Cross sectional shape effects on the performance of post-heated reinforced concrete columns wrapped with FRP composites

An experimental study was conducted to examine how the cross sectional shape affects the strength and ductility of post-heated reinforced concrete columns wrapped with unidirectional fibre reinforced polymer (FRP). Seventeen columns were tested under axial compression. The main variables investigated were the cross sectional shape of the columns, the presence of heat damage and the type of FRP used for repair. The columns were placed into three groups defined by columns without being subjected to heat, post-heated columns and post-heated and repaired columns. The test results showed that the load carrying capacity of post-heated FRP wrapped columns was significantly affected by the column’s original cross sectional shape. For circular sections the strength of post-heated columns was restored up to, or greater than, its original pre-heated strength. However, the strength of post-heated GFRP or CFRP wrapped square columns was recovered to some extent but not to the level of its original pre-heated strength. It was also found that the increase in the ductility of circular columns was more pronounced compared to square columns after wrapping with FRP. For all damaged columns the use of FRP did not restore the column’s stiffness which was lost due to damage caused by heating.     

Behaviour of FRP wrapped normal strength concrete columns under eccentric loading

Eccentric loads are common for columns in buildings and other types of structures. Columns that are in the border of buildings, especially corner columns and columns near opening are usually subject to a combination of axial load and bending moment, thus creating an equivalent eccentric load. Fibre Reinforced Polymers have been used in strengthening/retrofitting columns and other types of elements. The results by and large are satisfactory. Most of the research studies undertaken in strengthening columns are based on concentrically loaded columns. In effect, the behaviour of FRP wrapped columns under the influence of eccentric loading is less known compared to concentrically loaded columns. This paper presents results of testing six normal strength concrete columns under eccentric loading. The columns are wrapped with different number of layers of FRP. Results show that wrapping a column with an adequate number of FRP layers will result in higher strength, ductility and energy absorption than a column reinforced with steel bars.     

The behaviour of FRP wrapped HSC columns under different eccentric loads

The majority of columns are subjected to a combination of an axial load and a bending moment in one or two directions. With a few exceptions, most of the research in the area of FRP wrapped columns have concentrated on the behaviour of concentrically loaded columns. This paper presents results of testing nine reinforced high strength concrete columns. The column specimens are circular in shape with 205 mm diameter and 925 mm height. Concrete compressive strength was 65 MPa. All columns were reinforced with steel. Three columns were not wrapped, three columns were wrapped with three layers of carbon FRP and three columns were wrapped with three layers of E-Glass FRP. From each of the three groups, one column was tested concentrically, one column was tested with a 25 mm eccentric load and one column was tested with a 50 mm eccentric load. Results of testing the columns have shown that the carbon FRP is most effective in increasing the strength and ductility of columns.     

Behavior of corrosion-damaged RC columns wrapped with FRP under combined flexural and axial loading

This paper presents the results of a research program aimed at investigating the effectiveness of carbon fiber-reinforced polymers (CFRP) to upgrade corrosion-damaged eccentrically loaded reinforced concrete (RC) columns. A total of 16 square RC columns with end corbels were constructed. Test specimen had an overall length of 1200 mm whereas each end corbel had a cross section of and a length of 350 mm. The specimen in the test region was having longitudinal steel ratio of 1.9%. The damaged specimens were exposed to 30 days of accelerated corrosion that corresponded to a steel mass loss of about 4.25%. The main test parameters were the CFRP repair scheme (no wrapping, full-wrapping, and partial-wrapping) and the eccentricity-to-section height (e/h) ratio (0.3, 0.43, 0.57, and 0.86). The strength of the damaged columns fully wrapped with CFRP was up to 40% higher than that of the control undamaged columns. The strength gain was inversely proportional to the eccentricity ratio. Partial CFRP-wrapping was 8% less effective than full CFRP-wrapping at nominal e/h of 0.3. At higher e/h values, the confinement level had a negligible effect on the columns’ strength. An analytical model was then proposed to predict the columns’ strength under eccentric loading. A comparative analysis between predicted and experimental results demonstrated the model’s accuracy and reliability.     

Axial stress–strain relationship for FRP confined circular and rectangular concrete columns

A general mathematical model is developed to describe the stress–strain (f_c–ε_c) relationship of FRP confined concrete. The relationship is applicable to both circular and rectangular columns, and accounts for the main parameters that influence the stress–strain response. These include the area and material properties of the external FRP wraps, the aspect ratio of rectangular column sections, the corner radius used for FRP application, and the volumetric ratio and configuration of internal transverse steel. The proposed model reproduced accurately experimental results of stress–strain or load–deformation response of circular and rectangular columns. In addition to its importance in evaluating the effect of FRP confinement on the ultimate axial strength of concrete columns, the developed f_c–ε_c relationship can be employed very efficiently and effectively for analyzing the response of FRP confined concrete under different types of load application.     

Repair and strengthening of reinforced concrete square columns using ferrocement jackets

A series of 10 one-third scale square reinforced concrete column specimens were cast; preloaded under axial compression up to various fractions (0%, 60%, 80%, and 100%) of its ultimate load; repaired using ferrocement jackets containing two layers of Welded Wire Mesh (WWM) encapsulated in high strength mortar; and then retested to failure. The overall response of the specimens was investigated in terms of load carrying capacity, axial displacement, axial stress and strain, lateral displacement, and ductility. The test results indicated that jacketing reinforced concrete square columns with this form of ferrocement provided about 33% and 26% increases in axial load capacity and axial stiffness, respectively, compared to the control columns. The test results also indicated that repairing similar reinforced concrete columns (after preloading them to failure) with the same ferrocement jacket almost restored their original load capacity and stiffness. Furthermore, the repaired columns failed in a ductile manner compared to the brittle failure exhibited by the control columns.     

玄武岩纤维布约束高温损伤混凝土方柱轴压力学性能试验

对36个玄武岩纤维布增强聚合物基复合材料（BFRP）约束的高温损伤混凝土方柱和15个不同高温损伤的对比试件进行了轴压试验。试验表明,玄武岩纤维布横向约束能改变高温损伤后混凝土方柱的破坏形态,显著提高混凝土方柱的轴压强度和变形能力。其中三层玄武岩纤维布包裹的200℃、400℃、600℃和800℃高温损伤混凝土方柱轴压强度分别提高了48%、130%、206%和389%,轴向变形分别提高了433%、344%、319%和251%。采用典型的纤维增强聚合物基复合材料（FRP）约束常温未损伤混凝土轴压力学性能的设计模型预测FRP约束高温损伤混凝土的轴压强度和变形时存在较大的偏差。通过构建柱状膜结构静水压力平衡模型和约束混凝土方柱与FRP体积应变能平衡模型,分别改进了FRP约束混凝土方柱轴压极限应力和极限应变计算模型的基本形式。基于该基本形式和试验数据,分别确定了BFRP约束高温损伤混凝土方柱轴压极限应力和极限应变计算中与温度相关的参量,提出了适用于高温损伤混凝土方柱的轴压极限应力和极限应变的设计模型。      

Effect of fiber orientation on the structural behavior of FRP wrapped concrete cylinders

In this study, 27 concrete cylinders with a diameter of 152.4 and a height of 304.8 mm were prepared. Among them, 18 cylinders were wrapped using two layers of fiber reinforced polymer (FRP) with six fiber orientations; six cylinders were wrapped using four layers of FRP with fibers in axial or hoop direction only; the remaining three cylinders were used as control. The FRP used was E-glass fiber reinforced ultraviolet (UV) curing vinyl ester. Fifteen coupon specimens were prepared to experimentally determine the tensile strength of the FRP with fibers oriented at 0°, 45°, and 90° from the loading direction. Co-axial compression tests were conducted on the wrapped cylinders and control cylinders. The test results were compared with existing confinement models. It is found that the strength, ductility, and failure mode of FRP wrapped concrete cylinders depend on the fiber orientation and wall thickness. Fibers oriented at a certain angle in between the hoop direction and axial direction may result in strength lower than fibers along hoop or axial direction. A larger database is desired in order to refine the existing design-oriented confinement models.     

Post-repair performance of eccentrically loaded RC columns wrapped with CFRP composites

This paper presents the results of a research program for examining the post-repair performance of eccentrically loaded corrosion-damaged reinforced concrete (RC) columns wrapped with carbon fiber reinforced polymer (CFRP) composites. The specimens, except a control undamaged group, were initially exposed to accelerated corrosion for 30 days using an impressed current technique. Following the initial corrosion, the damaged specimens were either repaired with full or partial CFRP wrapping systems or kept unrepaired. A group from the damaged specimens were further exposed to 60 days of corrosion exposure. All specimens were tested to failure under various eccentric loading with a nominal eccentricity-to-section height ratio (e/h) in the range of 0.3–0.86. Test results showed that full CFRP wrapping system effectively reduced the post-repair corrosion rate relative to that of the unwrapped specimens whereas partial CFRP wrapping had almost no effect on the steel mass loss. The strengths of the damaged specimens fully wrapped with CFRP were higher than those of the control specimens at the end of the post-repair corrosion phase. The strengths of the damaged partially wrapped specimens were higher than those of the control at nominal e/h values 0.43. At higher e/h values, the strengths of the partially wrapped specimens were lower than those of the control but still higher than those of the damaged unrepaired specimens. An analytical model that accounts for the confinement effect of the CFRP and the change in geometry under eccentric loading was employed to predict the columns’ strength. The model’s predictions were validated against test results.     

FRP strengthening of steel and steel-concrete composite structures: an analytical approach

A recent technique for strengthening steel and steel-concrete composite structures by the use of externally bonded Fiber Reinforced Polymer (FRP) sheets, to increase the flexural capacity of the structural element, is described. Several researches developed FRP strengthening of reinforced concrete and masonry structures, but few experimental studies about steel and steel-concrete composite elements are available. Some examples of guidelines for the design and construction of externally bonded FRP systems for strengthening existing metal structures are available, but the method used to predict the flexural behaviour of FRP strengthened elements is usually based on the hypothesis of elastic behaviour of materials and FRP laminate is mainly considered only under the tensile flange. In this paper, an analytical procedure to predict the flexural behaviour of FRP strengthened steel and steel-concrete composite elements, based on cross-sectional behaviour and taking into account the non-linear behaviour of the materials with any configuration of FRP reinforcement, is given. Analytical predictions are compared with some experimental results available in the literature on the flexural behaviour of FRP strengthened steel and steel-concrete composite elements, showing good agreement of the results, even in the non-linear phase, until failure.     

Evaluating and proposing models of circular concrete columns confined with different FRP composites

Commonly used fiber-reinforced polymer (FRP) includes Carbon, Glass, and Aramid FRP composites. Meanwhile, some new FRPs such as PBO (Polypara-phenylene-Benzo-bis-Oxazole), PET (Polyethylene Terephthalate/Polyester), Dyneema, and Basalt have been gradually applied in recent years. Over the past 20 years, there has been extensive research on modeling of stress–strain response of confined concrete using the common types of FRP. In this study, most popular and recent models are investigated to evaluate their general practical application in predicting the response of FRP-confined concrete with strain-hardening performance without any restriction on the fiber used. The aim of the study is twofold. In case of different types of FRP composites, providing equivalent confinement modulus (lateral stiffness), five models are employed to find the FRP-confined concrete stress–strain relationship of three scale-model circular columns. Second ascending part (second stiffness) of the stress–strain relationship of FRP-confined concrete with strain-hardening performance is evaluated in the light of available database from the existing literature using these analytical models. The results showed that the examined models do not satisfy the fact that the slope of the second ascending branch of the stress–strain curve of FRP-confined concrete is independent of its types, provided the design confinement modulus is the same. A comparison of predicted values of the second stiffness with the collected test results of 257 cylinder specimens confined with the common types of FRP composites revealed the necessity for a more accurate model. Based on the discussion of the features and accuracy of these models, a model considering the effect of FRP lateral stiffness is proposed. Because of the variability observed in the test data, however, it appears impossible to develop simple empirical models based on the current database with less than approximately 27% mean absolute error for the second stiffness, and 16% mean absolute error for ultimate strength.     

喷射FRP加固震损钢筋混凝土柱抗震性能试验

通过12组72件喷射纤维/树脂复合材料(FRP)试样的拉伸强度试验,研究了纤维种类、树脂基体材料、纤维体积分数、纤维混杂比及纤维长度等因素对喷射FRP拉伸强度、弹性模量和断裂伸长率等性能的影响。通过8根钢筋混凝土(RC)柱试件的拟静力试验,研究了喷射玄武岩纤维/树脂复合材料(BFRP)和混杂玄武岩-碳纤维/树脂复合材料(BF-CFRP)加固震损RC柱的抗震性能,分析了喷射FRP层厚度、纤维混杂比、柱预损程度和柱轴压比等对加固试件的极限承载力、抗侧变形能力、刚度退化特征和滞回特性的影响。结果表明:玻璃纤维与乙烯基酯树脂基体的协同工作性能最优,而玄武岩纤维具有耐久性高、延性好、与乙烯基酯树脂基体协同工作性能好等优良性能,可以作为玻璃纤维的良好替代品;玄武岩纤维混杂少量比例的碳纤维作为树脂基体增强材料,可以有效提高喷射FRP的拉伸强度和变形性能;震损RC柱经喷射FRP加固后,可以基本恢复其震损前设计极限承载力,并有效提高其延性和耗能能力。该加固方法可以对地震区已震损RC柱进行快速加固,有效防止整体结构在余震中发生倒塌等严重破坏。      

An experimental study on the effect of environmental exposures and corrosion on RC columns with FRP composite jackets

The objective of this study was to evaluate the effects of various environmental conditions on the long-term behavior of reinforced concrete (RC) columns strengthened with carbon fibre reinforced polymer (CFRP) and glass fibre reinforced polymer (GFRP) sheets. Small-scale RC columns were manufactured in the laboratory and conditioned under accelerated environmental cycling and accelerated corrosion process of reinforcing bars. Then, uni-axial compressive failure tests were conducted in order to evaluate the change of mechanical properties of the test columns due to the environmental effects. The results revealed that the mechanical properties of RC column system (RC + FRP) were altered due to the environmental conditioning and the corrosion of steel reinforcement, and each type of environmental conditions had its unique effects and features.     

FRP与混凝土组合结构研究综述

提出一种在塑性铰区域采用高延性纤维增强水泥基复合材料(ECC)替代混凝土来改善FRP筋-钢筋增强混凝土柱抗震性能的新方法。对FRP筋-钢筋增强ECC-混凝土构件进行了低周往复荷载试验,系统地考察了基体材料、筋材种类、轴压比对构件破坏模态、裂缝模式、承载力、残余变形、延性和耗能能力的影响。结果表明,将ECC替代塑性铰区域混凝土能够有效避免FRP筋的受压屈曲,进而显著提升组合柱的抗震性能。与钢筋增强ECC-混凝土组合柱相比,复合筋增强ECC-混凝土组合柱的残余变形明显更小,且屈服后的刚度更高。随着轴压比的增大,构件极限强度升高但变形能力降低。通过有限元参数分析可知,组合柱的承载力和变形能力均随着ECC抗压强度及总配筋率的增大而增大;在总配筋率不变的情况下,FRP筋占比越高,构件的延性越好。     

Torsional and flexural buckling of composite FRP columns with cruciform sections considering local instabilities

In this paper, a semi-analytical finite strip method is developed for the prediction of torsional and flexural buckling stresses of composite FRP columns under pure compression. Numerical finite strip results will be compared with those obtained from closed-form equations for doubly symmetric open thin-walled FRP sections. The accuracy of the proposed finite strip method in determining critical flexural and torsional stresses of FRP columns will be assessed. Among the composite FRP columns with doubly symmetric open sections, buckling behavior of stiffened and unstiffened FRP cruciform sections will be evaluated and case studies performed. The effect of material properties and longitudinal stiffeners applied at the end of the web-plate and flange-plate on buckling modes of composite FRP cruciform sections is also reviewed.     

Effect of imperfections in the bond on the strength of FRP wrapped cylindrical concrete columns

Effect of imperfections at the interface between concrete and FRP on the strength of FRP confined axially loaded cylindrical concrete columns is investigated, experimentally and numerically. It is seen that the presence of imperfections facilitates localization of deformation, adversely affects the confining capacity of FRP, and reduces the failure load. The influence of size, location and orientation of imperfection on failure load is studied: the orientation and location are found to be more important than size. Critical locations and orientations of the imperfection are found and explained in terms of the mechanics of shear banding in pressure-sensitive elasto-plastic materials.     

Flexural ductility of high-strength concrete columns with minimal confinement

The use of high-strength concrete (HSC) instead of normal-strength concrete (NSC) in columns has the advantage of allowing the column size to be reduced and is thus becoming popular. However, since HSC is more brittle than NSC, its use could result in undesirable brittle failure. To evaluate the ductility of columns, nonlinear moment–curvature analysis taking into account the stress-path dependence of the steel reinforcement is required. Based on such analysis, a parametric study has been conducted to investigate the effects of various factors on the ductility of columns. The results revealed that the effect of concrete strength is dependent on the axial stress level (axial load to area ratio) and axial load level (axial load to capacity ratio). At the same axial stress level, the use of HSC has little or basically no adverse effect on the ductility but if the same axial load level is maintained to reduce the column size, the use of HSC would significantly reduce the ductility. Finally, two formulas for direct evaluation of the ductility of columns are developed.     

GBT formulation to analyse the buckling behaviour of FRP composite open-section thin-walled columns

This paper presents a Generalised Beam Theory (GBT) formulation to analyse the local and global buckling behaviour of FRP (fibre-reinforced polymer) composite thin-walled columns with arbitrary open cross-sections, which takes into account both shear deformation and cross-section deformation effects. After describing the steps and procedures involved in performing the GBT cross-section analysis of an arbitrarily branched composite (laminate plate) thin-walled member, the paper addresses the numerical implementation of the proposed GBT formulation, carried out by means of the finite element method (GBT-based beam element) – particular attention is devoted to the derivation of the element linear and geometric stiffness matrices, which incorporate all the material coupling effects. In order to illustrate the application and capabilities of the proposed formulation and implementation, several numerical results are presented and discussed, dealing with the local and global buckling behaviour of FRP composite I-section columns with different ply orientations and stacking sequences. Taking advantage of the GBT modal features, deep insight is acquired on the complex composite member buckling mechanics, namely those involving bending–torsion or global–local coupling effects. In particular, one investigates the influence of (i) the constitutive assumption regarding the transverse extension occurring in the cross-section composite walls and (ii) the distribution of pre-buckling normal stresses (due to axial compression) on the buckling behaviour of I-section columns. For validation purposes, the above results are compared with values recently reported in the literature and estimates obtained from shell finite element analyses.