Lateral strain modeling, analysis and design of confined concrete

Abstract

This thesis presents a lateral strain model of confined concrete, which makes no assumptions on the properties of confining materials and is therefore generally applicable regardless of the type of confinement. It can be applied to concrete members or columns confined by fiber reinforced polymer (FRP), steel tubes or steel tubes wrapped with FRP or steel rings/spirals, provided the appropriate stress-strain models of the confining materials are incorporated in the analysis. The lateral strain model can also be used to directly evaluate the lateral strain of confined concrete with non-cylindrical section or under eccentric loading where the lateral strain and confining stress could vary across the section using finite element analysis. The first part of the thesis presents the details of the derivations of the lateral strain model of confined concrete. A total of 95 actively confined concrete specimens and 34 passively confined concrete specimens covering a wide range of concrete strength from 25 to 120 MPa and confining stress from 0 to 0.99 times the concrete strength tested by other researchers in the published literature have been investigated for the influence of concrete strength, confining stress and axial strain on the lateral strain of confined concrete. Based on the regression analysis, the influence of concrete strength, confining stress and axial strain is accurately and consistently captured and a constitutive model for predicting the lateral strain of confined concrete under different levels of confinement from pre-crack and elastic state to post-crack and inelastic state is proposed. In the second part of the thesis, theoretical models integrating the proposed lateral strain model, axial stress-strain model of confined concrete, stress-strain model of steel tube, FRP, steel rings/spirals have been proposed to analyze the axial stress-strain curves of a total of 174 FRP confined concrete, 138 concrete-filled steel tubes (CFST) without external confinement, 89 CFST with FRP jackets and 98 CFST with external steel rings/spirals. The lateral strain, confining stress can be obtained by an iterative procedure and the axial stress/load can be evaluated by substituting the so obtained confining stress into the axial stress-strain model of confined concrete. It is observed that the theoretical models can predict the axial and lateral stress-strain behaviour of FRP confined concrete, CFST, and CFST with external confinement accurately and consistently. Finally, a parametric study has been carried out by varying a series of structural parameters such as concrete strength, steel tube yield strength and thickness-to-diameter ratios, FRP confining stiffness and rupture strain, and steel rings/spirals confining stiffness to study the effects on strength and ductility. From the data generated by the parametric study, design formulas for predicting the yield and ultimate strength of FRP confined concrete, CFST with or without external confinements are proposed and compared with existing codes and design charts for evaluating the minimum FRP confining stiffness or steel rings/spirals thickness-to-diameter ratio to achieve different levels of ductility for CFST with various concrete strength are presented. Moreover, the minimum confining stiffness of external confinement to avoid delamination is evaluated for design guidance.