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.     

Evaluation of corrosion activity in FRP repaired RC beams

This paper presents the results of an experimental study to evaluate the corrosion activity in reinforced concrete beams repaired with fibre reinforced polymer (FRP) sheets. Ten beam specimens (152 × 254 × 3200 mm) were constructed. One specimen was neither strengthened nor corroded to serve as a reference. Three specimens were corroded and not repaired. The remaining six beams were corroded and repaired with FRP sheets. The FRP sheets were applied after the main reinforcing bars were corroded to a 5.5% mass loss. Following the FRP repair, some specimens were subjected to further corrosion to investigate their post-repair performance. The corrosion activity was evaluated using non-destructive and destructive techniques. The non-destructive techniques included half-cell potential measurements. The destructive techniques included evaluation of the mass loss of the main reinforcing bars. The experimental results showed that the corrosion potential decreased with the progress of corrosion, and the FRP repair caused a higher rate of decrease in the corrosion potential with time than that observed when FRP was not provided. Results showed that mass loss of the main reinforcing bars due to corrosion was reduced by up to 16% because of FRP repair.     

Crack damage mitigation and shear behavior of shear-dominant reinforced concrete beams repaired with strain-hardening cement-based composite

This research aims to study: (1) the crack damage mitigation and shear behavior of reinforced concrete (RC) beams that have been repaired using strain-hardening cement-based composite (SHCC) via experimental testing and (2) the contribution of the SHCC layers to the shear strength of the repaired RC beams via predictions. Five cantilever RC beams with a shear span-to-depth ratio of 2.8 were subjected to cyclic concentrated loading. The study variables include two types of tensile performance of the SHCC (with low or high strength in tension) and two repair methods (patching and layering). The experimental results show that the use of a SHCC layer leads to a substantial increase in the shear strength and ductility of the RC beams after the peak load. During the tests, all of the SHCC repaired beams showed delamination along the interface between the concrete and SHCC, and the shear resistance started to drop. However, the results also indicate that SHCC layers can be effective repair material for enhancing the control of cracking to help protect the concrete from the migration of aggressive agents in severe environments. In order to predict the shear strength of RC beams that have been repaired with SHCC, two methods were used in this study; one is based on Dinh's proposed model that considers the shear strength in both the compression and tension zones, and the other method considers the shear strength of the reinforcement, such as a stirrup or fiber-reinforced polymer (FRP) laminate that considers only the tensile strength across cracks. These two methods were able to predict the contribution of the SHCC layer to the shear strength of the RC beams, and the predicted shear strength values were very similar between the two methods.     

Structural behaviour of RC beams with external flexural and flexural–shear strengthening by FRP sheets

This paper presents experimental research on reinforced concrete (RC) beams with external flexural and flexural–shear strengthening by fibre reinforced polymer (FRP) sheets consisting of carbon FRP (CFRP) and glass FRP (GFRP). The work carried out has examined both the flexural and flexural–shear strengthening capacities of retrofitted RC beams and has indicated how different strengthening arrangements of CFRP and GFRP sheets affect behaviour of the RC beams strengthened. Research output shows that the flexural–shear strengthening arrangement is much more effective than the flexural one in enhancing the stiffness, the ultimate strength and hardening behaviour of the RC beam. In addition theoretical calculations are developed to estimate the bending and shear capacities of the beams tested, which are compared with the corresponding experimental results.     

Improving shear capacity of RC T-beams using CFRP composites subjected to cyclic load

Various methods are developed for strengthening reinforced concrete beams against shear. Nowadays, external bonding of various composite members to RC beams was very popular and successfully technique internationally. This study present test results on strengthening of shear deficient RC beams by external bonding of carbon fiber reinforced polymer (CFRP) straps. Six RC beams with a T section were tested under cyclic loading in the experimental program. Width of the CFRP straps, arrangements of straps along the shear span, and anchorage technique that were applied at the ends of straps was the main parameters that were investigated during experimental study. Inclined CFRP straps were bonded along the shear spans of shear deficient beams for strengthening against shear by using epoxy. Arrangements and width of the inclined CFRP straps were the main parameters that were changed among the specimens. The test results confirmed that all CFRP arrangements improved the strength and stiffness of the specimens significantly. The failure mode, and ductility of specimens were proved to differ according to the CFRP strap width and arrangement along the beam. Experimental results were compared with the analytical approaches that were suggested by ACI-440 Committee report.     

Numerical evaluation of shear strengthening performance of CFRP sheets/strips and sprayed epoxy coating repair systems

A numerical study is conducted to evaluate the shear strengthening performance of two repair systems: CFRP sheets/strips and a sprayed epoxy coating. Micromechanical constitutive models for the CFRP sheets/strips and sprayed FRP coating proposed by Liang et al. [Liang Z, Lee HK, Suaris W. Micromechanics-based constitutive modeling for unidirectional laminated composites. Int J Solids Struct 2006;43:5674–89] and Lee et al. [Lee HK, Avila G, Montanez C. Numerical study on retrofit and strengthening performance of sprayed fiber-reinforced polymer. Eng Struct 2005;27:1476–87] and Lee and Simunovic [Lee HK, Simunovic S. Modeling of progressive damage in aligned and randomly oriented discontinuous fiber polymer matrix composites. Composites: Part B 2000;31:77–86] in conjunction with damage models, are implemented into the finite element code ABAQUS to solve boundary value problems. Using the implemented computational model, numerical simulations of four-point bending tests on concrete beams repaired with the repair systems are conducted to quantify their strengthening abilities. The numerical tests yield load–deflection curves from which the shear strengthening performance of the repair systems is evaluated. Furthermore, the present prediction is compared with available experimental data to assess the accuracy of the proposed computational model.     

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.     

Flexural behavior of reinforced concrete (RC) beams retrofitted with hybrid fiber reinforced polymers (FRPs) under sustaining loads

This paper reports experimental studies of reinforced concrete (RC) beams retrofitted with new hybrid fiber reinforced polymer (FRP) system consisting carbon FRP (CFRP) and glass FRP (GFRP). The objective of this study is to examine effect of hybrid FRPs on structural behavior of retrofitted RC beams and to investigate if different sequences of CFRP and GFRP sheets of the hybrid FRPs have influences on improvement of strengthening RC beams. Toward that goal, 14 RC beams are fabricated and retrofitted with hybrid FRPs having different combinations of CFRP and GFRP sheets. The beams are loaded with different magnitudes prior to retrofitting in order to investigate the effect of initial loading on the flexural behavior of the retrofitted beam. The main test variables are sequences of attaching hybrid FRP layers and magnitudes of preloads. Under loaded condition, beams are retrofitted with two or three layers of hybrid FRPs, then the load increases until the beams reach failure. Test results conclude that strengthening effects of hybrid FRPs on ductility and stiffness of RC beams depend on orders of FRP layers.     

Effectiveness of advanced composites in repairing heat-damaged RC columns

Thirteen rectangular RC column specimens, constructed at 1/3 scale, were tested under axial loading to investigate the use of advanced composites in repairing heat-induced damage. Eleven of the column specimens were subjected to elevated temperatures of 500 °C for 3 h. Nine heat-damaged columns were repaired using carbon fiber reinforced polymer (CFRP) sheets and plates. The effects of wrapping configuration, thickness of wrapping sheets, inclusion of plates as externally-bonded longitudinal reinforcement and the area of plates were examined using seven repair schemes. Test results confirmed that elevated temperatures adversely affect the axial load resistance and axial stiffness of the columns while increasing their toughness. Buckling under pure compressive loads was evident in heat-damaged columns except in those repaired using longitudinal CFRP plates. Partial wrapping with unidirectional CFRP sheets was found ineffective in augmenting the axial load capacity and stiffness of the damaged columns whereas full wrapping increased their axial load resistance and toughness. Using externally-bonded longitudinal CFRP plates, confined with circumferential wraps, significantly enhanced the initial axial stiffness and axial load resistance of the damaged columns. However, none of the seven repair schemes investigated in this study managed to regain the original axial stiffness and load resistance of the undamaged columns.     

Fatigue behavior of RC T-beams strengthened in shear with EB CFRP L-shaped laminates

The use of externally bonded carbon fiber-reinforced polymer (EB-CFRP) to strengthen deficient reinforced concrete (RC) beams has gained in popularity and has become a viable and cost-effective method. Fatigue behavior of RC beams strengthened with FRP is a complex issue due to the multiple variables that affect it (applied load range, frequency, number of cycles). Very few research studies have been conducted in shear under cyclic loading. The use of prefabricated CFRP L-shaped laminates (plates) for strengthening RC beams under static loading has proven to be technically feasible and very efficient. This study aimed to examine the fatigue performance of RC T-beams strengthened in shear for increased service load using prefabricated CFRP L-shaped laminates. The investigation involved six laboratory tests performed on full-size 4520 mm-long T-beams. The specimens were subjected to fatigue loading up to six million load cycles at a rate of 3 Hz. Two categories of specimens (unstrengthened and strengthened) and three different transverse-steel reinforcement ratios (Series S0, S1, and S3) were considered. Test results were compared with the upper fatigue limits specified by codes and standards. The specimens that did not fail in fatigue were then subjected to static loading up to failure. The test results confirmed the feasibility of using CFRP L-shaped laminates to extend the service life of RC T-beams subjected to fatigue loading. The overall response was characterized by an accelerated rate of damage accumulation during the early cycles, followed by a stable phase in which the rate slowed significantly. In addition, the strains in the stirrups decreased after the specimens were strengthened with CFRP, despite the higher applied fatigue loading. Moreover, the addition of L-shaped laminates enhanced the shear capacity of the specimens and changed the failure mode from brittle to ductile under static loading. Finally, the presence of transverse steel in strengthened beams resulted in a substantially reduced gain in shear resistance due to CFRP, confirming the existence of an interaction between the transverse steel and the CFRP.     

Effect of damaged concrete cover on the behavior of corroded concrete beams repaired with CFRP sheets

Fiber reinforced polymer matrix composites are being increasingly considered for use in civil infrastructure. These materials have found the maximum current use as materials for rapid and cost-effective rehabilitation (retrofit, repair and strengthening) of deteriorating and under-strength structural concrete components. This paper presents the experimental results of damaged/repaired reinforced concrete (RC) beams. The experimental program consisted of RC rectangular beam specimens exposed to accelerated corrosion. The corrosion rate was varied from 5% to 15% which represents loss in cross-sectional area of the steel reinforcement in the tension side. Half of the damaged beams were repaired by bonding Carbon Fiber Reinforced Polymer (CFRP) sheets to the tension side to restore the strength loss due to corrosion. The other half of the beams was first cleaned by removing spalled concrete cover and rusted bars were thoroughly cleaned. A new layer of concrete was cast to replace the removed spalled concrete cover. Then the CFRP sheets were attached to the new concrete layer. Corroded beams showed lower stiffness and strength than control (uncorroded) beams. Strength of damaged beams due to corrosion was restored to the undamaged state when repaired with CFRP sheets for all repaired beams, but the stiffness was almost unchanged. However, the repaired beams with replaced concrete cover layer resisted higher loads on average of 13% compared to damaged beams repaired without replacing the concrete cover layer.     

A fracture-based model for FRP debonding in strengthened beams

This paper presents an experimental and analytical research study aimed at understanding and modeling of debonding failures in fiber reinforced polymer (FRP) strengthened reinforced concrete (RC) beams. The experimental program investigated debonding failure modes and mechanisms in beams strengthened in shear and/or flexure and tested under monotonic loading. A newly developed fracture mechanics based model considers the global energy balance of the system and predicts the FRP debonding failure load by characterizing the dominant mechanisms of energy dissipation during debonding. Validation of the model is performed using experimental data from several independent research studies and a design procedure is outlined.     

Assessment of CFRP strengthened RC beams through dynamic tests

The use of Fiber Reinforced Polymers (FRPs) for strengthening damaged RC beams has become common practice over recent years. Two methods adopted for repairing or strengthening such beams are FRP plates/sheets glued onto their concrete surface and FRP rods or strips inserted into grooves.This paper investigates experimental vibration monitoring of strengthening according to the two aforementioned methods through dynamic tests on six RC beam models strengthened using carbon FRP. Three beams were strengthened applying CFRP sheets on the tensile cracked surface after loading and three beams were strengthened by near surface mounted (NSM) CFRP rods. The experimental results include both the static tests to create damage and the dynamic tests of strengthened, measuring natural vibration modes and frequency values for free end beams.Comparison between experimental dynamic response and static behavior established that vibration monitoring is a convenient, non-destructive method for assessing strengthened beams under service loads. Further studies and tests must be developed in order to solve the issues that emerged following the analysis of the experimental data obtained.     

FRP加固金属拉伸构件的性能分析

与传统的金属结构加固方法相比,粘贴纤维增强复合材料(FRP)加固具有明显的优势。FRP与金属构件之间的粘结应力会影响加固效果。根据平衡微分方程,对拉伸金属构件对称粘贴FRP加固后的复合构件进行应力分析,得到了剪应力沿复合构件厚度方向的分布,推导出FRP与金属构件之间的粘结应力,得到了FRP有效粘结长度的计算公式。通过碳纤维布双面加固钢板的静力拉伸试验,考察了碳纤维布与钢板之间的粘结应力分布,并对碳纤维布的有效粘结长度进行了分析。理论公式的计算结果与试验结果吻合较好,表明计算方法具有较高的精度。     

Shear creep of epoxy at the concrete–FRP interfaces

This paper presents the results of experimental and analytical investigations of the long-term behavior of the epoxy used at the concrete–FRP interfaces. Double shear long-term test was performed on specimens composed of concrete blocks bonded to FRP sheets using epoxy. Three test replicates were examined under sustained shear stress for up to six month time period with two primary parameters: the shear stress level and the epoxy thickness. The investigation showed that shear stress level might have significant effect on the long-term behavior of epoxy at the concrete–FRP interfaces. Based on the experimental results, creep characteristics of epoxy in the concrete–FRP interfaces were evaluated. Finite element analysis was performed incorporating these creep characteristics to investigate the long-term deformations and stress redistributions in the test specimens. It was recognized that creep of epoxy might result in stress-redistribution at the concrete–FRP interfaces.     

预应力FRP加固混凝土结构技术研究与应用

介绍了笔者进行的预应力芳纶纤维布和碳纤维筋加固混凝土结构的一些主要研究成果,内容包括:预应力芳纶纤维布永久锚具的开发;预应力芳纶纤维布的应力松弛损失研究;预应力芳纶纤维布加固混凝土梁的受弯、受剪性能研究;温度对芳纶纤维布配套粘结材料的力学性能影响研究;体外预应力碳纤维筋局部加固混凝土梁的力学性能研究;碳纤维筋预应力粗纤维混凝土梁的抗震性能研究;预应力纤维布加固混凝土结构的工程应用等。     

Stress–strain and deflection relationships of RC beam bonded with FRPs under sustained load

Fiber-reinforced polymer (FRP) systems that have a strong resistance against long-term deformation must provide improved serviceability to reinforced concrete (RC) members under sustained loads. Consequently, there is a need to develop a method for accurately predicting the time-dependent behavior of RC beams that are externally bonded with FRPs. However, there are very few previous studies that have been carried out or experimental results available, on the time-dependent behavior of RC beams externally bonded with FRP. In order to enable a reasonable prediction, correlations should first be clarified between the stress–strain relationship of the concrete, the reinforcement and the FRP that changes over time. By using these correlations, deflections under sustained loads should then be forecast. In this study, RC beams were fabricated for this purpose. Carbon reinforced polymer (CFRP) and glass reinforced polymer (GFRP) materials were bonded to the tension face of the two respective RC beams. The beams were then placed under sustained loads for 300 days. For the specimens that were externally bonded with FRPs and for the conventional specimen, the strain of the compression and tension reinforcement and the strain of FRP and deflection were measured respectively for comparison. In order to theoretically predict the time-dependent behavior of the RC Beam externally bonded with FRPs, creep coefficients for concrete and shrinkage strains were calculated by using the CEB-FIP and the ACI-209 Codes. For the method used to forecast the stress–strain relationships of the concrete, reinforcement and FRPs that change over time were theoretically clarified and were then compared with the experimental results. The deflection of the RC Beams externally bonded with FRP was predicted by using the ACI 318 Standard, EMM, AEMM, Branson’s method, and Mayer’s method. They were also compared to the experimental results. Subsequently, in the case of RC Beams externally bonded with FRPs under sustained loads, the proposed method proved that it is possible to accurately predict long-term deformations.     

U型FRP加固钢筋混凝土梁受剪剥离性能的有限元分析

采用FRP布对梁进行抗剪加固,可以有效的解决梁因配箍率不足而导致的受剪承载力偏低的问题。根据文献[1]中7根试验梁的参数,针对工程中常用的U型FRP受剪加固形式,建立三维有限元分析模型,采用商业有限元计算软件ANSYS,数值模拟了加载全过程和受剪剥离受力性能,根据试验结果确定了FRP-混凝土界面粘结剥离强度,并建议了合适的裂面剪力传递系数。根据有限元分析结果,作者又进一步研究了U型FRP布的应变分布、分担剪力的贡献、剥离破坏的过程,以及加固量、FRP类型和粘贴面积率对加固梁受剪承载力的影响。在有限元分析的基础上结合试验结果,建议了U型粘贴加固的受剪剥离承载力计算方法。     

The effect of surface preparation on the bond interface between FRP sheets and concrete members

Fiber-reinforced polymer (FRP) composite wraps have been established as an effective method for rehabilitation and strengthening of concrete structures. They are being increasingly used as an alternative to steel for reinforcing and strengthening of concrete structures. This paper presents the experimental and analytical results of the influence of concrete surface treatment and the type of FRP sheets on the bonding strength of concrete-FRP sheet. The FRP sheets were bonded to concrete beams in two opposite sides using an epoxy resin. Variables included the type of fiber (C1, C5, and GE) and the surface treatment (water jet and sanding). With changing the surface treatment of concrete surface preparation and the type of fiber sheets, the bonding strength, bonding load–strain response and failure modes were investigated. The concrete specimens with surface roughened with water jet showed much better bonding strength than those roughened with an ordinary sander. Equations for predicting the bond load failure of concrete specimens externally bonded with carbon and glass fiber sheets compared well with those of experimental.     

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.