FRP-confined RC columns : analysis, behavior and design

Abstract

A very popular application of FRP composites is to provide confinement to RC columns to enhance their load carrying capacity and ductility. This method of strengthening is based on the well-known phenomenon that the axial compressive strength and ultimate axial compressive strain of concrete can be significantly increased through lateral confinement. Despite the increasing popularity of this strengthening technique, relevant design provisions in most of the existing design guidelines for external strengthening of RC structures using FRP composites are only applicable to the design of short columns subjected to concentric compression. A proper design procedure for FRP-confined RC columns is urgently needed to facilitate wider practical applications. Against this background, this thesis is concerned with the development of a rational design procedure for FRP-confined RC columns to correct the deficiency in existing design guidelines. The thesis presents a systematic study covering the behavior and modeling of FRP-confined concrete as well as the analysis and design of FRP-confined RC columns. A series of axial compression tests on FRP-confined concrete cylinders was conducted first to gain a good understanding of the stress-strain behavior of FRP-confined concrete, which is fundamental and essential to the analysis and design of FRP-confined RC columns. Stress-strain models for FRP-confined concrete of different levels of sophistication were next developed as a prerequisite for the analysis of FRP-confined RC columns. Subsequently, a simple but accurate stress-strain model for FRP-confined concrete was incorporated into a conventional section analysis procedure to develop design equations for short FRP-confined RC columns with a negligible slenderness effect. Finally, two theoretical models of different levels of sophistication were developed to deal with the slenderness effect in slender FRP-confined RC columns. The rigorous theoretical model was used to develop a slenderness limit expression to differentiate short columns from slender columns while the simple theoretical model was used to develop design equations for slender columns. The results of the present study led to a comprehensive design procedure that includes a set of design equations for short columns, a simple expression to separate short columns from slender columns, and a set of design equations for slender columns. The present study is limited to circular columns, but the framework presented in the present study can be readily extended to FRP-confined rectangular RC columns when an accurate stress-strain model for FRP-confined concrete in rectangular columns becomes available. The present study has been partially motivated by the need to formulate design provisions for the Chinese Code for the Structural Use of FRP Composites in Construction, which is currently being finalized. This new code has been developed within the framework of the current Chinese Code for Design of Concrete Structures (GB-50010 2002). Therefore, some of the considerations in the present study follow the specifications given in GB-50010 (2002) and these considerations are highlighted where appropriate throughout the thesis.