Mechanical Simulation of the Passive Confinement of Reinforced Concrete for Design

Abstract

Reinforcement that acts to passively confine concrete, such as stirrups, steel tubes, FRP (fibre reinforced polymer) wraps or a combination of the above can enhance the strength and, more importantly, the ductility of concrete members, allowing for greater absorption of energy and consequently ductile failure. Research to quantify the stress-strain response of confined concrete has largely resulted in empirical or semi-empirical modification factors that are applied to the stress-strain relationships for unconfined or actively confined concrete. These approaches appear, however, to be the result of seemingly disparate research conducted to develop safe approaches for design purposes. As a result, the approaches often yield conservative predictions of performance within the bounds of the dataset from which they were calibrated, but poor performance when extended outside these bounds. This presents a particular challenge for the application of new types of confinement reinforcement material as expensive member tests for different size and concrete strength specimens are required and the whole procedure has to be repeated for each type of new material.. In this thesis, a generic mechanics-based model is proposed for the passive stress/strain of concrete that can incorporate: any type of confinement reinforcement; rectangular or circular cross-sections; different specimen sizes; and different concrete strengths. This approach is based on the direct application of fundamental partial-interaction shear-friction and bond-slip mechanics rather than the empirical modification of unconfined material properties. The benefit of this approach is that it is based directly on fundamental material properties that are obtained from simple material tests and, therefore, it can rapidly and inexpensively be extended to new types of confinement without the need for member level calibration testing. Additionally, simplified closed-form solutions for the proposed approach are developed for use in the design of members. This thesis first investigates the confinement reinforcement behaviour including debonding, yielding, fracture or a combination of the above and the corresponding closed-form equations are proposed. Then the shear friction material properties are derived from actively confined cylinder tests as well as shear-sliding tests and are simplified to a linear form. After which the bond-slip material properties for different types of confinement reinforcement are summarised. Having gathered all these fundamental material properties, the stress/strain response of confined concrete is quantified and corresponding simplified closed-form solutions are proposed for rectangular and circular cross-section members respectively. Next, closed-form solutions of the passive stress/strain of concentrically loaded specimens are simplified to rectangular stress blocks for flexural analysis. From which the closed-form solution based on the segmental analysis approach is used to quantify the beam ductility by deriving the moment/rotation of a hinge. Finally, the above proposed approach is extended to steel tube confined concrete for which the passive stress/strain incorporates shrinkage and the results are simplified to rectangular stress blocks that can be used in flexural analyses.