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.     

Analytical Evaluation of Seismic Performance of RC Frames Rehabilitated Using FRP for Increased Ductility of Members

Many reinforced concrete (RC) frame structures designed according to pre1970 strength-based codes are susceptible to abrupt strength deterioration once the shear capacity of the columns is reached. Fiber composites are used to increase the shear strength of existing RC columns and beams by wrapping or partially wrapping the members. Increasing the shear strength can alter the failure mode to be more ductile with higher energy dissipation and interstorey drift ratio capacities. The objective of this study was to analytically evaluate the effect of varying distributions of fiber-reinforced polymer (FRP) rehabilitation on the seismic performance of three existing RC frames with different heights when subjected to three types of scaled ground motion records. The FRP wrapping is designed to increase the displacement ductility of frame members to reach certain values representing moderate ductility and high ductility levels. These values were assumed based on previous experimental work conducted on members wrapped using FRP. The study also investigates the effect of the selected element’s force–displacement backbone curve on the capacities of the structures with respect to maximum interstory drift ratio, maximum peak ground acceleration, or peak ground velocity resisted by the frames, maximum storey shear-to-weight ratio and maximum energy dissipation. It was found that for low-rise buildings, the FRP rehabilitation of columns only was effective in enhancing the seismic performance; while for high-rise ones, rehabilitation of columns only was not as effective as rehabilitation of both columns and beams. Ignoring representing the postpeak strength degradation in the hysteretic nonlinear model of FRP-rehabilitated RC members was found to lead to erroneous overestimation of the seismic performance of the structure.     

Review of Design Guidelines for FRP Confinement of Reinforced Concrete Columns of Noncircular Cross Sections

Current international design guidelines provide predictive design equations for the strengthening of reinforced concrete (RC) columns of both circular and prismatic cross sections by means of fiber-reinforced polymer (FRP) confinement and subjected to pure axial loading. Extensive studies (experimental and analytical) have been conducted on columns with circular cross sections, and limited studies have been conducted on members with noncircular cross sections. In fact, the majority of available research work has been on small-scale, plain concrete specimens. In this review paper, four design guidelines are introduced, and a comparative study is presented. This study is based on the increment of concrete compressive strength and ductility and includes the experimental results from six RC columns of different cross-sectional shapes. The observed outcomes are used to identify and remark upon the limits beyond the ones specifically stated by each of the guides and that reflect the absence of effects not considered in current models. The purpose of this study is to present a constructive critical review of the state-of-the-art design methodologies available for the case of FRP-confined concrete RC columns and to indicate a direction for future developments.     

Fiber-Reinforced Plastic Jackets for Ductility Enhancement of Reinforced Concrete Bridge Columns with Poor Lap-Splice Detailing

This paper presents an inclusive testing program conducted on scaled models of reinforced concrete (RC) bridge columns with insufficient lap-splice length. Thirteen half-scale circular and square column samples were tested in flexure under lateral cyclic loading. Three columns were tested in the as-built configuration whereas ten samples were tested after being retrofitted with different composite-jacket systems. A brittle failure was observed in the as-built samples due to bond deterioration of the lap-spliced longitudinal reinforcement. The jacketed circular columns demonstrated a significant improvement in their cyclic performance. Yet, tests conducted on square jacketed columns showed a limited improvement in clamping on the lap-splice region and for enhancing the ductility of the column.     

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.     

Behavior of Cyclically Loaded Squat Reinforced Concrete Bridge Columns Upgraded with Advanced Composite-Material Jackets

This paper summarizes comprehensive experimental studies on scaled models of squat bridge columns repaired and retrofitted with advanced composite-material jackets. In the experimental program, a total of 14 half-scale squat circular and rectangular reinforced concrete columns were tested under fully reversed cyclic shear in a double bending configuration. In order to provide a basis for comparison, a total of three as-built columns were tested. Another 10 column samples were tested after being retrofitted with different composite jacket systems. One circular as-built column was repaired after failure. The repair process involved both crack injection as well as addition of carbon/epoxy composite jacket. The repaired column was then retested and evaluated. Experimental results showed that all as-built columns developed an unstable behavior and failed in brittle shear mode. The common failure mode for all retrofitted samples was due to flexure with significant improvement in the column ductility. The repaired column demonstrated ductility enhancement over the as-built sample.     

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.     

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.     

Confinement Effectiveness in Rectangular Concrete Columns with Fiber Reinforced Polymer Wraps

An innovative mechanistic based method for passive confinement efficiency estimation is proposed based on the extension to rectangular sections of the pulley model previously proposed by the writers. A refined finite element model was developed using a nonlinear concrete constitutive law in order to analyze stresses in columns passively confined with fiber reinforced polymer wraps. Rectangular and square cross sections of variable corner radii were investigated with reference to a circular cross section. Results showed an increase in corner stresses with sharper corner radii, a localization of failure at the corners, and a decrease in confinement effectiveness with an increase in the rectangularity of the cross section or an increase in corner sharpness. A rigorous numerical method for calculating geometric confinement efficiency factors is proposed and typical factors are calculated and compared with the predictions of the simple pulley model showing good agreement.     

Rectangular Filament-Wound Glass Fiber Reinforced Polymer Tubes Filled with Concrete under Flexural and Axial Loading: Analytical Modeling

This paper presents an analytical model to predict the behavior of concrete-filled rectangular fiber reinforced polymer (FRP) tubes (CFRFTs), subjected to bending and axial loads. The model accounts for different laminate structures of the flange and web of the tube. Gradual reduction of stiffness, resulting from progressive failure of FRP layers oriented at various angles is considered through the ultimate laminate failure approach. The model adopts cracked section analysis, using layer-by-layer approach and accounts for totally and partially filled tubes. The model predicts the moment–curvature responses of beams, load–strain responses of columns, and complete interaction curves of beam–columns. The model is verified using experimental results and is used to study the effects of laminate structure, hybrid laminates, thickness of the tube and optimization of partially filled tubes. Comparisons of CFRFT with conventional reinforced concrete (RC) sections showed that CFRFT could provide axial load–bending moment interaction curves comparable to those of RC sections of similar reinforcement index. Also, providing a small fraction of carbon fibers in the flanges could substantially improve flexural performance. The first ply failure approach could highly underestimate the strength of CFRFT.     

Active Confinement of Reinforced Concrete Bridge Columns Using Shape Memory Alloys

This paper presents experimental and analytical work conducted to explore the feasibility of using an innovative technique for seismic retrofitting of RC bridge columns using shape memory alloys (SMAs) spirals. The high recovery stress associated with the shape recovery of SMAs is being sought in this study as an easy and reliable method to apply external active confining pressure on RC bridge columns to improve their ductility. Uniaxial compression tests of concrete cylinders confined with SMA spirals show a significant improvement in the concrete strength and ductility even under small confining pressure. The experimental results are used to calibrate the concrete constitutive model used in the analytical study. Analytical models of bridge columns retrofitted with SMA spirals and carbon fiber-reinforced polymer (CFRP) sheets are studied under displacement-controlled cyclic loading and a suite of strong earthquake records. The analytical results proves the superiority of the proposed technique using SMA spirals to CFRP sheets in terms of enhancing the strength and effective stiffness and reducing the concrete damage and residual drifts of retrofitted columns.     

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.     

Debonding in RC Beams Shear Strengthened with Complete FRP Wraps

Substantial research has been conducted on the shear strengthening of reinforced concrete (RC) beams with bonded fiber reinforced polymer (FRP) strips. The beams may be strengthened in various ways: complete FRP wraps covering the whole cross section (i.e., complete wrapping), FRP U jackets covering the two sides and the tension face (i.e., U jacketing), and FRP strips bonded to the sides only (i.e., side bonding). Shear failure of such strengthened beams is generally in one of two modes: FRP rupture and debonding. The former mode governs in almost all beams with complete FRP wraps and some beams with U jackets, while the latter mode governs in all beams with side strips and U jackets. In RC beams strengthened with complete wraps, referred to as FRP wrapped beams, the shear failure process usually starts with the debonding of FRP from the sides of the beam near the critical shear crack, but ultimate failure is by rupture of the FRP. Most previous research has been concerned with the ultimate failure of FRP wrapped beams when FRP ruptures. However, debonding of FRP from the sides is at least a serviceability limit state and may also be taken as the ultimate limit state. This paper presents an experimental study on this debonding failure state in which a total of 18 beams were tested. The paper focuses on the distribution of strains in the FRP strips intersected by the critical shear crack, and the shear capacity at debonding. A simple model is proposed to predict the contribution of FRP to the shear capacity of the beam at the complete debonding of the critical FRP strip.     

Strengthening RC Beams in Flexure Using New Hybrid FRP Sheet/Ductile Anchor System

This paper explores a new hybrid fiber-reinforced polymer (FRP) sheet/ductile anchor system for rehabilitation of reinforced concrete (RC) beams. The advantages of the proposed strengthening method is that it overcomes the problem of low ductility that is associated with brittle failure mode in conventional methods of strengthening beams using epoxy-bonded FRP sheets. The proposed system leads to a ductile failure mode by triggering yielding to occur in a steel anchor system (steel links) rather than by rupture or debonding of FRP sheets, which is sudden in nature. Four half-scale RC T-beams were tested under four-point bending. Three retrofitted beams were strengthened using one layer of carbon FRP sheet. The results of the two beams that were strengthened with the new hybrid FRP sheet/ductile anchor system were compared with the results from the beam strengthened with conventional FRP bonding method and the control beam. The results show the effectiveness of the proposed strengthening system in increasing flexural capacity and ductility of RC beams.     

Hysteretic Behavior of Bridge Columns with FRP-Jacketed Lap Splices Designed for Moderate Ductility Enhancement

This paper presents test results of six specimens representing older bridge columns with inadequate reinforcement detailing consisting of short lap splices at the base and widely spaced transverse reinforcement. Four of these specimens were rehabilitated using fiber-reinforced polymer (FRP) jackets of two different composite materials (carbon and aramid) to avoid premature failure of the lapped bars after a limited number of postyield cycles. The test results indicate that thin FRP jackets can be used to avoid failure of short lap splices at moderate displacement ductilities. Displacement capacities consistent with expected demands in regions of moderate or low seismicity were achieved after jacket retrofitting. The hysteretic behavior of rehabilitated columns was assessed with emphasizing issues related to variation of stiffness and damping ratio as a function of ductility demand for this class of columns. Equations that account for the effect of axial load level on estimates of effective stiffness and damping as a function of displacement ductility are proposed for this class of columns.     

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.     

Bond Strength of Tension Lap Splices in High-Strength Concrete Beams Strengthened with Glass Fiber Reinforced Polymer Wraps

This paper presents the experimental results of the first phase of a study undertaken at the American University of Beirut to examine the effectiveness of fiber reinforced polymer (FRP) wraps to confine steel reinforcement in a tension lap splice region anchored in high-strength reinforced-concrete beams. Seven beam specimens were constructed. The specimens were reinforced on the tension side with three deformed bars spliced at midspan. The splice region was devoid of any transverse reinforcement to allow a full examination of the FRP wrap contribution. Glass fiber reinforced polymer (GFRP) sheets were used. The main test variables were the GFRP configuration in the splice region (one strip, two strips, or a continuous strip), and the number of layers of the GFRP wraps placed around the splice region (one layer or two layers). All GFRP wraps were U-shaped. Except for the epoxy adhesive, no other anchorage mechanism or bonding procedure was applied for the GFRP wraps on the concrete beam. Following the application of the GFRP wraps, the beams were tested in positive bending. The test results demonstrated that GFRP wraps were effective in enhancing the bond strength and ductility of failure mode of the tension lap splices, especially when continuous strips were applied over the splice region.     

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.     

Probabilistic Seismic Demand Models and Fragility Estimates for Reinforced Concrete Bridges with Two-Column Bents

Probabilistic models are developed to predict the deformation and shear demands due to seismic excitation on reinforced concrete (RC) columns in bridges with two-column bents. A Bayesian methodology is used to develop the models. The models are unbiased and properly account for the predominant uncertainties, including model errors, arising from a potentially inaccurate model form or missing variables, measurement errors, and statistical uncertainty. The probabilistic models developed are akin to deterministic demand models and procedures commonly used in practice, but they have additional correction terms that explicitly describe the inherent systematic and random errors. Through the use of a set of “explanatory” functions, terms that correct the bias in the existing deterministic demand models are identified. These explanatory functions provide insight into the underlying behavioral phenomena and provide a means to select ground motion parameters that are most relevant to the seismic demands. The approach takes into account information gained from scientific/engineering laws, observational data from laboratory experiments, and simulated data from numerical dynamic responses. The demand models are combined with previously developed probabilistic capacity models for RC bridge columns to objectively estimate the seismic vulnerability of bridge components and systems. The vulnerability is expressed in terms of the conditional probability (or fragility) that a demand quantity (deformation or shear) will be greater than or equal to the corresponding capacity. Fragility estimates are developed for an example RC bridge with two-column bents, designed based on the current specifications for California. Fragility estimates are computed at the individual column, bent, and bridge system levels, as a function of the spectral acceleration and the ratio between the peak ground velocity and the peak ground acceleration.