Quantifying the compressive ductility of concrete in RC members through shear friction mechanics.

Abstract

This thesis contains a series of journal papers in which the compressive ductility of concrete in RC members has been quantified through shear friction mechanics. Firstly, the size dependent stress‐strain models for unconfined and actively confined concrete are derived based on the fundamental mechanics of shear friction theory. At this stage, the shear friction properties, that is the relationship between the shear stress, normal stress, crack widening and interface slip across the sliding plane, are not specifically required. It is shown how the stress‐strain from cylinder tests of one specific length can be modified to determine that for any size of cylinder. Moreover, it is shown that the proposed approach can be used to make existing generic stress/axial‐strain relationships size dependent and these size dependent relationships can be directly used to determine the corresponding size dependent stress/lateral‐strain relationship. Being mechanics based, size dependent stress‐strain models reduce the reliance on vast experimental testing as only one size of specimen needs be tested to obtain stress‐strain relationships for all sizes. Secondly, the shear friction properties, that is the relationship between the shear stress, normal stress, crack widening and interface slip across the sliding plane is derived and presented in a generic form suitable for application. These generic shear‐friction material properties are then used to simulate and quantify the shear‐sliding behaviour of initially uncracked concrete generally obtained directly from relatively expensive tests. In addition, it is also shown how these shear‐sliding capacities can then be used to quantify the shear capacity of RC beams without stirrups and without the need for size factors as the mechanics based approach automatically, through mechanics, allows for member size. Thirdly, the generic shear‐friction material properties derived in Chapter 3 are used to simulate passive confinement in FRP confined cylinders. Importantly, two distinct cylinder failure modes have been identified and examined: that of the circumferential wedge that is common in standard cylinders with aspect ratios of 2:1; and that of the single sliding plane that occurs at higher aspect ratios. It shows the mechanics solutions for the influence of specimen size, that is both diameter and height, on the stress‐strain relationship of axially loaded FRP confined concrete cylindrical specimens and how small scale FRP wrapped specimens suitable for compression testing can be designed so that the stress/strain relationship of the full scale member under pure compression can be extracted from those of the small test specimen. Finally, a series test of steel tube confined concrete columns is designed to verify the accuracy of the size effect expressions proposed in previous chapters. Importantly, it shows that because the standard material test always adopts small scale 2:1 aspect ratio specimens, the majority failure mode in material test specimens is the circumferential wedge failure. Consequently it is for this wedge failure mode that most axial‐stress/global-axial‐strain relationships are developed. However, similar to the specimens studied in this test program, the aspect ratio of most practical steel tube confinement columns is more than 2. Hence only in a minority of cases does the circumferential wedge failure occur in practice. Therefore, the empirical or semi‐empirical equations developed from small scale concrete specimens are not truly representative of the actual behaviour of full‐scale columns which have aspect ratios markedly different from the 2:1 ratio most commonly tested.