Bond strength and effective bond length of FRP sheets/plates bonded to concrete considering the type of adhesive layer

Recent experimental results of the FRP–concrete bonded joint using flexible adhesive showed that the most popular analytical models available in the literature underestimate the bond strength and the effective bond length of these experiments. Most of these existing models need to be modified to consider the type of adhesive layer. Consequently, the bond strength model proposed by Chen and Teng (2001) has been modified to consider the type of adhesive layer. An extensive database consisting of about 100 test results of FRP–concrete joint has been assembled to examine the validity of the proposed model taking the type of adhesive layer into consideration. The modified bond strength model is accurately capable of predicting the bond strength and the effective bond length.     

Experimental investigation into bond behavior of CFRP sheets attached to concrete using EBR and EBROG techniques

Over the last decade, an extreme increase in the application of fiber reinforced polymers (FRPs) for strengthening of reinforced concrete (RC) structures has been observed. The most common technique for strengthening of RC members utilizing FRP reinforcements is externally bonded reinforcement (EBR) technique. Despite certain benefits of the technique such as simple and rapid installation, the main problem which has greatly hampered the use of EBR method is premature debonding of FRP composite from concrete substrate. Recently, grooving method (GM) has been introduced as an alternative to conventional EBR technique. Grooving with the special technique of externally bonded reinforcement on grooves (EBROG) has yielded promising results in postponing or, in some cases, completely elimination of undesirable debonding failure in flexural/shear strengthened RC beams. Consequently, the main intention of the current study is to make a comparison between FRP-to-concrete bond behavior of EBR and EBROG techniques by means of single-shear bond tests. To do so, CFRP sheets were adhered to 16 concrete prism specimens using EBR and EBROG techniques. The specimens were then subjected to single-shear bond test and the results were compared. A non-contact, full field deformation measurement technique, i.e. particle image velocimetry (PIV) was utilized to investigate the bond behavior of the strengthened specimens. Successive digital images were taken from each specimen undergoing deformation during the test process. Images were then analyzed utilizing PIV method and load–slip behavior as well as slip and strain profiles along the strengthening CFRP strips were reported. Experimental results of the current study strongly verify the capability of GM for strengthening RC members to completely eliminate the debonding failure.     

Numerical evaluation of shear strengthening performance of CFRP sheets/strips and sprayed epoxy coating repair systems

A numerical study is conducted to evaluate the shear strengthening performance of two repair systems: CFRP sheets/strips and a sprayed epoxy coating. Micromechanical constitutive models for the CFRP sheets/strips and sprayed FRP coating proposed by Liang et al. [Liang Z, Lee HK, Suaris W. Micromechanics-based constitutive modeling for unidirectional laminated composites. Int J Solids Struct 2006;43:5674–89] and Lee et al. [Lee HK, Avila G, Montanez C. Numerical study on retrofit and strengthening performance of sprayed fiber-reinforced polymer. Eng Struct 2005;27:1476–87] and Lee and Simunovic [Lee HK, Simunovic S. Modeling of progressive damage in aligned and randomly oriented discontinuous fiber polymer matrix composites. Composites: Part B 2000;31:77–86] in conjunction with damage models, are implemented into the finite element code ABAQUS to solve boundary value problems. Using the implemented computational model, numerical simulations of four-point bending tests on concrete beams repaired with the repair systems are conducted to quantify their strengthening abilities. The numerical tests yield load–deflection curves from which the shear strengthening performance of the repair systems is evaluated. Furthermore, the present prediction is compared with available experimental data to assess the accuracy of the proposed computational model.     

Critical Analysis of Fibre‐Reinforced Polymer Near‐Surface Mounted Double‐shear Pull‐out Tests

The study of the bond behaviour between fibre‐reinforced polymer (FRP) systems and concrete is an issue that nowadays attracts many researchers. The scientific community dedicated to the research of FRP reinforcement has been conducting numerous experimental programmes aiming to assess the local bond–slip law of the FRP–adhesive–concrete connection. This paper reports the relevant results obtained by the Structural Composite Research Group of Minho University in the scope of an international Round Robin Test. The suitability of the recommended test setup to derive a local bond constitutive law for modelling the bond behaviour of near‐surface mounted reinforcement systems is discussed based on a deep interpretation of the results.     

Bond failure performances between near-surface mounted FRP bars and concrete for flexural strengthening concrete structures

Near-surface mounted (NSM) fiber reinforced polymer (FRP) has been established as an effective technique for strengthening concrete member. In preview literatures, bond failure was observed usually in the strengthened beam test for increasing flexural capacity. Bond behavior is of primary importance for the transfer of stress between the concrete and the FRP reinforcement to develop composite action. In this paper, a total of 22 tests were conducted to study the bond failure performance between NSM FRP bars and concrete besides only one test as a comparison. Failure modes, load–deflection curves, strain distribution of FRP bars, and local bond stresses at the FRP-epoxy adhesive interface from the tests were analyzed in detail. Some of the factors expected to affect bond performance were presented, namely: diameter of FRP bars, type to FRP material, concrete compressive strength and bonded length. The test results reported in this paper should be useful for further establishing local bond–slip constitute relationship and further verification of numerical simulation models, in addition to gaining a better understanding of bond failures for flexural strengthening concrete structures with NSM FRP bars.     

Bond between FRP composites and concrete: Assessment of design procedures and analytical models

The use of fibre reinforced polymers (FRPs) to strengthen reinforced concrete (RC) structures has gained a wide popularity in the last decades. Although many experimental and analytical studies are available in literature, some issues are still under discussion in the research communities. Since the typical failure mode of FRP–concrete joints is reported to be debonding of the composite from the concrete substrate [1], the estimation of the bond strength between FRP and concrete substrate represents a key issue for the proper use of this technology. For this reason, several analytical models for the evaluation of the FRP–concrete bond strength and few models for the estimation of the effective bond length were proposed (some of them are included in design codes/recommendations/guidelines); however they were not assessed by means of an appropriate experimental database.This work shows an assessment of twenty analytical models for the evaluation of the FRP–concrete bond strength. The assessment is based on the analysis of a wide experimental database collected from the literature. The results are provided distinguishing between the test setup adopted (single or double shear test, bending test) and the material used (post impregnated sheets or pre impregnated laminates). The accuracy of each model was evaluated by means of a simplified statistical analysis. The influence of the test setup and basic material on the accuracy of the model used was analysed as well. Lastly, the accuracy of twelve available models in providing an estimation of the effective bond length was also assessed.     

Friction coefficient between FRP pultruded profiles and concrete

This paper presents the experimental results of a direct shear test to determine the friction coefficient between fibre reinforced polymer (FRP) pultruded profiles and concrete. The FRP pultruded profile used in this study was glass fibre reinforced polymer (GFRP) I-section. The specimens were composed of a concrete block and a coupon of the GFRP pultruded profile. The experiment was conducted by using the direct shear test method. The parameters investigated included the type of concrete (self-compacting concrete and normal concrete) and the compressive strength of the concrete, as well as the different components (web and flange) of the I-section. The test results verify that the bond behaviour between the concrete and the GFRP pultruded profiles mainly depends on two factors, the friction stress and the adhesion stress. The friction coefficient between the FRP pultruded profiles and the concrete was between 0.5 and 0.6 when the normal stress fluctuated between 0.5 and 2 MPa, and the adhesion stress was about 0.2 MPa. The compressive strength of the concrete and the different components of the I-section have little effect on the friction coefficient, however, the type of the concrete significantly affects this coefficient.     

U型FRP加固钢筋混凝土梁受剪剥离性能的有限元分析

采用FRP布对梁进行抗剪加固,可以有效的解决梁因配箍率不足而导致的受剪承载力偏低的问题。根据文献[1]中7根试验梁的参数,针对工程中常用的U型FRP受剪加固形式,建立三维有限元分析模型,采用商业有限元计算软件ANSYS,数值模拟了加载全过程和受剪剥离受力性能,根据试验结果确定了FRP-混凝土界面粘结剥离强度,并建议了合适的裂面剪力传递系数。根据有限元分析结果,作者又进一步研究了U型FRP布的应变分布、分担剪力的贡献、剥离破坏的过程,以及加固量、FRP类型和粘贴面积率对加固梁受剪承载力的影响。在有限元分析的基础上结合试验结果,建议了U型粘贴加固的受剪剥离承载力计算方法。     

Influence of FRP anchor fan configuration and dowel angle on anchoring FRP plates

To enhance the strain capacity of fibre-reinforced polymer (FRP) plates which have been bonded onto reinforced concrete (RC) members for strengthening purposes, FRP anchors can be utilised. Research on the characterisation of FRP anchors is still quite limited though despite the increasing use of FRP anchors in practice. In order to reduce such a knowledge gap, this paper reports the results of 30 single-shear FRP-to-concrete joint tests of which 26 joints were anchored with FRP anchors of differing geometric configurations and four joints were unanchored controls. More specifically for the anchored joints, the connection of the FRP anchor to the FRP plate via so called fan fibres was varied in addition to the angle of anchor insertion and these test parameters represent fundamentally important anchor components which have not been researched to date. Failure modes, joint strengths, load–slip responses as well as FRP plate strain distributions are reported and a relationship relating the influence of anchor insertion angle to joint strength is provided. A maximum increase in joint strength of 160% on average above the unanchored control joints was achieved. In addition, the maximum strain resisted by the FRP plate, relative to its elongation capacity, was increased on average from 25% for unanchored control joints to 67% for some anchored joints.     

Effect of confinement using steel, FRC, or FRP on the bond stress-slip response of steel bars under cyclic loading

The results of experimental investigation of the local bond stress-slip response of steel bars embedded in confined concrete and subjected to cyclic loading are presented. Different types of confinement and their effect on the bond stress-slip response were evaluated and compared. These included internal confinement by ordinary transverse steel ties or steel fiber reinforcement, and external confinement by fiber reinforced polymer (FRP) composites. Beam specimens with spliced reinforcement at midspan were tested. The test parameters included the size of the steel bars, the ratio of concrete cover to bar diameter, and the amount of confinement. Without confinement, the specimens suffered significant bond deterioration and loss in load resistance in the first or second load cycle after bond splitting. Confining the concrete with transverse steel, steel fiber reinforcement or FRP composites within the splice region increased the bond strength and reduced the bond degradation with the number of loading cycles, leading to significant improvement in seismic performance. The envelope curve of the cyclic bond stress-slip response showed very good agreement with earlier test results obtained under static load conditions and the results predicted using an analytical model of the local bond stress-slip response of steel bars embedded in confined concrete.     

抗剪加固FRP与混凝土界面粘结性能的试验研究

为了深入理解抗剪加固FRP与混凝土界面的粘结性能,该文对侧面粘贴FRP的21根预置裂缝小梁进行了试验研究。试验设计考虑了FRP粘结长度、宽度、厚度及FRP纤维方向与裂缝张开方向的角度(简称“FRP纤维受拉角度”,用θ表示)对FRP与混凝土界面粘结性能的影响。试验结果表明:1) 所用的预置裂缝小梁可以有效对FRP与混凝土的界面粘结性能进行试验研究;2) FRP粘结长度、宽度、厚度及纤维受拉角度等因素对粘结强度有明显影响;3) FRP应变沿宽度方向的分布是不均匀的,这主要是由于裂缝不均匀开展导致FRP在裂缝开展快的一边先剥离。     

Prediction of debonding strength for masonry elements retrofitted with FRP composites using neuro fuzzy and neural network approaches

This paper proposes application of neuro fuzzy and neural network for predicting debonding strength of retrofitted masonry elements. In order to achieve high-fidelity model, this study uses extensive experimental databases for bond test results between Fiber Reinforced Polymer (FRP) and masonry elements by collecting existing bond test subassemblage tests from the literature. Various influential parameters that affect debonding resistance including thickness of the FRP strip, width of the FRP strip, elastics modulus of the FRP, bonded length, tensile strength of the masonry block and width of the masonry block are considered as input parameters to the artificial neural network (ANN) and adaptive neuro fuzzy inference system (ANFIS). Test results of the ANN and ANFIS models were compared with multiple nonlinear regression, multiple linear regression and existing bond strength models. The accuracy of the optimal MNLR model was increased by 39% and 23% with respect to RMSE and MAE criteria using ANFIS. The comparison results indicated that the ANN and ANFIS models performed better than the other models and could be successfully used for prediction of debonding strength of retrofitted masonry elements.     

Fe modelling of CFRP-concrete interface subjected to cyclic temperature,humidity and mechanical stress

Modelling of environmental effects on the bond surface between FRP composites and concrete is crucial for the purpose of life prediction of strengthened members. The service performance of composite systems depends on the degree of chemical and physical aging of the system. Physical aging is a reversible process which is related mainly to temperature fluctuations. Chemical aging is the permanent molecular level degradation of fibres, resin or interfaces due to long-term exposure to the environment. The resulting strength reduction and the system behaviour due to aging are important in evaluating the strengthening of civil engineering infrastructure. In this paper, a finite element model is developed to predict the interface properties after long-term exposure to the cyclic temperature and the constant humidity. Effects of sustained loading are also taken into account. The predicted strength, cracks pattern, failure mode and strain variation along the bond line agree well with test results.     

考虑拉伸刚化效应的FRP筋/混凝土轴拉构件变形计算方法

考虑拉伸刚化效应是精确计算纤维增强树脂复合材料(FRP)筋/混凝土构件变形和裂缝的基础。提出了考虑拉伸刚化效应的FRP筋/混凝土拉伸构件变形计算的解析方法。首先,对修正Eligehausen黏结滑移模型(修正BPE模型)进行简化提出四线性黏结-滑移模型。根据该模型推导了拉伸构件在不同拉伸荷载阶段的FRP筋、混凝土应力和变形及黏结力和滑移量的分布表达式。结合混凝土开裂判别方法,提出了FRP筋/混凝土拉伸构件的全过程变形计算方法。通过与已有文献试验结果对比验证了本文方法的准确性。对影响拉伸刚化的一些参数进行了敏感性分析。结果表明,混凝土强度和配筋率对拉伸刚化效应影响不大,FRP筋弹性模量是影响拉伸刚化效应的主要因素。      

Behavior of square concrete columns reinforced with Prefabricated Cage System

Behavior of normal strength concrete columns reinforced with a new reinforcement, termed Prefabricated Cage System (PCS) is investigated. A total of 16 small-scale PCS and rebar reinforced column specimens were constructed and tested under monotonic axial displacement. The experimental results indicate that the overall behavior of rebar and PCS reinforced specimens are comparable prior to achieving the peak column load. In general, PCS specimens are more ductile and absorb more energy than similar rebar specimens after the peak load is reached. The effect of different parameters, such as the steel tube thickness, number of longitudinal reinforcements, transverse steel spacing, and crossties on the specimens’ strength and deformation capacity are investigated. A confined concrete model is proposed and used to predict the load–displacement response. The theoretical load–displacement relations obtained from the proposed model are compared with those obtained from the Mander et al. (J Struct Eng 114(8):1804–1826, 1988) confinement model.     

Reinforcement of concrete beam–column connections with hybrid FRP sheet

The paper describes the results of tests on prototype size reinforced concrete frame specimens which were designed to represent the column–beam connections in plane frames. The tests were devised to investigate the influence of fibre reinforced plastic (FRP) reinforcement applied to external surfaces adjacent to the beam–column connection on the behaviour of the test specimens under static loading. Of particular interest under static loading was the influence of FRP reinforcement on the strength and stiffness of beam–column connection. As a key to the study, the hybrid FRP composites of E-glass woven roving (WR) and plain carbon cloth, combined with chopped strand mat (CSM), glass fiber tape (GFT) with a vinyl-ester resin were designed to externally reinforce the joint of the concrete frame. The results show that retrofitting critical sections of concrete frames with FRP reinforcement can provide signification strengthening and stiffening to concrete frames and improve their behaviour under different types of loading. The selections of types of FRP and the architecture of composites in order to improve the bonding and strength of the retro-fitting were also discussed.     

Fatigue behaviour characterization of the fibre-matrix interface

The fracture behaviour of FRP composite materials is significantly influenced by the behaviour of the fibre-matrix interfacial bond. Thus far interfacial bond mechanical characterization has been based upon the critical strength and critical fracture energy of debonding. Characterization of the fatigue behaviour of the interfacial debonding process, however, may be more valuable for composite design and fibre-matrix selection. A fracture mechanics model of interfacial bond fatigue based on the mode II strain energy release rate (G _II) is presented. An expression forG _II is derived for a single fibre in matrix cylinder model. By fitting the model to single fibre pull-out fatigue test data, fatigue crack propagation plots for specific fibre-matrix combinations can be drawn. These should prove useful for the development of fatigue resistant FRP composite materials.     

海水浸泡对FRP筋-珊瑚混凝土粘结性能的影响

开展了30℃海水浸泡条件下玻璃纤维增强树脂基复合材料（GFRP）筋、碳纤维增强树脂基复合材料（CFRP）筋与珊瑚混凝土粘结性能的试验研究,分析了纤维增强树脂基复合材料（FRP）筋-珊瑚混凝土粘结滑移曲线特征、破坏形态及粘结强度变化。试验结果表明,海水浸泡后FRP筋力学性能和粘结性能均表现为不同程度的降低。随浸泡时间增加,GFRP筋表层树脂与纤维间的孔隙率明显增大,并逐渐出现脱粘现象,纤维本身遭受到侵蚀,而CFRP筋仅表面基体有少许损伤,其耐久性明显优于GFRP筋;FRP筋-珊瑚混凝土粘结强度呈现出先增加后减小的趋势,且后期下降速率逐渐变小,部分GFRP筋-珊瑚混凝土试件的破坏模式逐渐由筋被拔出转变为筋材断裂;增加保护层厚度能有效地减缓海水对GFRP筋的侵蚀,有利于保持GFRP筋-珊瑚混凝土间的粘结性能。     

Numerical modelling of the effects of elevated service temperatures on the debonding process of FRP-to-concrete bonded joints

There are many issues concerning the performance behaviour of FRP-to-concrete interfaces at elevated service temperatures (EST). At EST, i.e. slightly above the glass transition temperature (T_g), some properties associated with the FRP composites, such as the stiffness, strength or the bond characteristics, degrade. This is a crucial issue and there are only a few studies that take into account such effects on FRP-to-concrete interfaces at EST. This paper examines, through a numerical analysis, the performance of FRP-to-concrete bonded joints at EST using a new discrete model based on truss elements and shear springs. The External Bonded Reinforcement (EBR) systems subjected to EST are analyzed. The numerical discrete model was implemented in a MATLAB routine and the bond–slip curves of the interfaces at EST were obtained from a model found in literature. The numerical results revealed that the interface at EST behaves similarly to one with two equal mechanical loads applied at both ends of the FRP plate. The load–slip curves or bond stresses, strains or slippages along the bonded length obtained from several bond–slip curves at different temperatures were obtained. Two different single-lap shear tests were simulated at steady-state (steady temperature followed by load increase) and transient state (steady load followed by temperature increase). Regarding the influence of the temperature on the adhesion between the FRP and concrete, the results showed that an increase in the temperature at an earlier situation, i.e. during a period where temperature had no influence in the concrete deformations, leads to an increase in the effective bond length of the interface affecting the initial strength of the interface.