Pedram Sadeghian

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.

Effect of Fiber Orientation on Compressive Behavior of CFRP-confined Concrete Columns

This article presents the results of experimental studies on concrete cylinders confined with CFRP composites. Thirty small-scale specimens (150 × 300 mm2) were subjected to uniaxial compression up to failure and their stress—strain behaviors were studied. Various parameters such as wrap thickness and fiber orientation were considered. Different wrap thicknesses (1, 2, 3, and 4 layers), fiber orientation of 0°, 90°, ±45°, and their combinations were investigated. The results demonstrated significant enhancement in the compressive strength, stiffness, and ductility of the CFRP-confined concrete cylinders as compared to plain cases. An analytical model for ultimate stress and strain of confined concrete has been proposed.

Bond-Slip Analytical Formulation toward Optimal Embedment of Concrete-Filled Circular FRP Tubes into Concrete Footings

This paper presents a robust analytical model for a moment connection of concrete-filled fiber reinforced-polymer (FRP) tubes (CFFTs) to concrete footings. The CFFT connection is based on a simple approach of direct embedment into the footing, thereby eliminating the need for connection rebar or mechanical devices. The CFFT is externally subjected to lateral and axial loads, resembling practical applications such as piles affixed to pile caps, bridge columns, or utility poles. The model adopts the concepts of equilibrium, deformations compatibility, and nonlinear concrete stress-strain behavior. It also employs a “bond stress-slip” relation that can be obtained from simple push-through tests on some of the commercially used tubes. The model can predict the critical embedment length Xcr , which is the minimum length required to achieve material failure of the CFFT outside the footing, and bond failure inside the footing, simultaneously. If the actual embedment length is less than Xcr , bond failure occurs ...

Numerical Modeling of Concrete Cylinders Confined with CFRP Composites

This article presents the results of numerical analyses of concrete cylinders (i.e., columns) confined in composites jackets. The specimens were subjected to uniaxial compression and the analysis was carried out using a non-linear finite element method. Various parameters such as wrap thickness, fiber orientation, concrete strength, and interfacial bonding were considered. Three different wrap thicknesses (0.9, 1.8, and 2.7mm), fiber orientation of 0°, ±15°, ±30°, and ±45° with respect to the hoop direction, and concrete strength values ranging from 20 MPa to 40 MPa were investigated. The finite element analysis results were in good agreement with experimental data presented by other researchers. The numerical analysis results demonstrated significant enhancement in the compressive strength, stiffness, and ductility of the CFRP-wrapped concrete cylinders compared to unconfined concrete cylinders. The stress—strain response of cylinders was greatly affected by analysis parameters.

A Rational Approach toward Strain Efficiency Factor of Fiber-Reinforced Polymer-Wrapped Concrete Columns (with Appendix)

This paper aims to provide a rational approach to the concept of circumferential strain efficiency factor kₑ of circular concrete columns wrapped with fiber-reinforced polymer (FRP) jackets. The variable kₑ reflects the reduction in the FRP hoop rupture strain observed in experiments, relative to the true rupture strain of the FRP material obtained from standard coupon tests or manufacturer datasheets. Currently, the ACI 440.2R-08 design guide uses a constant empirical kₑ factor of 0.55 for any column, regardless of geometric or mechanical properties. A rational approach is proposed to calculate kₑ. It employs a simplified Tsai-Wu interaction failure envelope of FRP laminates and acknowledges the biaxial state of stresses in the jacket. A large database of 454 circular concrete specimens wrapped with unidirectional FRP fabrics has been compiled from literature and subjected to rigorous statistical evaluations. While the proposed model performed quite similarly to the current ACI 440.2R-08 in that it did not lead to significant improvement in prediction, it has the advantage of offering analytical rational bases to the observed phenomenon. It also distinguishes between columns of different geometric and mechanical properties, leading to a variable kₑ of 0.05 to 0.98 with a 0.65 average, which is consistent with experimental observations and more logical than the constant value of 0.55.

Effect of Fiber Orientation on Nonlinear Behavior of CFRP Composites

Fiber orientation is one of the important parameters that affect strength and ductility of FRP-confined concrete columns. A number of studies have been conducted on various fiber orientations and wrap thicknesses, but have not been carried out as a comprehensive study on tensile properties of FRP with various fiber orientations and their effects on confined concrete. This article presents the results of experimental studies about mechanical properties of CFRP composites. In this study, 24 coupons of CFRP composites were prepared and tested in axial tension under displacement control mode. Eight-ply configurations were prepared with fibers oriented at 08, ±458, and 908 from the axial direction and with 1, 2, 3, and 4 plies. It was found that stress-strain behavior basically depended on fiber orientation. The behavior in on-axis orientation was shown to be perfectly linear-elastic with brittle rupture, but in off-axis orientation it was shown to be fully nonlinear with high ductility. An analytical model has been proposed for the nonlinear behavior of composites with off-axis orientation.

Experimental and Analytical Behavior of Sandwich Composite Beams: Comparison of Natural and Synthetic Materials

In this study, the flexural behavior of sandwich composite beams made of fiber-reinforced polymer (FRP) skins and light-weight cores are studied. The focus is on the comparison of natural and synthetic fiber and core materials. Two types of fiber materials, namely glass and flax fibers, as well as two types of core materials, namely polypropylene honeycomb and cork, are considered. A total of 105 small-scale sandwich beam specimens (50 mm wide) were prepared and tested under four-point bending. Test parameters were fiber types (flax and glass fibers), core materials (cork ad honeycomb), skin layers (0, 1, and 2 layers), core thicknesses (6–25 mm), and beam spans (150 and 300 mm). The load–deflection behavior, peak load, initial stiffness, and failure mode of the specimens are evaluated. Moreover, the flexural stiffness, shear rigidity, and core shear modulus of the sandwich composites are computed based on the test results of the two spans. An analytical model is also implemented to compute the flexural stiffness, core shear strength, and skin normal stress of the sandwich composites. Overall, the natural fiber and cork materials showed a promising and comparable structural performance with their synthetic counterparts.

Strengthening Slender Reinforced Concrete Columns Using High-Modulus Bonded Longitudinal Reinforcement for Buckling Control

AbstractThis paper introduces a model for strengthening slender reinforced concrete columns. The proposed technique aims at controlling second-order lateral deflections using longitudinal high-modulus bonded reinforcement, thereby altering the loading path to intercept the axial load-bending moment (P-M) interaction curve at a higher axial capacity. With the availability of high and ultra-high-modulus carbon fiber–reinforced polymer (CFRP) plates, this approach should be quite efficient according to Euler’s buckling rule, in which column strength is stiffness-controlled. This approach is different from the classical transverse-wrapping method for confinement, a technique that achieves strengthening by enlarging the (P-M) diagram in the compression-controlled region. The proposed model accounts for concrete nonlinearity in compression, cracking in tension, steel rebar plasticity, and certainly geometric nonlinearity, in addition to the possibility of premature CFRP-debonding failure in tension and the lowe...

Closed-Form Model and Parametric Study on Connection of Concrete-Filled FRP Tubes to Concrete Footings by Direct Embedment

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) have been introduced as a new system for piles, columns, and poles. A simple moment connection based on direct embedment of the CFFT into concrete footings or pile caps, without using dowel-bar reinforcement, has been proposed by the authors. Robust analytical models to predict the critical embedment length (Xcr) were also developed and experimentally validated. In this paper, a comprehensive parametric study is carried out using the models developed earlier along with a newly developed closed-form model for the general case of axial loading, bending, and shear applied to the CFFT member. The parameters studied are the diameter (D), thickness (t), length outside the footing (L), and laminate structure of the FRP tube, as well as the tube-concrete interface bond strength (τmax⁡), concrete compressive strength in the CFFT (fct′) and footing (fc′), and the magnitude and eccentricity of axial compressive or tensile loads. It was shown that increasin...

Testing and Modeling of a New Moment Connection of Concrete-Filled FRP Tubes to Footings under Monotonic and Cyclic Loadings

This study explores a new moment connection of concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) to concrete footing. The tube is tightly fitted and adhesively bonded to a short reinforced concrete stub protruding from the footing, which facilitates concrete filling of the tube without the need for shoring. To establish the critical stub length (Xcr), specimens with heavily steel-reinforced stubs varying in length from 0.5D to 2.0D, where D = diameter of the CFFT, were fabricated and tested in flexure by using a cantilever setup. The Xcr required to achieve flexural failure of the CFFT was 1.05D. Additional specimens with a sufficient stub length of 1.5D were then fabricated to examine the effect on strength and ductility of the steel reinforcement ratio (ρs) in the stub. The optimal ρs of the stub required for the CFFT to reach flexural failure was 3.2%. Finally, the effect of low-cycle reversed bending fatigue was studied with and without an axial compression load. Remarkable ductility associat...