| Fiber reinforced polymer (FRP) composites have been increasingly applied to strengthen concrete structures due to their high strength-to-weight ratio and high corrosion resistance. One important application of FRP is the rehabilitation and strengthening for the concrete columns. At present, most investigations focus on the axial behavior of small size concrete cylinders (e.g.(?)150×300mm) and many models are proposed on test data of such size cylinders. However, large size concrete columns are more popularly used in practical engineering. It is unclear whether the conclusions and models developed on small scale cylinders are appropriate for large scale columns. Therefore, size effect of FRP-confined concrete cylinders in axial compression is still an open issue. Although some investigators studied how cylinder size affects the compressive behavior, comprehensive analysis including strength gain fcc’/fco’,ductility εcc/εco and normalized stress-strain relationship has not been conducted yet, or only test data of one size cylinder has been treated as the reference value in their studies. This paper aims to clarify the size effect of carbon fiber reinforced polymer (CFRP)-confined concrete cylinders in axial compression, including the influence of cylinder size on fcc’/fco’,εcc/εco and normalized stress-strain relationship as well as volumetric strain and dilation rate. Subsequently, both an analysis-oriented and a design-oriented stress-strain model are proposed based on the conclusion of non-size effect for FRP-confined cylinders. The research results in this dissertation are as follows:(1) A total of12CFRP-confined cylinders and12unconfined cylinders were conducted in this experiment. All the cylinders have three sizes including (?)100×200mm,(?)200×400mm and (?)300×600mm. The small-, medium-and large-size cylinders are wrapped with1,2and3plies CFRP to give the same lateral confining stress for each specimen. The experimental results show that there is no size effect for strength gain fcc’/fco’, ductility εcc/εco normalized stress-strain relationship as well as volumetric strain and ultimate dilation rate.(2) Based on the assumption of stress-strain relationship being independent on stress path, an analysis-oriented stress-strain model, containing a theoretical strength model for actively confined concrete with highest precision and a simplified axial strain-lateral strain relationship originally proposed by Pantazopoulou et al and later improved by Lee et al, is developed. According to the conclusion made by Lee and Hegemier that the secant stiffness of stress-strain curve is independent of lateral confinement, the axial and lateral stress-strain relationship of FRP-confined cylinders is obtained through incremental calculation. In addition, the strain efficiency factor kε=0.66is determined by the statistical analysis based on extensive test data. And a predicted strength model is developed through the regression of418calculating values of fcc’ regarding kε0.66as the ultimate state of calculation.(3) The proposed design-oriented stress-strain model in this paper consists of a parabolic curve followed by a straight line. The superiority of the design model is the predicted precision of the ultimate state (i.e., fcc’ and εcc). The empirical strength model is proposed by the regression of418test points of fcc’ and the ultimate strain model is developed through the relationship between ultimate Poisson’s ratio vu and ultimate strain scc of FRP-confined concrete. The developed design model and other design models are compared in the prediction of strength fcc’, ultimate strain scc, the slope of the second portion of the stress-strain curve E2and the area below the stress-strain curve, which shows the high accuracy for the design model in this paper. |
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