PSI - Issue 64

Ravina Sriram et al. / Procedia Structural Integrity 64 (2024) 2051–2058 Author name / Structural Integrity Procedia 00 (2019) 000–000

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The latter method listed in our successful applications, namely increased reinforcement to the sagging and/or hogging moment regions, can be achieved either with conventional passive (non-prestressed) steel reinforcements, or with more prestressed iron-based shape memory alloys. Iron-based shape memory alloy (Fe-SMA) are recently being used in construction market to strengthen and prestress existing concrete structures. The Fe-SMAs are deformed when heated and return to their initial shape when cooled down and thus can be used to prestress concrete structures, reduce crack widths, and existing deformations cost-effectively. This case study examines how reinforcing concrete slabs with iron-based shape memory alloys (Fe-SMA) affect the punching shear capacity of flat slab systems. Two specific applications are presented: 1. Strengthening the concrete slabs with Fe-SMA bars in the hogging moment region to improve the slab’s punching resistance. 2. Strengthening the concrete slabs with Fe-SMA bars in der sagging bending region to improve the slab’s punching resistance 2. Literature Review Numerous studies have studied the use of Fe-SMAs to strengthen components of civil structures, such as slabs (Schranz, 2021), beams (Shahverdi et al., 2022) and walls (Rezapour et al., 2021). It relies on the so-called "shape memory effect," which allows a (pre)stress to build up if the material cannot return to its original shape after being subjected to increased temperature (Janke, 2005). It simplifies and speeds up prestressing without hydraulic jacks, reducing frictional losses. Cost-competitive with conventional methods, its ease of use and decreasing costs make it promising for future applications. There are many techniques to strengthen flat slabs against punching shear failure, but no “one-size-fits-all” solution. The optimal solution is governed by limitations imposed by accessibility, architectural or space requirements and cost considerations. Generally, in all cases, pre-damage of the slab due to cracking needs to be partially removed via temporary supports such as bracings, so that the strengthening can be immediately activated. Different techniques such as post-installed shear reinforcements (Ruiz, Miguel Fernández et al., 2011), concrete overlays (Čereš & Gajdošová, 2021) or adding steel or concrete mushrooms (Muttoni, 2008) have been successfully applied within the industry since long time. Even though the influence of the amount of sagging reinforcement on punching strength is currently not included in design codes, there has been results indicating its positive effect on the punching shear resistance with a plastic design approach (Einpaul et al., 2015). 3. Methodology The critical shear crack theory (CSCT) is a well-established technique that relates punching resistance to slab rotation, which is implemented into the fib Model Code (Walraven & Dieteren, 2023) within different levels of approximation (LoA). In LoA IV, the punching resistance is calculated in terms of the relative slab-column rotation (ψ) determined with nonlinear analysis of the structure and accounting for cracking, tension-stiffening effects and yielding of the reinforcement (Béton (fib), 2020). This is an iterative approach and the junction point of the action and resistance as a function of slab-column rotation gives the punching shear resistance. Punching shear resistance is primarily determined by the resistance of the first concrete compression diagonal V Rd,cc0 , the resistance of concrete and steel components in the support area V Rd,cs0 , and the resistance of concrete outside the punching shear reinforcement V Rd,c1 . The smallest of these three resistances is the critical punching shear resistance.