Behaviour and modelling of large-scale FRP-confined rectangular and curvilinearized rectangular RC columns

Abstract

Existing studies have demonstrated that fibre reinforced polymer (FRP) jacketing/wrapping is highly effective in retrofitting/strengthening reinforced circular concrete (RC) columns with, but it is much less effective for rectangular RC columns (including square columns as a special case) due to the non-uniform FRP confinement in the latter columns. While FRP confinement effectiveness in rectangular columns can be enhanced by rounding the sharp corners, its benefit is limited, particularly for large RC columns, as the corner radius is limited by the presence of internal steel reinforcement. The behavior of FRP-confined concrete in circular columns has been extensively studied over the last two decades and is now well understood. By contrast, much less is known about the behavior of FRP-confined concrete in rectangular columns. Existing studies on FRP-confined rectangular concrete columns have generally been limited to small-scale specimens subjected to concentric axial compression. A number of issues (e.g., the effects of column size, load eccentricity and column slenderness) concerning the behavior of large FRP-confined rectangular RC columns have not been properly addressed. Against this background, this thesis presents a systematic study, conducted both experimentally and theoretically, for the development of an in-depth understanding of the stress-strain behaviour of FRP-confined concrete in large rectangular RC columns subjected to either concentric loading or eccentric loading. In addition, the thesis is concerned with a novel shape modification method named the section curvilinearization method (or simply the SC method) for large rectangular RC columns, in which the flat sides of a rectangular column are modified into slightly-curved sides before FRP wrapping. The behaviour and modelling of FRP-confined concrete in the resulting curvilinearized rectangular columns under either concentric or eccentric loading are also a main focus of this thesis. The first objective of the research programme was to investigate the stress-strain behaviour of FRP-confined concrete in large rectangular RC columns under either concentric loading or eccentric loading. Both laboratory testing and theoretical analysis were carried out. Various factors, including the section aspect ratio, strain gradient over the section (i.e., load eccentricity), column slenderness ratio, FRP confinement level, and loading direction, were systematically examined. In addition, the size effect in these columns was investigated through comparisons between the present test results and predictions using existing stress-strain models established based on small-scale columns. The second objective of the research programme was to investigate the stress-strain behaviour of FRP-confined concrete in large curvilinearized rectangular RC columns. Two experimental studies of such columns under concentric loading and eccentric loading were carried out respectively. The effects of edge rise-to-span ratio and corner radius ratio of the curved edges were given particular attention; these two factors had not been properly investigated in the existing studies. The effects of section aspect ratio, load eccentricity, column slenderness ratio, and the column size were also systematically examined. A new design-oriented stress-strain model for FRP-confined concrete in curvilinearized rectangular columns under concentric loading was then proposed based on the test results. The final objective of the research programme was to verify the applicability and accuracy of the design equations in the Chinese Technical Code for Infrastructure Application of FRP Composites for large FRP-confined rectangular RC columns (including curvilinearized columns) using the test results presented in this thesis.