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.     

Bond behaviour of FRP-to-clay brick masonry joints

The out-of-plane bending and in-plane shear response of unreinforced modern clay brick masonry walls retrofitted with fibre-reinforced polymer (FRP) strips is often governed by debonding failure mechanisms. Hence, it is necessary to quantify the fundamental interface bond–slip model, which describes the debonding behaviour of the FRP-to-masonry interface. This paper presents the results of a series of 29 pull tests investigating the use of externally bonded (EB) and near surface mounted (NSM) retrofitting techniques. Test variables included: surface preparation; geometric properties; location of FRP (relative to perpend joints and cores); bonding agent of bed joints (mortar and quick drying paste); bonding method for glass fibre sheets (plate bonding and dry lay-up); and FRP material. A discussion of the test results and preliminary practical recommendations are also provided. A model used to predict the intermediate crack debonding resistance was verified against the test data. The model is generic in that it is applicable to both the EB and NSM retrofitting techniques. This generic model was shown to give very good ultimate strength predictions for the series of 29 pull tests conducted as part of this research.     

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.     

Torsional strengthening of rectangular and flanged beams using carbon fibre-reinforced-polymers – Experimental study

The present experimental investigation deals with the torsional strengthening of concrete beams without stirrups using epoxy-bonded carbon fibre-reinforced-polymer (FRP) sheets and strips as external transverse reinforcement. The experimental program comprises 14 rectangular and T-shaped beams tested under pure torsion. Based on the measured values of the torsional moment at cracking and at ultimate, the corresponding twists, the behavioural curves and the failure modes of the beams, useful concluding remarks are indicated. The strengthened rectangular beams using full wrapping with continuous FRP sheets performed enhanced torsional behaviour and higher capacity than the strengthened beams with FRP strips. U-jacketed flanged beams exhibited premature debonding failure and substantial reductions of the potential torsional capabilities are reported. In general, FRP fabrics could effectively be used as external torsional reinforcement in under-reinforced concrete elements without steel transverse reinforcement.     

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.     

Numerical simulation of performance of near-surface mounted FRP-upgraded beam–column sub-assemblages under cyclic loading

This study deals with the performance of the upgrading schemes for the existing gravity load designed (GLD) reinforced concrete (RC) beam–column sub-assemblages using near-surface mounted (NSM) fibre-reinforced polymer (FRP) bars. In this study, exterior beam–column sub-assemblage of a general RC-framed structure has been considered. Numerical investigations of the sub-assemblages have been carried out under cyclic loading using nonlinear finite element analysis. Experimentally validated numerical models have been used for evaluating the performance of various upgrading schemes using NSM bars. Cyclic behaviour of reinforcement, concrete modelling based on fracture energy, bond–slip relations between concrete and steel reinforcement have been incorporated. The study also includes numerical investigation of crack and failure patterns, ultimate load-carrying capacity, strain comparisons and formation of plastic hinges, load–displacement hysteresis, energy dissipation and ductility. Seismic performance in terms of energy dissipation and development of strain in beam bar shows that some of the upgraded schemes are found to be comparable to the seismically designed ductile specimens. The findings of this study would be helpful to the practising and design engineers for developing detailing criteria for newly designed – or strengthening of deficient – reinforced concrete structure.     

Effective methods of using CFRP bars in shear strengthening of concrete girders

This paper presents the results of an experimental study that investigated the shear strength contribution of carbon fiber reinforced polymer (CFRP) bars attached with concrete beams using a near surface mounted (NSM) technique. In this research, four concrete beams were cast with regular steel reinforcement in flexure. The control beam had typical shear steel and the other three beams were strengthened in shear with CFRP bars. Strain gauges were attached with the shear reinforcement of all four beams at various shear critical locations. Strains during loading to failure of the beams were recorded using a data acquisition system. The performance of the NSM technique was found to be very effective with no occurrence of delamination, debonding or fracture of FRP. Effective strains in the NSM CFRP bars were determined through analyzing the collected strain data. A new formula to calculate the nominal shear strength provided by NSM CFRP bars has also been proposed.     

A new single-shear set-up for stable debonding of FRP–concrete joints

A new experimental set-up to perform single-lap shear debonding tests on fiber reinforced polymer (FRP) reinforcements bonded to concrete is presented. Back sides of both FRP reinforcement (sheet or plate) and concrete specimen are fixed to an external restraining system and the force is applied to the other side of FRP reinforcement. Hence, a stable debonding process can be experimentally followed, corresponding to the transition between two limit states of perfect bonding and fully debonded reinforcement. Both strain gauges along the FRP reinforcement and LVDT transducers have been used. Tests have been performed on both plates and sheets bonded to concrete specimens with different surface preparations before adhesive application. Shear stress–slips data have been computed from the experimental FRP strain measures. The parameters of a non-linear interface law have been then calibrated and compared with analogous results from experimental data obtained with a conventional set-up. A numerical bond–slip model has been finally used to simulate the experimental tests, adopting the above-mentioned law for the FRP–concrete interface. Numerical results in good agreement with experimental results have been obtained.     

Partially bonded near-surface-mounted CFRP bars for strengthened concrete T-beams

A partially bonded strengthening approach for reinforced concrete (RC) beams utilizing near-surface-mounted (NSM) carbon fiber reinforced polymer (CFRP) bars was investigated with the specific objective of improving deformability. A total of six RC T-beams strengthened with NSM CFRP bars of various unbonded lengths were tested. Test results showed a decrease of the stiffness at the post-yield stage of the load–deflection response in the partially bonded beams. This is caused by the delayed increase of the FRP strain within the unbonded length. As a result the beam deformability was increased as the unbonded length increased at the same applied load. Internal slip of the FRP bar and gradual concrete failure were observed near the ultimate state, which caused a complicate nonlinear behavior of the beams. An analytical model is proposed to address the complete beam behavior including the effect of slip of FRP reinforcement and gradual concrete crushing. This model was developed based on the compatibility of deformation of the partially bonded system and was able to represent the ultimate behavior of the beams well.     

Derivation of the bond–slip characteristics for FRP plated steel members

In order to understand the behaviour of steel members retrofitted using adhesively bonded fibre reinforced polymer (FRP) plates, the bond–slip characteristics of the adhesive joint between the FRP and steel must first be established. This is important so that debonding does not occur whilst the members are in service. Previously, purely empirical research on establishing the bond–slip characteristics involved strain gauging the length of the FRP plate, which can lead to a major scatter of results as well as underestimating the peak shear stress. The subject of this paper is to describe a technique for quantifying the bond–slip characteristics based on a structural mechanics approach and which does not require strain gauging the plate. Two types of pull tests and a partial-interaction numerical model are used to quantify the major characteristics of the bond–slip and two types of adhesive are used to illustrate the approach.     

无腹筋锈蚀钢筋混凝土梁承载能力的计算

锈蚀钢筋混凝土梁的承载力不仅与纵向钢筋的截面损失有关 ,而且和钢筋与混凝土之间的粘结强度的降低、混凝土保护层中出现的纵向锈胀裂缝有关。本文先考虑了由于钢筋的截面损失引起的钢筋混凝土梁的抗弯承载力的降低 ;再在梁 拱共同作用抵抗剪力的机制上 ,计算了无腹筋锈蚀钢筋混凝土梁的抗剪承载力 ,进而得到了无腹筋锈蚀钢筋混凝土梁的承载力及其相应的破坏模式。对一实例的计算结果表明 ,当混凝土保护层出现纵向锈胀裂缝后 ,钢筋与混凝土之间的极限粘结强度相应降低 ,梁的破坏模式由受弯破坏转向受剪破坏 ,承载能力有较大的降低。同时 ,锚固区的粘结强度的降低 ,导致梁也可能发生粘结锚固破坏。     

嵌入式CFRP板条-混凝土界面粘结性能的试验研究

嵌入式(NSM)加固法是FRP材料加固混凝土结构的一种新的应用形式。通过CFRP板条嵌入混凝土试块的界面单剪试验,研究粘结长度对界面粘结性能的影响。根据实测应变分布,得到沿粘结长度界面粘结应力分布,并通过计算分析,对局部粘结滑移关系进行了初步探讨。     

钢-连续纤维复合筋嵌入式加固RC梁承载力分析

新型钢-连续纤维复合筋（SFCB）是一种以普通钢筋为内芯,外包纵向连续纤维的新型筋材。SFCB由于良好的力学性能、高耐久性和高性价比而在嵌入式加固中具有独特的优势。对SFCB嵌入式加固RC梁的承载力分析方法进行介绍,首先根据平截面假定及力的平衡,提出了SFCB嵌入式加固钢筋混凝土梁非粘结破坏时的受弯承载力计算方法;然后对嵌入式加固RC梁始于加载点附近开始的剥离破坏现象进行了理论分析,并给出了是否会发生粘结剥离破坏的判别方法和极限承载力的计算方法;最后,将计算结果与嵌入式加固RC梁试验结果进行了比较,认为该方法对破坏模式和极限承载力的预测均具有较好的精度。     

Numerical Analysis of Debonding Mechanisms in FRP-Strengthened RC Beams

Abstract:   Fiber-reinforced polymer (FRP) composites have been increasingly used as externally bonded reinforcement in lieu of their steel counterpart in the rehabilitation and retrofit of existing concrete structures. Without proper understanding of interfacial fracture behavior and failure mechanisms, it is impossible to efficiently develop an effective and rational FRP bonding technique. This article is mainly focused on clarifying the debonding behavior and failure mechanisms caused by different types of flexural crack distributions in FRP-strengthened R/C beams, which has not been solved so far. Using a discrete crack model for concrete crack propagation and a bilinear bond–slip relationship with softening behavior to represent FRP–concrete interfacial behavior, a nonlinear fracture mechanics-based finite-element analysis is performed to investigate the effects of crack spacing and interfacial parameters such as stiffness, local bond strength, and fracture energy on the initiation and propagation of the debonding and the structural performance. It is shown that the debonding behavior and load-carrying capacity are significantly influenced by two important factors: interfacial fracture energy and crack spacing in relation to the effective transfer length of FRP sheets. Based on the numerical results, some suggestions concerning the effect of interfacial properties are made as practical design aids.     

爆炸荷载作用下外贴FRP加固钢筋混凝土双向板试验研究

通过集团装药隔土爆炸荷载作用下4块外贴FRP条带加固钢筋混凝土双向板和1块普通板的对比试验,考察了裂缝的产生、开展过程及分布形状,分析了FRP加固板的荷载、位移、加速度、钢筋和混凝土以及FRP应变动力响应时程,研究了FRP加固板的抗爆破坏特征。研究结果表明：外贴FRP条带加固能有效延缓混凝土的开裂,限制裂缝的开展,改善钢筋混凝土板的抗爆性能;外贴FRP条带加固后,RC双向板的跨中位移响应、混凝土和钢筋应变响应明显降低,结构的抗爆炸冲击波能力得到明显提高;外贴FRP条带加固双向板在爆炸冲击荷载作用下的破坏形态有受弯破坏和弯曲屈服后的剪切破坏,外贴FRP条带在极限状态时发生了剥离及断裂破坏。图12表6参10     

Strengthening of one-way spanning RC slabs with cutouts using FRP composites

This paper reports the results of tests on fibre reinforced polymer (FRP) strengthened one-way spanning reinforced concrete (RC) slabs with central cutouts. Four wide slabs with cutouts were tested in addition to two narrow slabs without cutouts. Different positions of applied line loads for the slabs with cutouts resulted in different slab bending action and hence different FRP behaviour for the strengthened slabs. All FRP-strengthened slabs achieved a higher load-carrying capacity than their unstrengthened control counterparts. In addition, all strengthened slabs failed by debonding initiating at intermediate cracks (IC debonding) and in the case of the slabs with cutouts, the critical cracks were diagonal and originated from the corners of the cutout. The ability of the FRP to redistribute stresses around the cutout, the failure mechanisms, as well as pre- and post-debonding behaviour of the strengthened slabs was therefore assessed for different load application positions. Strains on the FRP, concrete and internal steel reinforcement, as well as deflections at different positions on the slab surfaces are also reported. An analytical model, which is based on the ultimate moment of resistance about critical crack lines, is also reported and it its predictions are found to correlate well with the experimental results. The analytical model is able to capture the different slab bending actions in addition to the debonding failure of the strengthened slabs.     

HB FRP加固混凝土结构组合界面黏结特性

为揭示HB-FRP(hybrid bonded fiber reinforced polymer)加固混凝土结构多作用组合工作机制,设计了5组黏结作用组合试验.基于实测荷载-滑移关系、应变分布、黏结-滑移关系开展了界面黏结特性研究,提出组合界面黏结-滑移统一模型和黏结荷载表达式.结果表明:HB-FRP加固混凝土组合作用可拆分为FRP黏结混凝土、侧压力和FRP黏结钢板;组合界面的黏结应力发展不同步,FRP板下表面与混凝土的剥离早于FRP板上表面与钢板的剥离,叠合工作时序不同;由侧压力引起的界面摩擦应力随界面应力的发展而增加并趋于稳定;理论模型结果与试验结果具有较好的一致性,可用于计算组合界面的黏结荷载.     

纤维增强塑料筋混凝土粘结滑移本构模型

纤维增强塑料筋与钢筋性能的差异 ,使纤维增强塑料筋混凝土的粘结性能与钢筋混凝土的粘结性能存在明显不同 ,因此 ,研究塑料筋混凝土粘结滑移本构模型 ,对推广塑料筋混凝土结构在工程中的应用具有重要的理论意义和实用价值。在总结国内外已有的纤维增强塑料筋与混凝土粘结滑移本构模型的基础上 ,提出了粘结滑移的连续曲线本构模型。该模型以粘结滑移曲线的三个关键点为基础 ,物理概念明确、光滑连续 ,并与试验结果吻合良好     

超高性能混凝土有腹筋梁受剪性能及受剪承载力研究

为了研究超高性能混凝土（UHPC）有腹筋梁的受剪性能,对7根UHPC梁进行了受剪性能试验,变化参数包括剪跨比、纵筋配筋率、配箍率、钢纤维掺量等。试验结果表明：UHPC有腹筋梁的破坏形态有弯曲屈服后的剪切破坏和剪压破坏,破坏时梁表面呈现斜向多条裂缝形态;箍筋可以提高UHPC梁开裂后刚度,钢纤维和箍筋均可以提高UHPC梁的变形能力和受剪承载力,足够的箍筋和钢纤维共同作用可以进一步提高UHPC梁的延性;配箍率增加,梁腹部会出现较密的短斜裂缝。提出了UHPC有腹筋梁受剪承载力计算模型,其中包括剪压区混凝土、斜裂缝处钢纤维、箍筋及纵筋销栓作用对于梁受剪承载力的贡献,模型计算值与试验值吻合良好。     

Modeling of Tension Stiffening Behavior in FRP‐Strengthened RC Members Based on Rigid Body Spring Networks

Abstract: Allowing for the tension stiffening effects resulting from the bond between steel reinforcement and surrounding concrete leads to effective deformation analysis of reinforced concrete (RC) members when using a nonlinear finite element analysis modeled on the smeared crack concept. Nowadays, externally bonded fiber reinforced polymer (FRP) composites are widely used for strengthening existing RC structures. However, it remains unclear to what extent the tension stiffening of postcracking concrete is quantitatively influenced by the addition of FRP composites, as a result of the bond between the FRP and the concrete substrate. This article presents a discrete model, which is based on rigid body spring networks (RBSN), for investigating the tension stiffening behavior of concrete in FRP‐strengthened RC tensile members. A two‐parameter fracture energy‐based model was deployed to represent the bond‐slip behavior of the FRP‐to‐concrete interface. The reliability of the RBSN model was verified through comparisons with previous test results. Further parametric analysis indicates that the tension stiffening of concrete is hardly influenced by the addition of FRP composites before the yield of steel reinforcement has occurred although concrete crack patterns and crack widths may be influenced by the bond‐slip behavior of the FRP‐to‐concrete interface.