800 ACI Structural Journal/November-December 2009 ACI Structural Journal, V. 106, No. 6, November-December 2009. MS No. S-2007-308.R3 received February 10, 2009, and reviewed under Institute publication policies. Copyright © 2009, American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion including author’s closure, if any, will be published in the September- October 2010 ACI Structural Journal if the discussion is received by May 1, 2010. ACI STRUCTURAL JOURNAL TECHNICAL PAPER This study addresses the extension of a failure criterion developed for rock data to small or near full-size circular concrete columns confined by different materials. Initially, a wide range database on small-scale specimens confined by fiber-reinforced polymer (FRP) sheets or tubes and analytical studies were evaluated to predict the ultimate compressive strength. Then, the reinforced concrete (RC) short-columns retrofitted with FRP jacket were examined through available data. The research further covers middle- or near full- scale columns confined by FRP composites and steel reinforcement. All of these estimations were conducted within a unified approach to the failure criterion. Thus, the ultimate compressive strength for different confining materials can be predicted easily and systematically. Keywords: confinement; fiber-reinforced polymers; reinforced concrete; sheet; strength; tube. INTRODUCTION Confinement of concrete is an efficient technique to improve the load-carrying capacity and ductility of concrete columns. The strength and deformability of concrete significantly increase by applying lateral confining pressure. 1-8 The lateral confining action is generally induced by passively restraining the lateral expansion of the concrete through a closely spaced stirrup, spiral or hoop reinforcement, and, recently, fiber-reinforced polymer (FRP) composites. Numerous studies have shown that wrapping FRP sheets and encased tubes around circular columns is also a very effective technique to enhance their axial strength, shear strength, ductility, and seismic performance. 5-8 FRP composites are suitable for use in coastal and marine structures, as well as civil infrastructure facilities, due to their superior properties such as high strength-to-weight ratio, high tensile strength and modulus, corrosion resistance, and durability. Most empirical confinement models were established based on the Mohr-Coulomb failure criterion. 1,2,4,8-12 It is obvious, however, that the Mohr-Coulomb criterion is valid only in the compression region, whereas Hoek-Brown’s failure criterion from rock mechanics exists both in compressive and tensile regions to complete it (Fig. 1). This study addresses the extension of Hoek-Brown’s failure criterion 13 to concrete, which is developed essentially based on rock data, by covering several confinement materials. Most recently, the extension to concrete of this failure criterion has been successfully verified for the prediction of the ultimate strength in the high-strength concrete (HSC) specimens under active confining pressure. 14 The two main purposes for this study are 1) to apply this failure criterion to FRP- wrapped and FRP-encased short columns. Herein, the applicability of Hoek-Brown’s criterion to FRP-confined (that is, carbon-fiber-reinforced polymer [CFRP], glass fiber-reinforced polymer [GFRP], and aramid fiber-reinforced polymer [AFRP]) concrete is validated through a wide-range experimental database (from 7 to 170 MPa [1015 to 24,656 psi]) compiled from the actual literature; and 2) this failure criterion may provide a practical tool and unified approach to predict the ultimate strength for other confinement types (that is, spiral, hoop, or spiral + FRP). Furthermore, in this approach, the ultimate strength of concrete confined with transverse spiral/hoop reinforcement or FRP jacket may be predicted for near full-scale columns as well as short columns. For all the cases under consideration, the ultimate strength can be practically estimated in terms of cylinder compressive strength. RESEARCH SIGNIFICANCE The Hoek-Brown failure criterion, developed essentially based on rocks, may be further applied to concrete confined by several confinement types actively or passively (steel hoops, spirals, FRP sheets, and tubes). Within a unified approach, it may be available to extend the failure criterion up to FRP-confined concrete, reinforced concrete (RC), and RC retrofitted with FRP jacket. The ultimate compressive strength, not only for short columns but also for middle- and near full-size columns, are evaluated within the same integrated approach. The failure criterion provides a tool to rapidly and reliably assess the ultimate strength by reducing time and test costs. Title no. 106-S74 Modified Failure Criterion to Predict Ultimate Strength of Circular Columns Confined by Different Materials by Z. Canan Girgin Fig. 1—Hoek-Brown and Mohr-Coulomb’s failure envelope to estimate ultimate strength under triaxial compression.