Usama Ebead

Fiber-Reinforced Polymer Strengthening of Two-Way Slabs

This paper experimentally evaluates the strengthening of 2-way slabs using fiber-reinforced polymers (FRPs). Two different types of FRP materials were evaluated: carbon FRP strips and glass FRP laminates. The dominating failure mode for 2-way slab, flexural, or punching shear is based on the slab steel reinforcement ratio. The reinforcement ratios were chosen to serve the purpose of demarcating the 2 possible modes of failure. The tested specimens were classified according to the purpose of strengthening into specimens strengthened in flexure and specimens strengthened in punching shear. Specimens strengthened in flexure had 2 steel reinforcement ratios: 0.35 and 0.5%. Results show that the flexural capacity of 2-way slabs can increase to an average of 35.5% over that of the reference (unstrengthened) specimen. An increase of the initial stiffness was achieved for flexural specimens; however, an apparent decrease in the overall ductility was evident. FRP materials can be used to increase the flexural capacity of 2-way slabs. However, an average decrease in the values of the energy absorption of approximately 30% for flexural strengthening specimens was observed. Specimens strengthened for punching shear have an original slab reinforcement ratio of 1.0%. A strengthening technique that combines the use of carbon FRP strips and steel bolts increases the strength of the slab by 9.0%. An analytical model for the analysis of FRP strengthening of 2-way slabs under flexure or punching shear is introduced.

Studies on Mechanically Fastened Fiber-Reinforced Polymer Strengthening Systems

This study uses experimental tests and finite element modeling to investigate the interfacial behavior between mechanically fastened (MF)-fiber reinforced polymer (FRP) composite and concrete substrates. A total of 23 direct shear tests were conducted on FRP strips MF to concrete. Based on the experimental results of one fastener FRP/concrete connection, bearing-slip models are developed to address the MF-FRP/concrete interfacial behavior. Finite element models are then presented to address the interfacial behavior between the FRP strips and the concrete substrate for both the MF-FRP/concrete direct shear specimens, as well as for MF-FRP-strengthened reinforced concrete beams. Results are presented in terms of ultimate load capacities, load-slip relationships, and strain distributions along the FRP strip. Good agreement is obtained between the numerical predictions and experimental data in terms of ultimate carrying capacities, failure modes and load-deflection relationships.

STRENGTHENING OF TWO-WAY SLABS USING STEEL PLATES

This paper introduces a strengthening technique of two-way slabs using steel plates and steel bolts. The effectiveness of two configurations of steel plates and four different arrangements of steel bolts were evaluated. The strengthening steel plates were extended to twice the slab depth around the column and acted as a drop panel of an equivalent concrete depth. Steel bolts were used as vertical shear reinforcement. Eight bolts were sufficient to transfer the horizontal forces from the steel plates to the concrete and the confining concrete sandwiched between the steel plates. The strengthened slabs showed an increase in stiffness and energy absorption. In addition, the ductility was slightly improved. The load-carrying capacity of the strengthened slabs was increased by 56.55, 57.76, and 64.56% over that of the control specimen with slabs that had eight, 12, and 16 bolts, respectively. The research presents a strengthening concept that can be used to strengthen two-way slabs in multistory structures. A simple approach that was based on the yield line theory showed good agreement with test results.

Tension-stiffening model for FRP-strenghened RC concrete two-way slabs

The main focus of this paper is to present a tension-stiffening model that is suitable for finite element analysis (FEA) aimed at investigating the effect of FRP strengthening on the tensile behaviour of concrete slabs. Available experimental results of the FRP-strengthened reinforced concrete slabs are used to calibrate the finite element model based on the ultimate load carrying capacity of the two-way slabs. The proposed tension-stiffening model is implemented into the constitutive concrete model defined in a general-purpose finite element code. Reinforced concrete behaviour in tension can signifcantly be changed due to strengthening. An overall increase in the post-peak stiffness based on the tensile stress-strain relationship is observed. A simplified bilinear model is introduced to define the behaviour of the FRP-strengthened concrete in tension. An expression of the fracture energy density is introduced to define the area under the concrete tensile stress-strain relationship. The tensile stress-strain relationship of concrete is referred to as the tension-stiffening model. It is shown numerically that the ultimate load capacity of two-way slab specimens is sensitive to the fracture energy density. Hence, a distinction has to be made between the definitions of the tension-stiffening model of FRP-strengthened and un-strengthened concrete. This distinction is the focus of this paper.RésuméLobjectif principal de cet article est de présenter un modèle approprié de raidissement en traction pour lanalyse déléments finis (AEF). Lanalyse est destinée pour létude de leffet du renforcement en polymère renforcé de fibres (PRF) sur le comportement en traction des dalles en béton armé. Les résultats expérimentaux des dalles renforcées sont employés pour calibrer le modèle déléments finis basés sur la capacité ultime des dalles bidirectionnelle. Le modèle de raidissement en traction proposé, est appliqué dans un code général délément finis. Le comportement du béton armé renforcé en traction peut être changé dune manière significative due au renforcement. On observe une augmentation en tension de la rigidité du poteau crête basée sur la relation contrainte-déformation. Un modèle bilinéaire simplifié est présenté pour définir le comportement du béton renforcé de PRF en traction. Une expression de la densité dénergie de rupture est présentée pour définir laire sous la courbe contrainte-déformation. La relation contrainte-déformation du béton en tension est appelée modèle raidissement en traction. On montre numériquement que la capacité de la charge ultime de spécimens bi-directionnels de dalle est affectée par densité dénergie de rupture. Par conséquent, une distinction doit être faite entre les définitions du modèle raidissement en traction du béton renforcé de PRF est béton non renforcé. Cette distinction est lobjet de cet article.

Hybrid Externally Bonded/Mechanically Fastened Fiber-Reinforced Polymer for RC Beam Strengthening

Externally bonded (EB) fiber-reinforced polymer (FRP) systems and mechanically fastened (MF) FRP strips can both be used to strengthen reinforced concrete (RC) structures, but each has some drawbacks. This paper proposes a new technique for the flexural strengthening of RC beams that combines the EB and MF FRP systems. The technique uses nylon anchors inserted inside the concrete prior to installing the fasteners. These anchors provide a better grasp of the fasteners with the concrete. The hybrid EB/MF-FRP-strengthened specimens showed higher load capacity and post-cracking stiffness than those of the corresponding MF-FRP counterparts. Extending the FRP strips for the entire beam span is necessary for achieving the noticeable enhancement of the load capacity and stiffness with this hybrid EB/MF-FRP system. The failure of the strengthened beams in the system is associated with diagonal cracks at the strip end locations due to fastener rotation and bearing damage.

Mechanically Fastened FRP-Strengthened Two-Way Concrete Slabs with and without Cutouts

The feasibility of strengthening concrete slabs in flexure, with and without cutouts, using the mechanically fastened MF FRP technique is investigated. Two series of large-scale reinforced concrete slabs are tested. The first series is comprised of five slabs without a cutout, and measuring 2,6002,600120 mm; the second series consists of four slabs of the same dimensions with a central cutout measuring 800800 mm. The mechanically fastened system is found to be a valid alternative to the externally bonded system resulting in a rapid, economic, and effective strengthening technique for two-way concrete slabs. The increases in ultimate capacities of the MF FRP-strengthened slabs range between 30 and 70% over those of the unstrengthened specimens. In addition, finite-element modeling of MF FRP-strengthened slabs is introduced in this study. The interfacial behavior between the MF FRPs and the concrete substrate is accounted for by using appropriate interfacial models. Very good agreement is obtained between the test results and the numerical predictions.

Analysis of the Load-Deformation Behaviour and Debonding for FRP-Strengthened Concrete Structures

Results from nonlinear finite element analyses of fibre reinforced polymer (FRP)-strengthened concrete beams and slabs are presented. The direct shear test, a basic application that provides insight into FRP-concrete interfacial behaviour, is also considered. The motivation for this work is the fact that, although there is a large amount of experimental data available on the FRP strengthening of concrete structures, a full understanding of the various load–deformation behaviours and debonding phenomenon is still lacking. The numerical models presented in this paper adopt a displacement-controlled solution and are capable of simulating FRP-strengthened beams either in shear or in flexure, as well as slabs strengthened using either passive or prestressed FRP laminates. Results of the different applications are presented and compared with published test data, and a very good agreement in terms of the ultimate load carrying capacities, load–deflection behaviour and modes of failure, is obtained.

Effectiveness of Fabric-Reinforced Cementitious Matrix in Strengthening Reinforced Concrete Beams

AbstractThis paper reports on the efficiency of fabric-reinforced cementitious matrix (FRCM) in enhancing the flexural capacity and deformational characteristics of RC beams. In the main experimental part of the paper, 12 RC beams, 2,500xa0mm long, 150xa0mm wide, and 260xa0mm deep, were fabricated. The beams had two different steel reinforcement ratios, namely, ρsD12=0.72% and ρsD16=1.27%, representing typical underreinforced beam sections. The strengthened beams utilized two FRCM types, carbon and polyparaphenylene benzobisoxazole (PBO) FRCM systems. In the second part of the work, tensile material characterization tests were performed on the FRCM coupons to determine the tensile characteristics of the FRCM composites. The beams were tested in flexure under four-point loading until failure. Two beams without FRCM strengthening were used as a benchmark. Six beams were externally strengthened using one, two, and three layers of carbon FRCM system. Four beams were strengthened with one and two layers of PBO FRCM ...

Modeling of Reinforced Concrete Slabs Strengthened with Fiber-Reinforced Polymer or Steel Plates

This study presents finite element models for strengthened two-way slab-column connections. Two different strengthening techniques are modeled using steel plates or fiber-reinforced polymer (FRP) strips or laminates. The connections are categorized according to the strengthening type as flexural or punching shear specimens. FRP-strengthened connections are subjected to central axial loads. The connections that use an assembly of steel plates and bolts are subjected to a central load or a combination of central load and moment. The three-dimensional assembly of the slab and column of the connections is simulated. The interfacial behavior between the FRP materials and concrete is taken into account. A user-defined subroutine for the microplane model for concrete is incorporated in the finite element package. Results are presented in terms of the ultimate load capacities, load-slip relationships, load-interfacial stress relationships, and stress distributions in the strengthening materials. The models presented in this paper showed good agreement with experimental results.

Numerical Modeling of Shear Strengthened Reinforced Concrete Beams Using Different Systems

This research aims at creating finite-element models for fiber-reinforced polymer (FRP) shear strengthened concrete beams. It is inspired by the fact that the determination of the structural behavior of shear strengthened beams requires advanced numerical methods of which results are substantiated by credible experimental findings. The models are developed here to assess the shear and interfacial types of behavior of beams strengthened using one of three different schemes, namely, externally bonded (EB), mechanically fastened (MF), and hybrid EB/MF FRP schemes. The interfacial behavior between the EB, MF, and hybrid EB/MF FRP and the concrete is accounted for using interface elements for both vertical and inclined FRP strips. A user-defined subroutine for the microplane constitutive law for the concrete material is incorporated in the model. Results are presented in terms of the ultimate load-carrying capacities, load-deflection relationships, and interfacial stress/slip distributions. Numerical results are validated against available experimental data and show reasonable agreement. Models for hypothetical cases of MF FRP strengthened beams are created to enrich the discussion on the interfacial bearing stress distributions.