Fiber-reinforced Polymer Jackets Research Articles

Static and cyclic response of low-strength recycled aggregate concrete strengthened using fiber-reinforced polymer

Concrete production using recycled aggregates (RAs) from construction and demolition waste is known to have a lower strength performance than natural aggregate concrete. Many studies have been carried out to improve the strength properties of recycled aggregate concrete (RAC). One widely accepted strategy is to use fiber-reinforced polymer (FRP) jackets as external confinement material to enhance the compressive strength and ductility performance of RAC, especially in the seismic retrofit of RAC members. Most existing research has focused on normal strength concrete subjected to monotonic loading. However, retrofitting or repair work is more common for concrete structures with low concrete strength. In addition, cyclic behavior research focused on FRP-confined low-strength RAC is also rare due to the lack of sufficient experimental data. In this study, 29 concrete specimens were cast using varying proportions of RAs mixed with natural coarse aggregates. In this case, poor quality natural aggregates were selected to achieve a low concrete strength. Thickness of the carbon FRP (CFRP) jacket, loading types (monotonic or cyclic), and the RAs replacement ratio were the main test parameters of this experimental work. The mechanical properties of CFRP-confined RAC were analyzed and discussed from the failure mode perspective; these included peak strength, ultimate strain, and cyclic stress-strain relationships. Test results show that strength enhancement by FRP confinement was much more pronounced in lower strength RAC than in normal strength concrete, whereas the ductility improvement was almost consistent. Confinement rigidity and recycled aggregate replacement ratio had little effect on cyclic stress-strain behavior, in terms of unloading and reloading paths, as well as in plastic strain. The adhered cement mortar on RAs weakened the impact brought by confinement when repeated unloading and reloading were applied. This was especially true during cyclic loading, which generated a large deformation.

Simplified HYMOD non-linear simulations of a full-scale multistory retrofitted RC structure that undergoes multiple cyclic excitations – An infill RC wall retrofitting study

Having the ability to assess the earthquake resistance of retrofitted reinforced concrete (RC) structures through accurate and objective nonlinear cyclic analysis is of great importance for both scientists and professional Civil Engineers. Full-scale RC structure simulations under ultimate limit state cyclic loading conditions through the use of 3D detail modeling techniques, is currently one of the most challenging modeling tasks that any research or commercial software can undertake. The excessive computational demand and the numerical instabilities that occur when dealing with this type of cyclic nonlinear numerical analysis, make this modeling approach impractical. The simplified hybrid modeling (HYMOD) approach is adopted in this work, which overcomes the above numerical limitations and it is used herein to illustrate the capabilities of the method in capturing the experimental results of a full-scale 4-storey RC building that was retrofitted with RC infill walls and carbon fiber reinforced polymer jacketing. This work has the aim to investigate the importance of numerically accounting for the damage that has developed at the concrete and steel domains during the analysis of problems that foresee consecutive cyclic loading tests. Based on the numerical findings, it was concluded that the proposed modeling approach was able to accurately capture the experimental data and predict the capacity degradation of the building specimen. Furthermore, the proposed method was used to numerically investigate different retrofitting configurations that foresaw the use of infill RC walls. The numerical experiments performed in this work demonstrate that the proposed modeling approach provides with the ability to study the cyclic mechanical behavior of full-scale RC structures under ultimate limit state conditions, thus paves the way in performing additional parametric investigations in determining the optimum retrofitting design of RC structures by using different types of interventions.

Behavior of large-scale FRP-confined rectangular RC columns under axial compression

Fiber-reinforced polymer (FRP) jacketing has become an attractive technique for strengthening/retrofitting reinforced concrete (RC) columns. Extensive research has been conducted on FRP-confined rectangular columns under axial compression, leading to a significant number of stress-strain models for FRP-confined concrete in these columns. However, most of these models have been developed based on test results of small-scale columns, so their applicability to large FRP-confined rectangular RC columns has yet to be properly validated. To this end, the present paper first presents the test results of an experimental study consisting of nine large-scale rectangular RC columns, including eight FRP-confined RC columns and one RC column without FRP jacketing as the control specimen, tested under axial compression. The experimental program examined the sectional corner radius and the FRP jacket thickness as the key test variables. Five representative design-oriented stress-strain models for FRP-confined concrete in rectangular columns, identified from critical reviews of the existing literature, are then assessed using the test results to examine their validity for these large-scale columns.

Experimental Study on Strengthening of RC Short Columns with BFRP Sheets

Earlier the methods adopted for the repair and rehabilitation were the external bonding of the damaged component with steel fibers and steel jackets. But the major problem that caused due to this steel components is corrosion. Fiber Reinforced Polymer (FRP) composite materials used for strengthening of components in the form of confinement is of considerable importance in civil engineering. This paper presents the results of axially loaded RC short columns which are strengthened by providing confinement with new material BFRP. The primary objective of this work is to examine the load carrying capacity of concrete columns confined with basalt fiber reinforced polymer jacket (BFRP) which is considered as an abstruse material for strengthening. BFRP materials are non-corrosive, non-magnetic, and chemical resistant and they are progressively being used for repair, rehabilitation, and strengthening of structures that are not stable enough to carry loads. The parameters considered in the study are full and partial wrapping and the number of layers of confinement. The results obtained from the experimental studies and the analytical models precisely shows that the BFRP confinements will upgrade the compressive strength of columns under the given load condition.

Stress-strain behavior of concrete in circular concrete columns partially wrapped with FRP strips

Fiber-reinforced polymer (FRP) wrapping has become an attractive strengthening technique for concrete columns. Within this strengthening technique, FRP jackets are wrapped around the concrete column with the fibers in the jacket being oriented in the hoop direction. In practice, the FRP jackets can be either continuous or discontinuous along the column height and thus the resulting column is referred to as FRP fully or partially wrapped concrete columns. Existing research has demonstrated that the FRP partial wrapping strengthening technique by discrete FRP strips (rings) is a promising and economic alternative to the FRP full wrapping strengthening technique. Although a number of experimental investigations have been conducted on FRP partially wrapped concrete columns, the stress-strain behavior of FRP-confined concrete in partially wrapped concrete columns is not yet completely understood. This paper presents an experimental program to investigate the axial compressive behavior of circular concrete columns partially wrapped with FRP strips. The test results are presented and compared with five existing representative stress-strain models to examine the reliability and accuracy of each model. It has been demonstrated that the Teng et al.’s (2007) model is superior to the other four representative models and it provides reasonably accurate predictions of the ultimate axial stress of FRP partially wrapped concrete while it usually underestimates the ultimate axial strain.

Overlap length for confinement of carbon and glass FRP-jacketed concrete columns

This paper presents the experimental results of a study on the occurrence of debonding along the overlap length of fiber reinforced polymer (FRP) confined concrete prior to rupture of the FRP jacket. Plain concrete cylinders, twenty-six with 152 mm diameter and 305 mm height, and four with 100 mm diameter and 200 mm height, with cylinder compressive concrete strength 19 MPa, were wrapped with carbon- and glass-FRP jackets and tested under monotonic axial compression. Different overlap configurations and different FRP application methods were investigated. The parameters found to most significantly affect the bond resistance along the overlap zone are the type of the FRP material and the curing age of the resin when dry lay-up application is used.

Comparative study on the behaviour of different infill materials for pre-fabricated fibre composite repair systems

Prefabricated fibre-reinforced polymer (FRP) composite jacket is now becoming an effective repair system for deteriorating piles and columns exposed to marine environment. This system works by providing grout infills between the annulus of the existing structure and the composite jacket. Few studies are however available on the optimal grouting materials that can effectively transfer the stresses between the existing structure and the FRP jacket. This study is investigating the effect of cementitious, concrete and epoxy-based grout infills on the structural behaviour of pre-fabricated glass-FRP (GFRP) tubes. The considered grouts have compressive strength and modulus of elasticity ranging from 10 MPa to 70 MPa and from 10 GPa to 35 GPa, respectively. The experimental results showed that the behaviour of the composite repair system is highly dependent on the modulus of elasticity and the compressive strength of the grout infill. The brittle failure behaviour of the cementitious and epoxy grouts led to localised failure in the FRP repair system while the progressive cracking and crushing of the concrete infill resulted in effective utilisation of the high strength properties of the composite materials. Theoretical analysis of the overall compressive behaviour has also been conducted and showed very good agreement with the experimental results.

Behavior of steel fiber-reinforced concrete-filled FRP tube columns: Experimental results and a finite element model

This paper presents the results of an experimental study together with the first finite element (FE) model for the compressive behavior of fiber-reinforced polymer (FRP)-confined steel fiber-reinforced concrete (SFRC). 73 existing experimental test results of FRP-confined and actively confined SFRC specimens tested under axial compression were initially assembled. Additional axial compression tests were conducted on 16 actively confined SFRC specimens to address the gaps in the existing test database to compile a reliable database for the FE modeling of FRP-confined SFRCs. The analysis of experimental test results revealed that the compressive behavior of FRP-confined SFRCs is influenced by the steel fiber volume fraction and aspect ratio. New expressions were developed for the hoop rupture strain of the FRP jacket, axial strain-lateral strain relationship of FRP-confined and actively confined SFRC, and relationship between the confining pressure and the compressive strength of actively confined SFRC by considering the influences of the volume fraction and aspect ratio of internal steel fibers. A recently developed concrete damage-plasticity model, which was shown to be the most accurate currently available model for confined plain concrete, was adopted for the prediction of the compressive behavior of FRP-confined SFRC. The failure surface and flow rule of the model were modified based on the results from actively and FRP-confined SFRC. The results show that model predictions of the axial stress-axial strain, lateral strain-axial strain, axial stress-volumetric strain, plastic volumetric strain-axial plastic strain, and plastic dilation angle-axial plastic strain relationships are in good agreement with the experimental results of FRP-confined SFRC. The new model provides improved accuracy over the best performing existing models of FRP-confined plain concrete in predicting the behavior of FRP-confined SFRC.

New axial Stress-Strain model of square concrete columns confined with lateral steel and FRP

Due to its efficiency, Fiber Reinforced Polymers (FRP) jacketing has become a common practice for confining concrete columns that need increased axial strength, axial ductility or shear capacities. The presence of confining lateral steel in columns, when externally retrofitted with FRP, changes the mechanism of concrete behavior. The interaction between lateral and longitudinal steel and FRP jacketing redistributes stress concentrations in concrete and yields more complex effective confined area compared to that of plain concrete confined with either FRP or lateral steel alone. Square columns confined with lateral steel and FRP are not well-addressed in the literature compared to circular columns. Therefore, this research focuses on square columns confined with lateral steel and FRP sheets through reviewing existing models in the literature and benchmarking their performance against experimental cases of square columns using axial stress-strain diagrams. Moreover, statistical analysis conducted for theoretical peak strength and ultimate strain obtained from the available models highlight their comparative performance which can be further improved. Therefore, this paper proposes a new model that overcomes shortcomings found in the reviewed models and predicts peak strength and ultimate strain more accurately.

Experimental Study of Full-Scale Corroded Steel Bridge Piles Repaired Underwater with Grout-Filled Fiber-Reinforced Polymer Jackets

AbstractSteel bridge piles, many of which have been in service for over half a century across the United States, are now deteriorating because of corrosion. They often require retrofitting to resto...

Comparative Analysis of Existing RC Columns Jacketed with CFRP or FRCC.

Reinforced concrete (RC) columns typical of existing structures often exhibit premature failures during seismic events (i.e., longitudinal bars buckling and shear interaction mechanisms) due to the poor quality concrete and the absence of proper seismic details in the potential plastic hinge region. The Fiber Reinforced Polymers (FRP) externally bonded reinforcement is known to be a valid technique to improve the shear capacity or the ductility of existing RC columns. However, few experimental tests have proven its effectiveness in the case of columns affected by shear interaction mechanisms. In this work, the behavior of existing RC columns with border line behavior between flexure and shear have been investigated in the case of poor quality concrete and light FRP strengthening with local jacketing and medium quality concrete and strong FRP strengthening with local jacketing, in order to highlight the effect of concrete strength on the effectiveness of the retrofit intervention. As an alternative to FRP jacketing; the effectiveness of the Fiber Reinforced Cementitious Composite (FRCC) jacketing for the seismic strengthening of columns with highly deteriorated concrete cover or columns already damaged by an earthquake is also evaluated. Six full-scale RC columns have been tested under cyclic loading: one was used as a control specimen; four were strengthened in the potential plastic hinge region with carbon FRP (CFRP); and one was fully jacketed with FRCC. The comparison between poor and medium quality concrete columns showed that the CFRP local jacketing is more effective in the case of poor quality concrete. The FRCC jacketing appears to be a sound repair strategy and a suitable alternative to the FRP jacketing in case of poor quality; however, more experimental research is needed for improving this retrofit technique.

On the slenderness and FRP confinement of eccentrically-loaded circular RC columns

This article examines the effects of column slenderness and internal confinement on the behavior of eccentrically-loaded circular RC columns strengthened with fiber reinforced polymer (FRP) sheets. Test parameters included column height, level of transverse reinforcing steel, absence/presence of FRP jackets, fiber orientation and stiffness of the jacket. Tests proved that FRP wraps can be effectively used to enhance the strength, toughness, ductility and deformation capacities of eccentrically loaded-columns and that their efficiency decreases with increased slenderness. Tests showed that variations in internal confinement have a lower impact on column behavior compared to the external FRP confinement. Negligible improvements in load-carrying capacities of columns confined with one hoop FRP sheet were encountered upon adding a longitudinal sheet. Conventional section analysis using material properties based on the stress–strain model adopted by the American Concrete Institute for FRP-confined concrete under combined axial compression and bending resulted in over estimation of axial column strengths.

Behavior and Three-Dimensional Finite Element Modeling of Circular Concrete Columns Partially Wrapped with FRP Strips.

Fiber-reinforced polymer (FRP) jacketing/wrapping has become an attractive strengthening technique for concrete columns. Wrapping an existing concrete column with continuous FRP jackets with the fiber in the jacket being oriented in the hoop direction is referred to as FRP full wrapping strengthening technique. In practice, however, strengthening concrete columns with vertically discontinuous FRP strips is also favored and this technique is referred to as FRP partial wrapping strengthening technique. Existing research has demonstrated that FRP partial wrapping strengthening technique is a promising and economical alternative to the FRP full wrapping strengthening technique. Although extensive experimental investigations have hitherto been conducted on partially FRP-confined concrete columns, the confinement mechanics of confined concrete in partially FRP-confined circular columns remains unclear. In this paper, an experimental program consisting of fifteen column specimens was conducted and the test results were presented. A reliable three-dimensional (3D) finite element (FE) approach for modeling of partially FRP-confined circular columns was established. In the proposed FE approach, an accurate plastic-damage model for concrete under multiaxial compression is employed. The accuracy of the proposed FE approach was verified by comparisons between the numerical results and the test results. Numerical results from the verified FE approach were then presented to gain an improved understanding of the behavior of confined concrete in partially FRP-confined concrete columns.

Influence of geometry and fiber properties on rupture strain of cylindrical FRP jackets under internal ICE pressure

This paper presents an study on the rupture strain of cylindrical fiber-reinforced polymer (FRP) jackets under internal ice pressure. A total of 45 cylindrical FRP jackets were prepared using three unidirectional carbon, glass, and basalt fabrics in three different internal diameters, namely 60, 114, and 216 mm, and one-, two-, and three-plies. Three jackets for each combination were typically tested and the average hoop rupture strains were obtained and compared to the rupture strain of flat coupons in the form of a strain efficiency factor. It was found that the strain efficiency factor ranged from 0.53 to 1.05 with an average of 0.77. A new analytical model was also developed based on the bi-axial state of stress in a cylindrical FRP jacket to obtain the rupture strain and strain efficiency factor of the FRP jacket using a closed-form solution. The model engaged four major parameters, namely: diameter, thickness, axial/transverse strength ratio, and Poisson’s ratio of the FRP jacket. The two latter parameters were eliminated after a parametric study to propose a simplified formula. The analytical and simplified models predicted the experimental strain efficiency factors with an average error of −3.4% and −4.6%, respectively.

Mechanical behavior of FRP confined steel tubular columns under impact

This paper presents experimental and analytical results of fiber reinforced polymer (FRP) confined steel tubular columns under transverse impact loads. Influences of applied impact energy, thickness of FRP jacket and impact position were discussed in detail, and then the impact responses of FRP confined steel tubes were compared with bare steel tubes. The test results revealed that the FRP jacket contributes to prevent outward buckling deformation of steel at the clamped end and inward buckling of steel at the impact position. For the given applied impact energy, specimens wrapped with one layer and three layers of FRP have the lower peak impact loads than those of the bare steel tubes, whereas specimens wrapped with five layers of FRP exhibit the higher peak impact loads. All the FRP confined steel tubular specimens displayed a longer duration time than the bare steel tubes under the same magnitude of impact energy, and the specimen wrapped with one layer of FRP had the longest duration time. In addition, increasing the applied impact energy leads to the increase of peak impact load and duration time, whereas increasing the distance of impact position from the clamped end results in the decrease of peak impact load and the increase of duration time. The dynamic analysis software Abaqus Explicit was used to simulate the mechanical behavior of FRP confined steel tubular columns, and the numerical results agreed well with the test data. Analytical solution for lateral displacement of an equivalent cantilever beam model subjected to impact load was derived out. Comparison of analytical and experimental results shows that the maximum displacement can be precisely predicted by the present theoretical model.

Deformation‐based method for the analysis of reinforced concrete‐ and fiber reinforced polymer‐jacketed columns subjected to axial compression

This paper introduces a generalized approach for the sectional analysis of jacketing, based on the compatibility of lateral deformation of a jacketed column and jacket wall. The method is suitable to determine the magnitude and distribution of confining stresses provided by reinforced concrete or fiber reinforced polymer jackets at any stress level, and to estimate the ultimate resistance of strengthened columns subjected to axial compression. After the discussion of the theoretical bases of the proposed method, a simplified application is introduced. For validation of applicability and accuracy to predict ultimate capacity, comparison with a large number of experimental results with square‐ and circular‐shaped columns is presented. According to the comparisons, the average of the absolute values of relative errors of the proposed method is 8.34%. The calculation results are evaluated qualitatively as well to highlight relevant issues for jacket detailing and to point out possible future directions of developing adequate design methods.

Flexural rotation capacity models fitted to test results using different statistical approaches

Practical models for the ultimate deformations of flexure‐controlled members were fitted to a database of thousands of tests. They cover beams, rectangular columns or walls, members with T‐, H‐, C‐, or box section, conforming or not to seismic design codes, with or without fiber reinforced polymer jackets, in monotonic or cyclic uniaxial flexure with axial force. Model parameters were statistically derived assigning to each test (a) a weight of unity; (b) weight inversely proportional to the number of tests in a testing campaign, or (c) weight inversely proportional to the root‐mean‐square error of tests in a campaign. Predictions correlate well to those of a physical model based on curvatures and a plastic hinge length. Analysis of variance suggests that the scatter of model predictions with respect to test results comes more from the testing itself than from the model. Intralab and interlab variabilities are quantified and pure error is estimated from tests of nominally identical specimens.

Simulating the lateral performance of FRP-confined RC circular columns using a new eccentric-based stress-strain model

In this study, a stress-strain model of fiber-reinforced polymers (FRPs)-confined concrete based on the lateral confinement stiffness was adopted to simulate the lateral response of RC columns retrofitted with external FRP jackets and tested under axial and lateral loads. The adopted model and other five-stress-strain models (established in former studies) were comparatively studied to simulate the seismic response of eight RC-circular columns retrofitted with FRP jackets and experimentally tested under both axial and lateral loads. Compared to the experimental results, the simulation results indicated that all stress-strain models could not identify properly the ultimate lateral displacements of the simulated columns. The adopted stress-strain model was revised to consider the effect of a key influential parameter (eccentricity ratio), which showed a critical impact on the simulation of the seismic response of RC-columns under combined bending and axial loadings. Finally, the proposed model was evaluated in predicting the lateral response of additional three columns and the simulation results exhibited a good agreement with the experimental results.

Behavior of partially and fully FRP-confined circularized square columns under axial compression

Fiber-reinforced polymer (FRP) jacketing has become an attractive strengthening technique for concrete columns. Existing research has demonstrated that FRP confinement is effective for circular columns, whereas it is less effective for square columns. The lower FRP confinement effectiveness in a square column is predominantly attributed to the non-uniform FRP confinement in the column, while the concrete in an FRP-confined circular column is uniformly confined. Although rounding the corners of a square column can enhance the effectiveness of FRP confinement, its benefit is still not satisfactory as the radius of rounded corners is usually limited owing to the presence of internal steel reinforcement. An appropriate approach to enhancing the confinement effectiveness of FRP strengthening technique for square columns is to circularize a square column before FRP jacketing. This paper aims to study the compressive behavior of circularized square columns (CSCs). A total of 33 column specimens were prepared and tested under axial compression in this paper. The column parameters include FRP wrapping schemes (including fully wrapped and partially wrapped), FRP volumetric ratio, sectional shapes and unconfined concrete strength. The test results have indicated that the section circularization of square columns can significantly improve the effectiveness of FRP confinement, and strengthening square columns using section circularization in combination with partial FRP confinement is a promising and economical alternative to the fully FRP strengthening technique. Comparisons between the theoretical predictions and the test results were conducted, and the accuracy of existing partially FRP-confined concrete models were also examined.

Monotonic Stress–Strain Behavior of Steel Rebars Embedded in FRP-Confined Concrete Including Buckling

Fiber-reinforced polymer (FRP) confining jackets offer an attractive solution for the seismic retrofit of RC columns. For the accurate prediction of strength and ductility of FRP-confined RC columns, it is necessary to understand interactions between the FRP jacket and the RC column at all deformation levels under seismic loading. In particular, when the transverse steel reinforcement consists of widely spaced steel stirrups or spirals, the longitudinal steel reinforcing bars (rebars) are likely to develop buckling deformations despite the lateral support provided by the FRP-confined concrete. This paper presents a theoretical study into the buckling behavior of longitudinal steel rebars embedded in FRP-confined concrete subjected to monotonic axial compression. In the theoretical model, a beam-on-elastic foundation idealization is used, where the steel rebars are laterally supported by elastic springs representing the FRP-confined cover concrete. To evaluate the stiffness of the elastic springs, a curved beam model is proposed. Predictions from the theoretical model are compared with test results, demonstrating the reliability of the theoretical model. A parametric study is then presented to examine the influence of three key factors (i.e., spring stiffness, yield strength of steel, and slenderness ratio of rebar) on the overall compressive stress–strain response of laterally supported steel rebars. Finally, an empirical compressive stress–strain model considering the effect of buckling deformation is proposed for steel rebars embedded in FRP-confined concrete subjected to monotonic axial compression and validated through comparisons with the test results.