PSI - Issue 64

Carmine Lima et al. / Procedia Structural Integrity 64 (2024) 849–856 Lima et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Ultra High Performance Concrete (UHPC) is nowadays widely used for construction or reinforcement of bridge deck slabs in steel-concrete composite beams (Hung et al., 2021). The number of studies on UHPC or Ultra High Performance-Fiber Reinforced Concrete (UHP-FRC) for more resilient and sustainable reinforced concrete (RC) structures has rapidly increased in the recent years due to the mechanics and durability properties of UHPC. The examination of the technical and scientific literature has highlighted that the use of this class of materials in the field of bridge construction is currently widespread both in the United States (UHPC Solutions, 2019; Graybeal et al., 2020; FHWA, 2021.a). In particular, the superior mechanical properties of UHPC or UHP-FRC well respond to the reinforcement needs of existing structures (Haber, 2012; FHWA, 2021.b), both with reference to piers (Farzad, 2018) and beam slabs (Martìn-Sanz et al., 2019). The use of cementitious materials of this class determines the possibility of sustainable interventions, also in environmental terms, due to the limited thicknesses of material required for such interventions (Kothari et al., 2020). The possibility of also repairing corrosion-induced effects of existing reinforcement is another potentially interesting aspect for the use of UHPC for bridge slabs (Zmetra et al., 2017). More generally, the peculiarities of the mechanical response of UHPC or UHP-FRC have required particular attention in the study of cracking behavior for beams subjected to negative bending (Zhang et al., 2020; Zhu et al., 2020). Moreover, alternative solutions have been proposed for the connection system in order to benefit from the characteristics of these high-performance concretes (Hu et al., 2020; Zhu et al., 2021). The mechanical characterization of the connection system plays a key role and the issue of quantifying the effect of concrete strength on the force-displacement response of connectors is the main subject of interest and research (An and Cederwall, 1996; Kim et al., 2013). The possibility of replacing the existing slab with a UHPC one also implies the needed for re-evaluating the effectiveness of the existing connection and, sometimes, replace it so that it has a geometry suitable for the new slab (Qi et al., 2019). An intervention technique based on the replacement of connectors has been studied both experimentally (Kruszewski et al., 2018.a) and from the point of view of design formulas (Kruszewski et al., 2018.b). More generally, other research concerns demountable connectors (Wang et al., 2017) or connection system for prefabricated slabs (Zhang et al., 2021). In addition, specific studies have been conducted with the aim of investigating the behavior of short-head pin connectors (Xu et al., 2022; Zhao et al., 2021) which would be compatible with UHPC slabs with reduced thickness. In order to contribute to the expansion of the available knowledge on the use of UHPC slabs in composite steel concrete beams, nine push-out tests have been carried out at the Structural Engineering Testing Hall (Str.Eng.T.H.) of the University of Salerno with the aim to characterize the mechanical behaviour of the shear connection between concrete slab and steel joist. For this purpose, three groups of specimens were realized, each of which characterized by a slab made of a different concrete and with different thickness, while Nelson-type headed stud shear connectors were employed in all those specimens. In the following, section 2 reports the complete description of the performed experimental campaign, while in section 3 the results of the push-out tests are compared in terms of force-slip curves obtained for the three groups of tested steel-concrete stubs. Finally, in view of future developments intended at understanding the influence of UHP FRC on the overall global behavior of steel-concrete composite bridge decks, the final part of section 3 reports a possible comparison of the obtained experimental data versus theoretical curves. 2. Experimental campaign 2.1. Concrete mixtures characteristics In order to produce the push-out samples, three concrete mixtures were conceived for the three group of different samples to be tested: - Ordinary concrete strength: strength class C25/30 (EN 1992-1-1, 2004) incorporating CEM II 42.5R (EN 197-1, 2011), water, gravel and natural sand (nominal diameter ranging between 0 mm and8 mm) and characterized by an average cubic compressive strength (EN 12390-3, 2019) equal to 32.16 MPa;