FRP Jackets Research Articles

Experimental investigation of the stress-strain behavior of FRP confined concrete prisms

One of the main applications of FRP composites is confining concrete columns. Hence identifying the cyclic and monotonic stress-strain behavior of confined concrete columns and the parameters influencing this behavior is inevitable. Two significant parameters affecting the stress-strain behavior are aspect ratio and corner radius. The present study aims to scrutinize the effects of corner radius and aspect ratio on different aspects of stress-strain behavior of FRP confined concrete specimens (rectangular, square and circular). Hence 44 FRP confined concrete specimens were tested and the results of the tests were investigated. The findings indicated that for specimens with different aspect ratios, the relationship between the ultimate stress and the corner radius is linear and the variations of the ultimate stress versus the corner radius decreases as a result of an increase in aspect ratio. It was also observed that increase of the corner radius results in increase of the compressive strength and ultimate axial strain and increase of the aspect ratio causes an increase of the ultimate axial strain but a decrease of the compressive strength. Investigation of the ultimate condition showed that the FRP hoop rupture strain is smaller in comparison with the one obtained from the tensile coupon test and also the ultimate axial strain and confined concrete strength are smaller when a prism is under monotonic loading. Other important results of this study were, an increase in the axial strain during the early stage of unloading paths and increase of the confining effect of FRP jacket with the increase and decrease of the corner radius and aspect ratio respectively, a decrease in the slope of reloading branches with cycle repetitions and the independence of this trend from the variations of the aspect ratio and corner radius and also quadric relationship between the number of each cycle and the plastic strain of the same cycle as well as the independence of this relationship from the aspect ratio and corner radius.

Shear strengthening of reinforced concrete T-beams under cyclic loading with TRM or FRP jackets

The paper presents a systematic investigation on the effectiveness of U-shaped textile-reinforced mortar (TRM) jackets as shear strengthening materials of reinforced concrete T-beams, by examining a number of parameters not studied before: cyclic loading; fixed support conditions; different types of textiles; different numbers of layers; anchors; the relative performance of TRM versus equivalent FRP systems; and different displacement amplitudes of the loading cycles. For beams without anchors it is concluded that the effectiveness of TRM increases non-proportionally with the number of layers and that, for the same total volume fraction of fibers in the jacket, one layer of textile is more effective than two. For beams with anchored TRM jackets it is concluded that the effectiveness of the strengthening system is quite high, as is the effectiveness of an anchored FRP system. The results are used to derive simple analytical models for the effective strain in anchored or non-anchored TRM jackets.

THE CONFINEMENT OF CONCRETE IN COMPRESSION USING CFRP COMPOSITES – EFFECTIVE DESIGN EQUATIONS

This paper presents the results of an experimental study on the behaviour of axially loaded short concrete columns, with different cross sections that have been externally strengthened with carbon fibre-reinforced polymer (CFRP) sheets. Six series, forming the total of 60 specimens, were subjected to axial compression. All the test specimens were loaded to failure in axial compression and investigated in both axial and transverse directions. According to the obtained test results, FRP-confined specimen failure occurs before the FRP reached the ultimate strain capacities. Thus, the failure occurs prematurely and the circumferential failure strain is lower than the ultimate strain obtained from the standard tensile testing of the FRP composite. In existing models for FRP-confined concrete, it is commonly assumed that the FRP ruptures when the hoop stress in the FRP jacket reaches its tensile strength from either flat coupon tests, which is herein referred to as the FRP material tensile strength. This phenomenon considerably affects the accuracy of the existing models for FRP-confined concrete. On the basis of the effective lateral confining pressure of the composite jacket and the effective circumferential FRP failure strain, new equations were proposed to predict the strength of FRP-confined concrete and corresponding strain for each of the cross section geometry used, circular and square. The estimations given by these equations were compared with the experimental ones and general conclusions were drawn.

Behavior of partially defected R.C columns strengthened using steel jackets

The aim of this work is to study the effect of partial strengthening of partially defected columns on its capacity, and global behavior. There are two types of defects on R.C columns, the first is decreasing the characteristic compressive strength of a part of R.C column, and the second is bad stirrup arrangement.External FRP jacketing and steel jacketing have been widely used for strengthening R.C columns. The cost effectiveness and convince of applying the steel jacketing strengthening technique have been demonstrated through various strengthening applications.Few studies have been carried out on partial strengthening of defected columns. In this research, the efficiency of short steel jackets for strengthening of R.C defected columns was studied. The experimental program consists of testing of seven R.C columns with dimensions 200*200*1500mm and having stirrups in top and bottom thirds only, while the middle third was without stirrups.The main parameters studied were the type of steel jacket used and height of partial strengthened part of column. One of the tested specimens was a control specimen and the other six were partially strengthened with different types of steel jackets such as using 4 steel angles at corners connected with straps, using external ties with different spacings, and using 4 steel plates with different thicknesses welded together and connected to column by anchor bolts. Finally, the experimental results were analyzed and compared with results obtained from finite element analysis using ANSYS program.

Effect of FRP wrapping on axial behavior of concrete and cyclic behavior of external RC beam column joints

The study focuses on the effect of confinement due to transverse reinforcement and FRP jacketing on the axial behavior of concrete and external beam-column joints (i.e., T-joints) before and after retrofitting. The effectiveness of confinement is firstly evaluated by stress — strain behavior of square prism specimen under pure axial compression. These specimens are prepared with different amount of transverse reinforcement and FRP wraps (number of layers). Secondly, the cyclic tests are also carried out on beam-column joint specimens, with two different configurations of reinforcement detailing and retrofitted by externally bonded FRP wrapping. One specimen is designed to comply with gravity load design codes and the other with detailing according to Indian Standards. GFRP wrapping is used for retrofitting of damaged joints with epoxy resin. GFRP jacketing comes out to be an effective technique to regain the strength and stiffness of damaged joints. Also significant gain in strength and strain is observed under pure axial compression.

Investigation of CFRP- and GFRP-confined concrete cylinders under monotonic and cyclic loading

Abstract Fiber reinforced polymer (FRP) composites have found increasingly wide applications in 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, which can enhance both the compressive strength and the ultimate axial strain of concrete. With this respect, the stress-strain behavior of FRP-confined concrete, under both monotonic and cyclic compression, needs to be properly understood and modeled. This paper presents details of an experimental work carried out on concrete cylinders wrapped with FRP materials and subjected to both monotonic and cyclic loading. A total number of 12 FRP confined concrete specimens and 10 control specimens with a diameter of 100 mm and a height of 200 mm were cast and cured under the same conditions, and two FRP materials (carbon fibers (CFRP) and glass fibers (GFRP)) were used for the construction of the FRP jackets. The effect of the type of confinement material, reinforcement ratio based on the jacket stiffness, and type of loading is examined. A model that predicts the behavior of confined concrete, which takes into account the stiffness and effectiveness of different confinement materials is also briefly introduced.

Experimental investigation and strength modeling of CFRP-confined concrete rectangular prisms under axial monotonic compression

The effectiveness of an FRP jacket for a rectangular concrete column depends mainly on the edge sharpness and aspect ratio of its cross-section. This is because the effective confined area of cross-section will change with the variation of these parameters. In this paper results of an experimental study on the monotonic behavior of CFRP-confined concrete rectangular prisms are presented and the effects of simultaneous changes of aspect ratio and edge sharpness on this behavior are investigated. Other variable which is considered in this study is the unconfined concrete strength. Also the experimental results are compared with the existing models for this type of specimens. The findings of the present study indicate that for both the square and rectangular specimens the strength of confined concrete increases almost linearly with the decrease of edge sharpness. Results also demonstrated that the strength gain of specimens with higher aspect ratios, are much smaller than those of square specimens. The comparison between the experimental results and existing stress–strain models shows that, still there are some deficiencies in the models and further research is required to develop a more accurate stress–strain model for rectangular columns confined by FRP composites.

Experimental Studies on the Mechanical Performances of Corroded Reinforced Concrete Columns Retrofitted with FRP

Due to the superiority of high corrosion resistance of FRP materials, more and more attentions have been attracted to the retrofitting of corroded reinforced concrete (RC) columns with FRP. This paper thus presents an experimental study on mechanical performances of corroded RC columns strengthened with FRP, focusing on the effects of different corrosion rates of the reinforcements and the retrofitting scheme. The effectiveness of externally bonded FRP to the corroded RC column to increase its load capacity and ductility is tested; the mechanical performances of the strengthened columns are theoretically investigated. The results indicate that the effectiveness of retrofitting the existing corroded RC columns with FRP jackets is much more significant than that of retrofitting the newly built columns with FRP jackets and externally wrapped with FRP jackets is much more effective to improving the structural performances of heavily corroded columns.

Effect of core compaction on deformation capacity of FRP-jacketed concrete columns

Well-established confinement models can adequately estimate the axial strain capacity of FRP-strengthened columns with low-to-moderate confinement, but there is a tendency for overestimation of the effect of FRP-induced confinement on deformation capacity in the range of high confining pressures. Results are particularly evident in the case of existing columns comprising low-strength concrete. For high ratios of lateral pressure over concrete uniaxial strength, concrete compaction due to collapse of the porous structure of the matrix leads to pulverization and volume reduction of the encased core. This, in turn, reduces the Poisson-like lateral dilation phenomena of concrete which engage the FRP jackets thereby reducing their effectiveness as confining devices. This phenomenon is modeled using principles of concrete plasticity and is integrated in established confinement models. Calculated values of deformation capacity using this approach are compared with reported experimental results from 496 specimens tested under compression, of which 169 were reinforced concrete columns, demonstrating significantly improved correlation and dependability of the analytical estimates.

Confining concrete members with FRP systems: Predictive vs design strain models

Several analytical models are available in the literature to predict the compressive strength and the ultimate strain of the FRP confined concrete. The reliability of such models is often affected by an inaccurate definition of the “effective” confining pressure exerted by the FRP jacket and/or by an incorrect evaluation of the reduced confinement effectiveness in the case of square and rectangular cross sections. In this paper, following a previous study on the compression strength, an investigation on the ultimate strain of the FRP confined concrete is presented. To this aim, a large database including results from compression tests performed on over 450 FRP confined concrete cylinders was considered, and some of the most accredited predictive strain models were recalibrated through best-fit analyses of the experimental data. Finally, a probabilistic procedure was applied in order to define design models; for this purpose only data sets for which the unconfined concrete strength was below 40MPa were considered.

Electrochemical measurements of cathodic protection for reinforced concrete piles in a marine environment using embedded corrosion monitoring sensors

This study developed a sensor to monitor the corrosion of reinforced concrete structures. Concrete pile specimens with embedded sensors were used to obtain data on corrosion and cathodic protection for bridge columns in a real marine environment. Corrosion potential, cathodic protection current density, concrete resistivity, and the degree of depolarization potential were measured with the embedded sensors in concrete pile specimens. The cathodic protection (CP) state was accurately monitored by sensors installed in underwater, tidal, splash, and atmospheric zones. The protection potential measurements confirmed that the CP by Zn-mesh sacrificial anode was fairly effective in the marine pile environment. The protection current densities in the tidal, splash zones were 2–3 times higher than those in underwater and atmospheric zones. The concrete resistivity in the tidal and splash zones was decreased through the installation of both mortar-embedded Zn-mesh (sacrificial anode) and outside an FRP jacket (cover). Considering the CP, the cathodic prevention was more effective than cathodic protection.

Experimental Investigation of Concrete Frames Infilled with rc for Seismic Rehabilitation

AbstractNew reinforced concrete (RC) walls are popular in seismic rehabilitation of RC buildings as they reduce seismic-deformation demands. Their effectiveness was experimentally investigated by pseudodynamically testing 3∶4 scaled models of three four-story frames infilled with RC. The specimens were considered part of a frame building, the rest of which was substructured in the test as elastic. Unlike in past tests, the wall web was not thinner than the frame members and the specimens were tall enough to be governed by flexure. Two code-conforming designs were tried for the connection of the web to the surrounding frame members, one of them being less labor-intensive. Behavior and failure were dominated by flexure, but open, u-shaped FRP jackets at the two edges of the composite wall were essential to prevent premature failure near the base because of poor detailing of the columns. Slippage/separation at interfaces between the web and surrounding frame members was minor for both connection details. In ...

Stress–strain model for concrete in FRP-confined steel tubular columns

Concrete-filled steel tubes (CFTs) are widely used as columns in many structural systems. In CFTs, degradation in steel confinement, strength and ductility can result from inelastic outward local buckling. To overcome this deficiency of CFTs, external confinement of CFTs with an FRP jacket has been explored in recent studies. This paper presents a theoretical model in an incremental–iterative form for circular FRP-confined CFTs (CCFTs) under monotonic axial compression, with the focus being on the stress–strain behavior of the confined concrete. The proposed stress–strain model for concrete in CCFTs is based on the same approach as that commonly adopted by existing models for FRP-confined concrete and includes three components: (a) an active-confinement model; (b) a lateral strain equation; and (c) equations for determining the total confining pressure from the steel tube and the FRP jacket. It is shown that the lateral dilation behavior of concrete in CCFTs differs significantly from that of FRP-confined concrete in the initial stage because the former experiences more severe micro-cracking than the latter in the initial stage of loading; this difference is reflected in the proposed model. In general, the predictions of the proposed model are in close agreement with existing test results. The proposed model provides a useful tool for a parametric study on the stress–strain behavior of confined concrete in CCFTs to produce results for the formulation of simple stress–strain model in closed-form expressions for design use.

Factors Affecting the Ultimate Condition of FRP-Wrapped Concrete Columns

AbstractFiber reinforced polymer (FRP)-wrapped circular concrete columns under axial compression usually fail as a result of FRP rupture in the hoop direction. The average measured hoop strain in the FRP jacket at failure of such columns, i.e., the apparent FRP rupture strain, is typically considerably lower than the ultimate tensile strain obtained from tensile tests of flat coupons. A large scatter also exists in the measured rupture strains over the surface of an FRP-wrapped column at ultimate failure. No rational model based on sound theories is currently available to explicitly consider the reduction of the apparent FRP rupture strain in design. This paper discusses the possible failure modes of FRP-wrapped concrete columns and the factors that may play a role in the initiation of failure. Further research needs and the possible consequences for the optimized design of FRP-wrapped concrete columns are discussed.

Analytical model for the effective strain in FRP-wrapped circular RC columns

The available models for predicting the ultimate capacity and stress–strain relationships of confined concrete rely on an assumed FRP ultimate strain value. It is commonly assumed that FRP fails when hoop strain in the jacket reaches its ultimate tensile strain determined according to flat coupon tests. However, experimental results of FRP confined concrete showed that in most cases, FRP experimental ultimate tensile strain is clearly not reached at the rupture of the FRP jacket. The discrepancies may include the shape and conditions of concrete substrate as well as defects or stress concentrations in FRP jackets (in particular, the multiaxial stress state in FRP wrapping due to the transfer of loads through the bond with concrete). The present paper covers only issues related to this latter aspect. Since the average absolute error of all confinement models showed a remarkable decrease when the effective FRP hoop strain (smaller than ultimate flat coupon strain) was inserted in the equations, an analytical model to directly evaluate the FRP strain efficiency factor as the strain ratio between the effective FRP hoop strain at failure and the flat coupon test outcomes has been formulated. Multiaxial failure criteria have been adopted (i.e. 3D Tsai–Hill criterion for FRP) in the model considering axial, circumferential and radial stresses. Results of theoretical analyses and experimental tests (experimental data available in literature) have been compared showing a good agreement.

Nonlinear failure prediction of concrete composite columns by a mixed finite element formulation

This study focuses on developing a mixed frame finite element formulation of reinforced concrete and FRP composite columns in order to give more accuracy not only to predict the global behavior of the structural system but also to predict the local damage in the cross-section. A hypo-elastic constitutive law of concrete is presented under the basis of a three-dimensional stress state in order to model the compressive behavior of confined concrete wrapped with FRP jackets. To predict the nonlinear load path-dependent confinement model of FRP-confined concrete, the strength enhancement of concrete was determined by the failure surface of concrete in a tri-axial stress state, and its corresponding peak strain was computed by the strain-enhancement factor proposed in this study. The behavior of FRP jacket was modeled using the two-dimensional classical lamination theory. The flexural behavior of concrete and composite members was defined using a nonlinear fiber cross-sectional approach. The results obtained by developed mixed finite element formulation were verified with the experiments of concrete composite columns and also were compared with a displacement-based finite element formulation. It is shown that the proposed formulation gives e more accurate results in the global behavior of the column system as well as in the local damage in the column sections.

Fiber-Reinforced Polymer Retrofitting of Predamaged Substandard RC Prismatic Members

An experimental study was conducted to investigate the efficiency of FRP jackets in upgrading the seismic behavior of lightly reinforced concrete prismatic members previously damaged under a combination of axial compression and a reversed cyclic lateral displacement history simulating earthquake effects. The test program comprises 13 cantilever prismatic specimens, which, owing to substandard reinforcing details representative of older construction practices in southern Europe, were susceptible to various undesirable modes of damage such as web-shear cracking, longitudinal bar buckling, or lap-splice failure. After repair, the specimens were retested using the same load combination. The efficiency of the repair options considered in the study, which refer to alternative strengthening systems (with glass or carbon wraps), was investigated with reference to the design parameters of the intervention, the type of the applied lateral displacement history, and the mode of failure that had occurred previously in the initial phase of the tests. The results provided valuable insight regarding participation of the FRP jackets in the various mechanisms of resistance, their ability to reverse the effect of initial damage, and to impart deformation capacity to the structural member.

Bond Stress–Slip Model for Steel Bars in Unconfined or Steel, FRC, or FRP Confined Concrete under Cyclic Loading

An experimentally based relationship of the cyclic local bond stress-slip response of steel bars corresponding to splitting mode of bond failure is developed. The relationship is composed of a monotonic envelope response and cyclic bond degrading response. The model accounts for both unconfined and moderately confined concrete and incorporates the effect of several critical bond parameters such as the diameter of steel bars, the ratio of concrete cover to bar diameter, the concrete compressive strength, the type of confinement such as steel ties, fiber-reinforced concrete (FRC), and fiber-reinforced polymer jackets, and the area or content of confining reinforcement. Results predicted by the model showed very good agreement with the test data. The bond-slip relationship can be utilized for evaluating the seismic behavior and bond degradation response of bond-critical regions in reinforced concrete members when confined internally with steel ties or fiber reinforced concrete or retrofitted using external FRP jackets.

Displacement-based seismic design of reinforced concrete columns strengthened by FRP jackets using a nonlinear flexural model

In the current research, a displacement-based seismic design scheme to retrofit reinforced concrete columns using FRP composite materials has been proposed. An accurate prediction for the nonlinear flexural analysis of FRP jacketed concrete members has been presented under multiaxial constitutive laws of concrete and composite materials. Through modification of the displacement coefficient method (DCM) and the direct displacement-based design method (DDM) of reinforced concrete structures, two algorithms for a performance-based seismic retrofit design of reinforced concrete columns with a FRP jacket have been newly introduced. From applications to retrofit design it is known that two methods are easy to apply in retrofit design and the DCM procedure underestimates the target displacement to compare with the DDM procedure.

Stress–strain model for concrete confined with CFRP jackets

Enhancement of strength and ductility is the main reason for the extensive use of FRP jackets to provide external confinement to reinforced concrete columns especially in seismic areas. Therefore, numerous researches have been carried out in order to provide a better description of the behavior of FRP-confined concrete for practical design purposes. Most of the existing models are based on the improved or modified forms of the well known empirical formula of Richart et al. derived from the tests on concrete cylinders. Compressive strength of concrete is predicted without a failure criterion for this modeling approach. There exist only a few models that employ the simplified or modified forms of five-parameter failure criterion proposed by William–Warnke. This paper primarily concentrates on the modeling of FRP-confined concrete using a practical failure surface based on only unconfined compressive strength of concrete. A large comparative analysis is accomplished for the existing test data of 127 cylindrical concrete specimens confined with CFRP jackets. The performance of five existing analytical models for the prediction of the compressive strength of FRP-confined concrete is evaluated leading to the detection of the proposed approach as the most accurate one through this comparative study. Moreover, the complete stress–strain curves of eight concrete cylinders obtained from four different experimental studies are plotted adopting both the stress–strain relation of Saenz and the predictions for compressive strength of the approach. Comparisons between the experimental and analytical results point out that the proposed approach provides satisfactory predictions for both the compressive strength and the stress–strain plots of CFRP-confined concrete.