Strain monitoring of RC members strengthened with smart NSM FRP bars

Fiber-reinforced polymer (FRP) bars can be used as internal reinforcement for new reinforced concrete (RC) structures and as near-surface mounted (NSM) reinforcement for the strengthening of RC structures. The NSM method is an emerging strengthening technique for RC structures, where FRP bars are embedded into grooves cut in the cover of RC members. In both cases, strain monitoring of the FRP bars is desirable either for the investigation of the structural behavior or for the long-term health monitoring of the structure. This paper presents a study in which fiber-optic sensors were embedded into glass FRP (GFRP) bars to produce smart GFRP bars for NSM applications. The manufacturing process of the smart FRP bars is illustrated and their performance in tensile, bond and beam flexural tests is examined to assess the effectiveness of these smart FRP bars for achieving the dual purpose of structural strengthening and strain monitoring. On the basis of the test results, the advantages and limitations of fiber-optic sensors compared to electrical strain gages in the strain monitoring of NSM FRP bars are discussed. The bond and beam test results also confirm the effectiveness of the NSM method for the strengthening of RC structures.     

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.     

FRP筋增强混凝土结构耐久性能研究进展

一般认为在恶劣服役环境下,可利用纤维增强复合材料（FRP）筋替代普通钢筋,从而有效提升混凝土结构的耐久性能。但研究表明,FRP筋并非完全免疫于服役环境,其在腐蚀性环境下的耐久性能一直是困扰FRP筋在土木工程中应用的关键问题之一。文章从三个方面系统梳理了国内外关于FRP筋增强混凝土结构耐久性的研究进展：①在试验研究方面,从FRP筋自身、FRP筋与混凝土黏结以及FRP筋增强混凝土构件三个层次,对近几十年来国内外有关FRP筋增强混凝土结构耐久性能试验研究的进展进行系统的梳理;②在长期性能预测方法方面,对现有的各类FRP筋长期性能预测模型和理论进行梳理;③在设计方法方面,总结目前各国有关FRP筋增强混凝土结构耐久性的设计方法,比较了规范间的差异。文章梳理工作对提升我国FRP筋增强混凝土结构设计水平和推广FRP筋的工程应用,具有重要的参考意义。     

Behavior of square and circular columns strengthened with aramidic or carbon fibers

The confinement of reinforced concrete (RC) columns with transverse reinforcement has been extensively studied and it is known that the response of concrete cylinders subjected to equivalent levels of pressure depends on how that lateral pressure is transmitted, and not on its magnitude alone. When confinement is, additionally to that given by steel stirrups, provided by external jackets of fiber reinforced polymers (FRP) the complexity and the scarcity of experimental data increase and tests to gain insight into the structural behavior and consequent design procedures are still required. The present study reports tests performed on axially loaded RC columns, with and without jackets. The FRP tested were made either of carbon fibers reinforced polymers (CFRP) or aramidic (AFRP) wraps and the geometry of the specimens included square and circular cross-sections. Comparison of gains of axial strength and ductility are presented and aspects of the variation of the lateral pressure and rupture of FRP jackets are examined.     

Stress–strain model for FRP-confined concrete under cyclic axial compression

One important application of fibre-reinforced polymer (FRP) composites in construction is as FRP jackets to confine concrete in the seismic retrofit of reinforced concrete (RC) structures, because FRP confinement can enhance both the compressive strength and ultimate strain of concrete. For the safe and economic design of FRP jackets, the stress–strain behaviour of FRP-confined concrete under cyclic compression needs to be properly understood and modelled. This paper presents a stress–strain model for FRP-confined concrete under cyclic axial compression. The model consists of the following major components: (a) a monotonic stress–strain model for FRP-confined concrete developed by the authors in a previous study for predicting the envelope curve; (b) new algebraic expressions for predicting unloading and reloading paths; and (c) predictive equations for determining the permanent strain and stress deterioration, with the effect of loading history duly accounted for. The capability and accuracy of the proposed model in predicting the complete stress–strain history of FRP-confined concrete under cyclic axial compression are demonstrated through comparisons between predictions of the proposed model and test results.     

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.     

Cyclic response of concrete members with bond-damaged zones repaired using concrete confinement

This paper presents the results of experimental investigation undertaken for evaluating the cyclic response of concrete members which have already experienced structural damage and total loss of load resistance due to splitting bond failure of the tensile reinforcement, and then repaired for upgrading their bond strength and flexural capacity. The original (intact) specimens consisted of beams reinforced with identical top and bottom spliced reinforcement and subjected to inelastic cyclic load reversals until total bond degradation and complete loss of flexural strength. The repair procedure consisted of removing the deteriorated concrete within the damaged splice zone, adding concrete confinement and casting new concrete. Three types of concrete confinement were investigated, namely, internal confinement by steel ties or wire mesh reinforcement, and external confinement by FRP laminates. It was found that repairing the bond-damaged zone through concrete confinement leads to substantial regain of flexural stiffness and strength up to or exceeding those for the original specimens, reduces the structural damage, and results in considerable improvement of the energy absorption and dissipation capacity under cyclic loading. The experimental results were discussed, and comparison between the experimental data and analytical predictions is undertaken.     

加筋混凝土梁延性系数计算方法

由于纤维增强复合材料(FRP)筋不存在屈服状态,传统的延性系数计算方法不适用于FRP筋混凝土梁和混合配筋(钢筋+FRP筋)混凝土梁。为了提出一个相对完善的、统一的加筋混凝土梁截面延性计算方法,在对既有各类加筋混凝土梁延性指标计算方法进行分析的基础上,从抗震对结构延性的要求出发,依据延性系数的定义与动力要求统一的原则,推导得出了加筋混凝土结构延性系数-地震力降低系数(μ-C)关系式。依据等位移下的μ-C关系式,提出了加筋混凝土梁延性系数的计算方法。通过对比延性系数计算值与既有试验值,证明了该方法的有效性。对混凝土及钢筋强度、混凝土极限压应变、截面有效配筋率和FRP筋配筋刚度比等影响加筋混凝土梁延性的因素进行了参数化分析。结果表明:加筋混凝土梁的延性随着混凝土强度和极限压应变的增加而提高,随着钢筋强度、有效配筋率和FRP筋配筋刚度比的提高而降低。     

Design-oriented stress–strain model for FRP-confined concrete

External confinement by the wrapping of FRP sheets (or FRP jacketing) provides a very effective method for the retrofit of reinforced concrete (RC) columns subject to either static or seismic loads. For the reliable and cost-effective design of FRP jackets, an accurate stress–strain model is required for FRP-confined concrete. In this paper, a new design-oriented stress–strain model is proposed for concrete confined by FRP wraps with fibres only or predominantly in the hoop direction based on a careful interpretation of existing test data and observations. This model is simple, so it is suitable for direct use in design, but in the meantime, it captures all the main characteristics of the stress–strain behavior of concrete confined by different types of FRP. In addition, for unconfined concrete, this model reduces directly to idealized stress–strain curves in existing design codes. In the development of this model, a number of important issues including the actual hoop strains in FRP jackets at rupture, the sufficiency of FRP confinement for a significant strength enhancement, and the effect of jacket stiffness on the ultimate axial strain, were all carefully examined and appropriately resolved. The predictions of the model are shown to agree well with test data.     

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.     

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.     

TESTS ON STRUCTURALLY DEFICIENT RC SLABS STRENGTHENED WITH FIBRE REINFORCED POLYMER (FRP) COMPOSITES

This paper reports the results of a series of tests on fibre reinforced polymer (FRP) strengthened reinforced concrete (RC) slabs, which were recently undertaken at the University of Technology,Sydney. The slabs were reinforced with high-strength low-ductile steel reinforcement and strengthened with either carbon FRP (CFRP) or glass FRP (GFRP) composites. The unstrengthened control slabs failed by fracture of the steel tension reinforcement while the FRP strengthened slabs failed by debonding of the FRP followed by rupture of the tension steel. The FRP-strengthened slabs were stronger than their unstrengthened counterparts and displayed considerable ductility.     

震后碳纤维布加固钢筋混凝土构件的设计步骤

碳纤维布对于限制设备破坏以及不合格的钢筋混凝土构件的破坏具有很大优势,已经被广泛地用于建筑物抗震的局部修复。通过系统评估各种被加固构件的抗力机理和握裹力作用,探讨了抗震修复／增强技术有关的设计问题和细节准则,包括：屈曲、剪力、互搭接头承载力、塑性铰性能以及与植入钢筋有关的变形能力和包裹应变能力。     

Fiber element modeling for seismic performance of bridge columns made of concrete-filled FRP tubes

Concrete-filled fiber reinforced polymer (FRP) tubes (CFFT) are an alternative to reinforced concrete, providing for rapid construction with comparable strengths and higher ductility. Despite encouraging test data, it is not clear as to whether traditional analytical tools may be used for prediction of structural performance of CFFT, especially in seismic applications. This paper reports on modeling of CFFT either as cast-in-place reinforced or precast post-tensioned column in conjunction with a reinforced concrete (RC) footing. The model is verified against two earlier experimental programs at the member-level and subassembly-level. The model was then used to conduct a parametric study of different column configurations under a constant axial load and a reversed cyclic lateral load. Moreover, seismic performance of a typical CFFT column is compared with its RC counterpart under three different ground acceleration records. The study shows that seismic analysis of CFFT columns is possible using available analytical tools for conventional RC columns. Also, CFFT columns demonstrate superior performance over their RC counterparts in response to wide-ranging ground acceleration records. Fiber architecture of the FRP tube could be optimized for strength and ductility. Internal steel reinforcement and a minimum thickness of FRP tube are deemed necessary to provide adequate ductility and system integrity in seismic applications.     

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.     

Finite element analysis of thin GFRC panels reinforced with FRP

Glass fibre reinforced concrete (GFRC) incorporating fibre reinforced polymer (FRP) bar reinforcement is potentially an ideal composite material for the manufacture of thin structural elements due to its superior durability over GFRC containing conventional steel reinforcement. GFRC without any bar reinforcement has only been used for small units and short spans due to its relatively low flexural strength. Until now, no work has been reported on the use of FRP bars in GFRC. The first part of the paper deals with the stress–strain characteristics of GFRC. In the second part the bond strength of GFRC with both steel and FRP reinforcing bars is determined from a series of 24 pullout tests from which the characteristics of the local bond stress–slip response was established. The results show that the bond of FRP bars in GFRC is, in general, better than the bond in normal concrete, and that conventional numerical models can be used to model the behaviour. The last part of the paper investigates the performance of a 3 m span thin GFRC permanent formwork panel, reinforced with FRP, both experimentally and analytically with finite element (FE) analysis. It is concluded that the behaviour of thin GFRC elements incorporating FRP reinforcement can be predicted by FE analysis in which the GFRC stress–strain characteristics and bond characteristics are modelled with robust spring elements.     

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.     

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.     

Deflection and crack-width prediction of concrete beams reinforced with glass FRP rods

Concrete beams reinforced with fiber reinforced polymer (FRP) bars exhibit large deflections and crack widths as compared to concrete beams reinforced with steel due to the low modulus of elasticity of FRP. Current design methods for predicting deflections at service load and crack widths developed in concrete structures reinforced with steel bars may not be used for concrete structures reinforced with FRP bars. Thus, the ACI 440 Committee has provided design guidelines for concrete beams reinforced with FRP bars. Verification of the ACI 440 methods for predicting deflections and crack widths for glass fiber reinforced polymer reinforced concrete beams are presented in this paper. In addition, improvement to the crack width equation was suggested to account for 2 layers of reinforcement. This study shows that ACI 440.1R-01 can be effectively used to predict deflections in concrete beams reinforced with FRP bars and crack width in beams with one-layer FRP bars. However, when FRP bars are placed in two layers, ACI 440.1R-01 can be used after some parameters are modified. Six full concrete beams reinforced with different GFRP reinforcement ratios were load tested and the measured deflections and crack widths were analyzed and compared with those predicted by the proposed models. The experimental results compared well with those proposed by the model.     

部分配置FRP筋框支剪力墙抗震性能研究

针对全钢筋混凝土框支剪力墙在工程中所存在的问题,提出了采用比强度高、耐腐蚀性能好、自重轻但弹性模量低并具有线弹性性能的纤维增强塑料(FRP)筋来替换该剪力墙中部分钢筋的建议.通过拟静力试验及非线性有限元数值分析,比较了1榀全钢筋混凝土框支剪力墙试件(FSW-1)和1榀部分配置FRP筋框支剪力墙试件(FSW-4)的裂缝发展规律和破坏模式,及其承载能力、延性性能和滞回特征.结果表明:部分配置FRP筋框支剪力墙结构具有较高的承载能力和较好的抗震性能;非线性有限元分析结果与试验结果吻合较好.