in UHPFRC has a significant influence on the post-cracking behaviour, especially in terms of enhancing the ductility of the material and energy absorption capacity. There are also studies that have investigated the hybrid effect and demonstrated a substantial increase in fracture energy, more stable post-cracking response, and effectively reduced stress concentration at the crack tip, thereby slowing the rate of crack propagation [7,8]. Most existing studies in the literature predominantly focus on the use of 13 mm steel fibres and their effects at varying volume fractions [9,10]. These studies provide valuable insights into how different fibre dosages impact the mechanical behaviour of UHPFRC. However, research exploring the combined use of hybrid fibres, specifically the combination of 13 mm and 20 mm fibres, and the simultaneous variation of their volume fractions, are limited. Therefore, the objective of this research is to investigate the fracture behaviour of UHPFRC by analyzing the post-peak response to varying fibre volume percentages, as well as different combinations of 13 mm and 20 mm fibres. The study aims to provide a more comprehensive understanding of how hybrid fibres contribute to enhancing the toughness, ductility, and overall fracture performance of material beyond the cracking point.

GGBFS Ground granulated blast furnace slag FPZ Fracture process zone