Strengthening of Eccentrically Loaded Reinforced Concrete Columns with Fiber-Reinforced Polymer Wrapping System: Experimental Investigation and Analytical Modeling

This paper presents the results of experimental program and analytical modeling for performance evaluation of a fiber-reinforced polymer (FRP) wrapping system to upgrade eccentrically loaded reinforced concrete (RC) columns. A total of 12 RC columns with end corbels were tested. The test specimen had an overall length of 1,200?mm. Each end corbel had a cross section of 250×250?mm and a length of 350?mm. The specimen in the test region was 125×125?mm having a longitudinal steel ratio of 1.9%. Test parameters included confinement condition (no wrapping, full FRP wrapping, and partial FRP wrapping), and eccentricity-to-section height (e/h) ratio (0.3, 0.43, 0.57, and 0.86). Research findings indicated that the strength gain caused by FRP wrapping decreased as e/h was increased. Full FRP wrapping resulted in about 37% enhancement in compression strength at a nominal e/h of 0.3, whereas only 3% strength gain was recorded at a nominal e/h of 0.86. The compression strengths of the partially wrapped columns were on average 5% lower than those of the fully wrapped columns. A nonlinear, second-order analysis that accounts for the change in eccentricity caused by the lateral deformation was proposed to predict the columns strength. A comparison between analytical and experimental results of the present study in addition to data published in the literature demonstrated the accuracy and validity of the proposed analysis.     

Creep Analysis of Axially Loaded Fiber Reinforced Polymer-Confined Concrete Columns

An analytical model is developed to study the time-dependent behavior of concrete-filled fiber reinforced polymer (FRP) tubes (CFFT) and fiber-wrapped concrete columns (FWCC) under sustained axial loads. The model utilizes the double power law creep function for concrete in the framework of rate of flow method, and the linear viscoelastic creep model for FRP. It follows geometric compatibility and static equilibrium, and considers the effects of sealed concrete, multiaxial state of stresses, creep Poisson’s ratio, stress redistribution, variable creep stress history, and creep rupture. The model is verified against previous creep tests by the writers on FWCC and CFFT columns. It is then used to study the practical design parameters that may affect creep of FRP-confined concrete under service loads, or lead to creep rupture at high levels of sustained load. Creep of FWCC is shown to be close to that of sealed concrete of the same mix, as the effect of confinement on creep of concrete is not very significant. CFFT columns, on the other hand, creep much less than FWCC, mainly due to axial stress redistribution. As the stiffness of the tube increases relative to the concrete core, larger stress redistributions take place further reducing the creep. However, there is a threshold, beyond which, stiffer tubes would not significantly lower the creep of concrete. Creep rupture life expectancy of CFFT columns is shown to be quite acceptable.     

Model for Analysis of Short Columns of Concrete Confined by Fiber-Reinforced Polymer

This paper presents a numerical model for evaluating the behavior of axially loaded rectangular and cylindrical short columns of concrete confined by fiber-reinforced polymer (FRP) composites. The proposed formulation considers, for unconfined and confined compressed concrete, a uniaxial constitutive relation that utilizes the area strain as a parameter of measure of the material secant axial stiffness. For unconfined concrete, the model adopts an explicit relationship between axial strain and lateral strain, while for confined concrete, an implicit relation is considered. For this last case, the model employs a simple iterative-incremental approach that describes the entire stress-strain response of the columns. The behavior of the FRP is considered linear elastic until the rupture. To validate the model, a number of columns were analyzed and the numerical results were compared with experimental values published by other authors. This comparison between experimental and numerical results indicates that the model provides satisfactory predictions of the stress-strain response of the columns.     

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.     

Ductility of Reinforced Concrete Members Externally Wrapped with Fiber-Reinforced Polymer Sheets

The effectiveness of external wrapping with fiber-reinforced polymer for enhancing the curvature ductility of lightly reinforced concrete members is investigated. Referring to members with circular transverse cross sections, the performances in terms of both strength and ductility capacities are analyzed, and the predictive reliability of two different recent constitutive models, available in the literature and able to take into account the softening behavior of confined concrete, is checked. A parameter characterizing the effectiveness of the confining wrapping is proposed, and characteristic values are suggested. Moreover, referring to ductility increases due to confinement effects, a comparison is made between the predictions obtained using the constitutive models and simple expressions given in recent codes. Parametric analyses carried out highlight the importance of a definition of the limits of validity of expressions given in the literature for estimation of ductility increases in order to avoid nonconservative assessment.     

Rectangular Filament-Wound Glass Fiber Reinforced Polymer Tubes Filled with Concrete under Flexural and Axial Loading: Experimental Investigation

This paper presents results of an experimental investigation on three beams and five short columns, consisting of glass fiber reinforced polymer concrete-filled rectangular filament-wound tubes (CFRFTs). The tubes included fibers oriented at ±45° and 90° with respect to the longitudinal axis. Additional longitudinal fibers [0°] were provided in flanges for flexural rigidity. Beams included totally filled tubes and a tube partially filled with concrete, which had a central hole for reducing deadweight. The effect of reinforcement ratio was examined by using tubes of two different sizes. Flexural behavior of CFRFT was compared to concrete-filled rectangular steel tubes (CFRSTs) of similar reinforcement ratios. Short columns were tested under eccentricity ratios (e/h) of 0, 0.09, 0.18, and 0.24, where h is the section depth. Transverse strains were measured around the perimeter of concentrically loaded column to evaluate confinement effect. The study showed that CFRFT is a feasible system that could offer similar flexural strength to CFRST. The tube laminate structure and its progressive failure contribute to the slightly nonlinear behavior of beams. The CFRFT beam with inner hole had an overall strength-to-weight ratio, 77% higher than the totally filled beam, but failed in compression. Bulging of CFRFT columns has limited their confinement effectiveness.     

Strength and Ductility of Concrete Beams Reinforced with Carbon Fiber-Reinforced Polymer Plates and Steel

Seven concrete beams reinforced internally with varying amounts of steel and externally with precured carbon fiber-reinforced polymer (FRP) plates applied after the concrete had cracked under service loads were tested under four-point bending. Strains measured along the beam depth allowed computation of the beam curvature in the constant moment region. Results show that FRP is very effective for flexural strengthening. As the amount of steel increases, the additional strength provided by the carbon FRP plates decreases. Compared to a beam reinforced heavily with steel only, beams reinforced with both steel and carbon have adequate deformation capacity, in spite of their brittle mode of failure. Clamping or wrapping of the ends of the precured FRP plate enhances the capacity of adhesively bonded FRP anchorage. Design equations for anchorage, allowable stress, ductility, and amount of reinforcement are discussed.     

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.     

Comparative Study of Models on Confinement of Concrete Cylinders with Fiber-Reinforced Polymer Composites

The use of fiber-reinforced polymer (FRP) composites for strengthening and/or rehabilitation of concrete structures is gaining increasing popularity in the civil engineering community. One of the most attractive applications of FRP materials is their use as confining devices for concrete columns, which may result in remarkable increases of strength and ductility as indicated by numerous published experimental results. Despite a large research effort, a proper analytical tool to predict the behavior of FRP-confined concrete has not yet been established. Most of the available models are empirical in nature and have been calibrated against their own sets of experimental data. On the other hand, the experimental results available in the literature encompass a wide range of values of the significant variables. The objective of this work is a systematic assessment of the performance of the existing models on confinement of concrete columns with FRP materials. The study is conducted in the following steps: the experimental data on confinement of concrete cylinders with FRP available in the technical literature are classified according to the values of the significant variables; the existing empirical and analytical models are reviewed, pointing out their distinct features; the whole set of available experimental results is compared with the whole set of analytical models; and strengths and weaknesses of the various models are analyzed. Finally, a new equation is proposed to evaluate the axial strain at peak stress of FRP-confined concrete cylinders.     

3D Finite-Element Analysis of Substandard RC Columns Strengthened by Fiber-Reinforced Polymer Sheets

Numerical analyses are performed to predict the stress–strain behavior of square reinforced concrete columns strengthened by fiber-reinforced polymer (FRP) sheet confinement. The research focuses on the contribution of FRP sheets to the prevention of elastic buckling of longitudinal steel bars under compression, in cases of inadequate stirrup spacing. A new Drucker–Prager-type plasticity model is proposed for confined concrete and is used in constructed finite-element model. Suitable plasticity and elasticity models are used for steel reinforcing bars and fiber-reinforced polymers correspondingly. The finite-element analyses results are compared against published experimental results of columns subjected to axial compression, to validate the proposed finite-element model. Stress concentrations in concrete core and on FRP jacket are investigated considering circular or square sectioned, plain or reinforced concrete columns. Geometry of the section as well as the presence of steel bars and stirrups affect remarkably the variation and magnitude of stress on FRP as percentage of its tensile strength.     

Behavior of Concrete Beams Strengthened with Fiber-Reinforced Polymer Laminates under Impact Loading

Most of the research on application of composite materials in civil engineering during the past decade has concentrated on the behavior of structural elements under static loads. In engineering practice, there are many situations in which structures undergo impact or dynamic loading. In particular, the impact response of concrete beams strengthened with composite materials is of interest. This paper presents the results of an experimental investigation conducted to study the impact effects on concrete beams strengthened with fiber-reinforced polymer laminates. Two types of composite laminates, carbon and Kevlar, were bonded to the top and bottom faces of concrete beams with epoxy. Five beams were tested: two strengthened with Kevlar laminates, two strengthened with carbon laminates, and one unretrofitted beam as the control specimen. The impact load was applied by dropping a steel cylinder from a specified height onto the top face of the beam. The test results revealed that composite laminates significantly increased the capacity of the concrete beams to resist impact load. In addition, the laminates reduced the deflection and crack width. Comparing the test results of the beams strengthened with Kevlar and carbon laminates indicated that the gain in strength depends on the type, thickness, weight, and material properties of the composite laminate.     

Behavior of Large-Scale Rectangular Columns Confined with FRP Composites

This paper focuses on axially loaded, large-scale rectangular RC columns confined with fiber-reinforced polymer (FRP) wrapping. Experimental tests are conducted to obtain the stress-strain response and ultimate load for three field-size columns having different aspect ratios and/or corner radii. Effective transverse FRP failure strain and the effect of increasing confining action on the stress-strain behavior are examined. Existing strength models, the majority of which were developed for small-scale specimens, are applied to predict the structural response. Since some of them fail to adequately characterize the test data and others are complex and require significant calculation, a simple design-oriented model is developed. The new model is based on the confinement effectiveness coefficient, an aspect ratio coefficient, and a corner radius coefficient. It accurately predicts the axial ultimate strength of the large-scale columns at hand and, when applied to the small-scale columns studied by other investigators, produces reasonable results.     

Cyclic Response of Unbonded Posttensioned Precast Columns with Ductile Fiber-Reinforced Concrete

A precast segmental concrete bridge pier system is being investigated for use in seismic regions. The proposed system uses unbonded posttensioning (UBPT) to join the precast segments and has the option of using a ductile fiber-reinforced cement-based composite (DRFCC) in the precast segments at potential plastic hinging regions. The UBPT is expected to cause minimal residual displacements and a low amount of hysteretic energy dissipation. The DFRCC material is expected to add hysteretic energy dissipation and damage tolerance to the system. Small-scale experiments on cantilever columns using the proposed system were conducted. The two main variables were the material used in the plastic hinging region segment and the depth at which that segment was embedded in the column foundation. It was found that using DFRCC allowed the system to dissipate more hysteretic energy than traditional concrete up to drift levels of 3–6%. Furthermore, DFRCC maintained its integrity better than reinforced concrete under high cyclic tensile-compressive loads. The embedment depth of the bottom segment affected the extent of microcracking and hysteretic energy dissipation in the DFRCC. This research suggests that the proposed system may be promising for damage-tolerant structures in seismic regions.     

Experimental Investigation on Seismic Retrofitting of Square RC Columns by Carbon FRP Sheet Confinement Combined with Transverse Short Glass FRP Bars in Bored Holes

Jacketing is less effective to large square/rectangular RC columns due to the inability of the rectangular-shaped jacket in restraining the dilation of concrete in the middle of a straight side. A new retrofit method is proposed in this work by fiber reinforcing the surface concrete in the middle of a straight side. Fiber reinforcing is achieved by inserting small fiber-reinforced polymer (FRP) bars into the concrete in the plastic hinge zone. The inserted FRP bars act as horizontal reinforcement to increase the ductility of the concrete in a similar way as that in normal fiber-reinforced concrete. When this fiber reinforcing technique is combined with the conventional jacketing, the concrete in all parts of a cross section may be effectively confined. In this work, experimental tests were undertaken to investigate the effectiveness of this new retrofit technique. Six half-scaled columns were tested and the test results demonstrated the effectiveness of the method.     

Confinement Effectiveness in Rectangular Concrete Columns with Fiber Reinforced Polymer Wraps

An innovative mechanistic based method for passive confinement efficiency estimation is proposed based on the extension to rectangular sections of the pulley model previously proposed by the writers. A refined finite element model was developed using a nonlinear concrete constitutive law in order to analyze stresses in columns passively confined with fiber reinforced polymer wraps. Rectangular and square cross sections of variable corner radii were investigated with reference to a circular cross section. Results showed an increase in corner stresses with sharper corner radii, a localization of failure at the corners, and a decrease in confinement effectiveness with an increase in the rectangularity of the cross section or an increase in corner sharpness. A rigorous numerical method for calculating geometric confinement efficiency factors is proposed and typical factors are calculated and compared with the predictions of the simple pulley model showing good agreement.     

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.     

Flexural Response of Reinforced Concrete Beams Strengthened with End-Anchored Partially Bonded Carbon Fiber-Reinforced Polymer Strips

This paper presents the results of experimental and analytical studies carried out to investigate the flexural behavior of reinforced concrete beams strengthened with end-anchored partially bonded carbon fiber-reinforced polymer (CFRP) strips. A total of six beams, each 2400 mm long, 150 mm wide, and 250 mm deep with a tension steel reinforcement ratio of 1.18%, were tested. One beam was left unstrengthened as the control, another beam was strengthened with a fully bonded CFRP strip, and the remaining four beams were strengthened with partially bonded CFRP strips placed on the tension face of the beam and fixed at both ends using a mechanical anchor. The influence of varying the CFRP unbonded length (250 mm, 750 mm, 2×500 mm, and 1,250 mm) on the beam flexural response was studied. The experimental results revealed that end-anchored partially bonded CFRP strips significantly enhanced the ultimate capacity of the control beam and performed better than the fully bonded strip with no end-anchorage. This observation stresses the importance of end-anchorage in such strengthening schemes, especially considering that the end-anchored partially bonded CFRP strengthened beams showed similar flexural behavior trends. Finally, an inelastic section analysis procedure that takes into consideration the incompatibility of strains was developed to verify the obtained test results. The analysis produced good predictions of the experimental results in terms of the moment-curvature response and showed the effect of CFRP unbonded length on the strain of the internal tension steel.     

Shear Strengthening of Interior Slab–Column Connections Using Carbon Fiber-Reinforced Polymer Sheets

This paper presents the results of an experimental investigation undertaken to evaluate the punching shear capacity of interior slab–column connections, strengthened using flexible carbon fiber-reinforced polymer (CFRP) sheets. Sixteen square (670×670?mm) slab–column connections with different slab thicknesses (55 and 75 mm) and reinforcement ratios (1 and 1.5%) were tested. Twelve specimens were strengthened using CFRP sheets and the remaining four specimens were kept as controls. Without strengthening, all specimens were designed to experience punching shear failure. The CFRP sheets were bonded to the tension face of the specimens in two perpendicular directions parallel to the internal ordinary steel reinforcement. The test results clearly demonstrate that using CFRP leads to significant improvements in the flexural stiffness, flexural strength, and shear capacity of beam–column connections. Depending on the content of the ordinary reinforcement, thickness of the slab, and area of CFRP sheet, the flexural strength increased between 26 and 73% and the shear capacity increased between 17 and 45%. The measured stress in the CFRP sheets at nominal strength varied between 22 and 69% of the ultimate tensile strength of the fibers. Comparison with available prediction equations showed that the punching shear capacity can be predicted with reasonable accuracy if the contribution of CFRP reinforcement to the increase in flexural strength is accounted for. On the other hand, the code design expressions were conservative in predicting the capacity observed in the tests.     

Shape Effect on the Performance of Carbon Fiber Reinforced Polymer Wraps

At present, fiber reinforced polymer (FRP) composite materials are extensively used to strengthen concrete structures and a main application is wrapping compression members such as building and bridge columns for improved strength and ductility. In this case, FRP laminates are intended to provide confinement to the concrete and the cross section shape plays an important role on the effectiveness of the method. The primary purpose of this paper is to introduce a test device and a test method designed to determine the effect of corner radius on the strength of the FRP laminate and on the distribution of the resulting radial stress on the substrate material. Various curvatures were investigated. In the proposed device, they can be realized by using interchangeable inserts. Strain distribution around the corner, failure load, and failure mode of the FRP laminate were monitored and analyzed. The stress concentration in the laminate is studied numerically using the finite element method and compared with experimental results. The relationship between radial stress distribution and corner radius is determined to provide guidance in practical cases.     

Strengthening of Openings in One-Way Reinforced-Concrete Slabs Using Carbon Fiber-Reinforced Polymer Systems

Six prototype one-way RC slabs with openings were strengthened with externally bonded carbon fiber-reinforced polymer (CFRP) systems and subjected to concentrated line loads. The results were compared to those of a solid slab without opening and a slab with an unstrengthened opening. The CFRP system proved to be effective in enhancing the load-carrying capacity and stiffness of RC slabs with an opening, provided that premature failure due to CFRP debonding is excluded. An analytical model based on the modified yield line method is presented, in which four failure modes comprising one orthogonal and three nonorthogonal yield line patterns were considered. The analytical model predicts the load carrying capacity of the strengthened slabs very well. The opening width has a more prominent effect on the load-carrying capacity than does the opening length.