RC beams shear-strengthened with FRP: Stress distributions in the FRP reinforcement

Reinforced concrete (RC) beams may be strengthened for shear with externally bonded fibre reinforced polymer (FRP) composites through complete wrapping, U-jacketing or bonding on their sides only. The two main shear failure modes of such strengthened beams are FRP rupture and debonding. In both modes of failure, the contribution of the bonded FRP reinforcement to the shear capacity of the beam depends strongly on the stress (or strain) distribution in the FRP at the ultimate limit state. This paper presents a numerical study of the FRP stress distribution at debonding failure in U-jacketed or side-bonded beams using a rigorous FRP-to-concrete bond–slip model and assuming several different crack width distributions. Numerical results indicate that Chen and Teng’s early simple assumption [Chen JF, Teng JG. Shear capacity of FRP-strengthened RC beams: FRP debonding. Constr Build Mater 2003;17:27–41] for the stress distribution in the FRP results in satisfactory predictions for the effective FRP stress in most cases for both U-jacketed and side-bonded beams. However, it may become unconservative for side-bonded beams that have only light flexural steel reinforcement.     

混凝土梁外贴FRP抗剪加固承载力计算

剥离破坏是外贴FRP片材加固混凝土梁主要的破坏形式。回顾了对外贴FRP混凝土梁的试验研究、有限元分析和国内外现有的受剪剥离承载力计算公式。讨论了斜裂缝宽度分布规律,建立了FRP滑移分布模型,在此基础上分析了受剪剥离破坏时FRP的应力分布,讨论了FRP抗剪贡献与粘结长度、粘结方式等参数之间的关系。提出了受剪剥离承载力计算公式,与大量试验结果的对比表明,给出的设计建议公式与试验结果吻合良好,可供设计应用参考。     

钢筋混凝土T梁HB-FRP抗剪加固试验和数值模拟

基于室内模型试验,开展了混合黏结纤维增强复合材料(HB-FRP)抗剪加固钢筋混凝土T梁的受力性能研究.对未加固、外贴纤维增强复合材料(EB-FRP)加固及HB-FRP加固T梁进行了破坏性对比试验,并采用Abaqus软件建立了精细化有限元模型,对比分析了试验和数值计算结果,验证了有限元模型的准确性;在此基础上进行参数分析,研究了混凝土强度、箍筋间距、FRP条带数量及FRP厚度对加固梁抗剪承载能力的影响.结果 显示:HB-FRP抗剪加固梁的剪切裂缝间距要小于EB-FRP加固梁的裂缝间距;EB-FRP加固梁发生黏贴区域大面积剥离,而HB-FRP加固梁仅在相邻钢扣件间有裂缝的区域出现了剥离,钢扣件有效抑制了裂缝剥离扩展;HB-FRP抗剪加固梁的FRP应变水平为EB-FRP抗剪加固梁的2倍,表现出了较好的延性.综合考虑未加固梁、EB-FRP加固梁及HB-FRP加固梁的参数分析结果,对钢筋混凝土T梁抗剪承载能力的影响因素按照重要性降序为:混凝土强度、箍筋间距、FRP间距、FRP厚度.     

Strengthening of RC beams in shear using GFRP inclined strips – An experimental study

Though there have been a number of studies on shear strengthening of RC beams using externally bonded fiber reinforced polymer sheets, the behaviour of FRP strengthened beams in shear is not fully understood. This is partly due to various reinforcement configurations of sheets that can be used for shear strengthening and partly due to different failure modes a strengthened beam undergoes at ultimate state. Furthermore, the experimental data bank for shear strengthening of concrete beams using FRP remains relatively sparse due to which the design algorithms for computing the shear contribution of FRP are not yet clear. The objective of this study is to clarify the role of glass fiber reinforced polymer inclined strips epoxy bonded to the beam web for shear strengthening of reinforced concrete beams. Included in the study are effectiveness in terms of width and spacing of inclined GFRP strips, spacing of internal steel stirrups, and longitudinal steel rebar section on shear capacity of the RC beam. The study also aims to understand the shear contribution of concrete, shear strength due to steel bars and steel stirrups and the additional shear capacity due to glass fiber reinforced polymer strips in a RC beam. And also to study the failure modes, shear strengthening effect on ultimate force and load deflection behaviour of RC beams bonded externally with GFRP inclined strips on the shear region of the beam.     

Tests and design equations for FRP-strengthening in shear

The experimental/analytical study presented herewith arrived at developing a mechanics-based (as opposed to regression-based) model of the shear capacity of reinforced concrete beams, strengthened with externally bonded fibre reinforced polymers (FRP). The model is obtained through the following steps, with due consideration of the underlying physical mechanisms: (a) the generalised constitutive law of an FRP layer bonded to concrete is defined first, then, (b) the compatibility imposed by the shear crack opening and the appropriate boundary conditions – which depend on the strengthening configuration (either side bonding, U-jacketing or wrapping) – are included in the formulation, and, finally, (c) analytical expressions of the stress field in the FRP strip/sheet crossing a shear crack are obtained. Through these expressions, closed-form equations for the effective debonding strength of FRP strips/sheets are defined as function of, both, the adopted strengthening configuration, and of some basic geometric and mechanical parameters. The so-obtained FRP contribution is then added to those of concrete and steel, which, for the sake of comparison, have been considered as given by different codes. The equations accuracy has been verified by predicting – a priori, with the developed equations – the shear strength of experimentally tested r.c. beams, both collected from the literature and obtained from purposely carried out tests on under-designed real-scale beam specimens, strengthened with different FRP schemes. No a posteriori calibration of the model was performed. The prediction capability of the developed equations has been finally compared to other approaches available in the literature.     

Shear capacity of FRP-strengthened RC beams: FRP debonding

Many studies have been undertaken on shear strengthening of reinforced concrete (RC) beams by externally bonding fibre-reinforced polymer (FRP) composites. These studies have established clearly that such strengthened beams fail in shear mainly in one of two modes: FRP rupture; and FRP debonding, and have led to preliminary design proposals. This paper is concerned with the development of a simple, accurate and rational design proposal for the shear capacity of FRP-strengthened beams which fail by FRP debonding. Existing strength proposals are reviewed and their deficiencies highlighted. A new strength model is then developed. The model is validated against experimental data collected from the existing literature. Finally, a new design proposal is presented.     

Analytical model for the torsional behaviour of reinforced concrete beams retrofitted with FRP materials

An analytical approach for the prediction of the torsional response of reinforced concrete (RC) beams strengthened with fibre-reinforced-polymer (FRP) materials is described. The analysis method employs the combination of two different theoretical models: a smeared crack model for plain concrete in torsion for the elastic till the first cracking response and a properly modified truss model for the post-cracking response. The proposed method addresses the contribution of the externally bonded FRP materials to the torsional capacity of RC beams using specially developed equations in the well-known truss theory and utilizes softened and FRP-confined concrete stress–strain relationships. Detailed verification of this methodology is achieved through extensive comparisons between analytically predicted behaviour curves and experimentally obtained ones. The experimental data comprise a series of 12 tests in pure torsion and an additional database of experimental information for 24 specimens compiled from works around the world. These comparisons demonstrated that the proposed model is capable of adequately describing the full torsional response and of predicting with satisfactory accuracy the torsional moment at cracking and the ultimate torque capacity of FRP strengthened RC beams. Applications of the developed method as an assessment tool to strengthened beams with U-shaped FRP configurations and some first design examples that demonstrate the contribution of the FRP materials on the torsional response are also included.     

碳纤维增强复合网格-聚合物水泥砂浆加固RC梁的抗弯性能试验研究

为研究碳纤维增强复合(CFRP)网格和聚合物水泥砂浆(PCM)复合加固钢筋混凝土(RC)梁的抗弯性能,对5个RC梁试件进行抗弯性能试验,分析CFRP网格-PCM复合加固RC梁的抗弯破坏机理,研究网格不同层数和不同单位加固量对RC梁抗弯性能的影响。基于抗弯承载力的既有计算模型,引入剥离应变建立改良计算模型,并采用其他学者的9根FRP网格加固RC梁的受弯试验数据,验证改良计算模型的准确性。研究结果表明：CFRP网格-PCM对RC梁的抗弯加固效果明显,单位加固量较高的试件具有更高的承载能力,但其更易发生剥离破坏;在单位加固量相当的条件下,单层网格与双层网格呈现出相同的抗弯性能,双层网格重叠布置的加固方式是有效的;抗弯承载力的既有计算模型对试验结果拟合效果较差,所建立的改良计算模型拟合程度较好,能更好地反映CFRP网格-PCM复合加固层的实际受力状态。     

碳纤维增强复合网格-聚合物水泥砂浆加固RC梁抗剪性能试验研究

为研究碳纤维增强复合(CFRP)网格和聚合物水泥砂浆(PCM)复合加固工字形截面钢筋混凝土(RC)梁的抗剪性能,对3个试件进行抗剪性能试验和有限元模拟,分析了CFRP网格-PCM加固RC梁的抗剪破坏机理,研究了不同加固方式对试件抗剪性能的影响。研究结果表明：采用CFRP网格-PCM对RC梁进行抗剪加固可有效抑制斜裂缝的发展,能够较大幅度提高RC梁的抗剪承载力;相比仅腹部加固的试件,腹部和腋部都加固的试件的二次刚度、极限荷载均有所提高,且CFRP网格变形减小,与混凝土界面的黏结能力增强;在有限元分析中,采用混凝土的CDP模型和Spring2弹簧单元来预测CFRP网格加固混凝土的抗剪承载力是可行的;建立了基于杆状材料有效应变的抗剪计算方法,该方法可有效预测加固RC梁的抗剪承载力,为结构加固设计提供参考。     

在弯矩、剪力和反复扭矩复合作用下的碳纤维布加固RC箱梁抗扭性能试验研究

为研究碳纤维布加固弯矩、剪力和反复扭矩复合作用下的钢筋混凝土箱梁的抗扭性能,共设计制作了4根钢筋混凝土箱梁试件,其中3根采取碳纤维布加固、1根不加固作为对比试件。试验在自行研制的扭转试验装置上进行,对箱梁试件同步施加弯矩、剪力和反复扭矩作用。以加固方式和加固数量为主要研究参数,分析了箱梁试件的破坏机理、承载能力、变形能力和滞回性能等。通过各箱梁试件的碳纤维布和钢筋的应变变化规律,探讨了碳纤维布加固箱梁的抗扭工作机理;通过测得的各试件的扭矩-扭转角滞回曲线和骨架曲线,提出了碳纤维布加固钢筋混凝土箱梁的抗扭恢复力模型。从而为碳纤维布加固钢筋混凝土箱梁抗扭性能的理论研究和工程应用提供了重要的依据。     

织物增强混凝土加固RC梁的斜截面抗剪承载力试验研究

基于不同剪跨比的织物增强混凝土(TRC)加固RC梁的斜截面抗剪承载力试验,研究被加固梁的斜截面抗剪承载力及其破坏形态的变化,并研究剪跨区加固后梁的变形特点。试验结果表明,加固措施可以有效地提高梁的斜截面抗剪承载力,改善梁的斜截面破坏的脆性,对斜裂缝的发展有一定的约束作用,并减小梁在斜裂缝出现前的挠度。试验还表明,对剪跨比较大的RC梁,采用织物增强混凝土薄板加固后,其斜截面抗剪承载力提高幅度较大:当剪跨比为1时,加固后梁的斜截面极限抗剪承载力提高4%;当剪跨比为2.5时,加固后梁的斜截面极限抗剪承载力提高68%。在试验研究的基础上,给出TRC薄板加固梁的受剪承载力设计计算公式,公式计算结果与试验结果基本吻合。     

Investigation of the parameters affecting the behavior of RC beams strengthened with FRP

Three-point bending tests were carried out on nineteen Reinforced Concrete (RC) beams strengthened with FRP in the form of completely wrapping. The strip width to spacing ratios, FRP type, shear span to effective depth ratios, the number of FRP layers in shear, and the effect of stirrups spacing were the parameters investigated in the experimental study. The FRP contribution to strength on beams having the same strip width to spacing ratios could be affected by the shear span to effective depth ratios and stirrups spacing. The FRP contributions to strength were less on beams with stirrups in comparison to the tested beams without stirrups. Strengthening RC beams using FRP could change the failure modes of the beams compared to the reference beam. In addition to the experimental study, a number of equations used to predict the FRP contribution to the shear strength of the strengthened RC beams were assessed by using a limited number of beams available in the literature. The effective FRP strain is predicted by using test results, and this prediction is used to calculate the FRP contribution to shear strength in ACI 440.2R (2017) equation. Based on the statistical values of the data, the proposed equation has the lowest coefficient of variation (COV) value than the other equations.     

FRP补强加固RC梁粘结破坏机理研究

根据FRP(Fiberreinforcedplastic)补强加固RC梁的试验结果,总结了FRP补强加固钢筋混凝土受弯构件早期破坏形态的过程和特点。FRP加固钢筋混凝土抗弯构件粘结界面的剪应力的分布在纤维截断点处存在较高的应力集中,随着离截断点距离的增大剪应力分布逐渐趋于均匀,粘结锚固长度不足和过高的应力集中是造成FRP加固钢筋混凝土构件早期破坏的主要原因。采用“齿”状块体力学计算模型和混凝土裂缝理论分析了FRP加固钢筋混凝土受弯构件时需要的有效锚固长度,并通过简化修正得出了FRP加固钢筋混凝土受弯构件最小锚固长度的计算公式,提出了FRP的容许应变值和避免FRP早期破坏应采取的措施,可供FRP加固工程设计和施工参考。     

Finite element modelling of concrete cover separation failure in FRP plated RC beams

The technique of bonding fibre reinforced polymer (FRP) composites to the tension face or sides of reinforced concrete (RC) beams has become very popular for strengthening or retrofitting purposes. A distinct characteristic of such strengthened RC beams is that they very often fail due to various premature debonding failures. This paper presents a fracture mechanics based finite element analysis of debonding failures. Numerical results for an experimental beam are presented. Initial findings show that the method can successfully simulate the concrete cover separation failure mode in FRP strengthened RC beams.     

Behavior and capacity of RC beams strengthened in shear with NSM FRP reinforcement

A recent and promising method for shear strengthening of reinforced concrete (RC) members is the use of near-surface mounted (NSM) fiber-reinforced polymer (FRP) reinforcement. In the NSM method, the reinforcement is embedded in grooves cut onto the surface of the member to be strengthened and filled with an appropriate binding agent such as epoxy paste or cement grout. Only a few studies have been conducted to date on the use of NSM FRP reinforcement for shear strengthening of RC beams. These studies identified some critical failure modes related to debonding between the NSM reinforcement and the concrete substrate. However, more tests need to be conducted to identify all possible failure modes of strengthened beams. Moreover, virtually no test results are available on the behavior of shear-strengthened beams containing steel shear reinforcement, and on the effect of variables such as the type of epoxy used as groove filler. This paper illustrates a research program on shear strengthening of RC beams with NSM reinforcement, aimed at gaining more test results to fill the gaps in knowledge mentioned above. A number of beams were tested to analyze the influence on the structural behavior and failure mode of selected test parameters, i.e. type of NSM reinforcement (round bars and strips), spacing and inclination of the NSM reinforcement, and mechanical properties of the groove-filling epoxy. One beam strengthened in shear with externally bonded FRP laminates was also tested for comparison purposes. All beams had a limited amount of internal steel shear reinforcement to simulate a real strengthening situation. Test results are presented and discussed in the paper.     

FRP抗剪加固钢筋混凝土梁研究综述

对纤维增强复合材料(FRP)片材抗剪加固钢筋混凝土梁的研究现状进行了回顾和总结,归纳了FRP的抗剪加固方式,包括粘贴形式和锚固方法;将影响加固效果的主要因素概括为原梁条件和加固参数两大项,并讨论了若干分项因素的影响;探讨了加固梁的抗剪机理和破坏模式,列举了各国技术标准提供的和相关文献建议的多种抗剪承载力计算方法;指出了加强FRP片材端部锚固的重要性以及有待拓广和深入的应用基础研究方向,可为后续研究提供有益的参考。     

Flexural strengthening of reinforced lightweight polystyrene aggregate concrete beams with near-surface mounted GFRP bars

Application of near-surface mounted (NSM) fibre reinforced polymer (FRP) bars is emerging as a promising technology for increasing flexural and shear strength of deficient reinforced concrete (RC) members. In order for this technique to perform effectively, the structural behaviour of RC elements strengthened with NSM FRP bars needs to be fully characterized. This paper focuses on the characterization of flexural behaviour of RC members strengthened with NSM glass-FRP bars. Totally, 10 beams were tested using symmetrical two-point loads test. The parameters examined under the beam tests were type of concretes (lightweight polystyrene aggregate concrete and normal concrete), type of reinforcing bars (GFRP and steel), and type of adhesives. Flexural performance of the tested beams including modes of failure, moment–deflection response and ultimate moment capacity are presented and discussed in this paper. Results of this investigation showed that beams with NSM GFRP bars showed a reduction in ultimate deflection and an improvement in flexural stiffness and bending capacity, depending on the PA content of the beams. In general, beams strengthened with NSM GFRP bars overall showed a significant increase in ultimate moment ranging from 23% to 53% over the corresponding beams without NSM GFRP bars. The influence of epoxy type was found conspicuously dominated the moment–deflection response up to the peak moment. Besides, the ultimate moment of concrete beams reinforced with GFRP bars could be predicted satisfactorily using the equation provided in ACI 318-95 Building Code.     

复材网格-UHTCC复合增强钢筋混凝土梁抗弯性能试验研究

文中进行7根复材（FRP）网格增强超高韧性纤维水泥基（UHTCC）复合加固钢筋混凝土梁的抗弯性能试验,将FRP网格类型、FRP网格增强率、FRP-UHTCC复合层黏结长度作为试验变量,分析各变量对FRP-UHTCC复合增强混凝土梁弯曲性能的影响。在试验研究的基础上,给出FRP-UHTCC复合增强混凝土梁的抗弯承载力计算方法。试验结果表明,FRP-UHTCC复合层与混凝土间没有发生相对滑移现象,可以有效抑制加固层端部剥离破坏,加固梁的破坏模式为FRP网格中纵向纤维筋被拉断破坏。BFRP格栅与UHTCC黏结基体没有发生脱黏现象,优于BFRP编织网与UHTCC的黏结效果。随着FRP网格增强率的增大,加固梁的抗弯承载力得到显著提高。与未加固的普通混凝土梁相比,加固梁的开裂、屈服和极限荷载最大提高幅度分别为97%、35%和33%。计算结果表明,预测值与试验值吻合较好,可以有效地预测FRP-UHTCC复合增强混凝土梁的抗弯承载力。     

带锚FRP受剪加固梁非剥离破坏模式BP网络预测

对带可靠锚固FRP受剪加固混凝土梁的非剥离剪切破坏模式做了细化分类,即包括FRP断裂控制的破坏、受压区混凝土(达到极限应力状态)压碎控制的破坏、FRP断裂与混凝土压碎同步发生的界限破坏等3种模式;利用BP神经网络建立了带锚纤维受剪加固梁破坏模式的智能预测模型,与31根非剥离破坏加固梁试验的对比结果显示:模型总体精度达到90%,说明建立的破坏模式网络预测模型适用于带锚纤维受剪加固梁非剥离剪切破坏模式的判别。     

FRP片材增强钢筋混凝土梁刚度与变形计算

FRP（fiber reinforced polymer）片材增强钢筋混凝土梁纵筋屈服前刚度是其正常使用阶段的刚度,而纵筋屈服后刚度（又称二次刚度）则能够控制超载情况下梁的变形,因此,两个阶段的刚度都很重要。本文对FRP增强钢筋混凝土梁在受拉纵筋屈服前和屈服后两个阶段的刚度进行了理论分析,将纵筋屈服前后截面弯矩表达为受拉纵筋和FRP应变的函数,推导了增强材料应变不均匀系数的计算式,通过全过程分析得到了受压区混凝土边缘平均应变综合系数随截面弯矩的变化规律,发现屈服后阶段的变化规律明显不同于普通钢筋混凝土梁;通过对80根试验梁数据和40根模拟梁的全过程分析数据进行回归,得到了FRP增强钢筋混凝土梁的刚度和挠度计算式;将计算值与本文试验的实测值以及全过程分析结果进行对比,最后用文献中的试验数据进行了验证。结果表明,本文提出的刚度和挠度计算式具有较高的精度。