Confined Reinforced Concrete Columns Research Articles

Effects of structural size and corner radius on eccentric failure of rectangular CFRP-wrapped RC columns: Meso-scale modellings

The strengthening and retrofitting technique with fiber-reinforced polymer (FRP) composites on reinforced concrete (RC) columns has been widely popular. Compared with concentric compression behaviors of rectangular FRP-confined concrete columns, there are only limited investigations on their eccentric behaviors. Additionally, the applicability of existing design codes obtained from tested small-size columns in calculating load capacity of large-size columns has not been reasonably explained. In view of this, this paper developed a meso-scale numerical analysis method using a random aggregate model based on consideration of concrete heterogeneity. A series of rectangular carbon FRP (CFRP)-confined RC columns having different cross-sectional heights were modelled to investigate the influences of corner radius and size effect on their eccentric compression behaviors. Simulated results showed that there was a transformation of failure mode from brittle compression failure to ductile tensile failure with the increasing load eccentricity. An identical globally failure mode for rectangular columns having different cross-sectional heights was observed whereas crack widths became larger obviously with the increasing cross-sectional height. Regarding the axial load versus mid-height deflection relationship, the corner radius had a significant influence on maximum axial load and softening stage, especially at smaller load eccentricities. One can see that from nominal interaction diagrams, small-eccentrically-loaded columns exhibited more obvious size effect than large-eccentrically-loaded columns. The safety factor of load capacity decreased with the increasing cross-sectional height compared with existing design codes of ACI 440.2R-17 and GB 50608, illustrating that design codes appear to be not conservative for large-size specimens. Taking the influence of size effect into account, the design equations suggested by ACI 440.2R-17 were modified in this study, and the rationality of this correction methodology was verified against simulated data and results of tested large-size rectangular columns.

Analysis-oriented model for partially FRP-and-steel-confined circular RC columns under compression

Even though analysis-oriented models exist to simulate the axial and dilation behavior of reinforced concrete (RC) columns strengthened with fiber-reinforced-polymer (FRP) full confinement arrangements, a reliable model developed/calibrated for FRP partially imposed confinements is not yet available, identified as a research gap. Therefore, this paper is dedicated to the development of a new analysis-oriented model generalized for fully and partially confined RC columns under compression. In addition to vertical arching action phenomenon, the influence of the concrete expansion distribution along the column height on confining stress is considered in the establishment of the combined confinement from FRP strips and steel transverse reinforcements. A new unified dilation model is proposed, where the substantial effect of additional axial deformations induced by damage evolution in unwrapped zones is formulated by considering available experimental results. This model is coupled with an axial stress-strain formulation that includes a new failure surface function for simulating the dual confinement-induced enhancements, which are strongly dependent on the confinement stiffness. The developed model considers the influence of partially imposed confinement strategy on the axial and dilation behavior of RC columns, whose validation is demonstrated by simulating several experimental tests. Lastly, a parametric study is performed to evidence the dependence of FRP-steel confinement-induced enhancements on steel hoop and FRP spacing, and on the concrete compressive strength.

Modeling of confined circular RC columns using artificial neural network and finite element method

A Finite Element (FE) and an Artificial Neural Network (ANN) model were developed to investigate the effect of different parameters, such as the space between stirrups and the diameter of longitudinal steel, on the behavior of circular Reinforced-Concrete (RC) columns confined with fiber reinforced polymer (FRP) sheets using the actual hoop rupture strain of the FRP. The FE model was accomplished using the FE software ABAQUS 6.13 which incorporates the nonlinear behavior of concrete material, the bilinear stress–strain curve of steel, and the linear elastic behavior of FRP. It was found that there is a good agreement between the FE model (FEM) results and the experiments. The ANNs were trained and tested on an experimental database from the literature. The database contains the experimental ultimate FRP strain, and the ultimate load results of 92 FRP confined circular RC columns. The ANN results agreed well with the FEM results. The neural network results were carried out to develop empirical equations to predict the effective rupture FRP strain and the ultimate load for circular confined RC columns with R2 of 0.912 and 0.932, respectively. The proposed equations estimate the ultimate strain from experimental works of the FRP with a small error up to 20%. In addition, the predicted results from the proposed equations exhibited good accuracy compared with previous guidelines and experimental and FE results from the literature, and it can be easily used in engineering designs as well.

Performance of axially loaded square RC columns with single/double confinement layer(s) and strengthened with C-FRP wrapping under close-in blast

Very recently a new variant of reinforced concrete (RC) columns confined with a double layer of transverse steel has been introduced by researchers and investigated its performance under axial compression and seismic loading. Analysis of blast performance of such columns is of considerable interest. Most of the columns in a building structure are found to be of square or rectangular cross-section. Being axisymmetric, the circular columns perform better than square/rectangular ones under impulsive loadings. The stability of a structure supported mainly on columns subjected to such loadings depends upon the performance of the columns. Therefore, it is important to enhance the response of supporting RC compression members of square or rectangular cross-sections of a structure subjected to high intense loadings. In this research work, one 3000 mm long, 300 mm × 300 mm square RC column with conventional transverse steel carrying an axial working load of 950kN subjected to 82 kg-TNT-equivalent close-in blast loading at scaled distance 1.0 m/kg1/3 tested experimentally has been first analyzed using the high-fidelity dynamic computer code, ABAQUS/Explicit-v6.15. For comparison, another singly confined concrete column with seismic reinforcement over its entire length is also analyzed. By comparing the numerical results with the experimental results in the open literature, the accuracy of the numerical model is verified. To improve the response, doubly confined RC columns having outer transverse confining steel stirrups of square and inner ones’ diamond/circular geometry with different combinations of stirrup spacing have been considered and investigated. The columns are of the same area of compression steel and carry the same axial load (950kN). Carbon-fiber-reinforced-polymer (C-FRP) wrapping of 2 mm thickness is also considered and modeled using Hashin’s damage initiation criteria and energy-based damage evolution to investigate the blast resistance of the columns. Blast performance in terms of displacement, damage dissipation energy, concrete crushing, and cracking are compared and discussed.

Assessment of Standards and Codes Dedicated to CFRP Confinement of RC Columns.

Reinforced concrete (RC) columns are often placed under confinement to increase their strength and ductility. Carbon fiber reinforced polymer (CFRP) materials have recently been recognized as favorable confinement systems. At present, a number of national standards and codes dedicated to the design of concrete components strengthened with CFRP in general and CFRP confinement in particular are available. These sets of rules provide design equations for confined reinforced concrete columns with circular and rectangular cross sections. Most of the standards and codes exhibit significant differences, including the used predictive models, limitations, observed effects and covered loading conditions. In this paper, five international standards and design guidelines are introduced and discussed. The purpose is to present a constructive and critical assessment of the state-of-the-art design methodologies available for CFRP confined RC columns and to discuss effects not previously considered properly. Therefore, some recent research efforts and findings from the Leipzig University of Applied Sciences are also introduced. The obtained data is used for a comparative study of the guideline predictive equations. Furthermore, it is shown that some new findings concerning the rupture strength and the maximum strength plus accompanying axial strain of a CFRP confined column are suitable to improve the current guidelines.

FINITE ELEMENT ANALYSIS OF THE BEHAVIOUR OF REINFORCED CONCRETE COLUMNS CONFINED BY OVERLAPPING HOOPS SUBJECTED TO RAPID CONCENTRIC LOADING

The strain rate sensitivity of concrete material was discovered approximately one hundred years ago, and it has a marked effect on the behaviour of concrete members subjected to dynamic loadings such as strong earthquake and impact loading. Because of the great importance of the confined reinforced concrete (RC) columns in RC structures, the dynamic behaviour of the columns induced by the strain rate effect has been studied, but only few experiments and analyses have been conducted. To investigate the behaviour of overlapping hoop-confined square reinforced normal-strength concrete columns, considering the strain rate effect at a strain rate of 10-5/sec to 10-1/sec induced by earthquake excitation, an explicit dynamic finite element analysis (FEA) model was developed in ABAQUS to predict the behaviour of confined RC columns subjected to the rapid concentric loading. A locally modified stress-strain relation of confined concrete with the strain rate sensitivity of the concrete material and the confining effect of overlapping hoops were proposed to complete the simulation of the dynamic behaviour of concrete with the concrete plastic-constitutive model in ABAQUS. The finite element predictions are consistent with the existing test results. Based on the FEA model, a parametric investigation was conducted to capture more information about the behaviour of confined RC columns under varying loading rates

Seismic Behavior of Under Confined Square Reinforced Concrete Columns

The role of confining reinforcement in dissipating a large amount of energy in reinforced concrete (RC) elements at the time of seismic events has been well demonstrated in previous studies. In India, this practice of providing special confinement in RC elements began to be followed after the ductile detailing code came into being in the year 1993. Hence, the structures built before 1990s in seismicaly active areas had no ductile detailing, as the design recommendation of pre 90s era do not consider adequate confinement requirements. The Present experimental study is a part of an ongoing project of upgrading these old reinforced concrete structures, which had no special confinement and are located in seismically active regions. Three full-scale columns with different amount of transverse reinforcement (volumetric ratio of 1.31, 0.33, and 0.22) were tested in a specially made testing setup. One specimen was designed as per the current Indian guidelines for ductile detailing (IS 13920:2016), while the other two specimens were designed as per the guidelines which existed before the Code on ductile detailing was implemented. The response of the columns under quasi-static reverse lateral cyclic loading (at constant axial load) was recorded in terms of lateral load v/s deflection, moment v/s curvature, energy dissipation, stiffness reduction and different ductility parameters. As expected, the results of the study show inferior seismic performance of under-confined columns as indicated by the key strength and ductility parameters. The results have though quantified the strength and ductility deficit of these under confined RC columns, which shall be of great help in designing suitable retrofit for the existing structures constructed with no ductile detailing features.

Finite Element Analysis of Square RC Columns Confined by Different Configurations of Transverse Reinforcement

Introduction:Square reinforced concrete (RC) columns with the confinement effect of transverse reinforcement perform well in ductility and have been used widely in RC structures. Its behavior is the classic topic of anti-seismic and anti-collapse analysis of RC structures. With the advancement of the finite element (FE) analysis technology, the general-purpose simulation tools such as ABAQUS and ANSYS have been universally used to analyze the behavior of structures and members, where the material constitutive model is a key problem in the analysis.Methods:In this study, a new uniaxial compressive stress-strain curve of the confined concrete considering confinement effect of transverse reinforcement in square RC columns was proposed for the concrete damaged plasticity model in ABAQUS to solve the problem that there is no proper uniaxial compressive stress-strain curve for the concrete damaged plasticity model to describe the behavior of concrete confined by transverse reinforcement. Based on the proposed stress-strain relationship, a FE model was developed to analyze the behaviour of laterally confined RC columns under concentric loading.Results:The finite element model is able to predict the response of the confined RC columns from different experiments with reasonable accuracy. Finally, a parametric study was conducted in order to evaluate the effect of confinement reinforcement configuration on the behavior of core concrete in square section.

Strength and ductility of innovative hybrid NSM reinforced and FRP confined short RC columns under axial compression

Strengthening of concrete structures using Fiber Reinforced Polymer (FRP) composites has gained prominence in the recent years due to its various advantages. Reinforced Concrete (RC) columns are predominantly subjected to compression loading and often need strengthening to increase their strength and ductility. This paper explores an innovative hybrid strengthening technique where short RC square column elements are strengthened using both Near Surface Mounted (NSM) CFRP laminates and Externally Bonded (EB) CFRP fabrics for confinement. Ten square column elements of cross sectional dimension 230mm and height of 450mm were cast, strengthened and tested under pure axial compression to investigate the efficiency of different combinations of strengthening techniques. Existing constitutive models available from past literature were used to predict the behavior of FRP strengthened RC column elements. A close correlation was observed between the peak strength obtained from experimental and analytical values. Hybrid strengthening technique was found to be more efficient leading to higher increase in strength, stiffness and ductility as compared to only NSM strengthened or only CFRP confined RC columns.

Experimental investigation on the seismic performance of GFRP-wrapped thin-walled steel tube confined RC columns

In order to eliminate buckling in thin-walled steel tube confined reinforced concrete (RC) columns, the external fiber-reinforced polymer (FRP) composite wraps may be applied. This paper reports the details and results of a series of seismic tests on FRP-wrapped thin-walled steel tube confined RC columns. A total of six full-scale circular cantilever columns were tested under combined constant axial load and lateral cyclic displacement excursions. The test specimens consisted of a control RC column, a carbon FRP (CFRP) confined RC column, a glass FRP (GFRP) confined RC column, a thin-walled steel tube confined RC column, and two GFRP-wrapped thin-walled steel tube (GST) confined RC columns of varying axial load levels. The diameter and height of the RC columns were 400mm and 1600mm, respectively, and the outer diameter to thickness ratio of the steel tubes was 135. The seismic responses of the test specimens were compared in terms of failure mode, hysteric behavior, ductility, stiffness degradation, energy dissipation capacity and equivalent viscous damping ratio. The effects of the GFRP wraps and the steel tube on the behavior of the GST confined column were revealed respectively. Test results showed that the steel tube confined column experienced local buckling and abrupt welding seam splitting resulting from high biaxial stresses, whereas such premature failure was prevented in the GST confined RC columns. For the specimens designed and tested in this study, both components in the GST contributed much to the seismic performance of the GST confined column, while the thin-walled steel tube was more effective than the GFRP wraps in improving the ultimate drift ratio. In addition, as the axial compression ratio was increased from 0.2 to 0.45, there was almost no decrease in displacement ductility for the GST confined columns.

Using carbon nanotubes to improve strengthening efficiency of carbon fiber/epoxy composites confined RC columns

The effectiveness of using carbon nanotubes (CNTs) to enhance the strengthening efficiency of carbon fiber/epoxy composites confined reinforced concrete (RC) columns was investigated. A total of fourteen large scale rectangular RC columns were prepared and tested under concentric axial loading. The incorporation of CNTs within the carbon fiber/epoxy composites was achieved by modifying the epoxy resin using CNTs and/or coating the carbon fiber with CNTs enriched sizing agent. The failure modes, axial load–displacement responses, and microstructures of unwrapped, fully wrapped, and partially wrapped specimens were analyzed. Experimental results showed that using CNTs modified epoxy or CNTs enriched sizing agent improved the load carrying capacity and toughness of RC columns confined with carbon fiber/epoxy composites. Scanning electron microscopy (SEM) characterization showed that CNTs could improve the adhesion at the carbon fiber/epoxy interface and concrete/epoxy interface leading to improvement in the load transfer and load carrying capacity.

Application of fuzzy logic approach in predicting the lateral confinement coefficient for RC columns wrapped with CFRP

Worldwide ageing infrastructures which are vulnerable to seismic lateral loads and located in high seismicity regions have arrested the interest of many researchers to find alternative materials and techniques to strengthen in bending and shear, for example reinforced concrete (RC) beams, slabs, columns, etc. There are several strengthening/repair techniques and materials in literature. Although the method of strengthening concrete structures with fiber reinforced polymers (FRP) is a relatively new technique, it has existed for more than two decades. In this context, several confinement models have been developed for FRP-confined concrete for the prediction of stress–strain response and several researchers have developed various constitutive models to measure the increase in the axial strength of concrete due to the confinement effect of FRP laminates. In this study, RC columns wrapped with carbon FRP (CFRP) considering some existing confinement models in the literature have been investigated. Moreover, based on the experimental data set in the literature, a new artificial intelligence-based algorithm (a Mamdani-type fuzzy inference system) was implemented to model the strength enhancement of CFRP confined RC columns using fuzzy logic methodology. Fuzzy logic predicted results were compared with the outputs of a non-linear regression analysis-based exponential model derived in the scope of the present work. The best predictive performances of the models were assessed by means of various descriptive statistical indicators. The comparison of the proposed prognostic approach with existing empirical and experimental data exhibits a very good precision of the developed artificial intelligence-based model in predicting the lateral confinement coefficient in CFRP wrapped RC columns.

Design model for square RC columns under compression confined with CFRP

The enhancement of the mechanical behaviour of reinforced concrete (RC) columns with regard to axial compression is an up-to-date concern, namely if the strengthening of existing structures is to be considered. In view of this, external confinement with FRP systems has been tested in order to become a feasible technique, since it seems to have important advantages over other systems such as its high strength and stiffness in relation to weight and its improvement of strength and ductility while confining RC columns. Square columns confined with FRP show a more complex interpretation of their behaviour, when compared to circular columns. Accordingly, the present work includes the analysis of two experimental programs regarding axial compression on CFRP confined RC columns: one on circular and square specimens with different corner radii; the other on square specimens with side lengths ranging from medium to large. Based on this, modelling equations are proposed to predict maximum axial load, axial strain and lateral strain, as well as the entire behaviour until failure with curves of axial load–axial strain and axial load–lateral strain. The modelling results show that the analytical curves are in general agreement with the presented experimental curves for a wide range of dimensions.

FRP Wrapping for RC Columns with Varying Corner Radii

Abstract Existing reinforced concrete (RC) columns may be structurally deficient due to variety of reasons such as improper transverse reinforcement, flaws in structural design, insufficient load carrying capacity, etc. Carbon/Glass fibre reinforced polymer (FRP) confinement can be effectively used for strengthening the deficient RC columns. The effectiveness of FRP wrapping for RC columns mainly depend upon corner radius of the specimens as well as number of FRP layers used for the confinement. An attempt has been made hereby to investigate the experimental behaviour of GFRP wrapped small scale square RC columns with varying corner radii. Experimentally evaluated behaviour of GFRP wrapped RC columns is compared with the performance observed for non-wrapped RC columns. 15 RC columns having cross-sectional dimensions 125 mm × 125 mm and length of 1200 mm have been tested under axial compression. Three columns are unwrapped and have been designated as control specimens. Three columns each with corner radius equivalent to cover of 25 mm are wrapped with one and two layers of GFRP, respectively. To avoid a premature rupture of the GFRP composite, remaining six columns with corner radius of 5 mm have been wrapped with one and two layers of GFRP, respectively. GFRP wrapped columns go under higher axial displacement in order to gain higher compressive strength over the control column. The results showed that smoothening of the edges of square cross-section of RC columns play a significant role in delaying the rupture of the FRP composite at the edges. Corner radius equal to concrete cover gives better results in terms of ultimate load carrying capacity than the corner radius less than cover for confined RC columns.

Estimation of flexural capacity of quadrilateral FRP-confined RC columns using combined artificial neural network

This study presents the application of combined artificial neural networks (CANNs) for the flexural capacity estimation of quadrilateral fiber-reinforced polymer (FRP) confined reinforced concrete (RC) columns. A database on quadrilateral FRP confined RC columns subjected to axial load and moment was obtained from experimental studies in the literature; CANN models were built, trained and tested. Then the flexural capacities of quadrilateral FRP confined RC columns were determined using the developed CANN model. Single and combined ANN was used for the first time in the literature for the estimation of flexural capacities of non-circular fiber-reinforced polymer (FRP) confined reinforced concrete (RC) columns. The accuracies of the proposed ANN and CANN models were more satisfactory as compared to the existing conventional approaches in the literature. Moreover, the proposed CANN models’ results had lower prediction error than those of the single ANN model.

Performance of externally CFRP confined RC columns with changes in thickness of the wrap, slenderness of the column and shape of the section

The present study deals with the analysis of experimental results, in terms of load carrying capacity and strains, obtained from tests on reinforced concrete (RC) columns, strengthened with external carbon fiber reinforced polymer (CFRP) sheets. The experimental parameters include: number of wrap layers, slenderness of the columns (L/a or L/D) and shape of the section (circular and square). A total of 48 specimens were subjected to axial compression. All test specimens were loaded to failure in axial compression. Compressive stress, axial and hoop strains have been recorded to evaluate the stress-strain relationship, ultimate strength, stiffness, and ductility of the specimens. Results clearly demonstrate that composite wrapping can enhance the structural performance of RC columns in terms of both maximum strength and ductility. The effects of test parameters are evidenced and compared.

Experimental Investigations of Loading Rate Effects in Reinforced Concrete Columns

Seismic loading rates can significantly affect the behavior of reinforced concrete (RC) elements. However, few data are available to quantify these effects. Shaking table tests allow the study of loading rate phenomena; however, they suffer from difficulties in assessing causality (direct assessment of causes on effects) and are expensive to conduct. An alternative is to test individual RC elements by directly imparting high-velocity loading protocols. However, multiactuator setups are necessary to achieve seismically representative loading and boundary conditions, which entails particularly challenging control requirements. This investigation makes use of recent advances in real-time testing hardware to study the effects of loading rates on the structural response of lightly confined RC columns. A pioneering test setup in which three actuators are controlled independently at high velocities was used to test a series of columns until axial collapse. The experimental challenges and column behavior are discussed.

Modeling Confinement Efficiency of Reinforced Concrete Columns with Rectilinear Transverse Steel Using Artificial Neural Networks

Artificial neural networks have attracted considerable attention and have shown promise for modeling complex nonlinear relationships. This paper explores the use of artificial neural networks in predicting the confinement efficiency of concentrically loaded reinforced concrete (RC) columns with rectilinear transverse steel. Fifty-five experimental test results were collected from the literature of square columns tested under concentric loading. A multilayer-functional-link neural network was used for training and testing the experimental data. A comparison study between the neural network model and four parametric models is also carried out. It was found that the neural network model could reasonably capture the underlying behavior of confined RC columns. Moreover, compared with parametric models, the neural network approach provides better results. The close correlation between experimental and calculated values shows that neural network-based modeling is a practical method for predicting the confinement...