António Manuel Pinho Ramos

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One of the major disadvantages of conventional fibre-reinforced polymer (FRP) strengthening techniques is the premature debonding of the FRP, leading to an underutilization of the materials. The externally bonded reinforcement on grooves (EBROG) method, which has been proven successful in postponing debonding in several structural applications, is examined in this study for the first time for realistic conditions in flat slabs. To this end, two different layouts of the strengthening solution are tested under concentric monotonic loading: one representing roof-level slab-column connections in which carbon FRP (CFRP) sheets are laid on top of the joint region (cross layout); and another one representing intermediate floors, in which the aforementioned layout is not possible due to the presence of the column (grid layout). For each layout, two FRP bonding techniques are used: conventional externally bonded reinforcement (EBR) and EBROG. Another specimen, without FRP strengthening, is used as a reference. It is shown that the EBROG technique is effective in postponing debonding for both layouts. Compared to the specimens in which EBR was used, the load capacity was increased in case of EBROG by 36% when FRP sheets were bonded on top of the joint (cross layout) and by 15% when sheets were attached outside the joint region (grid layout). Debonding strains are shown to be significantly higher in the case of EBROG compared to EBR. The experimentally observed debonding strains were compared with code provisions and predictions of models from the literature. A simple calculation method giving reasonably good results for the load capacity of the FRP-strengthened specimens is presented.

Abstract Fiber reinforced polymer (FRP) composites can be efficient for flexural strengthening of flat slabs if debonding of the FRP is postponed. However, with the increase of the flexural capacity, the flat slab becomes more susceptible to punching shear failure. In this context, four flexural or simultaneous flexural and punching shear retrofitting systems are investigated in this study to strengthen a flexure-deficient flat slab. Externally Bonded Reinforcement on Grooves (EBROG) and externally bonded reinforcement (EBR) methods are used for flexural strengthening in two cases: slabs without punching shear reinforcement and with post-installed shear bolts as shear reinforcement. According to the results, flexural strengthening of the slab using the EBR and EBROG techniques increased its load capacity by 12% and 21%, respectively. Simultaneous flexural and shear strengthening of the slab using the EBROG technique was the most effective, leading to a 57% enhancement of the load capacity. For specimens whose failure was governed by punching, comparing the results with code predictions showed that Eurocode and ACI (and the respective guide documents fib bulletin 90 and ACI 440.2R) overestimated the capacity of these specimens. In cases where failure was governed by flexure, a simple application of the yield line theory predicted reasonably well the load capacity of the specimens.

The current research aims to study the behavior of thin reinforced concrete (RC) slabs under concentrated loads as well as to investigate the application of Critical Shear Crack Theory (CSCT) to such slabs. For this purpose, four square 100-mm-thick slabs were cast and subjected to concentrated punching monotonic loading. The experimental parameters were the flexural reinforcement ratio, 0.38% and 1.00%, and the presence or absence of shear headed stud reinforcement. It is shown that the failure criteria of CSCT describe reasonably well the observed failure modes and the ultimate loads of the specimens. However, attention is brought to some peculiarities in the analytical derivation of the load-rotation curve for thin lightly reinforced flat slabs, in which large deformations are experienced. Results showed that in such slabs, the behavior can be highly influenced by the post-yield stress-strain curve of the flexural steel reinforcement. As a result, the constitutive law of steel reinforcement should be explicitly taken into account in such cases. The versatility of CSCT to adapt to these conditions is demonstrated.

Abstract Fiber reinforced polymer (FRP) composites can be efficient for flexural strengthening of flat slabs if debonding of the FRP is postponed. However, with the increase of the flexural capacity, the flat slab becomes more susceptible to punching shear failure. In this context, four flexural or simultaneous flexural and punching shear retrofitting systems are investigated in this study to strengthen a flexure-deficient flat slab. Externally Bonded Reinforcement on Grooves (EBROG) and externally bonded reinforcement (EBR) methods are used for flexural strengthening in two cases: slabs without punching shear reinforcement and with post-installed shear bolts as shear reinforcement. According to the results, flexural strengthening of the slab using the EBR and EBROG techniques increased its load capacity by 12% and 21%, respectively. Simultaneous flexural and shear strengthening of the slab using the EBROG technique was the most effective, leading to a 57% enhancement of the load capacity. For specimens whose failure was governed by punching, comparing the results with code predictions showed that Eurocode and ACI (and the respective guide documents fib bulletin 90 and ACI 440.2R) overestimated the capacity of these specimens. In cases where failure was governed by flexure, a simple application of the yield line theory predicted reasonably well the load capacity of the specimens.

This work refers to experimental and 3D nonlinear FEA on punching. Numerical results were compared with experimental ones in order to benchmark the FE model and afterwards a parametric study was conducted, changing the reinforcement ratio, slab thickness, concrete strength and column dimensions, running a total of 360 models, where their effect on punching capacity is shown. EC2 and MC2010 provisions agreed approximately with experimental and FEA results. Based in the FEA results it is proposed an equation to predict the punching capacity with the introduction of fracture mechanics parameter, which was compared with several experimental results, giving good approximation.

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Abstract This paper analyses the performance of the experimental setups to assess the punching strength of slab-column connections in continuous flat slabs under vertical and horizontal loading. In the last years, several experimental campaigns have been performed to investigate the punching strength of slab-column connections, but most of the experimental tests concerned isolated slab-column connections. Among the few setups aimed at reproducing the eccentric punching failure in continuous flat slabs, the setup developed at the NOVA School of Science and Technology in Lisbon is considered in this paper. The performance of the Lisbon setup is assessed through nonlinear finite element analyses, calibrated on experimental data, by comparison with numerical results of a theoretical continuous setup. Then, the performance of the isolated setups, used in many researches and at the base of some international codes, is also evaluated through the same finite element model. Numerical analyses highlight that the setup developed in Lisbon could provide reliable ultimate rotations of continuous flat slab connections, but it underestimates the punching strength. Despite isolated setups lead to similar results when compared with the Lisbon setup, the latter seems to provide a better representation of a continuous slab-column connection. The numerical analyses presented in this paper have been performed assuming monotonic lateral loading.

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The present work reports experimental research carried out on repair and strengthening methods of flat slabs for punching. The repair and strengthening methods studied are: strengthening using transversal prestress, repair by substitution of the damaged concrete; and strengthening using steel beams as a column head, connected to the column and to the slab with epoxy resin and mechanical expansion anchors. Four experimental test slabs (AR1 to AR4) were produced and tested: two with transversal prestress, one only repaired and one strengthened with a steel column head. The execution process, its efficiency and design, are discussed.

The use of prestress in flat slabs is a common solution, mainly because it allows larger spans and thinner slabs. Nevertheless, smaller thicknesses near the slab-column connections, along with the superposition of high shear and flexural stresses, arise the question of the slab capacity to resist punching. The punching failure results from the superposition of shear and flexural stresses near the column, and is associated to the formation of a pyramidal plug of concrete which punches through the slab. It is a local and brittle failure. The use of prestress can increase the punching capacity of flat slabs-column connections.This work presents the experimental analysis of flat slab specimens with tendons under punching. Nine slabs were tested using unbonded prestress with high strength steel tendons. The influences on the punching capacity of the vertical component of the prestress forces resulting from inclined tendons near the column and their distance to the column are analysed. The in-plane compression force due to prestress was not applied to the slabs, in order to evaluate only the deviation force influence. This work aims to improve the understanding of the behaviour of prestressed flat slabs under punching load in order to properly evaluate the punching resistance of this kind of structures. The experimental punching loads are compared with the provisions of EC2, ACI 318-11 and MC2010. © 2014 Elsevier Ltd.

Flat slab structures are a very common structural solution nowadays, due to their architectural and economic advantages. However, flat slab-column connections may be vulnerable to punching failure, especially in an event of an earthquake, with potentially high human and economic losses. This type of structural solution is adequately covered by design codes and recommendations in North America, due to a large amount of experimental research carried out. In Europe the situation is different, missing specific guidance to flat slab design under earthquake action in most European codes. The ACI 318-14 prescriptive approach to the gravity shear ratio-drift ratio relationship shows good agreement with experimental results. Following a similar approach and, based in a databank containing cyclic horizontally loaded tests of slab-column connections found in literature, proposals are made applicable to EC2 and MC2010.

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The experimental analysis of nine prestressed flat slab models under punching is described and the results are compared with the recommendations of EC2 and MC90. The tests were performed on specimens at a scale 1/3, prestressed with unbonded high strength steel. In the first set, the specimens (AR2 to AR5 and AR7) were only subjected to in-plane compression, to evaluate the effect of the in-plane forces on the punching resistance. The second set of tests (AR8 to AR11) intended to study the effect of the vertical component of the tendon forces near the column in the punching resistance. This work aims to improve the understanding of the behaviour of prestressed flat slabs under punching load and the evaluation of the punching resistance.