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.     

盐腐蚀环境下内嵌FRP筋加固混凝土界面黏结性能试验研究

为了研究盐腐蚀环境下内嵌FRP筋加固混凝土界面的黏结性能,对27个内嵌FRP筋加固混凝土试件进行盐腐蚀后的单端拉拔试验,分析试件的受力过程和破坏模式,研究内嵌FRP筋黏结长度、腐蚀时间和FRP筋类型对界面黏结性能的影响。结果表明：盐腐蚀的试件破坏模式分为结构胶劈裂、FRP筋拉断和结构胶劈裂且FRP筋弯折等3种,且以结构胶劈裂破坏为主。盐腐蚀环境下内嵌FRP筋混凝土试件的黏结应力与黏结长度、破坏模式与腐蚀时间有关。盐腐蚀环境会影响混凝土、黏结材料及FRP筋的力学性能,加剧黏结界面失效破坏。腐蚀时间为30 d和90 d的内嵌BFRP筋加固混凝土试件的耐盐腐蚀能力高于内嵌GFRP筋加固混凝土试件的,腐蚀时间为60 d的内嵌GFRP筋加固混凝土试件的耐盐腐蚀能力优于内嵌BFRP筋试件的。根据试验数据拟合了盐腐蚀环境下内嵌FRP筋加固混凝土界面黏结-滑移本构关系,其拟合优度达到0.988 0。     

Experimental performances of RC beams strengthened with FRP materials

The present paper illustrates the results of an experimental program on Reinforced Concrete (RC) beams externally strengthened with carbon Fibre Reinforced Plastic (FRP) laminates and Near Surface Mounted (NSM) bars under monotonic and cyclic loads, the latter ones characterized by a low number of cycles in the elastic and post-elastic range. Comparisons between experimental and theoretical failure loads are discussed in detail.     

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。     

内嵌FRP加固钢筋混凝土梁的受弯承载力分析

对内嵌FRP加固矩形截面梁的三种弯曲破坏形式(钢筋屈服前混凝土压碎、钢筋屈服后混凝土压碎和钢筋屈服后FRP拉断)和两种界限破坏情形(钢筋屈服时混凝土压碎和FRP拉断时混凝土压碎)进行了极限状态分析,提出了各种弯曲破坏极限状态相应的承载力计算公式和界限条件,其计算结果与作者及国外已有试验实测值吻合较好。     

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.     

FRP嵌贴加固RC梁有限元分析

为了进一步研究表层嵌贴FRP抗弯加固钢筋混凝土梁的性能,本文结合了已有试验中的14根试件建立了三维非线性有限元模型,主要分析了加固梁的极限承载力、荷载-位移曲线以及破坏模式。计算结果表明:采用本文有限元方案分析的结果与试验结果吻合较好;嵌贴长度、CFRP板条宽度对于加固效果和破坏模式有重要影响。     

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。     

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。     

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。     

预应力碳纤维板加固钢筋混凝土梁预应力损失试验研究

通过25根预应力碳纤维板加固钢筋混凝土梁试验研究,分析了预应力加固梁制作各工序的预应力损失。试验结果表明:当预应力水平分别为20%和30%时,预应力碳纤维板加固梁的平均预应力损失(相对于初始预应力)分别为14.38%和15.36%。对碳纤维板采取超张拉和二次超张拉技术,可以有效减小预应力损失。基于国内外混凝土结构设计规范的预应力损失计算方法,将预应力损失分为三部分:碳纤维板和锚具间的滑移引起的预应力损失、混凝土弹性压缩变形引起的预应力损失和碳纤维板应力松弛造成的预应力损失。在试验基础上,提出了三部分预应力损失的计算表达式,为预应力碳纤维板的加固设计提供参考。     

预应力FRP加固混凝土受弯构件研究进展

由于普通黏结FRP加固混凝土受弯构件存在一些不足,预应力FRP加固法研究已成为工程结构领域的一个热点.对预应力FRP片材加固的研究现状进行了分类,指出了现有研究尚未开展的方向,可供加固研究参考.     

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.     

碳纤维筋表层嵌贴加固钢筋混凝土梁受弯性能试验研究

通过对4根内嵌碳纤维筋加固钢筋混凝土梁试验研究,在保证嵌入碳纤维筋锚同长度的前提下,尝试采用不同位置的嵌入方式,对不同部位嵌入对承载力提高的影响进行分析,并对嵌入式FRP筋承载力进行理论分析和承载力提高计算,提出了抗弯承载力增强计算方法.结果表明试验取得了较好的吻合效果.     

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.     

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。     

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。     

CFRP嵌入式加固混凝土梁抗弯性能试验研究

通过对3根对比梁和8根加固粱进行了试验研究,结果表明：CFRP布嵌入式加固能有效提高混凝土梁的抗弯承载力,另外,不同初始弯矩作用下加固梁的极限承载力几乎相同。