Effects of confining stiffness and rupture strain on performance of FRP confined concrete C.X. Dong a , A.K.H. Kwan a , J.C.M. Ho b,⇑ a Department of Civil Engineering, The University of Hong Kong, Hong Kong, China b School of Civil Engineering, The University of Queensland, Brisbane, Australia article info Article history: Received 7 November 2014 Revised 14 March 2015 Accepted 16 March 2015 Keywords: Confinement Confining stiffness FRP confined concrete Rupture strain abstract FRP (fiber reinforced polymer) confinement has been proven to be effective in enhancing the structural performance of concrete columns. However, the effectiveness of FRP confinement is dependent on vari- ous structural parameters, including the concrete strength, FRP confining stiffness and FRP rupture strain, and is not easy to predict as revealed by the large differences in the existing design formulas. Herein, a newly developed axial and lateral stress–strain model is used to evaluate the effects of these parameters on the yield strength/strain, ultimate strength/strain and ductility of FRP confined concrete. The new model is first verified by comparing with test results published in the literature and then used to perform a parametric study, based on which design formulas for estimating the performance of FRP confined concrete are developed. It will be seen at the end that the confining stiffness is more fundamental than the confining stress at rupture and that the rupture strain has independently significant effect on the ultimate strength/strain and ductility. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction FRP (fiber reinforced polymer) confinement of concrete struc- tures for enhancing their structural performance has become increasingly popular in the past two decades [1–3]. Particularly, the provision of FRP confinement at the potential plastic hinge zones of concrete columns can substantially improve the seismic performance of the concrete building, making FRP confinement an effective means of retrofitting old concrete buildings [4,5].A lot of effort has been spent on investigating the effectiveness of FRP confinement by testing numerous specimens with different combinations of structural parameters. However, the effects of concrete strength, FRP confining stiffness and FRP rupture strain on the strength and ductility of FRP confined concrete are fairly complicated and up to now, there are still no generally accepted guidelines for the design of FRP confinement. Extensive tests have been carried out to evaluate the effects of various types of FRP [6,7], FRP confining stiffness [8,9], fiber orientation [10,11], member size [12,13] and concrete strength [14–17] on the axial stress–strain behavior of FRP confined con- crete. Based on the different sets of test results, various criteria for distinguishing strain hardening and strain softening after yielding have been established [18–21], but large discrepancies exist between them. Moreover, some of the conclusions drawn by the different researchers and the various design formulas devel- oped do not agree with each other, probably because the different researchers were using limited test results covering different ranges of structural parameters in their separate studies. To resolve this issue, a more fundamental and rigorous theoretical analysis of how the confining stress varies during loading and how the various structural parameters would affect the axial stress–strain curve of FRP confined concrete is needed. Most previous studies, whether experimental or theoretical, just focused on the ultimate strength and ultimate strain of FRP confined concrete [19,22–27]. However, as some researchers have rightly pointed out [15], since it is difficult to fully utilize the enhanced ultimate strength and ultimate strain due to FRP con- finement, the yield strength and yield strain should be more important in the actual design of FRP confined concrete. It is only that generally, the proportional increases in yield strength and yield strain due to FRP confinement are much smaller than the corresponding increases in ultimate strength and ultimate strain. Somehow, although many design formulas have been developed for the prediction of ultimate strength and ultimate strain, there are still no design formulas ever developed for the prediction of the yield strength and yield strain (to the best of the authors’ knowledge). In fact, as will be explained later in this paper, it is http://dx.doi.org/10.1016/j.engstruct.2015.03.037 0141-0296/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: johnny.ho@uq.edu.au (J.C.M. Ho). Engineering Structures 97 (2015) 1–14 Contents lists available at ScienceDirect Engineering Structures journal homepage: www.elsevier.com/locate/engstruct