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.     

高强钢绞线网——聚合物砂浆加固混凝土及预应力混凝土梁的抗弯性能试验研究

该文侧重研究了采用高强钢绞线网——聚合物砂浆加固技术三面加固普通钢筋混凝土(RC)梁和预应力混凝土(PRC)梁的抗弯性能.进行了6 片RC 梁及5 片PRC 梁的抗弯静载试验,探讨了不同初始损伤程度、有效预应力大小和加载方式等对高强钢绞线网——渗透性聚合物砂浆加固梁的抗弯性能影响.结果表明:采用高强钢绞线网——聚合物砂浆加固技术加固RC/PRC梁能够有效抑制裂缝的开展,能有效地提高RC/PRC 加固梁的抗弯承载能力和抗弯刚度;重复加载会导致加固梁刚度一定程度的退化,初始损伤程度大小不会显著改变加固后RC/PRC梁的抗弯承载力;该文研究对于大量既有RC/PRC梁的抗弯加固具有一定的参考价值.     

碳纤维布加固已承受荷载的钢筋混凝土梁抗弯性能试验研究及抗弯承载力计算

进行了6根碳纤维布加固已承受荷载的钢筋混凝土梁和2根对比混凝土梁的抗弯性能试验研究,分析了碳纤维布加固已承受荷载的钢筋混凝土梁的破坏机理,研究了荷载历史对加固梁极限荷载的影响。试验结果表明,粘贴碳纤维布可以有效地提高加固梁的抗弯承载能力。无论荷载历史如何,只要梁承受的初始荷载相同,梁破坏时的极限荷载基本相同。梁端锚固对加固梁的极限荷载影响不明显。根据不同的破坏模式,提出了碳纤维布加固已承受荷载的钢筋混凝土梁的承载力计算方法,给出了工程实用计算公式。     

二次受力下自密实混凝土加固RC梁受弯性能研究

为模拟实际工程中加固构件的真实承载能力,实验在原混凝土构件持续受荷状态下,采用自密实混凝土对构件进行加固、养护。共进行了7根二次受力下自密实混凝土加固钢筋混凝土梁和2根对比梁的受弯性能试验,研究了不同初始受力水平、不同加固厚度及不同界面处理方式对加固钢筋混凝土梁抗弯承载力和截面刚度的影响。试验量测了构件裂缝分布形态、荷载-挠度曲线、钢筋应变发展规律等。结果表明:采用自密实混凝土加固钢筋混凝土梁,能有效地提高钢筋混凝土梁的抗弯承载力、截面刚度等性能;二次受力下自密实混凝土加固梁抗弯承载力随着初始受力水平的增大而降低。在试验结果的基础上,基于平截面假定,提出了二次受力下自密实混凝土加固梁钢筋滞后应变及抗弯承载力计算式,计算结果与试验结果吻合良好。     

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.     

Flexural retrofitting of RC buildings using GFRP/CFRP – A comparative study

The effectiveness of fibre reinforced polymers (FRPs) in retrofitting/repairing of the reinforced concrete (RC) components has been studied in the past to great detail. However, the seismic performance of RC structures retrofitted using FRP composites is yet to be scrutinised in terms of lateral resistance, ductility, and failure mechanism. This is of high importance if the retrofitted structures are to withstand higher seismic ground motions than they were designed for and/or pulse-type ground motions. In a comparative study, this paper reports on the results of an investigation into the flexural strengthening of RC buildings using glass/carbon fibre reinforced polymers (GFRP/CFRP). An 8-storey code-compliant RC building was considered as the case study to represent the medium-rise structures. With a slight intervention in the lateral displacement ductility and provision of the weak-beam strong-column design philosophy, the strengthening design strategy is aimed at increasing the lateral resistance. For this purpose, composite sheets are designed to be applied at the two end regions of all beams and columns on a practical flange-bonded scheme. The nonlinear pushover analysis with lumped plasticity approach was implemented in order to compare the seismic response of the original structure with the GFRP/CFRP retrofitted structures. Following validation of the adopted models, the force–deformation curves of the nonlinear plastic hinges are determined in a rigorous approach considering the material inelastic behaviour, reinforcement details, and dimensions of the members. While the nonlinear results confirm a significant increase in the lateral load carrying capacity using both composite materials, the CFRP improvement was as much as twice of the GFRP. However, the latter provides higher ductility.     

模拟不中断交通状况下粘贴碳纤维布加固钢筋混凝土梁试验研究

为了调查动荷载作用下碳纤维布与钢筋混凝土梁的粘贴性能及加固效果,进行了5根模拟交通荷载(等幅正弦波形动荷载)作用下粘贴碳纤维布加固钢筋混凝土梁和1根保持荷载下粘贴碳纤维布加固钢筋混凝土梁以及2根对比梁的试验研究。试验中考虑了混凝土等级、配筋率、有无锚固条、粘贴长度、荷载幅值5个变化参数。试验结果表明,在模拟交通荷载的作用下,碳纤维布加固的钢筋混凝土梁粘贴性能满足要求,粘贴效果良好。在模拟交通荷载后的静载作用下,试验梁的抗弯承载力提高较多,加固效果明显,进一步验证了粘贴效果。     

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.     

RC beams strengthened with composite material: a limit analysis approach and experimental study

This paper deals with the flexural strengthening of reinforced concrete beams by means of thin carbon fibre reinforced plastic (CFRP) plates. A simplified laminated plate model is used to describe the behaviour of three-layered plate in cylindrical bending which were subjected to third-point line loads. The upper bound theorem of limit analysis is used to approximate the ultimate load capacity for multi-layered plates and identify different collapse mechanisms. A reinforced concrete beam strengthened by CFRP is designed as a three-layered plate. Experimental results are obtained and a comparison with theoretical predictions made.     

Analytical study on RC beams strengthened for flexure with externally bonded FRP reinforcement

This paper presents analytical study on reinforced concrete (RC) beams strengthened for flexure with externally bonded fiber reinforced polymer (FRP) reinforcement. A simple yet rational numerical model is developed and proposed for this purpose. The model is based on cross-sectional analysis satisfying strain compatibility and equilibrium conditions. The moment–curvature relationship can be generated for an RC beam section using an incremental strain technique. The model can also generate the load–deflection relationship of the beam with respect to its configuration, loading system, and preloading conditions. The model can predict the flexural capacity of FRP-strengthened section based on full composite action and IC debonding failure modes. This also allows for designing the FRP strengthening area according to the desired failure mode. Various IC debonding criteria were adopted in the model and compared with test results from the literature. The result of comparison indicated that the accuracy of the model is dependent on the adopted IC debonding criterion. Furthermore, the model was verified against test data related to full composite action failure mode and good agreement was found.     

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。     

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.     

免拆UHPC模板RC梁受弯性能试验及承载力分析

对4根免拆超高性能混凝土（UHPC）模板钢筋混凝土（RC）梁和2根RC梁进行了受弯性能试验,试件变化参数为配筋率和保护层厚度,重点研究免拆UHPC模板RC梁的受力性能以及模板与后浇混凝土的剥离情况。结果表明:从开始加载到峰值荷载点,免拆UHPC模板与后浇混凝土界面没有发生任何滑移及剥离;峰值荷载后至构件破坏时,预制UHPC模板与后浇混凝土界面出现轻微剥离;免拆UHPC模板RC梁的开裂荷载较普通RC梁提高了近50%,屈服荷载、极限荷载提高约为10%。基于截面平衡条件、平截面假定以及UHPC、混凝土、钢筋的本构关系,建立了免拆UHPC模板RC梁的受弯承载力计算公式,公式计算值与试验值吻合较好。     

Optimizing the performance characteristics of beams strengthened with bonded CFRP laminates

Ductility is of fundamental importance in the design of concrete structures. With structures using conventional materials such as concrete and steel, ductility of a member as a whole can be satisfactorily defined in terms of deflection, curvature or energy absorption capacity as examplified by the area under the load-deflection curve. However, when structural members strengthened with externally bonded fibre reinforced polymer (FRP) laminates are considered, conventional definitions of ductility become less precise because of the brittle behaviour of FRP materials, and their resultant effect on the performance of the strengthened beam. Furthermore, such beams when stregthened without the provision of external anchorages will fail very suddenly, with abrupt debonding of the laminate and substantial loss of load capacity. This paper intends to propose a new criterion to evaluate the structural performance of such strengthened composite beams, and the efficiency of the external anchorage system. The design criterion termed, Peformance Factor, incorporates both the deformability and strength of composite beams. Unlike the concept of toughness as applied to materials, the Performance Factor incorporates the effect of numerous parameters which influence structural design. To examine the reliability of this parameter a series of eleven reinforced concrete beams were tested to evaluate the structural performance of beams strengthened with and without externally bonded carbon fibre reinforced polymer (CFRP) laminates, and with different types of internal reinforcement and external anchorage systems. The structural behaviour of these beams was then evaluated using the Performance Factor.     

智能CFRP加固RC梁荷载效应的实时模拟与测评

制备了4 根炭纤维复合材料(CFRP) 加固钢筋混凝土(RC) 实验梁, 并在梁内钢筋、混凝土及加固CFRP中预置了布拉格光栅光纤传感器(FBG) 和电阻应变片两种传感器。根据钢筋混凝土理论和ANSYS 有限元软件编制了实验梁受弯荷载效应模拟计算程序。实验表明, 实验梁在受弯承载过程中, 布拉格光栅光纤传感器与传统应变片有完全一致的线性关系; 模拟计算出的实验梁受拉钢筋、压区混凝土应变值及挠度与荷载的关系与CFRP 中FBG的实测值吻合较好。由于对既成RC 结构不能在内部装置传感器(会破坏结构降低抗力) , 采用智能CFRP 加固RC 结构可实现加固和实时健康测评双重功能。      

Experimental tests of RC beams strengthened with composite materials using IPN water-based resins

Composite materials have been used for both the structural strengthening of single RC elements and the complete retrofit of buildings for over 20 years. The composites mainly used are based on carbon, aramid or glass fibres, laminated with epoxy resins matrices. As known, one of the limitations of epoxy resins is the poor fire resistance of the matrix, and this can often influence how composite materials are used in building construction. In this research, an experimental investigation was carried out on reinforced concrete beams strengthened with unidirectional carbon fibre reinforcements and two-component resins of the Interpenetrated Polymer Network (IPN) water-based type that have greater thermal resistance than epoxy resins. In the experimental campaign, several beam specimens were reinforced using different strengthening configurations for bending and shear. In particular, the results obtained for the beams strengthened using water-based resins were compared with those for equivalent specimens strengthened with epoxy resins. The results showed good mechanical properties for the reinforcements with the IPN matrix, where the values of the collapse load were similar to those of the reinforcements with epoxy resins.     

Failure diagrams of FRP strengthened RC beams

Amongst various methods developed for strengthening and rehabilitation of reinforced concrete (RC) beams, external bonding of fibre reinforced plastic (FRP) strips to the beam has been widely accepted as an effective and convenient method. The experimental research on FRP strengthened RC beams has shown five most common modes, including (i) rupture of FRP strips; (ii) compression failure after yielding of steel; (iii) compression failure before yielding of steel; (iv) delamination of FRP strips due to crack; and (v) concrete cover separation. In this paper, a failure diagram is established to show the relationship and the transfer tendency among different failure modes for RC beams strengthened with FRP strips, and how failure modes change with FRP thickness and the distance from the end of FRP strips to the support. The idea behind the failure diagram is that the failure mode associated with the lowest strain in FRP or concrete by comparison is mostly likely to occur. The predictions based on the present failure diagram are compared to 33 experimental data from the literature and good agreement on failure mode and ultimate load has been obtained. Some discussion and recommendation for practical design are given.     

预应力FRP加固工程结构技术研究进展

围绕国内外预应力FRP加固技术的研究现状以及最新进展,从预应力FRP加固混凝土结构、预应力FRP加固钢结构、预应力FRP加固中关键技术的研究等方面进行了综述。试验研究表明：预应力FRP加固混凝土能显著提高构件的开裂荷载、屈服荷载和极限荷载,改善受弯构件在长期荷载的力学性能,提高构件的疲劳寿命;预应力CFRP加固钢梁后,其屈服荷载和极限荷载相对于对比梁都有明显的提高,其提高的程度随着预应力CFRP的用量和预应力水平的提高而增大;预应力CFRP加固对钢梁的刚度提高作用也比较明显,对低强度的钢材,提高效果更明显;采用预应力FRP加固工程结构的关键问题在于预应力的施加体系、预应力控制值、预应力损失和端部的锚固。     

NSM shear strengthening technique with CFRP laminates applied in high-strength concrete beams with or without pre-cracking

The effectiveness of the near surface mounted (NSM) shear strengthening technique with carbon fiber reinforced polymer (CFRP) laminates applied in high-strength concrete beams with a certain percentage of existing steel stirrups is assessed by experimental research. In this context, the influence of the following main parameters are investigated: (i) the percentage and the inclination of the CFRP laminates; (ii) the percentage of existing steel stirrups; (iii) the existence of cracks when the reinforced concrete (RC) beams are shear strengthened with NSM CFRP laminates. The results showed that the NSM shear strengthening technique with CFRP laminates is more effective when applied to RC beams of high-strength concrete, not only in terms of increasing the load carrying capacity of the beams, but also in assuring higher mobilization of the tensile properties of the CFRP. Inclined laminates were more effective than vertical laminates and the shear resistance of the beams has increased with the percentage of laminates. Pre-cracked RC beams strengthened with NSM CFRP laminates have presented a load carrying capacity similar to that of the homologous uncracked strengthened beams.     

Time-dependent behavior of RC beams strengthened with externally bonded FRP plates: interfacial stresses analysis

External bonding of fibre reinforced polymer (FRP) composites has becomes a popular technique for strengthening concrete structures all over the world. An important failure mode of such strengthened members is the debonding of the FRP plate from the concrete due to high interfacial stresses near the plate ends. For correctly installed FRP plate, failure will occur within the concrete. Accurate predictions of the interfacial stresses are prerequisite for designing against debonding failures. In particular, the interfacial stresses between a beam and soffit plate within the linear elastic range have been addressed by numerous analytical investigations. In this study, the time-dependent behavior of RC beams bonded with thin composite plate was investigated theoretically by including the effect of the adherend shear deformations. The time effects considered here are those that arise from shrinkage and creep deformations of the concrete. This paper presents an analytical model for the interfacial stresses between RC beam and a thin FRP plate bonded to its soffit. The influence of creep and shrinkage effect relative to the time of the casting and the time of the loading of the beams is taken into account. Numerical results from the present analysis are presented to illustrate the significance of time-dependent of adhesive stresses.