Behavior and modeling of compound concrete-filled FRP tubular (CCFFT) columns

Abstract

The recycling of construction and demolished concrete waste has attracted wide attention due to its great significance in sustainable development. A novel method of recycling waste concrete is to reuse large pieces of coarsely crushed demolition concrete (referred to as recycled concrete lumps or RCLs) for direct mixing with fresh concrete in new construction, leading to what is referred to as "compound concrete". This method avoids the complexity of recycling concrete as aggregate and enables a higher recycling ratio and a lower recycling cost. However, such compound concrete is much more heterogeneous than conventional concrete, and the weak interfaces between RCLs and fresh concrete may compromise performance. A new technique was proposed by Prof J.G. Teng to improve the properties of compound concrete through the provision of substantial confinement with an external fiber-reinforced polymer (FRP) tube. The resulting columns are referred to as compound concrete-filled FRP tubular (CCFFT) columns. Preliminary test results have shown that due to the confinement from the FRP tube, the axial stress-axial strain behavior of compound concrete in a CCFFT column is similar to that of the fresh concrete used in making the compound concrete even when the RCLs have a much lower compressive strength than the fresh concrete, indicating that the weaknesses of compound concrete can be largely eliminated through FRP confinement. However, this conclusion has been drawn from the limited results of only one series of tests on CCFFT columns under axial compression. Further research is needed to clarify the behavior of CCFFT columns under various loading conditions before this recycling technique can be effectively used in practice. Against this background, this thesis presents a systemic study on the stress-strain behavior of FRP-confined compound concrete in CCFFT columns under monotonic and cyclic axial compression, and the structural behavior of steel bar-reinforced CCFFT columns under eccentric loading. Following an introductory chapter and a literature review, Chapters 3 and 4 present the first part of the PhD research project, which is concerned with the behavior of FRP-confined compound concrete in CCFFT columns under monotonic axial compression. A total of 22 short CCFFT columns were tested to investigate the effects of FRP tube thickness, RCL mix ratio and strength difference between the fresh and the old concretes. The applicability of an existing stress-strain model previously developed for FRP-confined normal concrete in predicting the behavior of the test columns was examined. A three-dimensional (3D) finite element (FE) modelling approach was then developed, and its effectiveness and accuracy were demonstrated through comparisons with test results. The second part of the PhD research project was focused on the behavior of FRP-confined compound concrete under cyclic axial compression (Chapter 5). A total of 9 CCFFT columns were tested, and the effects of FRP tube thickness, RCL mix ratio, and loading scheme were investigated. The applicability of two existing cyclic stress-strain models previously developed for FRP-confined normal concrete was then examined. In the final part of this PhD research project, the behavior of steel reinforced CCFFT columns under eccentric loading was studied (Chapters 6 and 7). An experimental program involving 11 CCFFT columns subjected to eccentric loading was carried out. The effects of FRP tube thickness, RCL mix ratio, load eccentricity, and slenderness ratio were examined. The test results showed that the inclusion of RCLs had little effect on the response of eccentrically-loaded CCFFT columns. A theoretical column model incorporating existing stress-strain models for FRP-confined normal concrete was then employed to predict the test results. The applicability of design equations in the Chinese Technical Code for Infrastructure Application of FRP Composites was finally examined.