On the efficiency of flat slabs strengthening against punching using externally bonded fibre reinforced polymers

Abstract

Type of publication:Peer reviewed journal article FRP's can be effectively used to strengthen existing flat slabs against punching shear. The CSCT can be applied to predict the punching strength of strengthened slabs using FRP's. The strengthening efficiency is more pronounced in slabs with lower reinforcement ratio and larger column sizes. The strengthening efficiency is less pronounced in actual continuous flat slabs. a r t i c l e i n f o b s t r a c tOne possibility for strengthening existing flat slabs consists on gluing fibre reinforced polymers (FRPs) at the concrete surface. When applied on top of slab-column connections, this technique allows increasing the flexural stiffness and strength of the slab as well as its punching strength. Nevertheless, the higher punching strength is associated to a reduction on the deformation capacity of the slab-column connection, which can be detrimental for the overall behaviour of the structure (leading to a more brittle behaviour of the system). Design approaches for this strengthening technique are usually based on empirical formulas calibrated on the basis of the tests performed on isolated test specimens. However, some significant topics as the reduction on the deformation capacity or the influence of the whole slab (accounting for the reinforcement at mid-span) on the efficiency of the strengthening are neglected. In this paper, a critical review of this technique for strengthening against punching shear is investigated on the basis of the physical model proposed by the Critical Shear Crack Theory (CSCT). This approach allows taking into account the amount, layout and mechanical behaviour of the bonded FRP's in a consistent manner to estimate the punching strength and deformation capacity of strengthened slabs. The approach is first used to predict the punching strength of available test data, showing a good agreement. Then, it is applied in order to investigate strengthened continuous slabs, considering moment redistribution after concrete cracking and reinforcement yielding. This latter study provides valuable information regarding the differences between the behaviour of isolated test specimens and real strengthened flat slabs. The results show that empirical formulas calibrated on isolated specimens may overestimate the actual performance of FRP's strengthening. Finally, taking advantage of the physical model of the CSCT, the effect of the construction sequence on the punching shear strength is also evaluated, revealing the role of this issue which is also neglected in most empirical approaches.

The theoretical principles of the critical shear crack theory for punching shear failures and derivation of consistent closed‐form design expressions

“…Although detailed calculation of the load–rotation relationship can be performed (considering quadri‐linear moment–curvature diagrams incorporating cracking and tension‐stiffening effects), the use of a non‐linear parabolic law (derived from the quadri‐linear model) has shown to be efficient for design purposes in terms of accuracy and ease of use: where r s refers to the distance between the axis of the supported area and the line of zero radial moment, d is the effective depth, f y and E s are respectively the yield strength and the modulus of elasticity of flexural reinforcement, m s is the average acting bending moment in the support strip (see, e.g., Reference for its definition), m R is the average moment capacity in the support strip, and k m is a factor whose value depends on the level of refinement used to estimate the acting bending moment (value of 1.2 for refined analysis or 1.5 otherwise) . It can be noted that an advantage of this approach is that tailored load–rotation relationships can be developed for particular cases …”

Performance assessment of flat slabs strengthened with a bonded reinforced-concrete overlay

“…The aim of this paper is to propose an ad hoc design approach based on the CSCT. The authors have followed a similar method to that used by Faria et al (2014) for slabs strengthened with FRP strips on the upper face, but defined a new failure criterion and a new load-rotation curve ( Figure 4). Finally, the proposed method was validated by comparison with results of an experimental campaign carried out at the Department of Civil Engineering, Universidade NOVA de Lisboa (Fernandes et al, 2016).…”

“…For each curve inFigure 7, the fundamental parameters to consider are the uncracked stiffness, the critical moment that characterizes the change to the cracked phase, cross-section cracked stiffness (before and/or after yielding of existing reinforcement), and ultimate flexural moment. This approach was also used byFaria et al in (2014), concerning the strengthening of flat slabs with FRP strips. Appendix A and B provide all the elements required for the determination of the listed parameters.…”

Nachträgliche Durchstanzertüchtigung mittels Betonschrauben

“…In den letzten Jahren wurde eine Vielzahl von Methoden zur nachträglichen Durchstanzverstärkung entwickelt . Ein Überblick kann entnommen werden.…”