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.     

Modeling of debonding failure for RC beams strengthened in shear with NSM FRP reinforcement

The shear capacity of reinforced concrete members can be successfully increased using near-surface mounted (NSM) fiber-reinforced polymer (FRP) reinforcement. Tests conducted thus far have shown that failure is often controlled by diagonal tension associated to debonding between the NSM reinforcement and the concrete substrate. In absence of steel stirrups and/or when the spacing of the NSM reinforcement is large, debonding involves separately each of the bars crossed by the critical shear crack. In order for shear strengthening of beams with NSM reinforcement to be safely designed, an analytical model able to encompass the failure mode mentioned above must be developed. This paper presents two possible approaches, a simplified and a more sophisticated one, to predict the FRP contribution to the shear capacity. In the first approach, suitable for immediate design use, an ideally plastic bond–slip behavior of the NSM reinforcement is assumed, which implies a complete redistribution of the bond stresses along the reinforcement at ultimate. The second approach, implemented numerically, accounts for detailed bond–slip modeling of the NSM reinforcement, considering different types of local bond–slip laws calibrated during previous experimental investigations. It also takes advantage of an approach developed by previous researchers to evaluate the interaction between the contributions of steel stirrups and FRP reinforcement to the shear capacity. The paper illustrates the two models and compares their predictions, with the ultimate goal to evaluate whether the first simple model can be used expecting the same safety in predictions of the second model.     

FRP片材加固钢筋混凝土梁的研究进展

80年代以来,FRP(纤维增强复合材料)作为一种高性能的新型混凝土结构加固补强材料受到科研院所和工程界的广泛关注[1]。迄今为止,国内外关于FRP片材加固钢筋混凝土结构研究的试验和理论研究已相当丰富,并在工程界得到大量实践应用。本文着重介绍了FRP片材加固钢筋混凝土梁的抗弯性能、抗剪性能、以及裂缝、刚度研究,以期为FRP片材的进一步研究和应用提供参考。     

Fatigue-loading effect on RC beams strengthened with externally bonded FRP

External bonding of fiber-reinforced polymers (FRP) on concrete beams is particularly attractive for the strengthening of civil engineering structures in order to increase their strength and stiffness. Principles for design of such strengthening methods are now established and many guidelines exist. However, fatigue design procedure is still an ongoing research topic.This paper focuses on the damage behavior of FRP-strengthened reinforced concrete (RC) structures subjected to fatigue loading.In order to design bonded reinforcements, an iterative computational method based on section equilibrium and material properties (concrete, steel, adhesive and composite) has been previously developed by authors [1], [2], [3]. In the present study, this method is extended to describe the fatigue behavior of RC beams.A specific modeling coupled with an experimental investigation on large-scale beams made it possible to compare the theoretical and experimental fatigue behaviors of RC beams with and without composite reinforcements. The model is developed and calibrated using data of the literature or recorded during experiments specifically carried out for this study. Results showed that the beam deflection and the strain in each material could be calculated with a sufficient accuracy, so that the fatigue behavior of the FRP-strengthened beams was correctly estimated by the model.     

预应力纤维材料加固木梁研究

普通纤维加固木结构不能充分利用纤维材料的高强性能，对于构件的变形也不能起到很好的降低作用，而对纤维材料施加一定的预应力将很好解决这些问题，本对预应力纤维材料加固木梁进行了理论分析，给出在预应力条件下粘结应力的解析解，并提出简单实用的预应力施加方法——预弯法，为工程应用打下基础。     

预应力FRP布加固混凝土梁的承载能力分析

基于连续损伤理论,考虑胶层的剪切效应,建立了FRP加固修补混凝土结构的高阶剪切弯曲与面内变形耦合的非线性有限元分析模型。FRP加固修补的缺陷在于,失效主要是混凝土的破坏和胶层的失效,纤维布的性能并没有得到充分的发挥,采用将纤维布进行初始拉伸后贴于梁的下表面的方法对梁进行修补,可以改善加固修补效果;将纤维布预先进行3mm和5mm的拉伸并对加固过程进行了非线性有限元分析模拟,并与试验结果进行了对比,证明了此方法加固的有效性。     

预应力FRP布加固混凝土梁的承载能力分析

基于连续损伤理论，考虑胶层的剪切效应，建立了FRP加固修补混凝土结构的高阶剪切弯曲与面内变形耦合的非线性有限元分析模型。FRP加固修补的缺陷在于失效，失效主要是混凝土的破坏和胶层的失效，纤维布的性能并没有得到充分的发挥，采用将纤维布进行初始拉伸后贴于梁的下表面的方法对梁进行修补，可以改善加固修补效果；将纤维布预先进行3mm和5mm的拉伸并对加固过程进行了非线性有限元分析模拟，并与试验结果进行了对比，证明了此方法加固的有效性。     

FRP加固钢筋混凝土梁的变形简化计算分析

FRP由于其高强度、轻质量及防腐等特性,在结构加固改造中的应用越来越广泛。根据文献中的FRP加固钢筋混凝土梁的荷载—变形模型,通过计算梁在开裂、钢筋屈服及极限情况3个阶段的弯矩和变形,建立了基于Rasheed模型的三折线简化分析模型,从而使荷载—变形关系计算更加简便。对该模型的数值试验模拟算例分析结果表明,该模型与实际的荷载变形曲线能较好地吻合,为FRP的应用研究提供参考。     

预应力FRP布加固混凝土梁的承载能力分析

基于连续损伤理论，考虑胶层的剪切效应，建立了FRP加固修补混凝土结构的高阶剪切弯曲与面内变形耦合的非线性有限元分析模型。FRP加固修补的缺陷在于，失效主要是混凝土的破坏和胶层的失效，纤维布的性能并没有得到充分的发挥，采用将纤维布进行初始拉伸后贴于梁的下表面的方法对梁进行修补，可以改善加固修补效果：将纤维布预先进行3mm和5mm的拉伸并对加固过程进行了非线性有限元分析模拟，并与试验结果进行了对比，证明了此方法加固的有效性。     

FRP加固混凝土梁的非线性有限元分析

基于连续损伤理论，考虑胶层的剪切效应，建立了FRP加固修补混凝土结构的高阶剪切弯曲与面内变形耦合的非线性有限元分析模型。利用自编的有限元分析软件对GFRP加固混凝土梁进行了有限元分析。证明了该模型的有效性。利用此分析方法对未加固、初始加固及出现宏观裂纹后进行加固的混凝土渠进行分析对比，从而为实际工程的应用提供了理论依据。     

Unified analytical approach for determining shear capacity of RC beams strengthened with FRP

This paper presents a rational model to predict the ultimate load capacity of reinforced concrete (RC) beams strengthened by a combination of longitudinal and transverse fiber reinforced polymer (FRP) composite plates/sheets (flexure and shear strengthening system). The model is based on the truss analogy and the theory of plasticity and is opportunely refined in order to incorporate some critical aspects, such as variable angle crack, non-uniform FRP stress distribution over the shear crack, shear span/depth ratio. It is a general and unified model that allows consideration of all the main possible failure mechanisms of strengthened RC beams, related to flexural-shear interaction, shear web-crushing and pure flexural mechanisms. The model is validated against a large number of beam tests reported in the literature, involving a wide range of geometrical and mechanical characteristics. The numerical investigation shows a very satisfactory correlation between predicted and experimental data.     

纤维材料抗剪加固钢筋混凝土梁的影响因素与试验结果分折

本文讨论了多种因素对纤维布加固钢筋混凝土梁效果的影响,分析了不同因素的影响程度,并分析了纤维布对梁刚度、开裂荷载的影响,最后对纤维布的应变变化情况进行了研究。     

纤维材料抗剪加固钢筋混凝土梁的影响因素与试验结果分析

本文讨论了多种因素对纤维布加固钢筋混凝土梁效果的影响，分析了不同因素的影响程度，并分析了纤维布对梁刚度、开裂荷载的影响。最后对纤维布的应变变化情况进行了研究。     

Edge debonding in FRP strengthened beams: Stress versus energy failure criteria

In the present paper we analyse the edge debonding failure of a beam strengthened by a fibre reinforced polymer. As well known from the literature, a stress concentration is found at the edge of the reinforcement which triggers the debonding of the fibre reinforced polymer strip when the load reaches a certain critical threshold. Two failure criteria are proposed to study the debonding mechanism. The former is a stress assessment criterion, i.e. failure takes place whenever the maximum shearing stress reaches a limit value (the interfacial bond strength). The latter is an energy, fracture mechanics criterion, i.e. failure takes place as the strain energy release rate reaches a critical value (the interfacial fracture energy). It is argued that the energy criterion is more effective to address the edge debonding failure mode. However, under the assumption of shear lag behaviour for the adhesive layer between the beam and the reinforcement, a general rule linking the two approaches is set, thus providing the key to bypass the rather complicated energetic analysis. The final part of the paper is devoted to the crack instabilities that may occur after the debonding initiates, i.e. snap–back and snap-through phenomena. The size effect is then investigated by means of a dimensional analysis and a simplified formula providing the critical load is proposed that could be useful in engineering practice.     

钢筋混凝土梁TYFO复合纤维抗剪加固试验研究

介绍了带悬臂简支梁支座节点采用粘贴TYFO复合纤维进行抗剪加固的试验研究，通过对比试验，分析了在卸载或部分卸载情况下，加固对梁抗剪承载力的影响，并提出了一些加固设计建议。     

关于FRP加固钢筋混凝土梁跨中剥离应力计算方法的讨论

综述了几种FRP加固钢筋混凝土梁的跨中剥离应力的计算方法,说明对剥离块体进行有限元分析方法的优越性.     

HRB500级钢筋混凝土连续梁抗弯试验研究

在3根HRB500级钢筋混凝土连续梁受弯试验的基础上,分析了HRB500级钢筋和高强混凝土匹配下的梁的破坏形态、变形特点和承载性能。结果表明：其破坏特征、挠曲模式及截面应变分布与普通高强混凝土梁基本一致。梁裂缝宽度较规范计算的结果小,其塑性调幅能力不明显。     

粘贴片材加固混凝土梁的锚固端应力参数分析

在粘贴片材加固的混凝土梁的锚固端应力计算公式的基础上 ,对集中荷载和均布荷载作用下的梁进行参数分析。计算结果表明 ,最大锚固剪应力和最大剥离正应力主要和加强片厚度、弹性模量、长度以及粘胶层的厚度与弹性模量有关。     

粘贴FRP加固RC结构温度对粘结应力影响分析

目前对粘贴FRP加固系统粘结性能的研究大多集中在荷载作用下粘结应力的分析,而温度作用对粘结应力的影响在实际工程中通常被忽略了。该文针对粘贴FRP加固混凝土结构,建立了界面粘结应力分析的理论模型,并通过实例分析,阐明了FRP加固混凝土结构设计中温度应力作用的重要性。