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.     

Theoretical Model for Fiber-Reinforced Polymer-Confined Concrete

Fiber-reinforced polymer (FRP) composites have found increasingly wide applications in civil engineering due to their high strength-to-weight ratio and high corrosion resistance. One important application of FRP composites is as a confining material for concrete, particularly in the strengthening or seismic retrofit of existing reinforced concrete columns by the provision of a FRP jacket. FRP confinement can enhance both the compressive strength and the ultimate strain of concrete significantly. This paper presents a new stress–strain model for FRP-confined concrete in which the responses of the concrete core and the FRP jacket as well as their interaction are explicitly considered. Such a model is often referred to as an analysis-oriented model. The key novel feature of the proposed analysis-oriented model, compared to existing models of the same kind, is a more accurate and more widely applicable lateral strain equation based on a careful interpretation of the lateral deformation characteristics of unconfined, actively confined, and FRP-confined concrete. Through comparisons with independent test data, the proposed model is shown to be accurate not only for FRP-confined concrete but also for concrete confined with a steel tube, demonstrating the wide applicability of the model to concrete confined with different confining materials. The accuracy of the proposed model is also shown to be superior to existing analysis-oriented stress-strain models through comparisons with test data.     

Posttensioned FRP Composite Shells for Concrete Confinement

Experiments have shown that externally bonded fiber-reinforced polymer (FRP) jackets for square and rectangular columns are not as effective as they are for circular columns. The results of experiments on shape-modified concrete columns using posttensioned FRP shells are presented. Posttensioning was achieved by radially straining the precured FRP shell outwards to a substantial strain level, using expansive cement concrete, over a period of 60?days. The prefabricated FRP shell was also used as a stay-in-place formwork. The effectiveness of shape modification using posttensioned FRP shells is compared to FRP-confined original square and rectangular columns, as well as shape-modified columns with nonshrink grout and externally bonded FRP jackets. It is shown that shape modification with posttensioning of FRP shells, using expansive cement concrete, can change the confinement from passive to active and improve significantly the axial strength and ultimate compressive axial strain capacity of square and rectangular columns.     

Refinement of a Design-Oriented Stress–Strain Model for FRP-Confined Concrete

This paper presents the results of a recent study conducted to refine the design-oriented stress–strain model originally proposed by Lam and Teng for fiber-reinforced polymer (FRP)-confined concrete under axial compression. More accurate expressions for the ultimate axial strain and the compressive strength are proposed for use in this model. These new expressions are based on results from recent tests conducted by the writers’ group under well-defined conditions and on results from a parametric study using an accurate analysis-oriented stress–strain model for FRP-confined concrete. They allow the effects of confinement stiffness and the jacket strain capacity to be separately reflected and accounts for the effect of confinement stiffness explicitly instead of having it reflected only through the confinement ratio. The new expressions can be easily incorporated into Lam and Teng’s model for more accurate predictions. Based on these new expressions, two modified versions of Lam and Teng’s model are presented. The first version involves only the updating of the ultimate axial strain and compressive strength equations. The second version caters to stress–strain curves with a descending branch, which is not covered by the original model.     

Ultimate Condition of Fiber Reinforced Polymer-Confined Concrete

One important application of fiber reinforced polymer (FRP) composites is as a confining material for concrete in the retrofit of existing concrete columns by the provision of FRP jackets. Such jackets are commonly formed in a wet layup process, with the fibers being only or predominantly in the hoop direction. It has been well established in recent studies that the rupture strains/strengths of FRP measured in tests on such FRP-confined concrete cylinders fall substantially below those from flat coupon tensile tests, but the causes are unclear. This paper presents the results of a study that is aimed at clarifying these causes. To this end, the paper reports and compares the ultimate tensile strains of two types of FRP (carbon FRP and glass FRP) obtained from three types of tests—flat coupon tensile tests, ring splitting tests, and FRP-confined concrete cylinder tests. Based on comparisons of these test results, it can be concluded that the FRP hoop rupture strains in FRP-confined concrete cylinders are reduced below the ultimate tensile strains from flat coupon tests by at least three factors—(1) the curvature of the FRP jacket; (2) the deformation localization of the cracked concrete; and (3) the existence of an overlapping zone. While the first factor that reduces the in situ strain capacity of FRP on confined concrete is material dependent, the last two factors that result in a nonuniform strain distribution in the jacket are independent of the FRP material properties. The third effect reduces the average hoop rupture but does not affect the distribution of the confining pressure, as the FRP jacket is thicker in the overlapping zone.     

FRP-Confined Concrete Model

A uniaxial model for concrete confined with fiber-reinforced polymers (FRP), but also with steel jackets or conventional transverse reinforcement, is presented. The model, which is suitable to be inserted into fiber-type beam-column models, explicitly accounts for the continuous interaction with the confining device due to the lateral strain of concrete, through an incremental-iterative approach. The relation between the axial and lateral strains is implicitly derived through equilibrium between the (dilating) confined concrete and the confining device. This relation allows one to trace the state of strain in the jacket and to detect its failure. The model is compared with a set of experimental tests and shows very good agreement in both the stress-strain and the stress-lateral strain response. Evidence of the main aspects of the behavior of FRP-confined concrete is given that points out differences in the response when using fiberglass or carbonfiber. Predictive equations to determine the ultimate strength and strain of concrete confined with FRP are derived and tested on a number of experimental data.     

Experimental Study of Normal- and High-Strength Concrete Confined with Fiber-Reinforced Polymers

The experimental program reported here was conducted to gain insight into the behavior of concrete confined with fiber-reinforced polymers (FRPs). A total of 112 cylindrical concrete specimens, each 150 mm in diameter, 300 mm in height, and concrete strength up to 112 MPa, were tested under monotonic uniaxial compression. Test variables included amount of FRP, strength and stiffness of FRP, concrete strength, and the health of concrete at the time of strengthening. Results showed that, with an increase of the unconfined concrete strength, the strength enhancement, energy absorption capacity, ductility factor, and work (energy) index at rupture of FRP jackets all decreased remarkably. A positive correlation was found between concrete ductility and FRP rupture strain. A gradual post-peak failure of the specimens, observed previously from FRP-confined concrete columns tested at the University of Toronto, was also observed in some of the current tests. This ductile failure, attributed to the gradual unzipping failure of FRP jacket, is related to specimen size and is explained in terms of various confinement parameters.     

Short and Medium Term Durability Evaluation of FRP-Confined Circular Concrete

Evaluation of environmental durability of concrete circular specimens strengthened using externally applied fiber-reinforced-polymer (FRP) composites for confinement reinforcement was studied. FRP-confined concrete cylinder tests were carried out for various environmental exposure conditions including interior, exterior, and freeze–thaw cycling in saltwater. Two types of resin matrix-based FRP composite systems, including epoxy and urethane resin, utilizing either carbon or glass fibers were used. In addition, continuous single and double layer wrapping schemes were applied for confinement for each of the four FRP composite systems. The overall stress–strain behavior of FRP-confined concrete does not change fundamentally but different levels of exposure significantly affect its absolute stress–strain curve. The radial strain and corresponding axial strain at the point of zero volumetric strain is significantly affected by different levels of environmental exposure. It is proposed that the relative FRP composite effectiveness must be used to calculate the ultimate radial strain of FRP-confined concrete. It was found that for the four FRP systems used in this study the ultimate radial strain is not significantly affected by the type of exposure. The writers believe that the results shown in this paper help engineers to understand the short and midterm effects of the environment on FRP confined concrete; long term effects are still under investigation.     

Model of FRP-Confined Concrete Cylinders in Axial Compression

This paper introduces a dilatancy-based analytical model of the response of an axially loaded concrete cylinder, confined with a fiber-reinforced polymer (FRP) composite jacket. Model construction is based on the experimentally based observation that the relation between axial secant stiffness and the lateral (dilatancy) strain is effectively unique for cylinders with the same unconfined concrete strength, although the confinement levels may differ. Model development incorporates strength degradation of the concrete with dilatancy (lateral dilation); this feature allows one to demonstrate that the performance of FRP-confined concrete is consistent with the strength envelope obtained from triaxial tests. Model validation is accomplished by comparisons with existing test database and the new results on large-scale concrete cylinders. The results of the validation reveal good agreement with key response functions and parameters. The present study illustrates basic constitutive equations to model FRP-confined concrete in a more rational manner.     

Analytical Model for Circular Normal- and High-Strength Concrete Columns Confined with FRP

This paper presents a new incremental stress-strain model for fiber-reinforced polymer (FRP)-confined concrete. The model, able to accommodate concrete with a wide range of strength (25–110 MPa), is based on material properties, force equilibrium, and strain compatibility, and uses newly developed models for constantly confined concrete. An expression is proposed to calculate a FRP jacket rupture strain in columns. Beyond the initiation of rupture, gradual failure of a FRP jacket is modeled to account for the size effect on the FRP-confined concrete columns. This proposed constitutive model is unique in that it accommodates a wide range of concrete strength and uses an analytical rupture strain of a FRP jacket to predict the complete stress-strain curve. Small and large specimens tested by the authors and other researchers are used to validate the proposed model. Very good to excellent agreements have been achieved between the analytical and experimental responses.     

Performance Enhancement of Steel Columns Using Concrete-Filled Composite Jackets

This paper studies the cross-sectional behavior of steel columns strengthened with fiber-reinforced polymers (FRPs). The composite column is constructed by wrapping the steel I-section column with epoxy-saturated glass- and carbon-FRPs (GFRP and CFRP) sheets in the transverse direction and subsequently filling the voids between the FRP and the steel with concrete. Experimental tests were performed on stub columns under axial compression including one to three CFRP wraps. A corner treatment technique, to avoid stress concentration at the corners and to improve confinement efficiency, was also investigated. A simplified analytical model was developed to predict the axial behavior of the composite columns. Experimental results showed significant enhancement in the behavior of the composite columns primarily attributable to the confinement mechanism imposed by the FRP jacket and concrete. Increasing the corner radius resulted in higher compressive strength of the confined concrete and ultimate axial strain of the composite columns. Good agreement between the analytically developed axial load-displacement relationships and the test data indicates that the model can closely simulate the cross-sectional behavior of the composite columns.     

Axial Load Capacity of Concrete-Filled FRP Tube Columns: Experimental versus Theoretical Predictions

This paper presents the experimental and theoretical results of small and medium-scale concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) columns. A total of 23 CFFT specimens were tested under axial compression load. Five different types of new FRP tubes were used as stay-in-place formwork for the columns. The effects of the following parameters were examined: the FRP-confinement ratio, the unconfined concrete compressive strength, the presence of longitudinal steel reinforcement, and the height-to-diameter ratio. Comparisons between the experimental test results and the theoretical prediction values by the three North American codes and design guidelines (ACI 440.2R-08, CSA-S6-06, and CSA-S806-02) are performed in terms of confined concrete strength and ultimate load carrying capacity. The results of this investigation indicate that the design equations of the ACI 440.2R-08, CAN/CSA-S6-06, and CAN/CSA-S806-02 overestimate the factored axial load capacity of the short CFFT columns as compared to the yield and crack load levels. Also, the CAN/CSA-S6-06 and CAN/CSA-S806-02 confinement models showed conservative predictions, while the ACI 440.2R-08 was slightly less conservative. A new confinement model is proposed for the confined concrete compressive strength of the CFFT cylinders. Also, the design equations are modified to accurately predict the ultimate and yield load capacities of internally reinforced and unreinforced short CFFT columns. Two new factors are introduced in the modified equations, (kcc) accounts for the in-place-strength of CFFT columns to CFFT cylinder strength, and (kcr) accounts for the initiation of the steel yielding and concrete cracking for the FRP-confined columns.     

Masonry Confinement with Fiber-Reinforced Polymers

The application of fiber-reinforced polymer (FRP) as a means of increasing the axial capacity of masonry through confinement, a subject not addressed before, is investigated in this study. Four series of uniaxial compression tests, with a total of 42 specimens, were conducted on model masonry columns with these variables: number of layers, radius at the corners, cross-section aspect ratio, and type of fibers. It is concluded that, in general, FRP-confined masonry behaves very much like FRP-confined concrete. Confinement increases both the load-carrying capacity and the deformability of masonry almost linearly with the average confining stress. The uniaxial compression test results enabled the development of a simple confinement model for strength and ultimate strain of FRP-confined masonry. This model is consistent with the test results obtained here but should attract further experimental verification in the future to account for types of masonry materials other than those used in this study.     

Seismic Behavior of RC Columns with Bond-Critical Regions: Criteria for Bond Strengthening Using External FRP Jackets

In 2003, an experimental research program was initiated at the American University of Beirut with the objectives of (1) evaluating the effectiveness of external fiber-reinforced polymer (FRP) confinement in improving the bond strength of spliced reinforcement in reinforced-concrete (RC) columns and its implications on the lateral load capacity and ductility of the columns under seismic loading; and (2) establishing rational design criteria for bond strengthening of spliced reinforcement using external FRP jackets. This paper presents a discussion of recent experimental results dealing with rectangular columns and the results of a pilot study conducted on circular columns with particular emphasis on aspects related to the bond strength of the spliced column reinforcement. A nonlinear analysis model is developed for predicting the envelope load–drift response, taking into account the effect of FRP confinement on the stress–strain behavior of concrete in compression. Results predicted by the model showed excellent agreement with the test results. Design expressions of the bond strength of spliced bars in FRP-confined concrete were assessed against the current experimental data, and a criterion for seismic FRP strengthening of bond-critical regions in RC members is proposed.     

Bond Strength of Lap-Spliced Bars in Concrete Confined with Composite Jackets

The effectiveness of fiber-reinforced polymer (FRP) and textile-reinforced mortar (TRM) jackets was investigated experimentally and analytically in this study to confine old-type reinforced concrete (RC) columns with limited capacity because of bond failure at lap-splice regions. The local bond strength between lap-spliced bars and concrete was measured experimentally along the lap-splice region of six full-scale RC columns subjected to cyclic uniaxial flexure under constant axial load. The bond strength of the two column specimens tested without retrofitting was found to be in good agreement with the predictions given by two existing bond models. These models were modified to account for the contribution of composite material jacketing to the bond resistance between lap-spliced bars and concrete. The effectiveness of FRP and TRM jackets against splitting at lap splices was quantified as a function of jacket properties and geometry as well as in terms of the jacket effective strain, which was found to depend on the ratio of lap-splice length to bar diameter. Consequently, simple equations for calculating the bond strength of lap splices in members confined with composite materials (FRP or TRM) are proposed.     

Behavior of FRP Jacketed Concrete Columns under Eccentric Loading

This paper describes a study on the behavior of fiber-reinforced polymer (FRP) jacketed square concrete columns subjected to eccentric loading. The effect of strain gradient on the behavior of concrete columns confined by the FRP jacket was investigated through experimental and numerical analysis methods. Nine (108 × 108 × 305 mm) square concrete column stubs with zero, one, and two plies of unidirectional carbon FRP fabric were tested under axial compressive loading. In addition to the FRP jacket thickness, the effects of various eccentricities were examined. The nonlinear finite-element analysis results were compared and validated against the experimental test results. The results show that the FRP jacket can greatly enhance the strength and ductility of concrete columns under eccentric loading and that the strain gradient reduces the retrofit efficiency of the FRP jacket for concrete columns. Therefore, a smaller enhancement factor should be used in designing FRP-jacketed columns under eccentric loading. Furthermore, the nonlinear finite-element models established in this study can be used as templates for future research work on FRP-confined concrete columns.     

Numerical Modeling of Concrete Confined by Fiber-Reinforced Composites

Lateral confinement of reinforced concrete columns can significantly increase their lateral deflection capability and load-carrying capacity. While such retrofits were initially completed using steel jackets, fiber-reinforced polymer (FRP) composites have been used successfully and extensively for seismic and blast upgrades. Numerical modeling of such structures requires the use of a concrete material model that can accurately represent the volumetric behavior of concrete under triaxial stress states, to capture the interaction between concrete expansion and the resulting stress increase in the confining jacket. Test data by Suter and Pinzelli, Karbhari and Gao, and Mirmiran and Shahawy on concrete cylinders and prisms confined by aramid, carbon, and glass FRP sheets are analyzed numerically. The concrete material model used was developed for the study of the effect of blast loading on reinforced concrete structures and was verified and validated for a variety of triaxial stress paths. The numerical analyses closely reproduce the strength enhancements observed in the test specimens for various levels of confinement. The model also confirms the observed inefficiency of low levels of lateral confinement and the superior enhancement provided by circular cross sections as compared to rectangular ones.     

Design and Fabrication of FRP Grids for Aerospace and Civil Engineering Applications

This paper presents the results of experimental and analytical studies of the performance of composite grids as well as fiber-reinforced polymer (FRP) grid reinforced concrete columns. FRP grids and FRP grid confined concrete cylinders were instrumented and tested under uniaxial compressive loading. Test variables included types of composite materials and the spacing of composite circular ribs. It is shown that the proposed FRP grids can be constructed by filament winding, the process can be automated, and the manufacturing cost can be reduced. Results show that the proposed FRP grids have substantial ultimate load that make them attractive for use in aerospace applications, and that confinement of concrete by FRP grids can significantly enhance the strength, ductility, and energy absorption capacity of concrete as compared to steel confined concrete. Equations to predict the compressive strength and failure strain were developed. Comparisons between the experimental and analytical results indicate that the proposed models provide satisfactory predictions of ultimate compressive strength and failure strain.     

Design of FRP Jackets for Upgrade of Circular Bridge Piers

The upgrading of bridges located in seismic areas and built according to obsolete codes is becoming a priority task for highway administrations. Among the possible upgrading strategies, the use of fiber-reinforced plastic (FRP) jackets is gaining widespread acceptance. In this paper, a design equation is proposed to determine the optimal thickness of FRP jackets, to enhance the ductility of existing reinforced-concrete (RC) bridge piers with circular cross sections. The design procedure stems from the definition of an upgrading index, given as the ratio of the target to availability ductility at the pier base section, to be attained through FRP jacketing. The available ductility is that identified through the usual assessment procedures on the RC member set for upgrade, whereas the target ductility is evaluated based on the expected actions on the bridge. The upgrading index is initially defined in general terms and is subsequently extended to the case of piers built in seismic regions. It results in a simple expression in terms of easily computable quantities, such as the ultimate strain and the peak strength of concrete, before and after upgrading. A parametric study on old-code–designed bridge pier sections, upgraded with either glass or carbon fiber jackets, is performed based on a fiber-section model equipped with a newly developed FRP-confined concrete model. This study shows that the index, despite its simplicity, yields excellent predictions of the ductility increase obtained through FRP wrapping, and it is therefore used to develop a design equation. The equation allows the design of the optimal thickness of FRP jackets in terms of the desired upgrading index, mechanical characteristics of the selected composite material, and quantities defining the initial state of the pier section. The design procedure has been applied to available experimental tests of a scaled bridge pier wrapped with FRP and tested to failure, and it has been demonstrated to be very effective.