Axial Compressive Behavior of FRP-Concrete-Steel Double-Skin Tubular Columns Made of Normal- and High-Strength Concrete Togay Ozbakkaloglu 1 and Butje Louk Fanggi 2 Abstract: This paper presents the results of an experimental study that was undertaken to investigate the effects of key parameters on the compressive behavior of fiber-reinforced polymer (FRP)-concrete-steel double-skin tubular columns (DSTCs). A total of 24 normal-strength and high-strength concrete-filled DSTCs were manufactured and tested under axial compression. The key parameters examined included the concrete strength; thickness of FRP tube; diameter, strength, and thickness of inner steel tube; and presence (absence) of concrete filling inside it. The results indicate that both normal- and high-strength concretes in a DSTC system is confined effectively by FRP and steel tubes, resulting in a highly ductile compressive behavior. The results also indicate that increasing the inner steel tube diameter leads to an increase in the ultimate axial stress and strain of concrete in DSTCs. It is observed that the concrete filling of the inner steel tubes results in a slight decrease in the ultimate axial strain and a slight increase in ultimate stress of DSTCs. No clear influence of the strength of inner steel tube is observed on the ultimate condition 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. DOI: 10.1061/(ASCE)CC.1943-5614.0000401. © 2013 American Society of Civil Engineers. Author keywords: Fiber-reinforced polymers (FRP); Concrete; High-strength concrete (HSC); Confinement; Columns; FRP tubes; Steel tubes; Double-skin tubular columns (DSTCs). Introduction As an important application of fiber-reinforced polymer (FRP) composites, confinement of concrete with externally bonded FRP has received a great deal of attention over the last two decades. Numerous experimental studies have been conducted to examine the performance of FRP composites in retrofitting existing concrete columns (e.g., Rochette and Labossiere 2000; Chaallal et al. 2003; Lam and Teng 2004; Hadi 2006; Ilki et al. 2008; Ozcan et al. 2010; Wu and Wei 2010; Ozbakkaloglu and Akin 2012; Wang et al. 2012) and in the construction of new high-performance com- posites columns in the form of concrete-filled FRP tubes (CFFTs) (Seible et al. 1996; Mirmiran et al. 1998; Fam and Rizkalla 2001; Fam et al. 2005; Shao and Mirmiran 2005; Ozbakkaloglu and Saatcioglu 2006, 2007; Ozbakkaloglu and Oehlers 2008a, b; Mohamed and Masmoudi 2010; Ozbakkaloglu 2013a, b, c; Idris and Ozbakkaloglu 2013). More recently a new type of composite system was proposed by Teng et al. (2004) in the form of FRP-concrete-steel double-skin tubular columns (DSTCs). This composite system consists of a steel tube inside, an FRP tube outside with concrete in between, and it combines the advantages of all three materials to achieve a high-performance structural member. A series of axial compres- sion and flexure tests have been conducted by the research group lead by Teng (Teng et al. 2005, 2007, 2010; Yu et al. 2006; Wong et. al 2008; Yu and Teng 2010; Yu et al. 2010; Xie et al. 2011) to investigate the performance of FRP-concrete-steel DST stub columns and beams. Following these, Han et al. (2010) reported on a study in which a few DST beam-column specimens were tested under cyclic loading. The results of these early tests have demonstrated that the DST beam and column systems provide very effective confinement to concrete, which in turn leads to a highly ductile member behavior. These studies have also demonstrated that the behavior of DSTCs is different from previously studied column forms, including CFFTs, concrete-filled steel tubes (CFSTs), and concrete-filled steel double-skin tubes (CFSDS). Very much like that of FRP, the popularity of high-strength concrete (HSC) in the construction industry has been on a steady incline during the last two decades because of the superior perfor- mance and economy offered by HSC over normal-strength concrete (NSC) in a large number of structural engineering applications. The use of high-strength concrete in the construction of new composite columns such as CFFTs and DSTCs is particularly attractive be- cause the effective combination of these high-strength materials (i.e., HSC, steel and FRP) results in high-performance structural members. However, apart from the six axial compression test specimens reported in Teng et al. (2010), all of the existing studies on DSTCs have been concerned with NSC, and additional studies are required to better understand the compressive behavior of HSC DSTCs. To contribute towards this end, this paper presents the results of an experimental program that focused on the axial compressive behavior of FRP-HSC-steel DSTCs. The study was aimed at investigating the influence of critical columns parameters on the performance of DSTCs constructed of carbon FRP external tubes. The results of the experimental program are first presented and followed by a discussion on the influence of the key parameters on the behavior of DSTCs. 1 Senior Lecturer, School of Civil, Environmental and Mining Engineer- ing, Univ. of Adelaide, Adelaide, SA 5005, Australia (corresponding author). E-mail: togay.ozbakkaloglu@adelaide.edu.au 2 Ph.D. Student, School of Civil, Environmental and Mining Engineer- ing, Univ. of Adelaide, Adelaide, SA 5005, Australia. Note. This manuscript was submitted on January 9, 2013; approved on June 5, 2013; published online on June 7, 2013. Discussion period open until March 15, 2014; separate discussions must be submitted for individual papers. This paper is part of the Journal of Composites for Con- struction, © ASCE, ISSN 1090-0268/04013027(13)/$25.00. © ASCE 04013027-1 J. Compos. Constr. J. Compos. Constr. 2014.18. Downloaded from ascelibrary.org by ADELAIDE, UNIVERSITY OF on 02/09/14. Copyright ASCE. For personal use only; all rights reserved.