FRP-plated RC Beams Research Articles

An investigation about debonding mechanisms in FRP-strengthened RC structural elements by using a cohesive/volumetric modeling technique

Several existing fracture modeling strategies for the debonding mechanism simulation in strengthened structural elements rely on simplified cohesive approaches for concrete/FRP interfaces thus neglecting the prediction of the complex concrete nonlinear behavior. In this work, a cohesive/volumetric modeling technique, based on a diffuse interface fracture approach, is proposed to in-depth investigate the debonding mechanisms in FRP-plated RC beams and subsequently validated through experimental results. This model is used in combination with elastoplastic truss elements for the reinforcing bars, for which a bond-slip behavior with surrounding concrete is admitted, and with additional cohesive elements along the adhesive/concrete and adhesive/plate material interfaces. Moreover, a novel hybrid continuation strategy has been proposed to capture potential snapback behaviors in the equilibrium path usually observed during the brittle failure of existing retrofitted structures. Failure simulations have been performed to assess the capabilities of the adopted fracture approach to predict the load-carrying capacity and the related debonding phenomena in real-scale strengthened RC elements, also providing detailed interfacial stress analyses in the FRP system. The reliability and the effectiveness of the proposed fracture approach have been demonstrated by means of suitable comparisons with available experimental results.

Effect of the plate bondstress-slip property on the flexural strength of FRP Plated RC beams using a displacement-based approach

This study presents the theoretical results of a displacement-based approach for CFRP plated RC beams while varying the bondstress-slip relationship of the plate to see the influence of this material property on the global behavior. The experimental programme included three ordinary and three plated RC beams that had similar dimensions and similar material properties. From the displacement-based analysis, it was shown that the theoretical and experimental results for the ordinary RC beams were almost identical. However, for the CFRP plated RC beams, it was shown that using a linear-descending bondstress-slip model for the FRP plate in the analysis underestimated the flexural capacity of the plated beam as the theoretical results were about 75% of the experimental values. Moreover, through a parametric study, it was shown that adding an ascending branch to the FRP plate’s bondstress-slip property (e.g., using a bilinear bondstress-slip model) results in better correlation with the experimental values at service loads in comparison to using only a linear-descending branch without an ascending component. In addition, it was also shown that adding a frictional component to the bondstress-slip model of the FRP plate to allow the force in the plate to build up as debonding ensues gives closer results to the experimental values observed in the laboratory. The accurate simulation of the FRP plate’s bondstress-slip material property was seen to greatly affect the plated member’s capacity and ductility behavior.

Flexural strength of FRP plated RC beams using a partial-interaction displacement-based approach

Designing FRP plated RC beams using full-interaction moment-curvature analysis would often suggest that the RC structure is brittle with debonding often occurring prior to yielding of the reinforcement steel. For that reason, researchers have looked into a displacement-based approach that takes into account the member debonding mechanism. The force in the plate within the debonded region was assumed to remain at the intermediate crack debonding force, PIC, and the ultimate strength was determined by considering compatibility of displacements along the member length. However, from laboratory testing, it is seen that the force in the externally bonded FRP plate keeps building up until failure occurs. Therefore, in this study, an extension to the displacement-based approach developed by previous researchers for FRP plated beams is presented where the residual bondstress of the plate within the debonded region is incorporated in the analysis. This is achieved by adopting a bond-slip model with a residual shear component that allows for the force in the plate to increase beyond PIC. The ultimate strength of the FRP plated beams is determined when the plate displacement matches that of the concrete near the plate end. A comparison with the experimental results of seven adhesively plated beams shows that incorporating the residual bondstress of the externally bonded plate yields significant improvement in accuracy and give better correlation with experimental findings.

A study of concrete cover separation failure in FRP-plated RC beams via an inter-element fracture approach

The phenomenon of concrete cover separation failure in FRP strengthened RC elements, unlike plate-end and mid-span debonding failure modes, has been not analyzed in depth in the literature and simplified strength models are mainly available, which possess a limited predictive capability. To this end in the present work, a novel numerical approach to investigate cover separation in FRP-plated RC beams is proposed, based on an inter-element cohesive fracture approach able to simulate multiple crack onset, propagation and coalescence in concrete structures, used in combination with an embedded truss model for taking into account the interaction between concrete cracks and tensile steel rebars. This approach has been validated by performing complete failure simulations for benchmark crack propagation examples, and subsequently has been exploited for predicting the load-carrying capacity and the related failure mode of real-scale retrofitted RC elements. Suitable comparisons with available experimental results have clearly shown the reliability and the effectiveness (in terms of numerical accuracy) of the proposed fracture approach.

Inclined FRP U-jackets for enhancing structural performance of FRP-plated RC beams suffering from IC debonding

Intermediate crack debonding (i.e., IC debonding) commonly controls the failure of FRP-plated RC beams. Such IC debonding often occurs with the maximum strain of the FRP soffit plate being far lower than its rupture strain, thus leading to a low utilization of the high strength of expensive FRP material. It is of increasing interest of the research and engineering community to explore an effective anchorage for enhancing the structural performance of FRP-plated RC beams suffering from IC debonding. Inclined FRP U-jacketing is among the easiest and most effective options for this purpose. This paper presents an experimental study to systematically investigate the effect of inclined FRP U-jacketing on the structural performance of FRP-plated RC beams suffering from IC debonding. Eight full-scale RC beams were tested with the width, height and inclination of the U-jacket as the experimental variables. Findings from the present tests show that inclined FRP U-jacketing could successfully lead to concrete crushing failure from IC debonding in the control specimen without anchorage, and significant increase in the load and mid-span deflection of up to 55.8% and 229% over these of the control specimen.

Effect of load distribution on IC debonding in FRP-strengthened RC beams: Full-scale experiments

This paper presents an experimental study into the effect of load distribution on intermediate crack (IC) debonding in FRP-strengthened RC beams, an issue which has received very limited attention so far. Five large-scale FRP-plated RC beams were tested in two series to investigate the effects of shear span and load uniformity on IC debonding respectively. The tests indicated that increasing the load uniformity and shortening the shear span both had a beneficial effect on the maximum moment in the beam (i.e., mid-span moment) at IC debonding failure, and they led to an increase in the mid-span moment by up to 18.5%. The test results were next used to assess the accuracy of two existing IC debonding strength models which are applicable to different load distributions. The assessment results show that one of the approaches is rather conservative while the other approach significantly over-predicts the effect of load distribution.

Concrete Cover Separation in FRP-Plated RC Beams: Mitigation Using FRP U-Jackets

AbstractConcrete cover separation is a common failure mode of RC beams strengthened with a fiber-reinforced polymer (FRP) plate externally bonded to the tension face (FRP-plated RC beams). This failure mode initiates at the critical plate end and then propagates at the level of steel tension reinforcement in the direction of increasing moment. Plate-end anchorage by FRP U-jackets has been specified in some design guidelines as a mitigation measure to delay or suppress concrete cover separation, although its effectiveness is far from clear. This mitigation method is more attractive than other options because the same strengthening material is used and the installation procedure is simple. This paper presents the first systematic experimental study on the use of FRP U-jackets for mitigating plate-end concrete cover separation failure, with particular attention to the effect of inclination angle between the U-jacket and the beam axis. A total of 10 full-scale FRP-plated RC beams were tested. The test results...

General cohesive zone model for prediction of interfacial stresses induced by intermediate flexural crack of FRP-plated RC beams

In this paper, a general model is developed to predict the distribution of interfacial shear and normal stresses induced by intermediate flexural crack of FRP-plated reinforced concrete beam. For that, 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. The stress deformation relationship is generally referred to as bond-slip law since the deformation of the interface is mainly the relative displacement (slip) between the FRP plate and the reinforced concrete beam (RC beam). Adherent shear deformations have been included 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. To obtain interfacial normal stress, the adhesive layer is assumed in linearly elastic stage in normal direction. This assumption is adopted in this study, because the FRP plate and RC beam are in contact during the whole debonding process. A complementary experimental study is made in order to show the debonding process according to mechanical and geometrical characteristics of adhesive and FRP plate. Analytical results obtained through the closed-form of interface stresses are in good agreement with those given by numerical and experimental models. Finally, parametric studies are carried out to demonstrate the effect of the mechanical properties and thickness variations of FRP, concrete and adhesive on interface debonding.

Creep response of intermediate flexural cracking behavior of reinforced concrete beam strengthened with an externally bonded FRP plate

In this study, a creep response model for the long-term behavior of interfacial shear stresses induced by intermediate flexural crack of FRP-plated RC beams is developed. A 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 with time increments. The creep behavior of the RC beam, adhesive layer and FRP plate have been included by considering the time-dependent mechanical properties. The time-dependent stress-deformation relationship caused by creep is referred to as bond-slip law. Consequently, a new interfacial law which combines time-dependent mechanical properties of all retrofitted beam constituents is proposed by considering constant the total energy during the creep response with intermediate crack. Obtained results are in good agreement with those given in literature. A parametric study is carried out to demonstrate the effect of the mechanical properties and thickness variations of FRP, concrete and adhesive on interface debonding. Indeed, the creep behavior, activates the debonding process in long term. Size of the softening zone is reached quickly and the bearing capacity of the retrofitted beam will be relatively affected by the time increase.

Flexural Strength and Ductility of FRP-Plated RC Beams: Fundamental Mechanics Incorporating Local and Global IC Debonding

AbstractReinforced concrete (RC) beams and slabs are frequently strengthened or stiffened in flexure by adhesively bonding fiber-reinforced polymer (FRP) plates to their surfaces using a strain-based moment-curvature design technique. This design technique is generally based on the intermediate crack (IC) debonding strain of the FRP reinforcement, that is, on the start of IC debonding; from this analysis it is often deduced that FRP plating is ineffective at the ultimate limit state because FRP debonding occurs before yield of the steel reinforcement. In this paper, it is shown that the strain-based approach is generally a lower bound at the ultimate limit state. Instead, a displacement-based approach is described that shows that FRP plated beams can be designed to achieve a higher strength than that of the RC beam by itself no matter when IC debonding first occurs. The mechanics of the analysis approach developed here treat the FRP debonded plate as a FRP prestressing tendon with a force equal to the IC ...

Tensile stress-strain relaxation as a failure precursor for FRP-strengthened RC beams

Abstract The paper presents findings from an experimental investigation and finite element (FE) analysis of RC beams strengthened in flexure and shear with externally bonded carbon FRP composites. The work initially focuses on the brittle plate-end concrete cover failures of RC beams strengthened in flexure only. The strain measurements reveal that the ’plate-end debonding’ is preceded by tensile relaxation in the anchorage zone of the FRP plate as a result of tension softening within the concrete cover layer. Experimental work then dealt with the role of the side-bonded ’U’-shaped FRP strips in preventing the plate-end concrete cover failure and in increasing shear capacity of flexurally strengthened concrete beams. For the two strengthening configurations, with and without FRP strips, FE models developed with ADINA and ABAQUS were used to predict brittle failure through concrete. The non-linear FE analysis based on the smeared-crack constitutive model for concrete in tension produced the load-history of principal tensile stresses in concrete at the plate-end. This load-history was then used to define a design criterion for predicting the safe load levels against the plate-end concrete failure for FRP-plated RC beams.

Identification of intermediate debonding damage in FRP-plated RC beams based on multi-objective particle swarm optimization without updated baseline model

Fiber reinforced polymer composites (FRP) have found widespread usage in the repair and strengthening of concrete structures. FRP composites exhibit high strength-to-weight ratio, corrosion resistance, and are convenient to use in repair applications. Externally bonded FRP flexural strengthening of concrete beams is the most extended application of this technique. A common cause of failure in such members is associated with intermediate crack-induced debonding (IC debonding) of the FRP substrate from the concrete in an abrupt manner. Continuous monitoring of the concrete–FRP interface is essential to prevent IC debonding. Objective condition assessment and performance evaluation are challenging activities since they require some type of monitoring to track the response over a period of time. In this paper, a multi-objective model updating method integrated in the context of structural health monitoring is demonstrated as promising technology for the safety and reliability of this kind of strengthening technique. The proposed method, solved by a multi-objective extension of the particle swarm optimization method, is based on strain measurements under controlled loading. The use of permanently installed fiber Bragg grating (FBG) sensors embedded into the FRP–concrete interface or bonded onto the FRP strip together with the proposed methodology results in an automated method able to operate in an unsupervised mode.

Simulation of concrete cover separation failure in FRP plated RC beams

The flexural strength of a reinforced concrete beam can be increased by bonding a FRP sheet to the tension face. Nevertheless, this type of reinforcement may cause a premature debonding failure. This paper is concerned with the failure by concrete cover separation; in other words by peeling-off. Four series of FRP plated RC beams and one group of control beams were tested in a four-point bending setup where each group consisted of three beams. All strengthened beams failed by peeling-off. As a part of this study numerical analyses using the commercial program; Abaqus have been carried out in order to predict this type of failure. Comparisons between the predictions of the numerical model and test results show a very good agreement indicating that the ultimate load, the beam behavior and the failure mode can be closely predicted by using the proposed model.

Positive and negative moment behaviours of hybrid members comprising cellular GFRP bridge decking epoxy-bonded to reinforced concrete beams

This paper reports tests to failure of hybrid members comprising cellular GFRP decking connected to RC beams via an epoxy adhesive. RC beams of low crack widths under service loads are useful with GFRP decking owing to the economy of and modest expertise needed to install such beams. Key novelties of this investigation include the RC beams (steel, FRP and prestressed concrete beams have been used previously) with the GFRP decking, use of the epoxy-bonded (so chemical, an alternative to mechanical) connection between the GFRP decking and the concrete beams, and study of the positive and negative moment responses (for application to continuous bridges) of this hybrid form. The test results show the adhesive to be a significant deck-to-beam shear connector under positive and negative moments. Whether the deck was a tensile or compressive top chord, the primary failure mode, which occurred at high loads, was delamination within the GFRP at the deck’s flange-to-web joints, and was neither failure of the GFRP-adhesive interface nor of the adhesive between deck units. For the positive moment specimen there was also fracture of the concrete near the deck as observed for FRP-plated RC beams, despite the absence of flexural cracks from this near-deck concrete.

Analytical analysis of interfacial stresses in FRP-RC hybrid beams with time-dependent deformations of RC beam

In this paper, the effect of time-dependent deformations (such as shrinkage and creep) on the interfacial stresses between an RC beam and FRP plate is presented. For this end, a closed-form solution for such stresses in externally FRP plated RC beams including creep and shrinkage effects is presented. The developed model is formulated to predict the interfacial stresses at time ‘t’, in which the RC beams have been already subjected to creep and shrinkage effects. The adherend shear deformations have been included in the present theoretical analysis by assuming a parabolic shear stress through the thickness of the RC beam and the FRP panel. Contrary to some existing studies, the assumption that both RC beam and FRP panel have the same curvature is not used in the present investigation. This research is helpful for the understanding on mechanical behavior of the interface and design of the FRP-RC hybrid structures.

Behaviour of FRP-to-concrete interfaces between two adjacent cracks: A numerical investigation on the effect of bondline damage

The bond behaviour between FRP and concrete has been commonly studied using simple pull-off tests on bonded joints where an FRP plate bonded to a concrete prism is pulled at one end to induce debonding failure. Knowledge gained from such studies has been directly employed in predicting debonding failure in FRP-plated concrete beams induced by major flexural cracks, but significant differences exist between the two scenarios. The chief difference lies in the interaction between adjacent flexural cracks in a flexural member which is absent in a joint pull-off test. This interaction may be approximated using an FRP-to-concrete bonded joint where the FRP plate is pulled at both ends. This paper presents a numerical study into this bonded joint problem, with the main objective being to clarify the effect of bondline damage during slip reversals on the ultimate load. The study shows that such damage has a significant effect on the predicted bond behaviour and ultimate load when the ratio between the end loads is larger than 0.7, particularly when the bond length is reasonably large. An important implication of the present study is that in the modelling (e.g. finite element modelling) of debonding behaviour of FRP-plated RC beams where multiple cracks exist, the FRP-to-concrete interface should be represented using a bond-slip model with appropriate consideration of the damaged behaviour during slip reversals in order to achieve accurate predictions.

Finite-Element Modeling of Intermediate Crack Debonding in FRP-Plated RC Beams

Intermediate crack-induced debonding (IC debonding) is a common failure mode of RC beams strengthened with externally bonded fiber-reinforced polymer (FRP) reinforcement. Although extensive research has been carried out on IC debonding, much work is still needed to develop a better understanding of the failure mode and a more reliable strength model. This paper presents an advanced finite-element (FE) model on the basis of the smeared-crack approach for predicting IC debonding failure. Existing FE models of the same type are generally deficient in capturing localized cracks (both their pattern and widths). This deficiency is overcome in the proposed FE model through the accurate modeling of interfaces between the concrete and both the internal steel and the external FRP reinforcements. The capability and accuracy of the proposed model are demonstrated through comparisons of its predictions with selected test results. The importance of accurate modeling of localized cracking is also explained using numeric...

Simulation of Plastic Hinges in FRP-Plated RC Beams

Structural engineers inherently rely on the ductility of a member to absorb energy, give prior warning of failure, control the drift of columns, and redistribute moments in beams. However, quantifying the ductility of reinforced concrete members has been a seemingly difficult or even intractable structural engineering problem mainly because of the softening characteristics of concrete. In this paper, a numerical model of the hinge region of a reinforced concrete beam bonded with fiber reinforced fiber-reinforced polymer (FRP) tension face plates is described, which simulates four mechanisms that contribute to the rotation of the member and, significantly, defines the limit to rotation imposed by concrete softening.

Plate end debonding in FRP-plated RC beams—II: Strength model

RC beams strengthened in flexure by bonding an FRP or steel plate to the tension face can fail by debonding at or near the plate end in a number of different modes. This paper presents a simple, rationally-based predictive model for such plate end debonding failures. In this model, pure flexural debonding for a plate end located in a pure bending region and pure shear debonding for a plate end located in a high-shear zero (or low)-moment region are first dealt with. The general case of a plate end under the combined action of shear and bending is treated as the interaction of these two extreme conditions. The proposed model is shown to be accurate through comparisons with available test results. The model relates the debonding failure load to the shear capacity of the beam and a number of well-defined parameters, and can be easily incorporated in any national or international design codes and guidelines.

Plate end debonding in FRP-plated RC beams—I: Experiments

RC beams strengthened with a tension-face FRP plate (i.e. FRP-plated RC beams) often fails by debonding in one of several possible modes. This paper presents an experimental study on plate end debonding failures in FRP-plated RC beams, which was conducted to develop a better understanding of the behaviour and failure mechanisms for the subsequent development of a predictive model. The results of ten four-point bending tests and eleven three-point bending tests conducted on simply-supported FRP- or steel-plated RC beams are presented and discussed in detail. The test beams cover a variety of significant geometric and material parameters over a wide range. In these beams, the critical plate end was subjected to either bending or shear only. Most of the beams failed by debonding in the form of the separation of the concrete cover from the steel tension reinforcement (i.e. cover separation failure), and for both types of plate ends, the debonding failure load depends strongly on the stiffness of the “composite plate” composed of the bonded plate and the concrete cover or the bonded plate alone. For plate ends subjected to a high shear force but a low moment, cover separation failure often occurs following local shear-crack induced interfacial debonding between the plate and the concrete near the plate end. The shear resistance of the beam contributed by the concrete alone provides a lower bound to the debonding failure load of such plate ends, which can be significantly increased by the presence of internal steel shear reinforcement.