Performance of Axially Loaded Short Rectangular Columns Strengthened with Carbon Fiber-Reinforced Polymer Wrapping

This paper presents results of a comprehensive experimental investigation on the behavior of axially loaded short rectangular columns that have been strengthened with carbon fiber-reinforced polymer (CFRP) wrap. Six series, a total of 90 specimens, of uniaxial compression tests were conducted on rectangular and square short columns. The behavior of the specimens in the axial and transverse directions is investigated. The parameters considered in this study are (1) the concrete strength; (2) the aspect ratio of the cross section; and (3) the number of CFRP layers. The findings of this research can be summarized as follows: The CFRP wrapping enhances the compressive strength and the ductility of both square and rectangular columns, but to a lesser degree than that of circular columns. The ultimate strength and the ductility of the CFRP confined concrete increase with increasing number of confining layers. The increase in strength and ductility is more significant for lower strength concrete, representing poor or degraded concrete, than for normal-to-high strength concrete; that is, the maximum gain in strength that can be achieved for 3 ksi concrete wrapped columns is approximately 90%, as compared to only 30% for 6 ksi concrete wrapped columns. The CFRP confining jacket must be sufficiently stiff to develop appropriate confining forces at relatively low axial strain levels. The gain in compressive strength obtained by the CFRP confined concrete depends mainly on the relative stiffness of the CFRP jacket to the axial stiffness of the column.     

Slenderness Effects on Circular CFRP Confined Reinforced Concrete Columns

External bonding of circumferential fiber-reinforced polymer (FRP) wraps is a widely accepted technique to strengthen circular RC columns. To date, most of the tests performed on FRP strengthened columns have considered short, unreinforced, small-scale concrete cylinders, with height-to-diameter ratios of less than three, tested under concentric, monotonic, and axial load. In practice, most RC columns have height-to-diameter ratios considerably larger than three and are subjected to loads with at least minimal eccentricity. Results of an experimental program performed to study the effects of slenderness on carbon FRP (CFRP) wrapped circular RC columns under eccentric axial loads are presented. It is shown that CFRP wraps increase the strength and deformation capacity of slender columns, although the beneficial confining effects are proportionally greater for short columns, and that theoretical axial-flexural interaction diagrams developed using conventional sectional analysis (but incorporating a simple FRP confined concrete stress-strain model) provide conservative predictions for nonslender CFRP wrapped columns under eccentric loads. The use of longitudinal CFRP wraps to reduce lateral deflections and allow slender columns to achieve higher strengths, similar to otherwise identical nonslender columns, is also demonstrated.     

Assessing the Efficiency of CFRP Discrete Confinement Systems for Concrete Cylinders

Concrete columns requiring strengthening intervention always contain a certain percentage of steel hoops. Applying strips of wet layup carbon fiber-reinforced polymer (CFRP) sheets inbetween the existent steel hoops might, therefore, be an appropriate confinement technique with both technical and economic advantages, when full wrapping of a concrete column is taken as a basis of comparison. To assess the effectiveness of this discrete confinement strategy, circular cross-sectional concrete elements confined by distinct arrangements of strips of CFRP sheet are submitted to a direct compression load up to the failure point. The influence of the width of the strip, distance between strips, number of CFRP layers per strip, CFRP stiffness, and concrete strength class on the increase of the load carrying capacity and ductility of concrete columns, is evaluated. An analytical model is developed to predict the compressive stress-strain relationship of concrete columns confined by discrete and continuous CFRP arrangements. The main results of the experimental program are presented and analyzed and used to assess the model performance.     

Experimental and Parametric Study of Circular Short Columns Confined with CFRP Composites

This paper presents an experimental and nonlinear finite-element analysis (NLFEA) results of circular short reinforced concrete (RC) columns confined externally with carbon fiber-reinforced polymers (CFRP) subjected to pure axial loading. The experimental program involves the fabrication and testing of 55 specimens wrapped with different number and configuration of CFRP sheet layers in the transverse and longitudinal directions. In addition, the columns were modeled using NLFEA. After reasonable validation of NLFEA with the experimental test results of companion columns and available technical literature results, NLFEA was expanded to provide a parametric study of 96 columns that correlates the ultimate axial stress of CFRP-confined RC columns to unconfined strength of concrete (fco), the volumetric ratio of CFRP (ρf), and the size effect. Results indicated that the ultimate capacity and ductility increase with the increase in volumetric ratio of CFRP (ρf) and unconfined strength of concrete (fco). In addition, the results indicated that size effect exists and the confinement effectiveness was more pronounced for columns with low fco and ρf.     

Evaluation of As-Built, Retrofitted, and Repaired Shear-Critical Hollow Bridge Columns under Earthquake-Type Loading

The objective of this research is to investigate the seismic performance of as-built, retrofitted, and repaired hollow bridge columns with insufficient shear strength. Two as-built full-scale columns were first tested and repaired using carbon-fiber-reinforced polymer composites (CFRP) jackets and dog-bone-shaped bars and then retested. Another two columns having the same reinforcement as the as-built columns were retrofitted with CFRP jackets. In addition to the tests, the repairability of the failed hollow columns was investigated by analytical evaluation. The test results and analysis of the retrofitted columns showed that CFRP composites can effectively strengthen shear-critical hollow bridge columns and can successfully transform the failure mode from shear to flexure. The test results of the repaired circular columns show that dog-bone-shaped bars successfully repaired the flexural damage caused by the fractured longitudinal bars.     

Analytical Models and Guidelines for the Ductility Enhancement of Circular Reinforced Concrete Single-Column Bents Using Fiber-Reinforced Polymers

The target displacement ductility requirements for circular RC single-column bridge bents are considered using a proposed multifailure mode algorithm to determine the required thickness of fiber-reinforced polymer wraps (FRPs). The procedure is developed using two in-house computer algorithms, PACCC (plastic analysis of circular concrete columns) and PACCC-FRP, to generate a moment-curvature analysis using circular segment slices and subsequent failure mode predictions in single-column bents for both FRP-wrapped and unwrapped circular RC sections. The results of the study showed good comparison to published experimental tests at the ultimate force-deflection states of RC sections and against three commercial “software test beds.” The study uses PACCC-FRP to show that single columns experiencing a brittle failure may be retrofitted with FRP wraps in order to increase the displacement ductility and satisfy target ductility values within the ductility wrap envelope, or wrap-saturation level, as established herein.     

Effect of Fiber-Reinforced Polymer Confinement on Bond Strength of Reinforcement in Beam Anchorage Specimens

This paper reports on the fourth phase of a multiphase study undertaken at the American University of Beirut (AUB) to examine the effect of fiber-reinforced polymer (FRP) sheets in confining bond-critical regions in reinforced concrete beams. Results of the first three phases showed that glass- and carbon-fiber-reinforced polymer (GFRP and CFRP) sheets were effective in increasing the bond strength and improving the ductility of the mode of failure of tension lap splices in high-strength concrete (HSC) and normal-strength concrete (NSC) beams. The main objective of the fourth phase of the AUB study was to assess the effect of CFRP sheets in improving the serviceability and ultimate response of beam anchorage specimens. The added experimental data and the improved knowledge of the bond behavior of FRP confined concrete members will encourage the use of FRP technology to strengthen and retrofit bond anchorage zones. Ten beam anchorage specimens were tested in positive bending in two series. The variables were bar size, anchorage length, and concrete strength. For each bar size, anchorage length, and concrete strength, two companion specimens—identical except for whether the anchorage zone was wrapped with FRP sheets or not wrapped—were tested. The test results demonstrated that CFRP sheets were effective in enhancing the bond strength and ductility of anchorage zones in beam anchorage specimens where splitting failures were imminent.     

Seismic Retrofit of Hollow Rectangular Bridge Columns

The seismic performance of rectangular hollow bridge columns is a significant issue of the high-speed rail project in Taiwan. The flexural ductility and shear capacity of such columns with the configuration of lateral reinforcement used in Taiwan have been studied recently. This paper reports that hollow rectangular bridge columns retrofitted with fiber-reinforced polymer (FRP) sheets were tested under a constant axial load and a cyclically reversed horizontal load to investigate their seismic behavior, including flexural ductility, dissipated energy, and shear capacity. An analytical model was also developed to predict the moment-curvature curve of sections and the load-displacement relationship of columns. Based on the test results, the seismic behavior of such columns will be presented. The test results were also compared to the proposed analytical model. It was found that the ductility factors of the tested piers are in the range from 3.4 to 6.3, and the proposed analytical model can predict the load-displacement relationship of such columns with acceptable accuracy. All in all, FRP sheets can effectively improve both the ductility factor and shear capacity of hollow rectangular bridge columns.     

Axial Load-Bending Moment Diagrams of Carbon FRP Wrapped Hollow Core Reinforced Concrete Columns

Hollow core reinforced concrete columns are generally preferred in use to decrease the cost and weight/stiffnesss ratio of members, such as bridge columns and piles. With a simplified stress state assumption, strengthening a hollow core reinforced concrete column with fiber-reinforced polymer (FRP) wrapping provides a biaxial confinement to the concrete, which leads to a need of defining the effect of FRP wrapping on the strength and ductility of the hollow core reinforced concrete columns. In this study, two groups of four hollow core reinforced concrete columns (205?mm outer diameter, 56?mm hollow core diameter, and 925?mm height) were tested under concentric, eccentric (25 and 50?mm eccentricity) and bending loads to observe the effect of carbon FRP (CFRP) wrapping. All the columns had internal steel reinforcement. Half of the columns had three layers of circumferential CFRP wrapping, whereas the other half had no external confinement. Axial load-bending moment (P–M) diagrams of each group were drawn using the obtained experimental results for both groups. It was observed that, CFRP wrapped columns had higher load and moment carrying capacities than the other group. An analytical model is proposed for drawing the P–M diagram of CFRP wrapped hollow core reinforced concrete columns.     

Shear Retrofit of Hollow Bridge Piers with Carbon Fiber-Reinforced Polymer Sheets

Hollow bridge piers are currently being used in high-speed rail and highway projects in Taiwan. The flexural ductility and shear capacity of such piers with the configuration of lateral reinforcement used in Taiwan has recently been studied.?This paper reports that circular and rectangular hollow bridge piers retrofitted by carbon fiber-reinforced polymer (CFRP) sheets were tested under a constant axial load and a cyclic reversed horizontal load to investigate their seismic behavior, including flexural ductility, dissipated energy, and shear capacity. An analytical model is also developed to predict the moment-curvature relationship of sections and the lateral load-displacement relationship of piers. Based on the test results, the seismic behavior of such piers is presented. The test results are also compared with the proposed analytical model. It was found that the ductility factors of the tested piers ranged from 3.3 to 5.5 and that the proposed analytical model could predict the lateral load-displacement relationship of such piers with reasonable accuracy. All in all, CFRP sheets can effectively improve both the ductility factor and the shear capacity of hollow bridge piers.     

Behavior of Bond-Critical Regions Wrapped with Fiber-Reinforced Polymer Sheets in Normal and High-Strength Concrete

This paper reports on the third phase of a multiphase study undertaken at the American University of Beirut (AUB) to examine the effect of fiber-reinforced polymer (FRP) sheets in confining tension lap splice regions in reinforced concrete beams. Results of the first two phases showed that glass and carbon fiber-reinforced polymer (GFRP and CFRP) sheets were effective in increasing the bond strength and improving the ductility of the mode of failure of tension lap splices in high-strength concrete (HSC) beams with nominal concrete strength of 70 MPa. The experimental results of the two phases were used to propose a new FRP confinement parameter, Ktr,f, that accounts for the bond strength contribution of FRP sheets wrapping tension lap splice regions in HSC beams. In this third phase of the AUB study, the trend of the results of phases 1 and 2 and the validity of the analytical model proposed were verified if normal-strength concrete (NSC) is used instead of HSC. Seven beams with nominal concrete strength of 27.58 MPa (4 ksi) were tested in positive bending. Each beam was designed with a tension lap splice in a constant moment region in the midspan of the beam. The main test variables were the configuration (1 strip, 2 strips, or a continuous strip) and the number of layers (1 layer or 2 layers) of the CFRP sheets wrapping the splice region. The test results demonstrated that CFRP sheets were effective in enhancing the bond strength and ductility of failure mode of tension lap splices in NSC in a very similar way to HSC. In addition, the FRP confinement index proposed earlier for HSC was proven to be valid in the case of NSC.     

Retrofitting of Rectangular Columns with Deficient Lap Splices

The cyclic behavior of eight 0.4-scale reinforced concrete column specimens is investigated. The columns incorporated deficient design details to simulate bridge columns built in Washington State prior to 1971. Two columns were tested as reference specimens, five were tested after retrofitting using carbon fiber-reinforced polymer (CFRP), and one was tested after retrofitting using a conventional steel jacket. All the specimens were tested under constant gravity load and incrementally increasing lateral loading cycles. The specimens had rectangular cross sections with aspect ratios of 1.5 and 2.0. The parameters investigated included the amount of CFRP reinforcement, different retrofitting jacket configurations, and different retrofitting materials. For the as-built specimens, two modes of failure occurred, namely low cyclic fatigue of longitudinal reinforcement and lap splice failure. For the retrofitted specimens, no lap splice failure was observed. All the retrofitted specimens failed due to low cyclic fatigue failure of the longitudinal bars. The retrofitting measures improved the displacement ductility, energy dissipation, and equivalent viscous damping. In addition, increasing the amount of CFRP reinforcement improved the performance of the test specimens.     

Near-Surface-Mounted Composite System for Repair and Strengthening of Reinforced Concrete Columns Subjected to Axial Load and Biaxial Bending

This paper aims to examine the effectiveness of near-surface-mounted (NSM) glass fiber-reinforced polymer (GFRP) composite rebars in combination with external confinement with carbon fiber-reinforced polymer (CFRP) composite sheets to repair and strengthen reinforced concrete (RC) columns exposed to axial load and biaxial bending. Nine columns with a square cross section of 150×150??mm were constructed and tested under biaxial eccentric loading with equal eccentricity along each principal axis. Test parameters included load eccentricity, concrete grade, and level of the CFRP confinement used in combination with the NSM-GFRP reinforcement. The effectiveness of the NSM-GFRP reinforcement was greatly affected by the CFRP-confinement level and the load eccentricity. For columns with a high level of CFRP confinement, the gain in the load capacity attributable to the NSM-GFRP reinforcement was higher at a lower eccentricity. For columns with a low level of CFRP confinement, the gain in the load capacity attributable to the NSM-GFRP reinforcement was higher at a higher eccentricity. The enhancement in the load capacity was more pronounced in the columns with a lower concrete grade. An analytical model for predicting the load capacity of RC columns strengthened with NSM-GFRP rebars in combination with CFRP confinement under axial load and biaxial bending is introduced. The model accounts for the nonlinear behavior of materials and the change in geometry under biaxial eccentric loading. The model accuracy is demonstrated by comparing the model predictions with the experimental results.     

Variable Strain Ductility Ratio for Fiber-Reinforced Polymer-Confined Concrete

The encasement of concrete in fiber-reinforced polymer (FRP) composite jackets can significantly increase the compressive strength and strain ductility of concrete columns and the structural system of which the columns are a part, be it a building or a bridge. Due to the approximate bilinear compressive behavior of FRP-confined concrete, analysis and design of FRP-confined concrete members requires an accurate estimate of the performance enhancement due to the confinement provided by FRP composite jackets. An analytical model is presented for predicting the bilinear compressive behavior of concrete confined with either bonded or nonbonded FRP composite jackets. This article describes the basis of the model, which is a variable plastic strain ductility ratio. The variable plastic strain ductility ratio defines the increase in plastic compressive strain relative to the increase in the plastic compressive strength of the FRP-confined concrete, which is a function of the hoop stiffness of the confining FRP composite jacket, the plastic dilation rate, and the type of bond between the FRP composite and concrete.     

CFRP-Based Strengthening Technique to Increase the Flexural and Energy Dissipation Capacities of RC Columns

A strengthening technique, combining carbon fiber-reinforced polymer (CFRP) laminates and strips of wet layup CFRP sheet, is used to increase both the flexural and the energy dissipation capacities of reinforced concrete (RC) columns of square cross section of low to moderate concrete strength class, subjected to constant axial compressive load and increasing lateral cyclic loading. The laminates were applied according to the near surface mounted technique to increase the flexural resistance of the columns, while the strips of CFRP sheet were installed according to the externally bonded reinforcement technique to enhance the concrete confinement, particularly in the plastic hinge zone where they also offer resistance to the buckling and debonding of the laminates and longitudinal steel bars. The performance of this strengthening technique is assessed in undamaged RC columns and in columns that were subjected to intense damage. The influence of the concrete strength and percentage of longitudinal steel bars on the strengthening effectiveness is assessed. In the groups of RC columns of 8 MPa concrete compressive strength, this technique provided an increase of about 67% and 46% in terms of column’s load carrying capacity, when applied to undamaged and damaged columns, respectively. In terms of energy dissipation capacity, the increase ranged from 40%–87% in the undamaged columns, while a significant increase of about 39% was only observed in one of the damaged columns. In the column of moderate concrete compressive strength (29 MPa), the technique was even much more effective, since, when compared to the maximum load and energy dissipation capacity of the corresponding strengthened column of 8 MPa of average compressive strength, it provided an increase of 39% and 109%, respectively, showing its appropriateness for RC columns of buildings requiring upgrading against seismic events.     

Experimental and Computational Studies on High-Strength Concrete Circular Columns Confined by Aramid Fiber-Reinforced Polymer Sheets

The aim of this paper is to study the properties of high-strength concrete (HSC) circular columns confined by aramid fiber-reinforced polymer (AFRP) sheets under axial compression. A total of 60 specimens were tested, considering the following parameters: the compressive strength of concrete, the number of AFRP layers, and the form of AFRP wrapping. In addition, an analytical model for predicting the stress–strain curves is proposed based on the experimental results. Meanwhile, a three-dimensional nonlinear finite-element model with a Drucker–Prager plasticity model for the concrete core and an elastic model for the AFRP is developed by using the finite-element code ANSYS. It is demonstrated that the strength and ductility of the columns with continuous AFRP wrapping are increased greatly; whereas the strength of the columns with discontinuous AFRP wrapping is also increased, but the ductility is not always increased notably. The analytical model and the finite-element model are validated against the experimental results.     

Prestressed CFRP for Strengthening of Reinforced Concrete Structures: Recent Developments at Empa, Switzerland

In civil engineering today, only 20 to 30% of the strength of carbon-fiber-reinforced polymer (CFRP) strips is used when they are applied as externally bonded strips for flexural and shear strengthening or in confinement of reinforced concrete (RC) structural elements. The strips are better used when the CFRP material is prestressed. This offers several advantages, including reduced crack widths, reduced deflections, reduced stress in the internal steel, and possibly increased fatigue resistance. In this paper, recent developments in the field of RC strengthening using prestressed CFRP are presented. The paper focuses on developments in flexural and shear strengthening and column confinement made at the Swiss Federal Laboratory for Materials Testing and Research (Empa). Several innovative ideas have been successfully realized in the laboratory. For example, a gradient prestressing technique without end anchorage plates was developed and successfully applied to a 17?m RC bridge girder. A confinement technique using nonlaminated thermoplastic CFRP straps was also investigated and applied to 2?m high RC columns. These results are encouraging, although practical and theoretical problems remain to be solved before these techniques can be fully applied.     

Behavior of Gravity Load-Designed Rectangular Concrete Columns Confined with Fiber Reinforced Polymer Sheets

This study concentrates on analytical evaluation of the effect of external confinement using fiber reinforced polymers (FRP) sheets on the response of concrete rectangular columns designed for gravity load only and having spliced longitudinal reinforcement at the column base. A general analytical scheme for evaluating the strength capacity and ductility of the columns under combined flexural–axial loads was developed. The analysis takes into account the bond strength degradation of the spliced reinforcement with increase in lateral load by incorporating a generalized bond stress–slip law, and considers the effect of FRP confinement on the stress–strain response of concrete material. Particular emphasis is placed in the analysis on the slip response of the spliced bars and the consequent fixed end rotation that develops at the column base. Results predicted by the analysis showed very good agreement with limited experimental data. A parametric evaluation was carried out to evaluate the effect of different design and strength parameters on the column response under lateral load. Without confinement, the columns suffered premature bond failure and, consequently, low flexural strength capacity. Confining the concrete in the columns end zone at the splice location with FRP sheets enhanced the bond strength capacity of the spliced reinforcement, increased the steel stress that can be mobilized before bond failure occurs, and consequently improved the flexural strength capacity and ductility of the columns. A general design equation, expressed as a function of the main parameters that influence the bond strength capacity between spliced steel bars and FRP confined concrete, is proposed to calculate the area of FRP sheets needed for strengthening of the subject columns.     

Flexural Strengthening of RC Beams with Externally Bonded CFRP Systems: Test Results and 3D Nonlinear FE Analysis

This paper presents experimental results and a numerical analysis of the reinforced concrete (RC) beams strengthened in flexure with various externally bonded carbon fiber-reinforced polymer (CFRP) configurations. The aim of the experimental work was to investigate the parameters that may delay the intermediate crack debonding of the bottom CFRP laminate, and increase the load carrying capacity and CFRP strength utilization ratio. Ten rectangular RC specimens with a clear span of 4.2?m, categorized in two series, were tested to evaluate the effect of using the additional U-shaped CFRP systems on the intermediate crack debonding of the bottom laminate. Two different configurations of the additional systems were proposed, namely, continuous U-shaped wet layup sheets and spaced side-bonded CFRP L-shaped laminates. The fiber orientation effect of the side-bonded sheets was also investigated. A numerical analysis using an incremental nonlinear displacement-controlled 3D finite-element (FE) model was developed to investigate the flexural and CFRP/concrete interfacial responses of the tested beams. The finite-element model accounts for the orthotropic behavior of the CFRP laminates. An appropriate bond-slip model was adopted to characterize the behavior of the CFRP/concrete interface. Comparisons between the FE predictions and experimental results show very good agreement in terms of the load-deflection and load-strain relationships, ultimate capacities, and failure modes of the beams.