Behavior of FRP-confined normal- and high-strength concrete under cyclic axial compression

An important application of fiber-reinforced polymer (FRP) composites is as a confining material for concrete, both in the seismic retrofit of existing reinforced concrete columns and in the construction of concrete-filled FRP tubes as earthquake-resistant columns in new construction. The reliable design of these structural members against earthquake-induced forces necessitates a clear understanding of the stress-strain behavior of FRP-confined concrete under load cycles. This paper presents the results of an experimental study on the behavior of FRP-confined normal- and high-strength concrete under axial compression. A total of 24 aramid and carbon FRP-confined concrete cylinders with different concrete strengths and FRP jacket thicknesses were tested under monotonic and cyclic loading. Examination of the test results has led to a number of significant conclusions in regards to both the trend and ultimate condition of the axial stress-strain behavior of FRP-confined concrete. These results are presented, and a discussion is provided on the influence of the main test parameters in the observed behaviors. The results are also compared with two existing cyclic axial stress-strain models for FRP-confined concrete. © 2012 American Society of Civil Engineers.Togay Ozbakkaloglu and Emre Aki

Unified cyclic stress–strain model for normal and high strength concrete confined with FRP

Fiber reinforced polymer (FRP) has become increasingly popular as a confining material for concrete, both in the strengthening of existing columns where FRP wraps with fibers oriented completely or predominantly in the hoop direction are typically used, and in new construction where filament-wound FRP tubes with fibers oriented at desired angles to the longitudinal axis are typically used. For both types of applications, the stress-strain behavior of FRP-confined concrete under cyclic axial compression needs to be properly understood and modeled for the accurate simulation of such columns under seismic loading. This paper presents an improved cyclic stress-strain model for FRP-confined concrete on the basis of a critical assessment of an earlier model proposed by Lam and Teng in 2009 by making use of a database containing new test results of both concrete-filled FRP tubes (CFFTs) and concrete cylinders confined with an FRP wrap. The assessment reveals several deficiencies of Lam and Teng\u27s model due to the limited test results available to them. The proposed model corrects these deficiencies and is shown to provide reasonably accurate predictions for both concrete in CFFTs and concrete confined with an FRP wrap and for both normal strength concrete (NSC) and high strength concrete (HSC)

An experimental study on behvior of FRP-HSC-Steel double-skin tubular columns under concentric compression

This paper reports on part of an ongoing experimental program at The University of Adelaide on FRP-concrete-steel composite columns. The results from eight FRP-concrete-steel double-skin columns (DSTCs) that were tested under constant axial compression are presented. The key parameters examined included diameter, thickness, and strength of inner steel tube. The results of the experimental study indicate that concrete in a DSTC system is confined effectively by FRP and steel tubes. The results also indicate that increasing the inner steel tube diameter leads to an increase in the ultimate axial strength and strain of DSTCs. No clear influence of the strength of inner steel tube is observed on the ultimate condition of concrete in DSTCs. These results are presented together with a discussion on the influence of the key parameters on the compressive behavior of DSTCs.Togay Ozbakkaloglu and Butje Alfonsius Louk Fanggihttp://www.icsbm.org/2013

Square FRP-HSC-steel composite columns: Behavior under axial compression

Abstract not availableButje Alfonsius Louk Fanggi, Togay Ozbakkalogl

Compressive behavior of aramid FRP-HSC-steel double-skin tubular columns

This paper presents the results of an experimental study on the behavior of fiber reinforced polymer (FRP)-concrete-steel double-skin tubular columns (DSTCs) under concentric compression. Influence of column parameters was investigated experimentally through the test of 16 normal- and high-strength concrete-filled DSTCs and six concrete-filled FRP tubes (CFFTs). The column parameters examined included the thickness of FRP tube; the concrete strength; the diameter, thickness, and shape of inner steel tube. The results of the experimental study show that concrete in a DSTC system is confined effectively by FRP and steel tubes. Both the normal- and high-strength concrete DSTCs tested in the present study exhibited a highly ductile compressive behavior. The results also show that increasing the inner steel tube diameter leads to an increase in the ultimate axial stress and strain of concrete in DSTCs. It is found that, for a given nominal confinement ratio, an increase in the concrete strength results in a decrease in the ultimate axial strain of DSTCs. It is also observed that DSTCs with square inner steel tube confined concrete ineffectively. In addition, it is found that concrete in DSTCs developed similar ultimate axial stresses but higher ultimate axial strains compared to concrete in companion CFFTs. The results of the present study and those from the previously reported studies are then compared with the only existing stress-strain model proposed for DSTCs. Finally, a new design-oriented model that provides improved predictions of the ultimate conditions of concrete in DSTCs was proposed. © 2013 Elsevier Ltd. All rights reserved.Butje Alfonsius Louk Fanggi, Togay Ozbakkalogl

Axial compressive behavior of FRP-concrete-steel double-skin tubular columns made of normal- and high-strength concrete

Abstract not availableTogay Ozbakkaloglu and Butje Louk Fangg

Influence of inner steel tube properties on compressive behavior of FRP-HSC-steel double-skin tubular columns

This paper reports on part of an ongoing experimental program at The University of Adelaide on FRP-concrete-steel composite columns. A total of eight high-strength concrete double-skin tubular columns (DSTCs) were tested under axial compression. The column parameters examined included the diameter, thickness, and shape of inner steel tube. The results of the experimental study show that increasing the inner steel tube diameter leads to an increase in the ultimate axial stress and strain of concrete in DSTCs. The results also show that increasing inner steel tube thickness leads to an increase in the ultimate axial stress and strain of DSTCs. Furthermore, it is observed that concrete inside DSTCs with square inner steel tubes is not confined as effectively as concrete inside DSTCs with circular inner steel tubes.Butje Alfonsius Louk Fanggi and Togay Ozbakkalogl

Modeling the behavior of FRP-confined concrete using dynamic harmony search algorithm

The accurate prediction of ultimate conditions for fiber reinforced polymer (FRP)-confined concrete is essential for the reliable structural analysis and design of resulting structural members. Nonlinear mathematical models can be used for accurate calibration of strength and strain enhancement ratios of FRP-confined concrete. In this paper, a new procedure is proposed to calibrate the nonlinear mathematical functions, which involved the use of a dynamic harmony search (DHS) algorithm. The harmony memory is dynamically adjusted based on a novel pitch generation scheme using a dynamic bandwidth and random number with normal standard distribution in DHS. A new design-oriented confinement model is proposed based on three influential factors of FRP area ratio (ρa), lateral confinement stiffness ratio (ρE), and strain ratio (ρε). Five nonlinear mathematical design-oriented models are regressed on approximately 1000 axial compression tests of FRP-confined concrete in circular sections based on the proposed DHS algorithm. The proposed models for the prediction of the ultimate axial stress and strain of FRP-confined concrete are compared with the existing models. It has been shown that the DHS algorithm offers the best performance in terms of both accuracy and fast convergence rate in comparison with the other modified versions of harmony search algorithms for optimization problems. The proposed design-oriented model provides improved accuracy over the existing models.Behrooz Keshtegar, Togay Ozbakkaloglu, Aliakbar Gholampou

Nonlinear modeling of ultimate strength and strain of FRP-confined concrete using chaos control method

Abstract not availableBehrooz Keshtegar, Pedram Sadeghian, Aliakbar Gholampour, Togay Ozbakkalogl