FRP-confined Reinforced Concrete Columns Research Articles

Plastic hinge behavior of rectangular CFRP-confined RC columns: Meso-scale modelling and formulation

The ultimate rotation capacity, depending on the plastic hinge length, is a vital factor in the seismic design of flexural members. This paper focuses on the investigation of the plastic hinge length and size effect of rectangular carbon fiber-reinforced polymer (CFRP)-confined reinforced concrete (RC) columns. Firstly, considerable scatter and evident bias of the calculation results were found after evaluating existing models for calculating the plastic hinge length of FRP-confined RC columns. Accordingly, a meso-scale numerical approach was developed, with concrete heterogeneity considered, to investigate the influences of the confinement ratio and corner radius on the plastic hinge length and size effect of rectangular CFRP-confined RC columns. Results show that columns having different cross-sectional sizes exhibit similar failure patterns, and the region of lateral crack propagation for columns with lower confinement ratios is larger than that for columns with higher confinement ratios. The actual plastic hinge length is determined based on the yielding zone of reinforcement, the crushing zone of concrete, and the localization zone of curvature, while the localization zone of curvature is employed to determine the real plastic hinge zone. The size effect is found to exist in the plastic hinge length ratio, and it is weakened with the increase of the corner radius, which can be attributed to the fact that columns with higher corner ratios have more effective confinement. Taking into account the size effect, a new model for calculating the ultimate drift ratio of CFRP-confined RC columns was proposed based on an assumption of curvature distribution and presented accurate predictions.

Axial stress-strain model for concrete in partially FRP wrapped reinforced concrete columns

The application of fiber-reinforced polymer (FRP) composites has become a commonly-used technique for the strengthening/retrofitting of existing reinforced concrete (RC) columns. To date, extensive experimental studies have been conducted in this field, resulting in many stress-strain models of FRP-confined concrete for analysis or design use. Few models, however, are concerned with the concrete in FRP-confined RC columns, especially for the columns under partial FRP confinement. This paper first investigates the mechanical properties of the concrete under FRP strip-transverse steel reinforcement composite confinement via the assessment of existing analysis-oriented models (AOMs). Then, the peak stress of actively confined concrete and the dilation model in AOM are calibrated based on existing research, leading to a new AOM for FRP strip-steel confined concrete. Finally, a test database is assembled to evaluate the reliability of the new model in terms of the predictions of ultimate conditions and stress-strain curves. By comparing the experimental results and theoretical predictions, the proposed model exhibits higher accuracy for the FRP strip-steel confined concrete compared with the other existing models.

Behavior of large-scale FRP-confined square RC columns with UHP-ECC section curvilinearization under eccentric compression

Implementing section curvilinearization (SC) by curving flat sides into curved sides before fiber-reinforced polymer (FRP) jacketing has been proposed to enhance the FRP confinement effectiveness of FRP-confined rectangular concrete columns. In this paper, ultra-high performance engineered cementitious composite (UHP-ECC) is proposed as the added concrete for SC, with an aim to enhance both bending and axial strengths of concrete columns given that the UHP-ECC can contribute to resisting tensile stresses for columns. Thirteen large-scale FRP-confined square reinforced concrete (RC) columns with SC using UHP-ECC were tested under eccentric compression loading to explore their eccentric compressive behavior. The test parameters included the rise-to-span ratio, the FRP thickness, the eccentricity and the surface roughness of the old concrete. The test results show that FRP confinement with SC using UHP-ECC can substantially enhance the peak axial load and the bending moment than those without SC, while the ultimate lateral displacement is marginally increased. An increase in the rise-to-span ratio leads to an increase in the peak axial load.

Numerical Investigation of Axial Force–Bending Moment Interaction for FRP-Confined Reinforced Concrete Columns with Internal Steel Transverse Reinforcement

Externally bonded fiber-reinforced polymer (FRP) laminates are widely used in the retrofitting and rehabilitation of reinforced concrete (RC) columns because they can increase the axial and bending moment capacities of these columns through a confinement mechanism. The confinement produced by FRP laminates acts in addition to that exerted by the internal transverse steel, although the latter is often neglected in current design standards and guidelines. In this study, a recently developed FRP-and-steel-confined concrete constitutive model was employed to numerically investigate the axial force–bending moment interaction for FRP-confined RC columns modeled using finite-element (FE) analysis. The proposed model was first validated against experimental data available in the literature and then used to quantify, through an extensive parametric study, the effects of transverse steel confinement, FRP strength, FRP stiffness, FRP reinforcement ratio, column diameter, concrete compressive strength, and load eccentricity ratio on the strength of FRP-confined RC columns subject to combined axial compression and bending moment. It was found that the internal steel confinement can substantially enhance the strength of these columns, especially for low concrete compressive strengths, large cross sections, and small eccentricities. When design code provisions limiting concrete and FRP deformations were considered for eccentrically loaded columns, the contribution of the steel confinement increased for increasing FRP reinforcement ratio. Based on the parametric study results, this investigation proposed an extension to eccentric axial compression of two relative confinement coefficients, which were previously developed to describe the contribution of transverse steel confinement to the peak strength of FRP-confined RC columns subject to pure compression.

Compressive behavior degradation of FRP-confined RC columns exposed to a chlorine environment

Fiber-reinforced polymer (FRP) has been recognized as a promising method for strengthening reinforced concrete (RC) structures under a chlorine environment; however, it cannot prevent steel reinforcements from chloride-induced corrosion. Thus, the main objectives of this study were to (1) investigate the corrosion-induced performance degradation in FRP-confined RC columns under axial compression; (2) quantity the reduction of FRP rupture strain in FRP-confined RC columns subjected to a chlorine environment; and (3) propose a formula to estimate the load carrying capacity of FRP-confined RC columns considering the corrosion ratio. Twenty-eight columns with different FRP types (carbon fibre reinforced polymer (CFRP) and glass fibre reinforced polymer (GFRP)) and different layers of FRP (0, 1, 2, and 3) were conditioned under an accelerated corrosion process to obtain various corrosion ratios (0%, 5%, 10%, and 20%) and then tested under axial compression. The results showed that steel corrosion degraded the compressive response of FRP-confined RC columns, especially their deformation capacity. GFRP-confined RC columns generally had a better deformation capacity and were less vulnerable to steel corrosion than were CFRP-confined RC columns. Based on these test results, predictive equations for estimating the rupture strain of FRP were proposed as a function of corrosion ratio, which was introduced to modify an existing strength model of concrete confined with both FRP and steel stirrups. Finally, the ultimate load of FRP-confined RC columns with or without corrosion was predicted based on the modified concrete strength model.

COMPREHENSIVE DESIGN-ORIENTED MODEL FOR FRP-CONFINED CIRCULAR RC COLUMNS

Today, the use of fiber-reinforced polymers (FRPs) for the confinement of reinforced concrete (RC) columns is a state-of-the-art technic in the strengthening of existing structures. Thus far, various design models are present in literature to predict the stress-strain behavior and to allow for the ultimate limit state design of FRP-confined concrete. Often, the models proposed are scientific in nature only, they are just designed for plain concrete specimens, and they do not meet the demands necessary for practical use. Therefore, this paper presents a comprehensive design-oriented model for FRP-confined circular RC columns considering both the ultimate limit state (ULS) and the serviceability limit state (SLS). Beside equations to calculate the new ultimate confined concrete strength and the accompanying axial strain, the paper introduces an approach to generate a stress block simplifying the design procedure for columns subjected to combined compression and bending. Furthermore, an equation is presented to allow for the calculation of a combined partial safety factor taking into account the material safety factors of both the concrete of the column and the FRP system used. In order to realize a proper and coherent limit state method, the entire model uses characteristic material values and equations only. At last, the SLS is addressed with a simple approach limiting the maximal concrete compressive stress.

COMPREHENSIVE DESIGN-ORIENTED MODEL FOR FRP-CONFINED CIRCULAR RC COLUMNS

Today, the use of fiber-reinforced polymers (FRPs) for the confinement of reinforced concrete (RC) columns is a state-of-the-art technic in the strengthening of existing structures. Thus far, various design models are present in literature to predict the stress-strain behavior and to allow for the ultimate limit state design of FRP-confined concrete. Often, the models proposed are scientific in nature only, they are just designed for plain concrete specimens, and they do not meet the demands necessary for practical use. Therefore, this paper presents a comprehensive design-oriented model for FRP-confined circular RC columns considering both the ultimate limit state (ULS) and the serviceability limit state (SLS). Beside equations to calculate the new ultimate confined concrete strength and the accompanying axial strain, the paper introduces an approach to generate a stress block simplifying the design procedure for columns subjected to combined compression and bending. Furthermore, an equation is presented to allow for the calculation of a combined partial safety factor taking into account the material safety factors of both the concrete of the column and the FRP system used. In order to realize a proper and coherent limit state method, the entire model uses characteristic material values and equations only. At last, the SLS is addressed with a simple approach limiting the maximal concrete compressive stress.

Effect of Internal Stirrups on the Eccentric Compression Behavior of FRP-Confined RC Columns Based on Finite-Element Analysis

Fiber-reinforced polymers (FRPs) have been widely used in the retrofitting and rehabilitation of reinforced concrete (RC) columns because they can provide substantial lateral confinement to concrete. Conventionally, the presence of internal stirrups is neglected in the retrofitting design due to a lack of understanding on the effect of internal hoops when both external FRP and internal steel confinement exist, particularly under eccentric loading. In this study, a finite-element (FE) method is developed to investigate the eccentric compression behavior of FRP-confined RC columns. A reliable concrete plastic-damage model that accurately considers the confining stiffness ratio between the FRP and steel is proposed for the first time, which facilitates the study of a difficult problem: the behavior of RC columns with the dual confinement system under eccentric loading. The accuracy of the model is verified by test results. An extensive parametric study of various affecting factors is carried out to investigate the confinement mechanisms of the concrete columns in terms of the axial stress and confining pressure distributions, lateral principal stress ratio, local stress–strain response, and interaction between the FRP/steel hoops and concrete. It is found that the nonuniform confining pressures under eccentric compression are significantly affected by the internal stirrups. With the existence of discrete transverse steel reinforcement, the axial resistance of the concrete core is increased, thereby improving the ultimate load capacity of the columns. An ultimate axial load model for RC columns that considered combined FRP–steel confinement is subsequently proposed. More accurate results are obtained using the proposed model compared with those calculated from existing design codes.

Analytical Review on Eccentric Axial Compression Behavior of Short and Slender Circular RC Columns Strengthened Using CFRP.

Although reinforced concrete (RC) columns subjected to combined axial compression and flexural loads (i.e., eccentric load) are the most common structural members used in practice, research on FRP-confined circular RC columns subjected to eccentric axial compression has been very limited. More specifically, the available eccentric-loading models were mainly based on existing concentric stress–strain models of FRP-confined unreinforced concrete columns of small scale. The strength and ductility of FRP-strengthened slender circular RC columns predicted using these models showed significant errors. In light of such demand to date, this paper presents a stress–strain model for FRP-confined circular reinforced concrete (RC) columns under eccentric axial compression. The model is mainly based on observations of tests and results reported in the technical literature, in which 207 results of FRP-confined circular unreinforced and reinforced concrete columns were carefully studied and analyzed. A model for the axial-flexural interaction of FRP-confined concrete is also provided. Based on a full parametric analysis, a simple formula of the slenderness limit for FRP-strengthened RC columns is further provided. The proposed model considers the effects of key parameters such as longitudinal and hoop steel reinforcement, level of FRP hoop confinement, slenderness ratio, presence of longitudinal FRP wraps, and varying eccentricity ratio. The accuracy of the proposed model is finally validated through comparisons made between the predictions and the compiled test results.

Axial Compressive Strength Models of Eccentrically-Loaded Rectangular Reinforced Concrete Columns Confined with FRP.

The majority of experimental and analytical studies on fiber-reinforced polymer (FRP) confined concrete has largely concentrated on plain (unreinforced) small-scale concrete columns, on which the efficiency of strengthening is much higher compared with large-scale columns. Although reinforced concrete (RC) columns subjected to combined axial compression and flexural loads (i.e., eccentric compression) are the most common structural elements used in practice, research on eccentrically-loaded FRP-confined rectangular RC columns has been much more limited. More specifically, the limited research has generally been concerned with small-scale RC columns, and hence, the proposed eccentric-loading stress-strain models were mainly based on the existing concentric-loading models of FRP-confined concrete columns of small scale. In the light of such demand to date, this paper is aimed at developing a mathematical model to better predict the strength of FRP-confined rectangular RC columns. The strain distribution of FRP around the circumference of the rectangular sections was investigated to propose equations for the actual rupture strain of FRP wrapped in the horizontal and vertical directions. The model was accomplished using 230 results of 155 tested specimens compiled from 19 studies available in the technical literature. The test database covers an unconfined concrete strength ranging between 9.9 and 73.1 MPa, and section’s dimension ranging from 100–300 mm and 125–435 mm for the short and long sides, respectively. Other test parameters, such as aspect ratio, corner radius, internal hoop steel reinforcement, FRP wrapping layout, and number of FRP wraps were all considered in the model. The performance of the model shows a very good correlation with the test results.

A reliability-based approach and code calibration of FRP-Confined rectangular RC columns subjected to concentric loading

This study aims to present an approach to investigate the reliability levels of reinforced concrete (RC) columns confined by Fiber Reinforced Polymers (FRP) subjected to concentric loading. First, the statistical properties of random variables such as model error, FRP strain efficiency factor (FSE factor) and axial resistance of columns are investigated. Then in respect to the existing correlation between variables and extensive collected database, an appropriate probabilistic model for model error, FSE factor and compressive resistance of RC column is proposed. In addition, the effects of live-to-dead load ratio, reinforcement ratio and the confinement levels on the reliability of FRP-confined columns are studied through a reliability analysis. Furthermore, based on existing studies, reliability indices for various levels of confinement are calculated. Finally, a calibration analysis on safety factors of ACI 440.2R-17 guideline and shape factor of rectangular cross-section is presented.

Efficiency of insulation layers in fire protection of FRP-confined RC columns-numerical study

This paper addresses the efficiency of thermal insulation layers applied to protect structural elements strengthened by fiber-reinforced polymers (FRP) in the case of fire event. The paper presents numerical modeling and nonlinear analysis of reinforced concrete (RC) columns externally strengthened by FRP and protected by thermal insulation layers when subjected to elevated temperature specified by standard fire tests, in order to predict their residual capacity and fire endurance. The adopted numerical approach uses commercial software includes heat transfer, variation of thermal and mechanical properties of concrete, steel reinforcement, FRP and insulation material with elevated temperature. The numerical results show good agreement with published results of full-scale fire tests. A parametric study was conducted to investigate the influence of several variables on the structural response and residual capacity of insulated FRP-confined columns loaded by service loads when exposed to fire. The residual capacity of FRP-confined RC column was affected by concrete grade and insulation material and was shown to improve substantially by increasing the concrete cover and insulation layer thickness. By increasing the VG insulation layer thickness 15, 32, 44, 57 mm, the loss in column capacity after 5 hours of fire was 30%, 13%, 7% and 5%, respectively. The obtained results demonstrate the validity of the presented approach for estimation of fire endurance and residual strength, as an alternative for fire testing, and for design of fire protection layers for FRP-confined RC columns.

Behavior of large-scale FRP-confined rectangular RC columns under eccentric compression

Fiber-reinforced polymer (FRP) composites have become widely accepted in the strengthening or seismic retrofitting of reinforced concrete (RC) columns in practice. FRP-confined rectangular concrete columns under concentric axial compression have been extensively studied, leading to many stress-strain models (i.e., concentric-loading stress-strain models). Although RC columns in practical structures are commonly subjected to combined axial compression and bending (i.e., eccentric compression), existing research on eccentrically-loaded FRP-confined rectangular RC columns has been much more limited. More specifically, the limited research available has generally been concerned with small-scale RC columns, and the applicability of existing concentric-loading stress-strain models for FRP-confined concrete in the analysis of large-scale eccentrically-loaded rectangular RC columns has not been properly clarified. This paper presents the results of an experimental study including eight large-scale FRP-confined rectangular RC columns tested under eccentric compression. The following key test variables were carefully examined in the experimental program: the load eccentricity, the direction of bending, and the FRP jacket thickness. A theoretical column model is then presented for predicting the responses of the test columns. It is shown that the direct use of a concentric-loading stress-strain model for FRP-confined concrete in the column model leads to significant errors in predicting the ultimate deformation of the test columns.

Analytical Stress-Strain Model for FRP-Confined Rectangular RC Columns

One of the major applications of Fiber Reinforced Polymers (FRPs) in construction is in the confinement of reinforced concrete (RC) columns. The performance of FRP-confined concrete in circular columns has been extensively investigated in literature and the efficiency of the available models is nowadays considered to be satisfactory. However, the case of confinement of rectangular RC sections with FRPs is a more complex problem, the mechanism of which has not yet been adequately described. The aim of this work is to simplify the problem by proposing an iterative procedure based on the results of a three-dimensional finite element (3D FEM) analysis. An interesting finding is that the arching effect is not observed: indeed, the unconfined regions are partially confined and contribute a certain amount to the overall strength of the rectangular RC sections. Based on (a) a system of ‘generalized’ springs, (b) well-known stress-strain laws and (c) a failure criterion, a simplified mechanical model which gives the stress-strain behaviour of a rectangular RC section confined by FRPs under concentric load is proposed. The algorithm takes into account all parameters available to designers, such as corner rounding radius, stiffness of the FRP and concrete strength, while it can be easily understood and implemented. Its results are found to correlate adequately to recent experimental data yielded by large-scale tests on FRP-confined rectangular RC columns. Finally, in order to further evaluate the performance of this material model, it was implemented in the simulation of a series of experimental tests of FRP-retrofitted square RC columns under cyclic lateral loading simulating earthquake loads and simultaneous constant axial compression. In particular, all specimens were simulated using nonlinear fiber elements, in which the FRP-confined concrete was modelled using the aforementioned material model. Comparison between the numerical and experimental hysteresis of the column is indicative of the effectiveness of the implemented modelling.

Behavior of large-scale FRP-confined rectangular RC columns under axial compression

Fiber-reinforced polymer (FRP) jacketing has become an attractive technique for strengthening/retrofitting reinforced concrete (RC) columns. Extensive research has been conducted on FRP-confined rectangular columns under axial compression, leading to a significant number of stress-strain models for FRP-confined concrete in these columns. However, most of these models have been developed based on test results of small-scale columns, so their applicability to large FRP-confined rectangular RC columns has yet to be properly validated. To this end, the present paper first presents the test results of an experimental study consisting of nine large-scale rectangular RC columns, including eight FRP-confined RC columns and one RC column without FRP jacketing as the control specimen, tested under axial compression. The experimental program examined the sectional corner radius and the FRP jacket thickness as the key test variables. Five representative design-oriented stress-strain models for FRP-confined concrete in rectangular columns, identified from critical reviews of the existing literature, are then assessed using the test results to examine their validity for these large-scale columns.

Probabilistic assessment of FRP-confined reinforced concrete columns

This paper presents a reliability study on reinforced concrete (RC) columns retrofitted by Fibre Reinforced Polymers (FRP) confinement. First, variability of the compressive resistance of FRP-confined RC columns considering uncertainty in the model error, material properties, section dimensions, FRP strain efficiency factor and minimum load eccentricity as well as any existing correlation among random variables was studied. Then, reliability of FRP-confined RC columns designed based on the ACI 440 and the fib bulletin 14 guidelines was investigated. Results showed that in order to achieve the target reliability used in design of conventional RC columns, resistance reduction factors of these guidelines require modification. For the design of FRP-confined RC columns, a resistance reduction factor of 0.85 for the ACI 440 guideline and a partial safety factor of 1.45 for the fib bulletin 14 are suggested.

Displacement and plastic hinge length of FRP-confined circular reinforced concrete columns

Confinement of both existing and newly constructed reinforced concrete (RC) columns by fibre reinforced polymer (FRP) has been commonly used in recent decades. This is because of its ability to enhance the shear resistance and the ductility of the RC columns, which are the main parameters that govern the behaviour of RC columns under lateral loading. This paper presents a finite element (FE) model that was developed using the LS-DYNA program aimed at modelling the plastic hinge length (lp) for FRP-confined RC columns. A FE parametric study was conducted to investigate the effect of FRP-confinement on lp. Empirical models were proposed to predict lp and the ultimate drift ratio (δu) for FRP-confined RC columns and the results were compared with similar previous models. The proposed FE model was able to predict the plastic hinge region and lp value which can provide a simple way for designers to investigate the behaviour of FRP-confined columns during the design process. The proposed δu model reduced the average of errors (A) and standard deviation (SD) by 15.1%, 3.9%, respectively, compared to the best predictions by previous models.

Three-dimensional finite element analysis of reinforced concrete columns with FRP and/or steel confinement

The strength and ductility of reinforced concrete (RC) columns can be substantially enhanced though lateral confinement which may be provided by transverse steel reinforcement and/or a supplemental fiber-reinforced polymer (FRP) jacket. Despite extensive past research on confined concrete columns, most of the existing work has been either experimental or empirical, particularly when discrete steel hoops/spirals need to be considered. This paper instead is focused on the alternative approach of three-dimensional (3D) finite element (FE) analysis of circular FRP-confined RC columns, with the discrete nature of transverse steel reinforcement properly captured. The key to the success of such FE analysis lies in an accurate constitutive model for the concrete which is under 3D compressive stresses, and this is achieved in the present study by building on an accurate plastic-damage model recently proposed by the authors’ group. In implementing this plastic-damage model, a “local” stress–strain model for concrete under uniform confinement, obtained by resolving a number of issues associated with 3D FE modeling, is employed to generate data for the input parameters. The proposed FE approach is capable of providing accurate prediction for both FRP-confined RC columns and steel-confined RC columns as demonstrated through comparisons with existing test data. FE results obtained for steel-confined circular RC columns are also examined in detail to gain an improved understanding of the confinement mechanisms in these columns.

Simple and efficient finite element modeling of reinforced concrete columns confined with fiber-reinforced polymers

This paper presents a frame finite element (FE) that is able to accurately estimate the load-carrying capacity and ductility of reinforced concrete (RC) circular columns confined with externally-bonded fiber-reinforced polymers (FRP). This frame FE can model collapse mechanisms due to concrete crushing, reinforcement steel yielding, and FRP rupture. The adopted FE considers distributed plasticity with fiber discretization of the cross-sections in the context of a force-based formulation, and uses advanced nonlinear material constitutive models for reinforcing steel and unconfined, steel-confined, and FRP-confined concrete.The adopted frame FE is validated through a comparison between numerical simulations and experimental results available in the literature of the load-carrying capacity of FRP-confined RC columns subjected to axial load only, and both axial and lateral load. The adopted FE is suitable for efficient and accurate modeling and analysis of FRP-confined RC columns, and thus it represents a step toward enabling analysis of real-world large-scale structures containing FRP-confined RC columns, for which more accurate three-dimensional models could be computationally prohibitive.

Experimental testing and analytical modeling of CFRP-confined large circular RC columns subjected to cyclic axial compression

A physically informed analytical model which describes the cyclic axial stress–strain behavior of reinforced concrete (RC) columns confined with fiber-reinforced polymer (FRP) composite wraps is necessary for the non-linear analysis and seismic design of such columns. Although extensive studies have been conducted on the monotonic axial behavior of small sized FRP-confined unreinforced concrete columns, there is a lack of research on FRP-confined RC columns and also their cyclic axial response. As a consequence, there is a lack of cyclic axial stress–strain models which can enable seismic response to be reliably simulated by numerical means. Due to such knowledge gaps, this study reports the results of thirty CFRP-confined large-scale unreinforced and RC circular columns subjected to monotonic and cyclic axial compression loading. The test results indicate that the overall shape of the stress–strain curves, the peak compressive stress and strain at failure, the unloading/reloading paths, and also the plastic strain of CFRP-confined concrete are influenced by the CFRP wrap and internal hoop steel reinforcement. On the basis of the experimental results, a cyclic stress–strain model for CFRP-confined circular RC columns is then proposed. The proposed model consists of three main components, namely (i) a monotonically ascending portion to describe the envelope curve, (ii) a polynomial equation to describe unloading paths, and (iii) a straight line to describe reloading paths. The influence of internal hoop reinforcement on the monotonic model, as well as the unloading and reloading responses is also considered. The accuracy of the proposed model is validated with the test data reported in this paper as well as other test results of relevance extracted from the open literature.