Debonding Strength of Steel Beams Strengthened with CFRP Plates

This paper addresses the debonding strength of partial-length, adhesively bonded carbon fiber-reinforced polymer (CFRP) plates that are used to strengthen steel beams. Bonded CFRP plates tend to debond under static and fatigue loadings because of the very high stress field at the plate end. Such failures limit the application of CFRP plates. Static and fatigue tests show that the stress intensity factor governs the debonding strength. The steel/adhesive corner was the locus of debond initiation. The effects of the following parameters on stress intensity factors are discussed: plate thickness, plate modulus, bondline thickness, adhesive modulus, and adhesive spew-fillet angle. The stress intensity factors are calculated using the Betti’s law-based reciprocal work contour integral method (RWCIM). The parametric study results indicate that the stress intensity factors cannot be used to represent the severity of the corner as the adhesive spew-fillet angle (and singularity) changes. Therefore, the use of stress intensity factors as a failure criterion for the purpose of predicting debonding strength is limited to the same spew-fillet angle.     

Flexural Response of Steel Beams Strengthened with Partial-Length CFRP Plates

This paper presents the flexural behavior of rolled steel beams that were strengthened with partial-length, adhesive-bonded carbon fiber-reinforced polymer (CFRP) plates. The hybrid beams had two types of failure mode, depending on the length of the plate: (1) plate debonding in beams with short plates;?and (2) plate rupture at midspan in beams with long plates. The flexural behavior that was investigated includes the development of tensile stresses in the plate, the moment-curvature of the strengthened section, and the load-deflection of the strengthened beam. The analytical methods used include shear lag analysis, section analysis, and application of the virtual work principle. Agreement between the experimental results and the analytical predictions is discussed.     

Slenderness Effects on Circular CFRP Confined Reinforced Concrete Columns

External bonding of circumferential fiber-reinforced polymer (FRP) wraps is a widely accepted technique to strengthen circular RC columns. To date, most of the tests performed on FRP strengthened columns have considered short, unreinforced, small-scale concrete cylinders, with height-to-diameter ratios of less than three, tested under concentric, monotonic, and axial load. In practice, most RC columns have height-to-diameter ratios considerably larger than three and are subjected to loads with at least minimal eccentricity. Results of an experimental program performed to study the effects of slenderness on carbon FRP (CFRP) wrapped circular RC columns under eccentric axial loads are presented. It is shown that CFRP wraps increase the strength and deformation capacity of slender columns, although the beneficial confining effects are proportionally greater for short columns, and that theoretical axial-flexural interaction diagrams developed using conventional sectional analysis (but incorporating a simple FRP confined concrete stress-strain model) provide conservative predictions for nonslender CFRP wrapped columns under eccentric loads. The use of longitudinal CFRP wraps to reduce lateral deflections and allow slender columns to achieve higher strengths, similar to otherwise identical nonslender columns, is also demonstrated.     

Experimental Performance of RC Hollow Columns Confined with CFRP

Column jacketing with fiber-reinforced polymer (FRP) composite materials has been extensively investigated in the last decade to address the issue of seismic upgrade and retrofit of existing reinforced concrete (RC) columns. Researchers have mainly focused their attention on solid columns, while very little research has been done on hollow columns strengthened with FRP. To study the behavior of noncircular hollow cross sections subjected to combined axial load and bending and to contribute to the comprehension of the resistant mechanisms present in FRP confinement, a total of seven specimens have been tested. The present work is the first step in a broader endeavor aimed at evaluating the benefits generated by a FRP wrapping, computing (P-M) interaction diagrams for hollow columns confined with FRP, and defining design criteria for the strengthening of these elements using composite jackets. The theoretical analyses will also assess under which conditions the standard approaches for columns with solid cross sections could be extended to the case of hollow columns.     

Restraining Steel Brace Buckling Using a Carbon Fiber-Reinforced Polymer Composite System: Experiments and Computational Simulation

This paper presents an experimental and computational study of the buckling behavior of steel members strengthened with carbon fiber-reinforced polymer (CFRP) wraps. In the proposed strengthening system, steel members are first sandwiched within a core comprised of mortar or PVC blocks and then the entire system is wrapped with CFRP sheets. A matrix of specimens is tested under monotonic compression to investigate the parameters that influence system response. Test results show that the proposed strengthening method can provide enough lateral support to a steel bar member to allow it to reach yield in compression and to continue deforming inelastically beyond. Key failure modes are identified in the test program. Important parameters that influence behavior are also pinpointed and studied in more detail through a computational simulation model that is validated using the test data. Parameters identified as influential in the experimental and computational studies include: number of CFRP layers, core thickness, bond between CFRP layers and the core, bond between the core and the inner steel member, and strength of transverse sheets at the member ends.     

Local Buckling of Elastically Restrained Fiber-Reinforced Plastic Plates and its Application to Box Sections

An analytical study of local buckling of rectangular composite plates rotationally restrained elastically along unloaded edges and subjected to nonuniform in-plane axial action at simply supported loaded edges is presented. A variational formulation of the Ritz method is used to establish an eigenvalue problem, and by using combined harmonic and polynomial buckling deformation functions, which satisfy all the restrained boundary conditions, the explicit solution of plate local buckling coefficients is obtained. The explicit formulas for local buckling strength of orthotropic plates are simplified to the cases of isotropic plates, which are consistent with classical solutions. The elastically rotationally restrained plates are further treated as discrete plates or panels of fiber-reinforced plastic (FRP) box shapes, and by considering the effect of elastic restraints at the joint connections of flanges and webs, the local buckling strength of FRP box shapes is predicted. The theoretical predictions are in good agreement with transcendental solutions and finite-element eigenvalue analyses for local buckling of FRP box columns. The present explicit formulation can be applied to determine local buckling capacities of composite plates with elastic restraints along the unloaded edges and can be further used to predict the local buckling strength of FRP shapes.     

Fatigue Performance of Tension Steel Plates Strengthened with Prestressed CFRP Laminates

An experimental and analytical study was conducted to investigate the fatigue behavior of tension steel plates strengthened with prestressed carbon-fiber-reinforced polymer (CFRP) laminates. A simple fracture mechanics model was proposed to predict the fatigue life of reinforced specimens. Double-edge-notched specimens were precracked by fatigue loading and then strengthened by CFRP laminates at different prestressing levels. The effects of the applied stress range, CFRP stiffness, and prestressing level on the crack growth were investigated. Experimental results show that the increase of the prestressing level extends the fatigue life of a damaged steel plate to a large amount. The CFRP with the highest prestressing level performed best, prolonging fatigue life by as much as four times under 25% higher fatigue loading. Theoretically, predicted results were in a reasonable agreement with the experimental results. A parametric analysis was also performed to investigate the effects of the applied stress range and the prestressing level on the debonding behavior of the adhesive and on the secondary crack propagation.     

Fiber-Reinforced Polymer-Confined Circular Concrete Columns: Investigation of Size and Slenderness Effects

Laboratory investigations of the compressive behavior of fiber-reinforced polymer (FRP)-confined concrete columns have generally been carried out using relatively small-scale specimens, and the majority of theoretical models that have been developed so far are based on test data from such specimens. However, the use of small specimens may conceal possible scale effects. In this study, the influence of slenderness ratio and specimen size on axially loaded FRP-confined concrete columns was investigated experimentally, and the results have been compared to theoretical models and experimental results gathered from the published literature. The investigation aims to validate past results obtained from concrete cylinders and to verify existing empirical models as well. Three different specimen diameters and two slenderness (length-to-diameter) ratios, combined with two FRP-confinement materials, were varied as parameters. According to the statistical analysis of the results, it is shown that conventional FRP-confined concrete cylinders can effectively be used to model the axial behavior of short columns. Size effects, however, are clearly evident in very small ( ≈ 50?mm diameter) specimens. The usefulness of published results involving such small-scale specimens is therefore questionable, as is the validity of theoretical models and strength predictions based on test data from small-diameter specimens.     

Analytical Model of Circular CFRP Confined Concrete-Filled Steel Tubular Columns under Axial Compression

This study intends to provide a simplified analytical model of the laterally confined concrete filled steel tube (CCFT) column system which adopts carbon-fiber-reinforced polymer (CFRP) jackets in order to make up for major defects of the traditional concrete filled steel tube (CFT) column system. This CCFT analytical model, by adding one additional parameter for CFRP confinement to the CFT column analytical solution, is greatly simplified and expedites the analytical processes to explain the stress-strain relationship of the CCFT column system. In the study, several types of the CCFT column systems with different parameters are analyzed by the proposed simplified analytical model and its associated numerical program (USC-CFT). To verify the accuracy of the analytical model, this study compares the load-strain relationship calculated by USC-CFT both to the experimental results conducted by the traditional method and to the results calculated by the computer-aided finite element method (FEM) analysis method. This study shows equilibrium conditions, deformation compatibilities, constitutive models, and an analysis procedure used in the proposed simplified analytical solution and presents finite element models and analysis procedure used in FEM analysis.     

Simplified Model for Compressive Behavior of Concrete Columns Strengthened by Steel Angles and Strips

This paper presents an analytical model to determine the compressive response of concrete members, having a square transverse cross section and reinforced with steel angles at the corners and steel strips connecting the steel angles. The model evaluates the effects of transverse strips and steel angles on confinement pressures, including strength and buckling phenomena. Compressive response was evaluated in cases of directly and indirectly loaded elements. The data produced correlate well with available experimental data.     

Thermal Buckling Analysis of SMA Fiber-Reinforced Composite Plates Using Layerwise Model

Thermal buckling analysis of laminated smart composite plates subjected to uniform temperature distribution has been presented. Shape memory alloy (SMA) fibers whose material properties depend on temperature have been used as a smart material. A three-dimensional layerwise plate model has been employed in developing the system equations using variational approach. Finite-element method has been adopted for discretization of the laminate. Lagrangian interpolation functions have been used to approximate the displacement components along the thickness as well as in the in-plane direction. The actual variation of prebuckling stresses has been accounted for in the derivation of the geometric stiffness matrix of the laminates. An incremental load technique has been used in the analysis to take into account the nonlinearity in the material properties of the SMA arising due to their temperature dependence. The effects of thickness ratio, orthotropic ratio, fiber orientation, aspect ratio, stacking sequence and various boundary conditions on the critical buckling temperature have been examined in details. The results have been validated with those available in the literature.     

Performance Enhancement of Steel Columns Using Concrete-Filled Composite Jackets

This paper studies the cross-sectional behavior of steel columns strengthened with fiber-reinforced polymers (FRPs). The composite column is constructed by wrapping the steel I-section column with epoxy-saturated glass- and carbon-FRPs (GFRP and CFRP) sheets in the transverse direction and subsequently filling the voids between the FRP and the steel with concrete. Experimental tests were performed on stub columns under axial compression including one to three CFRP wraps. A corner treatment technique, to avoid stress concentration at the corners and to improve confinement efficiency, was also investigated. A simplified analytical model was developed to predict the axial behavior of the composite columns. Experimental results showed significant enhancement in the behavior of the composite columns primarily attributable to the confinement mechanism imposed by the FRP jacket and concrete. Increasing the corner radius resulted in higher compressive strength of the confined concrete and ultimate axial strain of the composite columns. Good agreement between the analytically developed axial load-displacement relationships and the test data indicates that the model can closely simulate the cross-sectional behavior of the composite columns.     

Exact Solutions for Buckling of Multispan Rectangular Plates

This paper presents exact solutions for buckling of multispan rectangular plates having two opposite edges simply supported and the other two edges being either free, simply supported, or clamped. The Levy solution procedure is employed to develop an analytical approach for buckling analysis of multispan plates. The Levy solution for each span is derived and the continuity along the interface of two spans is ensured through the implementation of the essential and natural boundary conditions at the interface. Extensive buckling factors, most of which are first-known exact solutions, are given in tabular and design chart forms for two- and three-unequal-span square plates subjected to uniaxial in-plane load in the x or y directions and biaxial in-plane load. The influence of the span ratios and plate boundary conditions on the buckling factors is discussed. Buckling factors are also obtained for two-, three-, and four-equal-span rectangular plates with various edge support conditions. The exact buckling solutions presented in this paper are of benchmark values for such plates.     

Experimental Study of Rectangular RC Columns Strengthened with CFRP Composites under Eccentric Loading

This paper presents the results of experimental studies on reinforced concrete columns strengthened with carbon fiber-reinforced polymer (CFRP) composites under the combination of axial load and bending moment. A total of seven large-scale specimens with rectangular cross section (200?mm×300?mm) were prepared and tested under eccentric compressive loading up to failure. The overall length of specimens with two haunched heads was 2,700 mm. Different FRP thicknesses of two, three, and five layers; fiber orientations of 0°, 45°, and 90°; and two eccentricities of 200 and 300 mm were investigated. The effects of these parameters on load-displacement and moment-curvature behaviors of the columns as well as the variation of longitudinal and transverse strains on different faces of the columns were studied. The results of the study demonstrated a significant enhancement on the performance of strengthened columns compared to unstrengthened columns.     

Repair of Steel Composite Beams with Carbon Fiber-Reinforced Polymer Plates

One significant cause of deterioration of steel bridge structures is the corrosion due to extensive use of deicing salts in winter weather. The investigation presented in this paper focused on the behavior of steel composite beams damaged intentionally at their tension flange to simulate corrosion and then repaired with carbon fiber-reinforced polymer (CFRP) plates attached to their tension areas side. Damage to the beams was induced by removing part of the bottom flange, which was varied between no damage and loss of 75% of the bottom flange. All beams were tested to failure to observe their behavior in the elastic, inelastic, and ultimate states. To help implement this strengthening technique, a nonlinear analytical procedure was also developed to predict the behavior of the section/member in the elastic, inelastic, and ultimate states. The test results showed a significant increase in the strength and stiffness of the repaired beams. Through the use of CFRP plates, all damaged beams were fully restored to their original (undamaged state) strength.     

Impact Response of Externally Strengthened Unreinforced Masonry Walls Using CFRP

Research reported herein investigates the out-of-plane impact resistance of unreinforced masonry (URM) walls strengthened with carbon fiber-reinforced polymer (CFRP) composites, externally applied in sheets to one face of the wall. Two analytical methods based on energy principle and wave propagation theory and a finite-element-based numerical model have been developed, assuming a perfect bond at composite–masonry interface with an equivalent stiffness of the system. Full-scale impact tests are conducted for verification purpose, where three 1.2?m tall URM concrete walls (one unstrengthened and two strengthened with continuous unidirectional and woven CFRP sheets) are vertically tested up to cracking using a pendulum drop-weight impact tester. The test results compare reasonably well with those obtained from the analyses and simulation. It is found that the energy and finite-element methods can provide reasonable estimates for peak impact force and wall deflection, whereas the wave propagation method is rather limited by its applicability. Parametric studies are conducted to examine the effect of impactor mass, velocity, amount of CFRP reinforcement, and property of masonry material using the developed models.     

Tests on the Ductility of Reinforced Concrete Beams Retrofitted with FRP and Steel Near-Surface Mounted Plates

Reinforced concrete beams are now commonly retrofitted using externally bonded (EB) fiber reinforced polymer (FRP) plates as the technique is both inexpensive and unobtrusive. However, tests have shown that EB carbon FRP plates tend to debond at low strains, which can severely limit the ductility or moment redistribution to such an extent that guidelines often preclude moment redistribution. This paper reports the moment redistribution achieved in tests on nine near full-scale two-span continuous reinforced concrete beams that were retrofitted with near-surface mounted (NSM) plates. The plates were either carbon FRP or high yield steel strips which were adhesively bonded within saw grooves cut into the concrete cover on the tension face or sides of the beam. It was found that the debonding strains of these NSM plates were considerably larger than those associated with EB plates and that substantial amounts of moment redistribution occurred. These tests suggest that NSM plates can be used to increase the strength of reinforced concrete structures with little, if any, loss of ductility.     

CFRP Composite Connector for Concrete Members

Steel plate connections are frequently used in tilt-up and precast concrete building construction to tie adjacent wall panels together for shear and overturning effects, and to provide continuous diaphragm chord connections for wind and seismic loading. These welded connectors perform poorly in regions of high seismicity and are vulnerable to corrosion. Until now, retrofit and repair strategies for in-plane shear transfer strengthening were limited to attaching steel sections across panel edges. In the present paper, an experimental program is described that utilizes carbon fiber reinforced plastic (CFRP) composites to develop a viable retrofit scheme for precast concrete shear walls and diaphragms. Nine full-scale precast wall panel assemblies with CFRP composite connectors have been tested. The results show that the CFRP composite connection is an effective solution for the seismic retrofit and repair of precast concrete wall assemblies and other precast concrete elements, such as horizontal diaphragms, that require in-plane shear transfer strengthening.     

Novel Technique for Inhibiting Buckling of Thin-Walled Steel Structures Using Pultruded Glass FRP Sections

The use of composite materials for strengthening the ailing infrastructure has been steadily gaining acceptance and market share. It can even be stated that this strengthening technique has become main stream in some applications such as strengthening concrete structures. The same cannot be said about steel structures; for which research on composite material strengthening is relatively new. Several challenges face strengthening steel structures using composite materials such as the need for high-modulus composites to improve the effectiveness of the strengthening system. This paper explores a new approach for strengthening steel structures by introducing additional stiffness to buckling-prone regions. The proposed technique relies on improving the out-of-plane stiffness of buckling-prone members by bonding pultruded fiber-reinforced polymer (FRP) sections as opposed to the commonly used approach that relies on in-plane FRP contribution. The paper presents results from an experimental investigation where shear-controlled beam specimens were tested to explore the feasibility of the proposed technique. Bar specimens were also tested in tension to compare between in-plane and out-of-plane contributions of FRP to the behavior and strength of thin steel plates. Based on the results, it can be concluded that this strengthening technique has great potential for altering failure modes by delaying the initiation of undesirable local buckling of thin steel plates. Recommendations for future research efforts are made to expand the knowledge base about this unexplored strengthening technique.     

Effect of Severe Environmental Exposures on CFRP Wrapped Concrete Columns

Deterioration of concrete structures caused by corrosion of reinforcing steel, aging, and weathering is a major problem in harsh environments such as coastal areas and cold regions. In addition, a hot environment, such as in the Arabian Gulf, is recognized as one of the most severe and aggressive environments that affects concrete durability. The purpose of this study is to investigate the effectiveness of strengthening plain concrete cylinders, subjected to extreme temperature variations, by wrapping with two layers of unidirectional carbon fiber-reinforced polymer (CFRP) sheets. Thirty-six plain concrete cylinders (150×300?mm) were tested. Nine specimens served as unstrengthened controls and the remaining cylinders were strengthened with two layers of CFRP sheets. Cylinders were subjected to high temperatures (45°C), to heating and cooling cycles (23 to 45°C), and to prolonged heat exposure (45°C). Some of the cylinders that were subjected to heating and cooling, were later subjected to freezing and thawing cycles, while others were submerged in fresh water or salt water. The specimens were loaded to failure under uniaxial compressive load and the axial and lateral deformations were monitored. High temperature exposure was not found to decrease the strength of the wrapped concrete cylinders.