Assessment of Eurocode-like design equations for the shear capacity of FRP RC members

Fibre reinforced polymer (FRP) bars represent an interesting alternative to conventional steel as internal reinforcement of reinforced concrete (RC) members where some properties such as durability, magnetic transparency, insulation, are of primary concern. The present paper focuses on the assessment of Eurocode-like design equations for the evaluation of the shear strength of FRP RC members, as proposed by the guidelines of the Italian Research Council CNR-DT 203 [CNR-DT 203/2006. Guide for the design and construction of concrete structures reinforced with fiber-reinforced polymer bars. National Research Council, Rome, Italy; 2006]. Both the concrete and the FRP stirrups contributions to shear are taken into account: the new equations derived with reference to Eurocode equations for shear of steel RC members are verified through comparison with the equations given by ACI, CSA and JSCE guidelines, considering a large database of members with and without shear reinforcement failed in shear.     

Assessment of available prediction models for the strength of FRP retrofitted RC beams

A reinforced concrete beam retrofitted with fibre reinforced polymer composites (FRP) to enhance its flexural capacity can experience several failure modes, namely flexural failure, end debond and midspan debond. The mechanism of those failures and available prediction models are first identified in this paper. The models are then assessed with an up to date database of beams reported in literature together with beams tested by the authors. The study verifies that beam theory can predict flexural failure well. The credibility of several methods to predict end debond and mid-span debond is also proved.     

IR thermography for interface analysis of FRP laminates externally bonded to RC beams

Non-destructive (ND) evaluation by thermographic analysis was applied to reinforced concrete (RC) beams strengthened with CFRP laminates, to assess the quality of the interface between reinforcement and substrate before and under loading, during laboratory bending tests. The proper algorithm for processing data acquired from the thermographic system was first selected by testing reduced-scale fiber reinforced polymer (FRP)-strengthened concrete slabs with artificial defects and anomalies, deliberately placed at the interface. Portable heating equipment was purposely set-up to allow continuous scanning along the loaded beams. Results showed that infrared thermography (IRT) can supply significant qualitative and quantitative information on bonding of FRP materials applied to structural substrates, for both preliminary and in-situ investigations, by means of a reliable low-time-consuming procedure. Cross-evaluation of crack patterns during bending tests and thermographic results are also presented.     

Effective strain of RC beams strengthened in shear with FRP

The failure modes of Reinforced Concrete (RC) beams strengthened in shear with Fiber Reinforced Polymer (FRP) sheets or strips are not well understood as much as those of RC beams reinforced with steel stirrups. When the beams are strengthened in shear with FRP composites, beams may fail due to crushing of the concrete before the FRP reaches its rupture strain. Therefore, the effective strain of the FRP plays an important role in predicting the shear strength of such beams. This paper presents the results of an analytical and experimental study on the performance of reinforced concrete beams strengthened in shear with FRP composites and internally reinforced with conventional steel stirrups. Ten RC beams strengthened with varying FRP reinforcement ratio, the type of fiber material (carbon or glass) and configuration (continuous sheets or strips) were tested. Comparisons between the observed and calculated effective strains of the FRP in the tested beams failing in shear showed reasonable agreement.     

Strengthening RC beams with composite fiber cement plate reinforced by prestressed FRP rods: Experimental and numerical analysis

This paper deals with the development of a new strengthening system for reinforced concrete beams with externally-bonded plate made of composite fiber cement reinforced by rebars made of fiber-reinforced plastic (FRP) [1]. The proposed strengthening material involves the preloading of FRP rod before mortar casting. The paper presents experimental and numerical analysis carried out on many large-scale beams strengthened by well-known reinforcement techniques, such as externally bonded Carbon Fiber-Reinforced Plastic (CFRP) plate and the Near Surface Mounted (NSM) technique, which are compared to the proposed new strengthening material through four-point bending tests. Results are analyzed with regard to the load-displacement curve, bending stiffness, cracking load, yield strength and failure load. The developed numerical model is in agreement with the experimental results. It clearly shows the effects of prestressed FRP rod on cracking mechanisms and internal strength distribution in the analyzed beams.     

Modeling of flexural behavior of RC beams strengthened with mechanically fastened FRP strips

An alternative to fiber reinforced polymer (FRP) materials adhesively bonded to the concrete substrate is the implementation of mechanically fastened FRP (MF-FRP) systems using steel anchors to secure the laminate to the substrate. The benefit of MF-FRP, compared to adhesive bonding for FRP flexural strengthening, is the speed of installation with unskilled labor, minimal or absent surface preparation under any meteorological condition and immediate use of the strengthened structures. Some of the potential shortcomings are: possible concrete damage during anchoring and limited opportunity of installation in the presence of congested internal reinforcement in the members to be strengthened. Laboratory testing and a number of field applications have shown the effectiveness of the MF-FRP method. In this paper, an analytical model is discussed for reinforced concrete (RC) members strengthened with MF-FRP strips. The model accounts for equilibrium, compatibility and constitutive relationships of the constituent materials; in particular, it accounts explicitly for the slip between the substrate surface and the FRP strip due to the behavior of the fasteners. The proposed flexural model, coupled with the computation algorithm, is able to predict the fundamentals of the behavior of RC flexural members strengthened with MF-FRP strips, in terms of both ultimate and serviceability limit states. Comparisons between the analytical predictions and the experimental results have been successfully performed.     

Genetic algorithm model for shear capacity of RC beams reinforced with externally bonded FRP

A variety of on-site construction applications using FRP materials have been realized worldwide. However, this technology is currently at a stage where its future widespread implementation and competitiveness will depend on the development of reliable design guidelines based on sound engineering principles. This paper presents simple, yet improved, equations to calculate the shear capacity of FRP bonded-reinforced concrete beams based on the genetic algorithms (GAs) approach applied to 212 experimental data points available in the open literature. The performance of the proposed equations was compared to that of commonly used shear design methods, namely the ACI 440, Eurocode (EC2), the Matthys Model, Colotti model and the ISIS Canada guidelines. Results demonstrate that the proposed equations better agree with the available experimental data than the existing models investigated. Moreover, a sensitivity analysis was carried out to investigate the effect of the shear span-to-depth ratio on the shear capacity contributed by concrete, the ultimate effective strain in FRP sheets, and the ultimate effective stress in transverse rebars. Results indicate that the shear span-to-depth ratio has a significant effect on the shear behaviour of FRP bonded-reinforced concrete beams.     

Experiments and analytical comparison of RC beams strengthened with CFRP composites

This paper deals with strengthening, upgrading, and rehabilitation of existing reinforced concrete structures using externally bonded composite materials. Five strengthened, retrofitted, or rehabilitated reinforced concrete beams are experimentally and analytically investigated. Emphasis in placed on the stress concentration that arises near the edge of the fiber reinforced plastic strip, the failure modes triggered by these edge effects, and the means for the prevention of such modes of failure. Three beams are tested with various edge configurations that include wrapping the edge region with vertical composite straps and special forms of the adhesive layer at its edge. The last two beams are preloaded up to failure before strengthening and the ability to rehabilitate members that endured progressive or even total damage is examined. The results reveal a significant improvement in the serviceability and strength of the tested beams and demonstrate that the method is suitable for the rehabilitation of severely damaged structural members. They also reveal the efficiency of the various edge designs and their ability to control the characteristic brittle failure modes. The analytical results are obtained through the Closed-Form High-Order model and are in good agreement with the experiment ones. The analytical and experimental results are also used for a preliminary quantitative evaluation of a fracture mechanics based failure criterion for the strengthened beam.     

Acoustic emission monitoring and numerical modeling of FRP delamination in RC beams with non-rectangular cross-section

A case study concerning both numerical modeling and in-situ monitoring of a retrofitted RC beam with non-rectangular cross-section is presented. Before retrofitting, non-destructive techniques, such as pull-out and impact tests, were used to estimate the mechanical parameters of concrete. At the same time, a long-term monitoring with the Acoustic Emission (AE) technique was carried out in order to investigate on creep effects and microcracking phenomena. Then, after a complete removal of the overload and retrofitting with FRP sheets, an in-situ loading test was performed. At that stage, the AE technique was again profitably used for the analysis of the cracking progression leading to FRP debonding. A numerical model of the structure is then proposed in the framework of the FE discretization with mechanical parameters estimated according to an inverse analysis on the monitored mechanical behavior of the structure before retrofitting. According to this model it is shown that, when the flexural inertia of the retrofitted beam is considerably higher than that of the unrepaired beam, snap-back instabilities can take place. Finally, considering the self-similarity between the acoustic emission phenomenon and seismicity, an analogy between the snap-back instability of the FRP delamination and that occurring during fault growth is proposed.

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Résumé Une étude de cas au sujet de modeler numérique et de surveiller in-situ d’un faisceau monté en rattrapage de RC avec la section transversale non-rectangulaire est présentée. Avant l’adaptation d’un faisceau, des techniques non destructives, telles que des essais à dégagement et à choc, ont été employées pour estimer les paramètres mécaniques du béton. En même temps, une surveillance à long terme avec la technique d’émission acoustique (AE) a été effectuée afin d’étudier sur des effets de fluage et des phénomènes de microfissuration. Puis, après un déplacement complet de la surcharge et l’adaptation ultérieure avec des feuilles de FRP, un essai in-situ de chargement a été réalisé. á cette étape, la technique d’AE a été encore profitablement employée pour l’analyse de la progression de fissures menant au décollement du FRP. On propose alors un modèle numérique de la structure dans le cadre de la discrétisation FE avec des paramètres mécaniques estimés selon une analyse inverse sur le comportement mécanique surveillé de la structure avant l’adaptation ultérieure. Selon ce modèle, on le démontre que, quand l’inertie flexural␣du faisceau monté en rattrapage est considérablement plus haute que cela de la structure non réparé, les instabilités de snap-back peuvent avoir lieu. En conclusion, vu l’auto similarité entre le phénomène d’émission et la séismicité acoustiques, on propose une analogie entre l’instabilité de snap-back du décollement de FRP et cela qui se produit pendant la croissance de défaut.

     

Behavior and performance of RC T-section deep beams externally strengthened in shear with CFRP sheets

A series of experimental tests were carried out to investigate the behavior and performance of reinforced concrete (RC) T-section deep beams strengthened in shear with CFRP sheets. Key variables evaluated in this study were strengthening length, fiber direction combination of CFRP sheets, and an anchorage using U-wrapped CFRP sheets. A total of 14 RC T-section deep beams were designed to be deficient in shear with a shear span-to-effective depth ratio (a/d) of 1.22. Crack patterns and behavior of the tested deep beams were observed during four-point loading tests. Except the CS-FL-HP specimen, almost all strengthened deep beams showed a shear–compression failure due to partial delamination of the CFRP sheets. From the load–displacement (p–u) curves, the effects of key variables on the shear performance of the strengthened deep beams were addressed. It was concluded from the test results that the key variables of strengthening length, fiber direction combination, and anchorage have significant influence on the shear performance of strengthened deep beams. In addition, a series of comparative studies between the present experimental data and theoretical results in accordance with the commonly applied design codes were made to evaluate the shear strength of a control beam and deep beams strengthened with CFRP sheets.     

Fracture-mechanics failure criteria for RC beams strengthened with FRP strips––a simplified approach

A simplified approximation approach for the evaluation of a fracture mechanics based criterion for the edge delamination failure of reinforced concrete beams strengthened with externally bonded composite materials is presented. The proposed approach is based on evaluation of the energy release rate (ERR) through the virtual crack extension method using various analytical and numerical stress analysis models. The investigated models include the high-order model, two types of “elastic foundation” or “springs” models, a simplified beam model, and finite elements analysis. The stress and displacement fields, the governing equations and their closed form solutions, and the expressions for the release rate of the total potential energy of the various models are presented. The proposed approach sets up the basis for an energetic failure criterion, in which the ERR is compared to the specific fracture energy of the bonded system. This criterion replaces the traditional allowable stress approach in describing the initiation and stable or unstable growth of the delamination crack. The capabilities of the proposed approach and its ability to evaluate the ERR through simplified and approximated models is investigated numerically. The accuracy of the simplified approach is numerically examined through comparison with the J-integral formulation. Numerical results in terms of stresses near the edge of the bonded strip, the ERR associated with initiation and growth of the interfacial crack, and the critical loads and crack lengths are presented. The paper closes with a summary and conclusions.     

Design equations for reinforced concrete members strengthened in shear with external FRP reinforcement formulated in an evolutionary multi-objective framework

Methods for predicting the shear capacity of FRP shear strengthened RC beams assume the traditional approach of superimposing the contribution of the FRP reinforcing to the contributions from the reinforcing steel and the concrete. These methods become the basis for most guides for the design of externally bonded FRP systems for strengthening concrete structures. The variations among them come from the way they account for the effect of basic shear design parameters on shear capacity. This paper presents a simple method for defining improved equations to calculate the shear capacity of reinforced concrete beams externally shear strengthened with FRP. For the first time, the equations are obtained in a multiobjective optimization framework solved by using genetic algorithms, resulting from considering simultaneously the experimental results of beams with and without FRP external reinforcement. The performance of the new proposed equations is compared to the predictions with some of the current shear design guidelines for strengthening concrete structures using FRPs. The proposed procedure is also reformulated as a constrained optimization problem to provide more conservative shear predictions.     

Fatigue lives of RC beams strengthened with CFRP at different temperatures under cyclic bending loads

Fatigue performance of reinforced concrete (RC) components strengthened with fibre reinforced polymer (FRP) is largely improved. However, temperature changes of service environment have a great effect on the fatigue behaviour of RC components strengthened with FRP. Concerning about temperature variations in subtropical areas such as South China, this paper analyses the fatigue behaviour of RC beams strengthened with carbon fibre laminate (CFL) from experimental studies and theoretical analysis under four different temperatures (5 °C, 20 °C, 50 °C, 80 °C) and five different stress levels (0.60, 0.66, 0.72, 0.78, 0.84). The paper discusses temperature fatigue behaviour of RC beams strengthened with CFL under cyclic bending loads in different service environments, and proposes a calculation formula of fatigue lives of RC beams strengthened with CFL under environmental temperatures and external forces coupling. Experimental results show that the formula not only effectively predicts the fatigue lives of the RC beams strengthened with CFL under environmental temperatures and bending loads coupling but also estimates the fatigue limits.     

Experimental study of moment redistribution in RC frames strengthened with CFRP sheets

The present paper describes an experimental investigation of moment redistribution of continuous beams in 6 two-span RC frames strengthened with CFRP laminates. Design variables were the number of CFRP layers, and the configuration of the laminates. To prevent debonding of the CFRP laminates at the end region and at the beam-column connection, U-shaped CFRP anchorages were provided for all of the frame specimens. Furthermore, mechanical anchorages of steel plate strips and bolts were added to the laminates in one frame of these specimens. Test results showed that a maximum moment redistribution of 56% occurred in the strengthened frames. Furthermore, the load carrying capacities of the strengthened frames increased from 20% to 38%, while the flexural capacities had an increase of 9% to 20% and 35% to 55% at the negative and positive moment regions, respectively.     

Effect of variations in the plate modulus of elasticity on the failure modes of FRP plated R.C. beams

In the absence of FRP plate/glue/concrete interface bond failure (i.e. interfacial debonding), eight possible flexural modes of failure are identified for reinforced concrete beams experiencing lateral loading, and strengthened in flexure with external FRP or steel plates glued to their soffits. All possible changes in such modes of failure, as a result of variations in the modulus of elasticity of the FRP material (assuming an associated constant value of ultimate tensile strength for the FRP plate in a given beam design), have been addressed in some detail, with a quantitative treatment of the critical values of the FRP modulus of elasticity associated with various failure mode transitions (i.e. changes).     

Cyclic responses of reinforced concrete composite columns strengthened in the plastic hinge region by HPFRC mortar

The brittleness of concrete raises several concerns due to the lack of strength and ductility in the plastic hinge region of reinforced concrete columns. In this study, in order to improve the seismic strength and performance of reinforced concrete columns, a new method of seismic strengthened reinforced concrete composite columns was attempted by applying High Performance Fiber Reinforced Cementitious composites (HPFRCs) instead of concrete locally in the plastic hinge region of the column. HPFRC has high-ductile tensile strains about 2–5% with sustaining the tensile stress after cracking and develops multiple micro-cracking behaviors. A series of column tests under cyclic lateral load combined with a constant axial load was carried out. Three specimens of reinforced concrete composite cantilever columns by applying the HPFRC instead of concrete locally in the column plastic hinge zone and one of a conventional reinforced concrete column were designed and manufactured. From the experiments, it was known that the developed HPFRC applied reinforced concrete columns not only improved cyclic lateral load and deformation capacities but also minimized bending and shear cracks in the flexural critical region of the reinforced concrete columns.     

Cohesive zone model for the prediction of interfacial shear stresses in a composite-plate RC beam with an intermediate flexural crack

In this paper, an analytical method is developed to predict the distribution of interfacial shear stresses in concrete beams strengthened by composite plates. Accurate predictions of such stresses are necessary when designing to prevent debonding induced by a central flexural crack in a FRP-plated reinforced concrete (RC) beam. In the present analysis, a new theoretical model based on the bi-linear cohesive zone model for intermediate crack-induced debonding is established, with the unique feature of unifying debonding initiation and growth. Adherent shear deformations have been included in the present theoretical analyses by assuming a parabolic shear stress through the thickness of the adherents, verifying the cubic variation of the longitudinal displacement function, whereas all existing solutions neglect this effect. The results obtained for interfacial shear stress distribution near the crack are compared to the Jialai Wang analytical model and the numerical solutions are based on finite element analysis. Parametric studies are carried out to demonstrate the effect of the mechanical properties and thickness variations of FRP, concrete and adhesive on interface debonding. Indeed, the softening zone size is considerably larger than that obtained by other models which neglect adherent shear deformations. However, loads at the limit of the softening and debonding stages are larger than those calculated without the thickness effect. Consequently, debonding at the interface becomes less apparent and the lifespan of our structure is greater.     

Shear capacity of reinforced concrete members strengthened in shear with FRP by using strut-and-tie models and genetic algorithms

Substantial research has been performed on the shear strengthening of reinforced concrete (RC) beams with externally bonded fibre reinforced polymers (FRP). However, referring to shear, many questions remain opened given the complexity of the failure mechanism of RC structures strengthened in shear with FRP. This paper is concerned with the development of a simple automatic procedure for predicting the shear capacity of RC beams shear strengthened with FRP. The proposed model is based on an extension of the strut-and-tie models used for the shear strength design of RC beams to the case of shear strengthened beams with FRP. By the formulation of an optimization problem solved by using genetic algorithms, the optimal configuration of the strut-and-tie mechanism of an FRP shear strengthened RC beam is determined. Furthermore, unlike the conventional truss approaches, in the optimal configuration, compressive struts are not enforced to be parallel, which represents more consistently the physical reality of the flow of forces. The proposed model is validated against experimental data collected from the existing literature and comparisons with predictions of some design proposals are also performed.     

A unified approach for the static and dynamic analyses of intermediate debonding in FRP-strengthened reinforced concrete beams

A spectral method is proposed for solving static and dynamic problems in FRP-strengthened reinforced concrete beams in a unified way. In order to appropriately simulate the debonding failure a mechanical model considering nonlinear stress–strain relationships for concrete and steel is used. The FRP-to-concrete interface is modelled using a realistic bilinear local bond-slip law. Numerical results with the proposed model for the interfacial shear stress distribution and the load–displacement response are derived for beams statically tested. Using the same spectral model the influence of interfacial delaminations on the dynamic characteristics of the structures is studied. The feasibility of the proposed method for performing dynamic analyses for high frequency excitations in a very simple and non-expensive way makes this study very useful in non-destructive testing analyses as a tool for diagnoses and detection of debonding in its initial stage by monitoring the change in dynamic characteristics.     

Web-bonded FRPs for relocation of plastic hinges away from the column face in exterior RC joints

In an RC building subjected to lateral loads, the beam–column joints constitute one of the critical regions. In existing frames, which were not adequately designed, a practical way of controlling plastic hinging and implement the strong-column weak-beam concept is through the use of a web-bonded FRP retrofitting system. This paper presents the results of an experimental and numerical study carried out in order to evaluate the ability of CFRP sheets in preventing the plastic hinge formation at the face of the column in exterior RC joints. Seven scaled-down RC exterior joints of a typical Ordinary Moment Resisting Frame are tested under moderately monotonic/cyclic loads. Two specimens are used as control while the other five are CFRP-strengthened/repaired of different lengths and thicknesses. The results show that carbon fibre sheets can effectively relocate the plastic hinge away from the face of the column. Non-linear numerical results using ANSYS are also presented and discussed.