FRP-confined curvilinearized square concrete columns under axial compression

Abstract

Confining concrete columns with fibre-reinforced polymer (FRP) jackets is now widely used as a strengthening method for concrete structures as it can significantly improve the compressive strength and ductility of concrete columns. Many existing studies have shown that FRP confinement is highly effective for concrete columns with a circular section but is much less effective for concrete columns with a rectangular or square section. The reduced confinement effectiveness for square/rectangular columns is due to the existence of flat sides and sharp corners in these columns. Although confinement effectiveness for these columns can be enhanced by rounding the corners, the radius of the rounded corners is limited by the existence of internal steel reinforcement. Therefore, rounding corners alone can only achieve limited improvement to the effectiveness of FRP confinement for square/rectangular columns.This thesis is concerned with a new shape modification method called the section curvilinearization method or simply the SC method. In this method, the straight sides of a square/rectangular column are modified into slightly-curved sides before FRP jacketing; the resulting columns are referred to as curvilinearized columns. Following an introductory chapter and a literature review chapter, this thesis presents the results of an MPhil research programme that included both experimental and theoretical investigations into the behaviour of FRP-confined curvilinearized concrete columns. The experimental work was undertaken to investigate the behaviour of FRP-confined curvilinearized square concrete columns under monotonic concentric compression. In particular, the effects of the significant parameters, including the rise-to-span ratio of the curved sides, the level of confinement, and the size of column, were examined. A numerical study into FRP-confined curvilinearized concrete columns is next presented. Finite element models were developed using ABAQUS based on a damaged plasticity model previously proposed by the authors' group. The finite element predictions are shown to be in close agreement with the test results. The finite element approach was then employed to investigate a number of issues that were not adequately covered by the tests. Finally, a design-oriented stress-strain model for FRP-confined concrete in curvilinearized square concrete columns was proposed for use in practical design.Based on the results and discussions presented in this thesis, it can be concluded that the SC method can significantly enhance the effectiveness of confinement as reflected by the axial stress-strain response and the compressive strength of the confined concrete, and the second-stage slope of the axial stress-axial strain curves is almost linearly related to the rise-to-span ratio. Both small-scale columns and large-scale columns were tested, and a comparison of their results indicates that the axial stress-strain relationship is little affected by column size.