A new ductility model of reinforced concrete beams strengthened using Fiber Reinforced Polymer reinforcement

Despite the superior performance of Fiber Reinforced Polymer (FRP) as compared with conventional steel bars in terms of high strength-to-weight ratio, corrosion resistance, and high fatigue performance, FRP strengthened beams exhibit lower ductility due to the linear elastic response of the FRP reinforcement. Several ductility and deformability models were developed in order to account for the elastic behavior, i.e. high elastic energy, of FRP when used for strengthening existing steel reinforced concrete or for new construction. In this paper, a new ductility expression that relates the deformability of a reinforced concrete (RC) structure strengthened using FRP to the energy dissipated, was developed. The developed expression also considers the type of loading, static or fatigue. The new expression was validated against experimental test results of RC beams strengthened using prestressed Near Surface Mounted (NSM) carbon FRP un-fatigued and fatigued beams. Furthermore, the ductility index at which the deformability of the structure equals the ratio of total energy dissipated to elastic energy, defined as the optimum ductility index, was investigated for both the un-fatigued and fatigued beams. The prestress strain corresponding to the optimum ductility index was found to be 2830 με (the strain value can be determined using an accepted arbitrary test such as monotonic test) while no optimum ductility was achieved for the case of the un-fatigued beams. It is noteworthy that the optimum ductility index is subject to the variability of design, beam geometry, and prestressing level. Therefore, the ductility evaluation of the NSM CFRP strengthened beams was meant to give only an insight into the problem and not to propose certain limits.     

Flexural behaviour of strengthened reinforced concrete beams

A large experimental research programme has investigated the flexural strength of simply supported reinforced concrete beams. The beams were first damaged so that they could be strengthened by means of jackets (cast-in-place shotcrete or pre-packed special mortar plus additional new reinforcement). This paper analyses the flexural strength of these beams. The behaviour in service and ultimate state as well as the bond characteristics are studied.

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Résumé Au cours d'un vaste programme expérimental, on a étudié la résistance en flexion de poutres en béton armé à appui simple. Ces poutres ont d'abord été endommagées, pour ensuite être renforcées par du béton projeté ou par du mortier spécial préconditionné, avec l'adjonction de nouvelles armatures. Cet article discute de la résistance en flexion de ces poutres. Le comportement en service et à l'état limite ultime, ainsi que les caractéristiques d'adhérence, sont étudiés.

     

Viscoelastic analysis of FRP strengthened reinforced concrete beams

External bonding of FRP plates or sheets has become a popular method for strengthening reinforced concrete structures. Stresses along the FRP-concrete interface are critical to the effectiveness of this technique because high stress concentration along the FRP-concrete interface can lead to the FRP debonding from the concrete beam. Although the short-term stress distribution along the FRP-concrete interface has been studied extensively, very few studies have been conducted on the long-term stress distribution, which closely simulates the behavior of the structure during the service-life. In this study, we develop a viscoelastic solution for the long-term interface stress distribution in a FRP plate strengthened reinforced concrete beam. In this solution, the RC beam and the FRP plate are modeled as elastic materials; while the adhesive layer is modeled as a viscoelastic material using the Standard Linear Solid model. Closed-form expressions of the interface stresses and deflection of the beam are obtained using Laplace transform and calculated using the Zakian’s numerical method. The validation of this viscoelastic solution is verified by finite element analysis using a subroutine UMAT based on the Standard Linear Solid model.     

Fiber volume fraction and ductility index of concrete beams

The mechanical response of fiber-reinforced concrete (FRC) beams depends on the amount of fibers, and the transition from brittle to ductile behavior in bending is related to a critical value of fiber volume fraction. Such quantity, which is mechanically equivalent to the minimum amount of steel rebars in reinforced concrete beams, can be defined according to the new approach proposed herein. It derives from the application of a general model and from the introduction of the so-called ductility index (DI). When FRC beams show a ductile behavior DI is positive, whereas DI is negative in the case of brittle response. Both the theoretical and experimental results prove the existence of a general linear relationship between DI and the fiber volume fraction. Accordingly, a new design-by-testing procedure can be used to determine the critical value of fiber volume fraction, which corresponds to a ductility index equal to zero.     

CFRP-PCPs复合筋嵌入加固钢筋混凝土梁裂缝试验及计算方法研究

通过5根嵌入不同张拉控制应力的碳纤维增强塑料预应力混凝土棱柱体(CFRP-PCPs)复合筋加固钢筋混凝土梁受弯试验,对比分析试验梁的裂缝分布与发展,得到最大裂缝宽度与平均裂缝宽度在静力荷载作用下的变化特性。结果表明: 嵌入CFRP-PCPs复合筋能有效的减少被加固钢筋混凝土梁的裂缝宽度和高度。在试验基础上,根据国家现行混凝土规范,对平均裂缝间距和最大裂缝宽度计算公式进行参数修正,建立了CFRP-PCPs复合筋嵌入加固钢筋混凝土梁最大裂缝宽度计算公式,计算值与试验值吻合较好。     

Performance of reinforced concrete beams strengthened by hybrid FRP laminates

In the last two decades, the use of advanced composite materials such as Fiber Reinforced Polymers (FRP) in strengthening reinforced concrete (RC) structural elements has been increasing. Research and design guidelines concluded that externally bonded FRP could increase the capacity of RC elements efficiently. However, the linear stress–strain characteristics of FRP up to failure and lack of yield plateau have a negative impact on the overall ductility of the strengthened RC elements. Use of hybrid FRP laminates, which consist of a combination of either carbon and glass fibers, or glass and aramid fibers, changes the behaviour of the material to a non-linear behaviour. This paper aims to study the performance of reinforced concrete beams strengthened by hybrid FRP laminates.

This paper presents an experimental program conducted to study the behaviour of RC beams strengthened with hybrid fiber reinforced polymer (HFRP) laminates. The program consists of a total of twelve T-beams with overall dimensions equal to 460 × 300 × 3250 mm. The beams were tested under cyclic loading up to failure to examine its flexural behaviour. Different reinforcement ratios, fiber directions, locations and combinations of carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer (GFRP) laminates were attached to the beams to determine the best strengthening scheme. Different percentages of steel reinforcement were also used. An analytical model based on the stress–strain characteristics of concrete, steel and FRP was adopted. Recommendations and design guidelines of RC beams strengthened by FRP and HFRP laminates are introduced.     

Sensitivity analysis of reinforced concrete beams strengthened with FRP laminates

Numerical procedures are proposed to predict the failure of reinforced concrete (RC) beams strengthened in flexure with fiber-reinforced polymeric (FRP) laminates. The framework of damage mechanics was used during the modeling. Numerical results were validated against experimental data obtained from 19 beams strengthened with different types of FRP. These beams failed by concrete crushing, cover failure and plate debonding. The numerical models were capable of predicting the experimentally observed load–deflection, failure load and failure modes. The sensitivity of the numerical results was studied. In particular, the effect of the concrete constitutive behavior and different modeling considerations was evaluated. It was found that the fracture energy of the concrete–repair interface plays a central part in predicting plate-debonding failures.     

Reinforced concrete beams strengthened with carbon fiber reinforced polymer by friction hybrid bond technique: Experimental investigation

Carbon fiber reinforced polymer (CFRP) can be used to strengthen the reinforced concrete (RC) beams. But premature debonding is the main failure model in ordinary bond technique, and the strengthening effect is limited. In order to improve bonding and restricting sliding displacement, Friction Hybrid Bonded FRP Technique (FHB-FRP) is developed. Six simple-span RC specimen beams with different strengthened methods were tested in four-point bending. The experiment results indicate that FRP debonding can be effectively prevented by the FHB-FRP strengthened beam. The ultimate load-carrying capacity of the specimen strengthened by FHB-FRP technique is able to increase by a factor of 2.13 times compared with the beam strengthened with ordinary bond technique (U-jacketing technique). In addition, the cracking and yielding loads are improved more significantly by FHB-FRP technique than U-jacketing technique. Specimens strengthened with FHB-FRP technique have cracks with a more limited distribution and width. Finally, the finite element method (FEM) is conducted to simulate the behavior of the test specimens. The results obtained from the finite element method are compared with experiment. Excellent agreements have been achieved in the comparison of results.     

Shear enhancement of reinforced concrete beams strengthened with FRP composite laminates

This paper presents the results of an experimental investigation on shear strength enhancement of reinforced concrete beams externally reinforced with fiber-reinforced polymer (FRP) composites. A total of nine full-scale beam specimens of three different classes, as-built (unstrengthened), repaired and retrofitted were tested in the experimental evaluation program. Three composite systems namely carbon/epoxy wet layup, E-glass/epoxy wet layup and carbon/epoxy precured strips were used for retrofit and repair evaluation. Experimental results indicated that the composite systems provided substantial increase in ultimate strength of repaired and strengthened beams as compared to the pre-cracked and as-built beam specimens. A comparative study of the experimental results with published analytical models, including the ACI 440 model, was also conducted in order to evaluate the different analytical models and identify the influencing factors on the shear behavior of FRP strengthened reinforced concrete beams. Comparison indicated that the shear span-to-depth ratio (a/d) is an important factor that actively controls the shear failure mode of beam and consequently influences on the shear strength enhancement.     

U形CFRP条带混锚加固混凝土梁抗剪试验

为解决纯粘贴U形纤维增强聚合物基复合材料（FRP）加固钢筋混凝土梁中FRP端部容易发生剥离破坏等问题,自主研发了对纤维布条带端部进行自锁锚固的方法和锚板,提出了端锚与粘贴并用的混锚U形条带抗剪加固方法。通过2根未加固梁、1根纯粘贴和2根混锚U形碳纤维增强聚合物基复合材料（CFRP）带抗剪加固梁的对比试验,证实了混锚抗剪加固的有效性：混锚能够对纤维带端部进行可靠锚固,阻止端部剥离破坏的发生,实现纤维拉断破坏,大幅度提高材料强度利用率。混锚加固在抑制混凝土梁斜裂缝开展、延缓箍筋屈服、提高箍筋和CFRP的极限应变以及提高抗剪承载力等多个方面的表现均明显优于纯粘贴加固。     

Shear strength of reinforced aerated concrete beams with shear reinforcement

The paper reports on the analysis of shear strength of reinforced beams made of autoclaved aerated concrete with shear reinforcement. The test data are taken from three different investigations from three countries, in Europe and Japan, and include 61 tests. The analysis of the test data results in regression expressions, suitably modified from a formula used for ordinary concrete members, and shows good agreement with test values. Appropriate expressions are suggested for design.

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Résumé Ce rapport présente l'analyse de la résistance au cisaillement de poutres en béton cellulaire armées (AAC), avec armatures d'effort tranchant. Les résultats d'essais sont pris parmi trois recherches différentes dans trois pays, en Europe et au Japon, et ils regroupent 61 essais. La charge dans la majorité des essais consistait en deux forces symétriques (voir Fig. 1). Dans quelques cas, il y avait une charge en ligne à mi-portée; dans d'autres cas, une charge en ligne était appliquée près d'un appui. Aucun essai n'a été exécuté avec une charge uniformément répartie. Une formule du type de l'équation 1 prenant en compte, les trois variables principales a été utilisée depuis longtemps dans le cas du béton armé sans armature d'effort tranchant. A partir de son développement théorique, cette formule a été adaptée par le passé au béton cellulaire d'ou l'équation 2. Pour les poutres avec armatures d'effort tranchant, l'équation 3 représente l'addition de la résistance au cisaillement due au béton à celle due aux armatures d'effort tranchant. Les résultats d'essais sont présentés dans le Tableau 2, y compris les résistances au cisaillement prédites à partir de l'équation 3 avec les coefficients de régression calculés en utilisant le programme SYSTAT sur un ordinateur personnel. Les résultats des régressions sont donnés dans le Tableau 4 et l'expression finale dans l'équation 4. Dans l'optique du dimensionnement, des valeurs minimales de la résistance ultime au cisaillement avec un seuil de confiance de 90% sont présentées dans l'équation 5. L'analyse rapportée dans cet article fournit des expressions fiables pour la prédiction de la résistance au cisaillement de poutres en béton cellulaire armées avec armatures d'effort tranchant.

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Research progress on the fatigue performance of RC beams strengthened in flexure using Fiber Reinforced Polymers

A comprehensive literature review on the fatigue performance of Reinforced Concrete (RC) beams strengthened using Fiber Reinforced Polymers (FRP) reinforcement is presented in this paper. Fatigue behavior of RC beams strengthened using FRP is a complicated topic due to the involvement of many variables such as, load limits, testing frequency, number of cycles, and concrete composition. Thus, a proper understanding of the fundamental fatigue characteristics of concrete, steel, and FRP materials would allow better understanding of the overall composite behavior when subjected to fatigue loading. In this paper, the fatigue properties of the constituent materials are reviewed. The recent development and the research progress are also presented and categorized based on the rationale of fatigue load application. The effects of pre-damage and environmental exposure on the fatigue behavior are also discussed. Recommendations for future research are presented.     

Behavior of CFRPC strengthened reinforced concrete beams with varying degrees of strengthening

This paper summarizes the results of experimental and analytical studies on the flexural strengthening of reinforced concrete beams by the external bonding of high-strength, light-weight carbon fiber reinforced polymer composite (CFRPC) laminates to the tension face of the beam. Four sets of beams, three with different amounts of CFRPC reinforcement by changing the width of CFRPC laminate, and one without CFRPC were tested in four-point bending over a span of 900 mm. The tests were carried out under displacement control. At least one beam in a set was extensively instrumented to monitor strains and deflections over the entire range of loading till the failure of the beam. The increase in strength and stiffness provided by the bonded laminate was assessed by varying the width of laminate. The results indicate that the flexural strength of beams was significantly increased as the width of laminate increased. Theoretical analysis using a computer program based on strain compatibility is presented to predict the ultimate strength and moment–deflection behavior of the beams. The comparison of the experimental results with theoretical values is also presented, along with an investigation of the beam failure modes.     

Flexural response predictions of reinforced concrete beams strengthened with prestressed CFRP plates

Existing experimental studies showed that the reinforced concrete (RC) beams strengthened with prestressed carbon fiber-reinforced polymer (CFRP) plates had three possible flexural failure modes (including the compression failure, tension failure and debonding failure) according to the CFRP reinforcement ratio. Theoretical formulas based on the compatibility of strains and equilibrium of forces were presented to predict the nominal flexural strength of strengthened beams under the three failure modes, respectively, and a limitation on the tensile strain level developed in the prestressed CFRP plate was proposed as the debonding failure occurred. In addition, the calculation methods for cracking moment, crack width and deflection of strengthened beams were provided with taking into account the contribution of prestressed CFRP plates. Experimental studies on five RC beams strengthened with prestressed CFRP plates and a nonlinear finite element parametric analysis were carried out to verify the proposed theoretical formulas. The available test results conducted by other researchers were also compared with the predicted values.     

Nonlinear finite element analysis of reinforced concrete beams strengthened by fiber-reinforced plastics

Numerical analyses are performed using the ABAQUS finite element program to predict the ultimate loading capacity of rectangular reinforced concrete beams strengthened by fiber-reinforced plastics applied at the bottom or on both sides of these beams. Nonlinear material behavior, as it relates to steel reinforcing bars, plain concrete, and fiber-reinforced plastics is simulated using appropriate constitutive models. The influences of fiber orientation, beam length and reinforcement ratios on the ultimate strength of the beams are investigated. It has been shown that the use of fiber-reinforced plastics can significantly increase the stiffnesses as well as the ultimate strengths of reinforced concrete beams. In addition, with the same fiber-reinforced plastics layer numbers, the ultimate strengths of beams strengthened by fiber-reinforced plastics at the bottom of the beams are much higher than those strengthened by fiber-reinforced plastics on both sides of the beams.     

Prestressed fiber reinforced concrete beams with reduced ratios of shear reinforcement

This paper presents an analysis of the influence of prestress and fibers on the shear behaviour of thin-walled I-section beams with reduced shear reinforcement ratio. Reduction of shear reinforcement in prestressed precast beams can make the reinforcement simpler and may increase the productivity in long line precasting beds. The use of short fibers can improve the shear strength and ductility. Nine concrete beams were built (six with prestressing forces) with three different mixtures: without fibers, with steel fibers, and with polypropylene fibers. Shear reinforcement ratios varied from 0 to 0.225% (geometric ratio). It was noted that prestressing increases cracking strength (both in bending and shear), extends the non-cracked area, and makes the compression struts less inclined. In the case of fiber reinforced concrete beams, control of cracking is more effective and consequently deflections are smaller. Ductility is also increased. Both fibers and prestressing reduce stresses in the stirrups and increase shear strength.     

Behaviour of reinforced concrete rectangular columns strengthened using GFRP

Rectangular columns are often used in bridge pier design, and they make up the majority of building columns. Columns in need of strengthening and retrofit are very common. This paper presents results of a comprehensive experimental investigation on the behavior of axially loaded rectangular columns that have been strengthened with glass fibre reinforced polymer (GFRP) wrap. This paper is intended to examine several aspects related to the use of glass FRP fabrics for strengthening rectangular columns subjected to axial compression. The objectives of the study are as follows: (1) to evaluate the effectiveness of external GFRP strengthening for rectangular Concrete Columns (2) to evaluate the effect of number of GFRP layers on the ultimate load and ductility of confined concrete and (3) to evaluate the effect of the aspect ratio of the column on the effectively confined cross-section. To cover a wide range of cross-sectional dimension ratios, three aspect ratios (a/b, where a and b are, respectively, the longer and shorter sides of the cross-section) were studied: a/b = 1.0, a/b = 1.25, and a/b = 1.66. Specimens with zero, one, and two layers of GFRP wrap were investigated. Totally nine specimens were subjected to axial compression which includes three control specimens. All the test specimens were loaded to failure in axial compression and the behavior of the specimens in the axial and transverse directions was investigated.     

Experimental and analytical investigation of reinforced high strength concrete continuous beams strengthened with fiber reinforced polymer

Carbon and glass fiber reinforced polymer (CFRP and GFRP) are two materials suitable for strengthening the reinforced concrete (RC) beams. Although many in situ RC beams are of continuous constructions, there has been very limited research on the behavior of such beams with externally applied FRP laminate. In addition, most design guidelines were developed for simply supported beams with external FRP laminates. This paper presents an experimental program conducted to study the flexural behavior and redistribution in moment of reinforced high strength concrete (RHSC) continuous beams strengthened with CFRP and GFRP sheets. Test results showed that with increasing the number of CFRP sheet layers, the ultimate strength increases, while the ductility, moment redistribution, and ultimate strain of CFRP sheet decrease. Also, by using the GFRP sheet in strengthening the continuous beam reduced loss in ductility and moment redistribution but it did not significantly increase ultimate strength of beam. The moment enhancement ratio of the strengthened continuous beams was significantly higher than the ultimate load enhancement ratio in the same beam. An analytical model for moment–curvature and load capacity are developed and used for the tested continuous beams in current and other similar studies. The stress–strain curves of concrete, steel and FRP were considered as integrity model. Stress–strain model of concrete is extended from Oztekin et al.’s model by modifying the ultimate strain. Also, new parameters of equivalent stress block are obtained for flexural calculation of RHSC beams. Good agreement between experiment and prediction values is achieved.     

Assessment of adhesive setting time in reinforced concrete beams strengthened with carbon fibre reinforced polymer laminates

The strengthened effectiveness and the performance capacity of repaired Reinforced Concrete (RC) structures with Carbon Fibre Reinforced Polymer (CFRP) sheets is dependent on the properties of the adhesive interface layer. Adhesive material requires a specific setting time to achieve the maximum design capacity. Adhesive producer provides technical data which demonstrates the increase with time of the capacity, up to the maximum. The aim of this study is to investigate the effect of the adhesive setting time on the modal parameters as an indication of the effectiveness of CFRP on repaired RC beams. Firstly, datum modal parameters were determined on the undamaged beam and subsequently the parameters were obtained when damaged was induced on the RC beam by application of load until the appearance of the first crack. Finally, the RC beam is repaired with externally bonded CFRP sheets, and modal parameters are once again applied after 0.5, 1, 2, 3, 5, 8, 11, 15 and 18 days. The comparison is made with the data based on half day results in order to monitor the change in the modal parameters corresponding to the adhesive setting time. The modal parameters where used as indicators for the effectiveness of CFRP are affected by the adhesive time as shown in this study. Results are compared with the adhesive technical data provided by the adhesive producer.