FRP Jackets Research Articles

Bond strengthening of lap spliced reinforcement using external FRP jackets: An effective technique for seismic retrofit of rectangular or circular RC columns

Confinement of concrete at the critical hinging regions is one of the most effective means for improving the seismic behavior of reinforced concrete (RC) members. Confinement reduces the bond degradation of the steel reinforcement and limits the concrete damage within the confined zone leading to increased energy absorption and dissipation capacity of the structure. Recent experimental and analytical studies undertaken on the use of FRP composites for seismic upgrading of RC members [FRPRCS08 Proceedings. In: Triantafillou TC, editor. 8th International symposium on fiber-reinforced polymer reinforcement for concrete structures. Patrace, Greece: University of Patrace; July 2007. p. 16–8] have clearly identified the advantages of this technology when compared to conventional strengthening methods. However, unfortunately, no guidelines have yet been developed by code committees for seismic retrofit using FRP composites. This paper provides a synthesis of the results of a series of experimental and analytical studies undertaken in the main part by the writer and in part by other investigators on the use of external FRP jackets for bond strengthening of developed/spliced steel bars in tension and its implications on the static and seismic response of concrete members. Results of the experimental programs are briefly discussed, and design expressions for evaluating the minimum thickness of FRP jacket required for seismic bond strengthening are presented and compared. The accuracy of the design expressions was verified against experimental data. In addition, a generalized model of the local bond stress–slip response of steel bars embedded in FRP confined concrete corresponding to splitting mode of bond failure is described. The model is composed of a monotonic envelope curve and bond degrading curve and can be used for evaluating the seismic behavior of the hinging zones in reinforced concrete members when confined externally with FRP jackets in comparison with the response of unconfined concrete.

Prediction of nonlinear bending behavior for FRP concrete beams based on multi-axial constitutive laws

This study focuses on a predictive model of the flexural behavior of FRP-confined concrete structural members. A hypoelasticity-based constitutive law of concrete is presented on the basis of a three-dimensional stress state in order to model the compressive behavior of confined concrete wrapped with FRP jackets. The strength enhancement of concrete was determined by the failure surface of concrete in a triaxial stress state, and its corresponding peak strain was computed by the strain-enhancement factor also proposed in this study. The behavior of FRP jackets is modeled using two-dimensional orthotropic lamination theory. Therefore, the newly proposed model is a nonlinear load path-dependent confinement model of FRP-confined concrete. To model FRP-confined concrete bending members, a procedure is presented for the prediction of the flexural behavior of FRP-confined concrete structural members using a nonlinear fiber cross-sectional approach. The developed model is validated with test results obtained from several bending experiments. The results demonstrate that the proposed model is able to capture the flexural behavior of FRP-confined concrete structural members as well as the axial and lateral strains of the sections.

FRP 보강 철근콘크리트기둥의 변위계수법에 의한 내진성능설계기법 개발

In the current research, the scheme of displacement-based seismic design for seismic retrofit of concrete structures using FRP composite materials has been proposed. An algorithm of the nonlinear flexural analysis of FRP composite concrete members has been presented under multiaxial constitutive laws of concrete and composite materials. An algorithm for performance-based seismic retrofit design of reinforced concrete columns with FRP jacket has been newly introduced to modify the displacement coefficient method used in reinforced concrete structures. From applications of retrofit design, the method are easy to apply in the practice of retrofit design and give practical prediction of nonlinear seismic performance evaluation of retrofitted structures.

Seismic Retrofitting of Columns with Lap Spliced Smooth Bars Through FRP or Concrete Jackets

The effectiveness of RC jacketing or FRP wrapping for seismic retrofitting of rectangular columns having smooth (plain) bars with 180° hooks lap-spliced at floor level is experimentally investigated. The relatively low deformation capacity and energy dissipation of five unretrofitted columns is found not to depend on lap length, if lapping is not less than 15 bar-diameters. Six columns cyclically tested up to ultimate deformation after RC concrete jacketing demonstrate force and deformation capacity and energy dissipation sufficient for earthquake resistance, regardless of the presence or length of lap splicing in the original column. Another ten columns cyclically tested to ultimate deformation after wrapping of the plastic hinge region with CFRP show that FRP wrapping of the splice region is more effective than concrete jackets for enhancement of the deformation and energy dissipation capacity of old-type columns with smooth bars lap-spliced at floor level, provided that wrapping extends over the member length sufficiently to preclude plastic hinging and early member failure outside the FRP-wrapped length of the column.

Detailing procedures for seismic rehabilitation of reinforced concrete members with fiber reinforced polymers

Fiber reinforced polymer jackets (FRP sheets) offer great advantages as confining devices of damaged or substandard reinforced concrete (r.c.) members and are already used extensively as local interventions in seismic rehabilitation of existing construction. The design issues and detailing rules related to this seismic repair/strengthening technique are explored by evaluating systematically the various mechanisms of resistance of the upgraded member and the jacket contribution. These include flexure, shear, lap-splice capacity, plastic hinge behavior, displacement capacity and jacket strain capacity associated with embedded bar buckling. A database of published experiments on r.c. beam/columns, tested under cyclic loading after being jacketed by FRPs was used as a point of reference in assessing the detailing rules and in calibrating design lower bound expressions. Criteria that should be considered for the upgrade design strategy in order to control the deformation demand of the structure when FRP jackets are used are also discussed in the paper.

직접변위기반설계법에 의한 철근콘크리트 기둥의 FRP 피복보강 내진성능설계법의 개발

본 연구에서는, 기존 철근 콘크리트 구조물에 적용된 직접 변위-기반 설계법을 적용 FRP 피복 보강된 성능개선 콘크리트 부재에 대한 정밀 비선형 휨 해석 및 내진성능설계의 구체적 알고리즘을 제시하였다. 비선형 휨 해석의 정밀 예측을 위하여 콘크리트 및 FRP 복합재료의 다축 구성관계를 고려하였으며, Chopra 등 (1999)이 제안한 직접 변위-기반 설계법(DDM)을 개선하여 철근콘크리트 기둥에 대한 성능개선을 위한 FRP 피복 보강을 위한 성능설계 알고리즘을 제시하였다. 제시된 직접 변위-기반 설계법은, 변위계수법과 비교하여, 비선형 거동이 큰 경우에도 목표 변위 성능 값에 대한 정확한 추정을 해준다. 이는 변위계수법이 항복 이전의 유효탄성계수를 사용하는 반면, 직접 변위-기반 설계법은 유효탄할선탄성계수를 고려하고 있어, 목표 변위에 따른 성능설계 평가에 있어서 보다 높은 연성비의 거동을 반영하고 있기 때문인 것으로 평가된다. In the current research, an algorithm of performance-based seismic retrofit design of reinforced concrete columns using FRP jacket has been proposed. For exact prediction of the nonlinear flexural analysis or FRP composite RC members, multiaxial constitutive laws of concrete and composite materials have been presented. For seismic retrofit design, an algorithm of direct displacement-based design method (DDM) proposed by Chopra and Goel (2001) has been newly applied to determine the design thickness of FRP jacket in seismic retrofit of reinforced concrete columns. To compare with the displacement coefficient method (DCM), the DDM gives an accurate prediction of the target displacement in highly nonlinear region, since the DCM uses the elastic stiffness before reaching the yield load as the effective stiffness but the DDM uses the secant stiffness.

Design of FRP-wrapped reinforced concrete columns for enhancing axial load carrying capacity

The paper proposed a design method and an experimental programme to evaluate the axial load strength of rectangular and square reinforced compression members confined with GFRP (glass fibre reinforced polymer plates) jackets and steel hoops. Three square and three rectangular columns were tested under axial compression up to failure. The test results clearly showed the efficiency of the jackets in enhancing the ultimate strain and strength of the columns. The design method was calibrated using data from the tests. Closed-form equations are proposed for calculating the axial load strength of columns confined with FRP jackets.

Durability based design of FRP jackets for seismic retrofit

Although FRP composites are increasingly being used for the rehabilitation of civil infrastructure, there is still a lack of well documented long-term durability data, and of design methodologies that explicitly consider effects of deterioration over time at a structural level. This paper provides results of an investigation aimed at assessing the effect of deterioration over time, at the materials level, on the effectiveness of FRP jackets used for seismic retrofit. Three different systems are investigated and results of accelerated testing are used to provide predictive equations for long-term performance of the material, which are then used to analyze effectiveness at the level of seismic retrofit through four specific cases. The effect of deterioration is expressed, for ease in comparison, to an increase in the required thickness of the jacket with expected service-life. Results using unexposed values for materials performance are compared to those obtained using the values recommended by ACI-440 procedures, as well as through the use of time-dependent materials degradation models. It is shown that the ACI recommendations for durability may be excessively conservative for the 50-year period considered herein. The use of the proposed predictive methodology for materials durability combined with the analytical tools for design of FRP jacket thickness are shown to not only enable a better assessment of required jacket thickness but can also enable assessment of the dominant mechanism controlling selection of thickness which can change with time of exposure. Results of the accelerated tests are also linked to field exposure results providing a set of correlation factors.

Fundamental Principles That Govern Retrofitting of Reinforced Concrete Columns by Steel and FRP Jacketing

Extensive investigations on seismic retrofitting of reinforced concrete columns have been undertaken in recent years and many retrofitting methods have been developed and reported in the literature. However contradictory results and conclusions are not rare especially for jacketing of rectilinear columns, and there is not a clear picture on the current state-of-the-art in the literature. This work is based on the review and assessment of more than 120 papers, and collection and analysis of more than 700 column tests. In combination with the authors' original theoretical studies and experimental testing, this work identifies the key factors that affect the effectiveness and deficiencies of different retrofitting techniques and provides engineers with a clearer picture on the capabilities of different retrofitting techniques, in particular how to improve the effectiveness of a jacketing system. From the theoretical studies, this work also provides researchers with fundamental insight and principles that govern the retrofitting of RC columns, which shed light on the development of new models and identification of further research subjects.

Seismic Upgrading Of Square And RectangularRC Columns Using FRP Wrapping

This paper presents the results of an experimental study to evaluate the effectiveness of using FRP wrapping for repair and seismic upgrading of square and rectangular RC columns in buildings. The factors affecting the performance of FRP wrapping in rectangular columns under the action of both axial and combined axial and lateral loads are considered in this paper. These factors include the rectangularity ratio of the column cross section, the thickness of the FRP jacket, and the use of carbon verses glass FRP for column jacketing. Techniques to improve the performance of strengthening rectangular columns were also proposed and evaluated in the paper. Such techniques include rounding the sharp edges of the columns, and transferring square into circular columns using mortar. A total of fourteen half-scale reinforced concrete columns, divided into two groups, are tested in this research. The first group of columns consists of three square columns and three rectangular columns which are tested under axial loads. The second group consists of four square columns and four rectangular columns tested under combined axial and lateral loads.

THE PERFORMANCE OF LIGHTWEIGHT CONCRETE CONFINED BY FIBER COMPOSITES

Degradation of infrastructure due to overloading and corrosion is a major concern. FRP materials are now increasingly being used in the construction industry. Confinement of circular concrete members using FRP is a popular application. As such, studying the performance of concrete members confined by FRP jackets is quite important for both rehabilitated and new structures. This thesis presents results of an experimental study on the performance of concrete cylinders confined with glass-FRP (GFRP) sheets, carbon-FRP (CFRP) sheets. While the subject of FRP-confinement of circular concrete columns has been extensively studied experimentally and analytically, more emphasis has typically been put on the characteristics of the FRP jackets in comparison to the concrete core properties. In this paper, the performance of the lightweight concrete core on FRP jackets is explored in order to simulate the worst case scenario in old structures that were built several decades ago, and maximize the damaging effect of concrete durability. This study simulates wrapping of circular columns with FRP sheets for retrofitting applications; concrete core with different strengths has been used to evaluate the different levels of confinement induced by GFRP and CFRP jackets of different thicknesses, specimens were tested to failure under axial compression, the study showed that wraps provide substantial increase in strength and ductility of different lightweight concretes. However, increasing the strength of concrete core shows a decreasing in the confined strength gain of tested specimens with any improvement in ductility.

FRP Jacketed and Shape-Modified Compression Members: II – Model

FRP Jacketed and Shape-Modified Compression Members: II – Model

FRP Jacketed and Shape-Modified Compression Members: I – Experimental Behavior

FRP Jacketed and Shape-Modified Compression Members: I – Experimental Behavior

Concrete or FRP Jacketing of Columns with Lap Splices for Seismic Rehabilitation

Seismic rehabilitation of rectangular columns with corner deformed bars lap-spliced at floor level through RC jacketing or CFRP wrapping, is experimentally studied. In three unretrofitted columns, deformation capacity and energy dissipation drop fast with lap length below 45-bar diameter. Three columns were cyclically tested after concrete jacketing of their full length and another nine after CFRP wrapping of the lap splice and plastic hinge region. Five CFRP layers were found to be more effective than two, but the improvement was not major. The positive effect of FRP wrapping on flexural resistance, ultimate deformation and energy dissipation declined with decreasing lap length. There is a limit to the improvement that can be obtained through FRP wrapping: if lapping is as short as 15-bar diameter, its adverse effects on force capacity and energy dissipation cannot be sufficiently removed by FRP wrapping. Overall, RC jacketing is more effective than FRP wrapping for the improvement of the deformation capacity of columns with 15-bar diameter lapping.

Compressive behavior of concrete externally confined by composite jackets: Part B: modeling

This paper presents an analytical model adapted to all FRP-confined concretes, including confined HPC. The development of this model is based on an experimental compressive behavior study using various concrete cylinders externally confined by carbon and glass FRP jackets described in part A of this paper. Different parameters such as the confinement level and the jackets mechanical properties are taken in to account. Proposed relationships allow to assess the ultimate capacity and the stress–strain mechanical relation of the confined concrete as a function of each materials parameters. Experimental results presented in the part A of this paper and those obtained by other authors are compared with the analytical results predicted with the described model. The stress–strain responses of all FRP-confined concrete obtained by the proposed model are in accordance with experimental data. The ultimate behavior is well predicted, for all confinement levels and all concrete types.

Recovery of seismic resistance in corrosion-damaged reinforced concrete through FRP jacketing

Corrosion of steel reinforcement is a critical issue in assessment of exposed reinforced concrete (r.c.) structures because its harmful influence on stiffness, strength and ductility may drastically limit their service life. In assessing residual strength several parameters need be considered: loss of flexural strength due to bar section loss, bursting pressure due to rust build-up, loss of bond due to reduction of rib height and the coefficient of friction. This paper explores the behaviour of corroded r.c. members under flexure-shear (seismic effects). An analytical model was developed to describe the mechanical implications of corrosion. The efficacy of FRP jackets in slowing down corrosion and for strength recovery is considered. Experiments from international literature were used to correlate the proposed methods and to derive practical expressions for assessment and redesign.

Compressive behavior of concrete externally confined by composite jackets. Part A: experimental study

This paper presents the results of an experimental investigation concerning the compressive behavior of concrete short columns externally confined by carbon and E-glass FRP jackets. The influences of different parameters such as the confinement level, the mechanical properties of the jackets and the compressive strength of the concrete core were studied. The results show that external confinement can significantly improve the ultimate strength and ductility of the specimens. The stress–strain response of confined concrete is bilinear: in the first linear zone, the behaviors of plain and confined concrete specimens are similar, whereas in the second pseudo-plastic branch, the behavior of confined concrete depends mainly on the jacket mechanicals properties. Test results demonstrate that the second part of (plastic) the stress–strain response depends on the jacket stiffness and that the ultimate strength of the jacket and the concrete strength are the most influential factors affecting the ultimate strength and strain of the confined concrete. Also, the study indicates that the confinement efficiency falls when the compressive strength of the concrete core increases.

Experimental evaluation of FRP jackets in upgrading RC corroded columns with substandard detailing

Reinforced concrete columns designed and built prior to the introduction of modern detailing practices are often susceptible to the compounded influences of reinforcement corrosion owing to carbonation or exposure to the marine environment, a circumstance that decimates even further the limited safety reserves of old-type construction. This problem was witnessed repeatedly during the recent earthquakes in Southern Europe. In this paper, the structural behavior of corrosion-damaged columns with reinforcement details representative of pre-1980s Codes of practice was studied through an experimental program comprising 21 prismatic specimens. Columns were upgraded by means of FRP jacketing after being conditioned to accelerated electrochemical corrosion, and were subsequently tested to failure under concentric compression; some specimens were subjected to a repeated cycle of corrosion conditioning after jacketing and prior to mechanical testing. Primary variables of the investigation were the jacket characteristics and the repair method applied. Parameters used to gauge the performance of the upgrade were, the efficacy of the jacket in recovering the strength and in enhancing the deformation capacity of the columns, as well as in slowing down the depletion of reinforcement during the post-repair corrosion cycle.

Assessment of Steel and Fiber Reinforced Plastic Jackets for Seismic Retrofit of Reinforced Concrete Columns with Structural Flares

An analytical and experimental study of reinforced concrete flared bridge columns was conducted to determine an appropriate retrofit method for columns with inadequate shear capacity under earthquake loads. Four 0.3-scale models were tested on a shake table to failure, one representing as-built columns and the other three retrofitted with jackets, one with steel, one with a glass fiber reinforced plastic ~FRP!, and the third with a carbon FRP jacket. The jackets were designed to keep the plastic hinge away from the column end. A new lay up method was developed for FRP fabrics used on nonprismatic members. The results showed that all jackets improved the seismic performance. The effectiveness of the jackets was nearly the same for all materials. The method to estimate shear capacity based on the guidelines of the California Department of Transportation and a modified version of that method were found to closely estimate the shear capacity. However, the method in the Federal Highway Administration seismic retrofit manual overestimated the shear capacity and was unconservative.

Design-oriented stress–strain model for FRP-confined concrete

External confinement by the wrapping of FRP sheets (or FRP jacketing) provides a very effective method for the retrofit of reinforced concrete (RC) columns subject to either static or seismic loads. For the reliable and cost-effective design of FRP jackets, an accurate stress–strain model is required for FRP-confined concrete. In this paper, a new design-oriented stress–strain model is proposed for concrete confined by FRP wraps with fibres only or predominantly in the hoop direction based on a careful interpretation of existing test data and observations. This model is simple, so it is suitable for direct use in design, but in the meantime, it captures all the main characteristics of the stress–strain behavior of concrete confined by different types of FRP. In addition, for unconfined concrete, this model reduces directly to idealized stress–strain curves in existing design codes. In the development of this model, a number of important issues including the actual hoop strains in FRP jackets at rupture, the sufficiency of FRP confinement for a significant strength enhancement, and the effect of jacket stiffness on the ultimate axial strain, were all carefully examined and appropriately resolved. The predictions of the model are shown to agree well with test data.