Concrete-Filled Square and Rectangular FRP Tubes under Axial Compression

This paper presents results of an experimental study on the behavior of square and rectangular concrete-filled fiber reinforced polymer (FRP) tubes (CFFTs) under concentric compression. FRP tubes were designed as column confinement reinforcement and were manufactured using unidirectional carbon fiber sheets with fibers oriented in the hoop direction. The effects of the thickness and corner radius of the tube, sectional aspect ratio, and concrete strength on the axial behavior of CFFTs were investigated experimentally. Test results indicate that FRP confinement leads to substantial improvement in the ductility of both square and rectangular columns. Confinement provided by the FRP tube may also improve the axial load-carrying capacity of the square and rectangular columns if the confinement effectiveness of the FRP tube is sufficiently high. The results also indicate that the confinement effectiveness of FRP tubes is higher in square columns than in rectangular columns, and in both sections the effectiveness of confinement increases with the corner radius. Furthermore, for a given confinement level, improvement observed on the axial behavior of concrete due to confinement decreases with increasing concrete strength.     

Analytical Model for FRP-Confined Circular Reinforced Concrete Columns

One disadvantage of most available stress–strain models for concrete confined with fiber-reinforced polymer (FRP) composites is that they do not take into consideration the interaction between the internal lateral steel reinforcement and the external FRP sheets. According to most structural concrete design codes, concrete columns must contain minimum amounts of longitudinal and transverse reinforcement. Therefore, concrete columns that have to be retrofitted (and therefore confined) with FRP sheets usually contain lateral steel. Hence, the retrofitted concrete column is under two actions of confinement: the action due to the FRP and that due to the steel ties. This paper presents a new designed-oriented confinement model for the axial and lateral behavior of circular concrete columns confined with steel ties, FRP composites, and both steel ties and FRP composites. Comparison with experimental results of confined concrete stress–strain curves shows good agreement between the test and predicted results.     

Modeling of Corroded FRP-Wrapped Reinforced Concrete Columns in Axial Compression

This paper discusses the mechanical behavior of reinforced concrete columns wrapped with fiber-reinforced polymer (FRP) sheets. A numerical routine was developed to predict the behavior of the columns using a step-by-step technique. The routine is based on an existing model and was modified to account for confinement provided by the traditional steel as well as the external FRP wraps. Several empirical equations for the confined concrete were calibrated with results from experimental tests from different published papers. The most accurate equation was incorporated into the routine to predict the stress-strain relation of the column up to failure. A different confinement to the outer concrete cover and the inner core was used to account for the FRP wraps and the transverse steel. The model was calibrated with experimental results from different experiments on FRP-wrapped reinforced concrete columns.The model was taken one step further by using it to predict the behavior of reinforced concrete columns, with a combination of steel corrosion and CFRP wraps. The columns modeled were subjected to harsh corrosive environment over 44 months. The model successfully predicted the load deformation in both axial and circumferential directions in corroded and intact columns, both wrapped and unwrapped, with good accuracy. The analysis forms a solid foundation for accurate evaluation of the effect of corrosion and wrapping on reinforced concrete columns.     

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.     

Seismic Behavior of Concrete Columns Reinforced by Steel-FRP Composite Bars

Steel-fiber-reinforced polymer (FRP) composite bars (SFCBs) are a novel reinforcement for concrete structures. Because of the FRP’s linear elastic characteristic and high ultimate strength, they can achieve a stable postyield stiffness even after the inner steel bar has yielded, which subsequently enables a performance-based seismic design to easily be implemented. In this study, lateral cyclic loading tests of concrete columns reinforced either by SFCBs or by ordinary steel bars were conducted with axial compression ratios of 0.12. The main variable parameters were the FRP type (basalt or carbon FRP) and the steel/FRP ratio of the SFCBs. The test results showed the following: (1)?compared with ordinary RC columns, SFCB-reinforced concrete columns had a stable postyield stiffness after the SFCB’s inner steel bar yielded; (2)?because of the postyield stiffness of the SFCB, the SFCB-reinforced concrete columns exhibited less column-base curvature demand than ordinary RC columns for a given column cap lateral deformation. Thus, reduced unloading residual deformation (i.e., higher postearthquake reparability) of SFCB columns could be achieved; (3)?the outer FRP type of SFCB had a direct influence on the performance of SFCB-reinforced concrete columns, and concrete columns reinforced with steel-basalt FRP (BFRP) composite bars exhibited better ductility (i.e., a longer effective length of postyield stiffness) and a smaller unloading residual deformation under the same unloading displacement when compared with steel-carbon FRP (CFRP) composite bar columns; (4)?the degradation of the unloading stiffness by an ordinary RC column based on the Takeda (TK) model was only suitable at a certain lateral displacement. In evaluating the reparability of important structures at the small plastic deformation stage, the TK model estimated a much smaller residual displacement, which is unsafe for important structures.     

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.     

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.     

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.     

Analytical Model for FRP Confinement of Concrete Columns with and without Internal Steel Reinforcement

The paper aims to contribute to a better understanding of the behavior of reinforced concrete columns confined with fiber-reinforced polymer (FRP) sheets. In particular, some new insights on interaction mechanisms between internal steel reinforcement and external FRP strengthening and their influence on efficiency of FRP confinement technique are given. In this context a procedure to generate the complete stress-strain response including new analytical proposals for (1) effective confinement pressure at failure; (2) peak stress; (3) ultimate stress; (4) ultimate axial strain; and (5) axial strain corresponding to peak stress for FRP confined elements with circular and rectangular cross sections, with and without internal steel reinforcement, is presented. Interaction mechanisms between internal steel reinforcement and external FRP strengthening, shown by some experimental results obtained at the University of Padova with accurate measurements, are taken into account in the analytical model. Four experimental databases regarding FRP confined concrete columns, with circular and rectangular cross section with and without steel reinforcement, are gathered for the assessment of some of the confinement models shown in literature and the new proposed model. The proposed model shows a good performance and analytical stress-strain curves approximate some available test results quite well.     

Effectiveness of Composites in Earthquake Damage Repair of Reinforced Concrete Flared Columns

A method to utilize fiber composites for rapid repair of earthquake damaged flared columns was developed. Two 0.4-scale reinforced concrete columns that had been tested to failure in previous research were used. Both columns had been subjected to slow cyclic loads and had failed due to low-cycle fatigue of the longitudinal bars. To repair the columns, the damaged concrete in and around the plastic hinge was removed and the steel bars were straightened. Low-shrinkage, high-strength concrete grout was placed in the column afterward. The broken longitudinal bars were not replaced. Rather, glass and carbon fiber reinforced polymer (FRP) sheets with fibers running in the axial direction of the column were added to provide flexural strength to the columns. Additionally, glass FRP sheets with horizontal fibers were attached on the column to provide confinement and shear strength. Cyclic tests of the repaired columns indicated that the method to restore the strength was effective. Analysis using conventional constitutive relationships led to a close estimate of the lateral load response of the models.     

Slender Steel Columns Strengthened Using High-Modulus CFRP Plates for Buckling Control

This paper presents the results of an experimental investigation into the behavior of slender steel columns strengthened using high-modulus (313?GPa), carbon fiber-reinforced polymer (CFRP) plates. Eighteen slender hollow structural section square column specimens, 44×44×3.2?mm, were concentrically loaded to failure. The effectiveness of CFRP was evaluated for different slenderness ratios (kL/r), namely, 46, 70, and 93. The maximum increases in ultimate load ranged from 6 to 71% and axial stiffness ranged from 10 to 17%, respectively, depending on kL/r. As kL/r reduced, the effectiveness of CFRP plates also reduced, and failure mode changed from CFRP plate crushing after occurrence of overall buckling, to debonding prior to, or just at, buckling. A simplified analytical model is proposed to predict the ultimate axial load of FRP-strengthened slender steel columns, based on the ANSI/AISC 360-05 provisions, which were modified to account for the transformed section properties and a failure criteria of FRP derived from the experimental results. It was shown that for a given FRP reinforcement ratio, there is a critical kL/r at the low end, below which FRP may not enhance the strength of the column.     

3D Finite-Element Analysis of Substandard RC Columns Strengthened by Fiber-Reinforced Polymer Sheets

Numerical analyses are performed to predict the stress–strain behavior of square reinforced concrete columns strengthened by fiber-reinforced polymer (FRP) sheet confinement. The research focuses on the contribution of FRP sheets to the prevention of elastic buckling of longitudinal steel bars under compression, in cases of inadequate stirrup spacing. A new Drucker–Prager-type plasticity model is proposed for confined concrete and is used in constructed finite-element model. Suitable plasticity and elasticity models are used for steel reinforcing bars and fiber-reinforced polymers correspondingly. The finite-element analyses results are compared against published experimental results of columns subjected to axial compression, to validate the proposed finite-element model. Stress concentrations in concrete core and on FRP jacket are investigated considering circular or square sectioned, plain or reinforced concrete columns. Geometry of the section as well as the presence of steel bars and stirrups affect remarkably the variation and magnitude of stress on FRP as percentage of its tensile strength.     

Investigation into the Effect of Bonding on FRP-Wrapped Cylindrical Concrete Columns

In this paper the failure of a three layer system comprising a concrete column, an intermediate epoxy layer, and fiber-reinforced polymer (FRP) confinement is investigated. We perform a series of numerical experiments to investigate how the failure loads and ultimate strains of axially loaded plain cement concrete (PCC) and reinforced cement concrete (RCC) columns change with the type of the bond between concrete and epoxy and between epoxy and FRP. Three types of interfacial behavior are considered: rigid, cohesive compliant, and unbonded contact. An idealized spring model for the resultant confinement stiffness is used to explain the effect the nature of the bond has on the results. It is found that the type of bond has a significant effect on the ultimate strength of PCC columns. The results also indicate that the presence of longitudinal and hoop steel reinforcement allows use of comparatively less stiff FRP sheets as confinement material for RCC columns.     

Confinement Effectiveness of FRP in Retrofitting Circular Concrete Columns under Simulated Seismic Load

The results of a research program that evaluated the confinement effectiveness of the type and the amount of fiber-reinforced polymer (FRP) used to retrofit circular concrete columns are presented. A total of 17 circular concrete columns were tested under combined lateral cyclic displacement excursions and constant axial load. It is demonstrated that a high axial load level has a detrimental effect and that a large aspect ratio has a positive effect on drift capacity. Compared with the performance of columns that are monotonically loaded until failure, three cycles of every displacement excursion significantly affect drift capacity. The energy dissipation capacity is controlled by FRP jacket confinement stiffness, especially under a high axial load level. The fracture strain of FRP material has no significant impact on the drift capacity of retrofitted circular concrete columns as long as the same confining pressure is provided, which differs from the common opinion that a larger FRP fracture strain is advantageous in seismic retrofitting. The amount of confining FRP greatly affects the length of the plastic hinge region and the drift capacity of FRP-retrofitted columns. A further increase in confinement after a critical value causes a reduction in the deformation capacity of the columns.     

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.     

Experimental Performance of RC Hollow Columns Confined with CFRP

Column jacketing with fiber-reinforced polymer (FRP) composite materials has been extensively investigated in the last decade to address the issue of seismic upgrade and retrofit of existing reinforced concrete (RC) columns. Researchers have mainly focused their attention on solid columns, while very little research has been done on hollow columns strengthened with FRP. To study the behavior of noncircular hollow cross sections subjected to combined axial load and bending and to contribute to the comprehension of the resistant mechanisms present in FRP confinement, a total of seven specimens have been tested. The present work is the first step in a broader endeavor aimed at evaluating the benefits generated by a FRP wrapping, computing (P-M) interaction diagrams for hollow columns confined with FRP, and defining design criteria for the strengthening of these elements using composite jackets. The theoretical analyses will also assess under which conditions the standard approaches for columns with solid cross sections could be extended to the case of hollow columns.     

Effects of Corrosion of Steel Reinforcement on RC Columns Wrapped with FRP Sheets

This study investigated the effectiveness of carbon fiber-reinforced polymer (CFRP) sheets in protecting reinforced concrete (RC) columns from corrosion of steel reinforcement. Thirty small-scale RC columns and four midscale RC columns were used in this study. The small-scale columns were used for a comprehensive parametric study, whereas the midscale columns were used to evaluate design guidelines proposed based on the results of the small-scale column tests. The test columns were conditioned under an accelerated corrosion process and then tested under uniaxial compression up to failure. The test results showed that although CFRP sheet wrapping decreased the corrosion rate, the corrosion of steel reinforcement could continue to occur, eventually showing a decrease in ultimate axial compression capacity. Design guidelines were proposed based on the small-scale RC column tests and evaluated through a comparison with the test results of midscale RC columns. The proposed design guidelines introduced a concept of effective area to account for the corrosion damage, such as internal cracking and cross-sectional loss of steel reinforcement.     

Effect of Geometric Discontinuities on Strains in FRP-Wrapped Columns

Extensive research on fiber-reinforced polymer (FRP) confinement strengthening for concrete columns has led to a good understanding of, and accurate models for, circular columns subjected to axial load. This research is being extended to noncircular and eccentrically loaded columns, steel tubular, and concrete-filled steel tubular columns. A significant gap in our understanding of all these columns, however, is that the apparent FRP strain at failure is significantly lower than the ultimate strain in a flat coupon test. There may be many factors contributing to this complex phenomenon. This paper shows that one of these factors is the geometrical discontinuities at the ends of the FRP wrap. The significance of the effect of discontinuities on the strain concentrations in the FRP wrap and the bonding adhesive is explored through an elastic finite-element analysis.     

Size Effect of Concrete Short Columns Confined with Aramid FRP Jackets

Most previous studies on concrete short columns confined with fiber-reinforced polymer (FRP) composites were based on small-scale testing, and size effect of the columns still has not been studied thoroughly. In this study, 99 confined concrete short columns wrapped with aramid FRP (AFRP) jackets and 36 unconfined concrete short columns with circular and square cross sections were tested under axial compressive loading. The circular specimens were divided into six groups, and the square specimens were divided into five groups, with each group containing different levels of the AFRP’s confinement. In each group, the specimens were geometrically similar to one another and had three different scaling dimensions. Statistical analyses were used to evaluate the size and interaction effects between the specimen size and the AFRP’s confinement, and a size-dependent model for predicting the strength of the columns was developed by modifying Baz?nt’s size-effect law. The experimental results showed that the size of a specimen had a significant effect on the strength of AFRP-confined concrete short columns, lesser effect on the axial stress-strain curves, and slight effect on the failure modes. The modified Baz?nt model was in good agreement with the experimental data.     

Structural Upgrading of Masonry Columns by Using Composite Reinforcements

Emerging techniques that use fiber-reinforced polymer (FRP) composites for strengthening and conservation of historic masonry are becoming increasingly accepted. In the last decades steel plates or wood frames were used for external confinement in containing the lateral dilation of masonry columns subjected to axial loads. In the last years FRP epoxy bonded strips or jackets were also employed to increase strength and ductility with encouraging results in terms of mechanical behavior and cost effectiveness. The behavior of masonry columns confined with FRP and subjected to axial compression is studied in this paper. An extended experimental investigation is presented in order to show the mechanical behavior of circular masonry columns built with calcareous blocks that may be commonly found in Italy and all over Europe in historical buildings. Different stacking schemes were used to build the columns, aiming to simulate the most common situations in existing masonry structures. Carbon FRP sheets were applied as external reinforcement; different amounts and different schemes of confining reinforcement were studied. The experiments include a new reinforcement technique made by using injected FRP bars through the columns cross section. Such a solution can be considered in place of a more traditional confinement, when external reinforcement must be avoided, or in addition to external reinforcement when an improved confinement effect is required. The structural behavior of masonry columns damaged under different levels of load and strengthened by using FRP reinforcements, was also investigated. Experimental results revealed the effectiveness of the FRP confinement for masonry columns, also for columns that were strongly predamaged before strengthening. A computation of the ultimate load was conducted using the Italian National Research Council recommendations to show an application of the design approach recently proposed in Italy. An existing analytical model, previously developed by the writers, was applied for computation of expected experimental values.