The objective of this research project is to conduct an experimental study of the behavior of carbon fiber-reinforced polymer (CFRP) materials for strengthening reinforced concrete structures. The test program consisted of 23 reinforced concrete columns strengthened with CFRP sheets under static eccentric compressive loading. It is shown that under large eccentric compression loading, ultimate capacity of the columns can be effectively increased with the longitudinal CFRP sheets, and the ductility factors of the columns can be increased with the transverse CFRP sheets. It was observed that the material strength utilization of transverse CFRP sheets was more sufficient than that of longitudinal CFRP. The transverse CFRP sheets were broken at failure of a column, but the maximum measured strain in the longitudinal CFRP was only about 5000µe at peak load.

This paper describes a series of tests on concrete cylinders confined by carbon fiber jackets. The primary variables in the investigation were the thickness of the jackets (and therefore the lateral confinement stress), the size of the cylinders, the loading speed, and the loading type (monotonic vs. cyclic), and the jacket type (individual layers or continuous wrap). Of these parameters, the lateral confining stress was found to have the greatest influence, but the coefficient for the concrete used was found to be slightly lower than the 4.1 that is implicit in ACI318-02. The cylinder size, the loading speed and the cyclic loading regimes were found to have essentially no influence on stress and strain at failure. The continuously wound jackets were found to initiate failure by delamination, rather than fracture of the fiber, and to lower the stress and strain at failure.

In order to evaluate the load carrying capacity and ductility of hinging zone ofthe concrete filled steel tubular (CFT) columns, tests are carried out on 12 circular speci-mens and 18 square specimens subjected to uniform bending under a constant axial load.The experimental parameters are: 1) depth to thickness ratio (D/t ratio) and width to thick-ness ratio (B/t ratio) of steel tube; 2) axial load ratio; 3) material strength; 4) deformationhistory and 5) annealing. One of the features of the test is the wide range of D/t and B/tratio. The range of D/t ratio of the circular CFT columns are 40.5-160 and the range of B/t ratio of the square CFT columns are 32.8-98.0, respectively. The experimental load-de-formation relations are compared with those of the elasto-plastic analysis based on theproposed stress-strain relationships established for the filled concrete and for the steel tube.The analytical results show good agreement with the test results for all specimens. Thisimplies that the proposed stress-strain relationships for CFT columns are useful to predictthe characteristics of the filled concrete and the steel tube.

Concepts in ductile design have led to an increased interest in understanding the role of confinement in improving the seismic performance of reinforced concrete members. While transverse reinforcement is regarded as a form of passive confinement in RC members, the observed increase in the strength of confined concrete is typically a function of the axial strain levels. Confinement models have been developed by numerous researchers to describe the stress-strain behavior of concrete as a function of certain key parameters that are related to the amount and type of transverse reinforcement. Accurate constitutive models of confined concrete are necessary for direct use in fiber-model based discretization of RC components or for indirect use in hysteresis based phenomenological models. This paper examines the relevance and importance of accurate confinement modeling in predicting the inelastic behavior of well-confined concrete columns. In particular, the influence of incorporating confinement effects in predicting the monotonic and cyclic response of RC columns is investigated. It is analytically demonstrated that the role of the longitudinal reinforcing bars play a more significant role in determining the overall force-deformation behavior of RC components. Detailed fiber-based discretizations that rely entirely on constitutive models are incapable of reproducing post-yield softening and deterioration because of their inability to incorporate complex large deformation behavior of both the longitudinal and the confining reinforcement. Approximate phenomenological models will continue to see widespread use in inelastic analysis of RC structures until these limitations of constitutive-based element models are overcome.

It has been widely known that concrete filled steel tubular (CFT) columns have much higher strength and deformation capacities than common reinforced concrete (RC) columns because of beneficent interactive confinement effect between the filled concrete and the steel tube. The confinement effect by steel tube furthermore contributes to improving ductility of high-strength concrete, and enables application of the CFT columns in high-rise buildings located on seismic regions. This paper describes the axial and the flexural behaviors of CFT columns with circular and square sections based on many experimental researches conducted in Japan. The emphasis of this paper will be placed on the stress-strain curve models for concrete in CFT columns, which play the fundamental role in assessing both of the axial and the flexural behaviors of the CFT columns.