Fiber-reinforced Polymer Confinement Research Articles

FRP-confined concrete members: Axial compression experiments and plasticity modelling

The behaviour and modelling of circular sections confined by Fibre Reinforced Polymer (FRP) reinforcement have been examined extensively recently. This study concentrates on FRP confined specimens with square sections under axial loading. An experimental programme was designed in which 101 prismatic concrete specimens of different strengths were tested. The specimens had dimensions of 200×200×320 mm. They were externally confined by carbon or glass FRP sheets in different confinement volumetric ratios. Monotonic as well as cyclic axial compressive loads were applied. The testing results indicate that square concrete sections, properly confined by FRP reinforcement, can achieve high levels of strength and ductility. It was found that glass FRP is less effective in terms of strength and ductility enhancement when compared with carbon FRP confinement of same axial rigidity for low volumetric ratios. A plasticity model capable of reproducing hardening–softening behaviour is also presented here to predict the response of the FRP confined specimens. The model generates the stress–strain curves for axially loaded square columns confined by FRP sheets or tubes. The predicted curves are compared successfully to the present as well as other published experimental data.

FRP-confined concrete under axial cyclic compression

Abstract One important application of fiber reinforced polymer (FRP) composites in construction is as FRP jackets to confine concrete in the seismic retrofit of reinforced concrete (RC) structures, as FRP confinement can enhance both the compressive strength and ultimate strain of concrete. For the safe and economic design of FRP jackets, the stress–strain behavior of FRP-confined concrete under cyclic compression needs to be properly understood and modeled. This paper presents the results of an experimental study on the behavior of FRP-confined concrete under cyclic compression. Test results obtained from CFRP-wrapped concrete cylinders are presented and examined, which allows a number of significant conclusions to be drawn, including the existence of an envelope curve and the cumulative effect of loading cycles. The results are also compared with two existing stress–strain models for FRP-confined concrete, one for monotonic loading and another one for cyclic loading. The monotonic stress–strain model of Lam and Teng is shown to be able to provide accurate predictions of the envelope curve, but the only existing cyclic stress–strain model is shown to require improvement.

Evaluation of ductility requirements in current design guidelines for FRP strengthening

Advanced composites are widely used for the strengthening of existing concrete structures. Current design guidelines give basic requirements on how to model the enhancement of structural performance of concrete members using surface bonded FRP (fibre reinforced polymer) reinforcement. With respect to this, it is of interest to evaluate the ductility requirements which are explicitly or implicitly imposed by design guides. Based on an evaluation of four major design guidelines in Europe, Japan and North-America, and a small parametric study, the ductility aspect of the design of FRP strengthened concrete members is verified. It appears that the ductility of flexural members strengthened with FRP should be considered with care, as reduced deformability is obtained at ultimate, though generally a minimum deformability is implicitly obtained in a proper design. At the other hand, ductility enhancement by means of FRP confinement is explicitly considered in the design guidelines.

Behaviour of FRP-jacketed circular steel tubes and cylindrical shells under axial compression

Fibre-reinforced polymer (FRP) jackets have been widely used to confine reinforced concrete (RC) columns for enhancement in both strength and ductility. This paper presents the results of a recent study in which the benefit of FRP confinement of hollow steel tubes was explored. Axial compression tests on FRP-confined steel tubes are first described. Finite element modelling of these tests is next discussed. Both the test and the numerical results show that FRP jacketing is a very promising technique for the retrofit and strengthening of circular hollow steel tubes. In addition, finite element results for FRP-jacketed thin cylindrical shells under combined axial compression and internal pressure are presented to show that FRP jacketing is also an effective strengthening method for such shells failing by elephant’s foot collapse near the base.

Comparison of Roles of FRP Sheets, Stirrups, and Steel Fibers in Confining Bond Critical Regions

Based on experimental data of tension lap splices confined with fiber reinforced polymer (FRP) sheets in normal and high strength concrete (HSC) specimens, a new FRP confinement parameter, Ktr,f , was recommended. It accounts for the increase in bond strength due to the presence of FRP sheets. In this paper, a correlation is presented between the confining effects of FRP flexible sheets, transverse reinforcement, and steel fibers to improve the bond capacity and ductility of the mode of failure of tension lap splices. The correlation is based on research programs conducted at the American University of Beirut in recent years using identical specimens except for the confinement method used: FRP sheets, transverse steel stirrups, or steel fibers. Other variables included the amount of confinement provided and concrete strength. Analysis of test results indicated that an equivalent improvement in bond strength of tension lap splices in normal and high strength concrete specimens is provided by an amount of F...

Ultimate behavior of axially loaded RC wall-like columns confined with GFRP

The assessment of the effectiveness of the fiber reinforced polymer (FRP) confinement on rectangular reinforced concrete (RC) columns with high aspect ratio (wall-like) still represents an unresolved issue. The present paper aims at providing more experimental evidence about the behavior of such members confined with both uni-directional and quadri-directional glass FRP laminates. Particular attention is devoted to issues related to the premature failure of confining fibers experimentally observed in wall-like columns. Test results on nine axially loaded columns are herein presented; emphasis is also given to the analysis of FRP strain profiles along the sides of the cross-section. The analysis of test results highlights that glass FRP (GFRP) confinement could determine significant strength and ductility increases; the discussion of failure modes points out that the failure of GFRP confined wall-like columns is controlled by the shape of the cross-section and occurs at transverse strains in the jacket much lower than those ultimate of the fibers. Theoretical–experimental comparisons are performed using some available models for strength prediction of such members.

Circular Columns Confined with FRP: Experimental versus Predictions of Models and Guidelines

This paper presents results of an experimental investigation on half-scale concrete columns retrofitted with externally bonded fiber reinforced polymers (FRP). It then reviews six selected confinement models and three North American design guidelines for circular columns retrofitted with FRP and provides an in-depth comparison between experimental test results, models predictions, and values obtained using the guidelines. The comparison, which is carried out in terms of the confined to unconfined strengths, but also in terms of the gain in the load carrying capacity, considers the following parameters: The FRP confinement ratio, the unconfined concrete strength, and the elastic and mechanical properties of FRP. The results of this investigation indicated that for columns made of lower strength concrete, existing models and guidelines overestimate the confined concrete strength, and hence, the load carrying capacity of the columns.

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.

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

This paper reports on the fourth phase of a multiphase study undertaken at the American University of Beirut (AUB) to examine the effect of fiber-reinforced polymer (FRP) sheets in confining bond-critical regions in reinforced concrete beams. Results of the first three phases showed that glass- and carbon-fiber-reinforced polymer (GFRP and CFRP) sheets were effective in increasing the bond strength and improving the ductility of the mode of failure of tension lap splices in high-strength concrete (HSC) and normal-strength concrete (NSC) beams. The main objective of the fourth phase of the AUB study was to assess the effect of CFRP sheets in improving the serviceability and ultimate response of beam anchorage specimens. The added experimental data and the improved knowledge of the bond behavior of FRP confined concrete members will encourage the use of FRP technology to strengthen and retrofit bond anchorage zones. Ten beam anchorage specimens were tested in positive bending in two series. The variables we...

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

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

Effect of Confinement on Bond Strength between Steel Bars and Concrete

This work experimentally examines the local bond aspect between steel bars and concrete confined with ordinary transverse steel. The test parameters included diameter of reinforcing bar, ratio of concrete cover to bar diameter, and area of transverse reinforcement. Results were compared with those of similar specimens for concrete confined either internally using steel fiber reinforcement or externally using fiber-reinforced polymer (FRP) sheets. Based on the comparisons, a unified expression for the local bond strength of confined concrete is derived, and a general model for the local bond stress-slip response is proposed and used to conduct an analytical evaluation of the effect of confinement on development/splice strength. Results predicted by the analysis were in good agreement with experimental results. For small development/splice lengths corresponding to local bond conditions, confining the concrete only slightly increases the local bond resistance but leads to considerable improvement in the ductility of bond failure. The corresponding ductility at the local level allows, for the practical range of development/splice lengths, more bar lugs to participate in resisting the applied bar force resulting in a more uniform bond stress distribution along the development/splice length and, consequently, a sizable increase in the average bond strength at bond failure as compared with plain unconfined concrete. The bond strength due to FRP confinement increases in proportion to the modulus of elasticity of the FRP material. For the same area of transverse reinforcement per unit length along the splice, taking into account the relative modulus of elasticity of the confining materials, external confinement of concrete using FRP sheets is more effective in increasing the development/splice strength than internal confinement with ordinary transverse steel. A general design expression is proposed to estimate the development/splice length of steel bars embedded in concrete confined with ordinary transverse steel, FRP, or steel-fiber reinforcement.

Concrete confined by FRP material: a plasticity approach

Carbon fiber-reinforced polymer (FRP) material has proved to be more efficient than other composites when applied to concrete columns as an external reinforcement. Because of its enhanced durability characteristics compared to glass or aramid, and its relatively high E-modulus, carbon FRP shows a higher confining performance. The behavior of 22 cylindrical 200×320 mm specimens (height to diameter ratio 1.6) that are externally wrapped by carbon FRP sheets in low volumetric ratios (0.23–0.7%) is presented. The specimens are subjected to axial monotonic load until failure occurs. Carbon FRP confinement, even in low volumetric ratios, seems to considerably increase the strength and especially the ductility of concrete. A constitutive model based on plasticity theory is applied. The model proposed and applied to steel-confined concrete in this paper is modified and calibrated so as to incorporate the dilation characteristics of the FRP-confined concrete. The model provides the stress–strain curves of axially loaded circular columns that are confined by FRP reinforcement. It can be used in FRP tube-encased concrete as well as in FRP sheet-wrapped concrete. Satisfactory correlation of experimental and analytical results is observed.

Static-pressure dependence of the cavitation erosion intensity in liquid oxygen

This study aims to investigate the structural behavior and failure modes of fiber-reinforced-polymer (FRP) confined concrete wrapped with different FRP arrangements. A total of twenty four specimens were cast and tested, with three of these specimens acting as reference specimens and the remaining specimens wrapped with different types of FRP (CFRP and GFRP) by different wrapping arrangements. They include fully wrapped, partially wrapped and non-uniformly wrapped concrete cylinders. The non-uniformly wrapped concrete cylinders provided higher compressive strengths and strain for FRP-confined concrete, in comparison with conventional fully wrapping arrangements. The effect of confinement level on the effectiveness of FRP confinement is also investigated. In addition, the partially wrapping arrangements changes the failure modes of the specimens and the angle of the failure surface.