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This paper presents experimental and analytical studies on long-term behavior of square CFT columns under central axial loading. Two loading cases are considered; (1) the load applied only at the inner concrete of the column and (2) the load applied simultaneously on both the concrete and the steel tube. Four specimens of square CFT columns were tested under the two loading cases, and basic creep test for two concrete specimens was performed to find out the creep properties of the inner concrete. Three-dimensional finite element analysis models were established and verified with the experimental results. The verification shows that the prediction for the long-term behavior of actual square CFT columns is possible from the three dimensional finite element modeling considering the bond behavior between steel tube and inner concrete. Also, experimental results and numerical calculations revealed that the bond stress Induced by the confinement pressure as well as the slip between inner concrete and steel tube were increased with time In the first loading case. However, the confinement by the loading Plate was decreased with time while increasing confinement effect by the steel tube was observed over time. In contrast no confinement effects occur in the second loading case.

Poisson's ratio due to multiaxial creep of concrete reported by existing experimental works was controversial. Poisson's ratio calculated from measured strain is very sensitive to small experimental error. This sensitivity make it difficult to find out whether the Poisson's ratio varies with time or remain constant, and whether the Poisson's ratio has different value with stress states or not. A new approach method is needed to resolve the discrepancy and obtain reliable results. This paper presents analytical study on multiaxial creep test results. Microplane model as a new approach method is applied to optimally fitting the test data extracted from experimental studies on multiaxial creep of concrete. Double-power law is used as a model to present volumetric and deviatoric creep evolutions on a microplane. Six parameters representing the volumetric and deviatoric compliance functions are determined from regression analysis and the optimum fits accurately describe the test data. Poisson's ratio is calculated from the optimum fits and its value varies with time. Regression analysis is also performed assuming that Poisson's ratio remains constant with time. Four parameters are determined for this condition, and the error between the optimum fits and the test data is slightly larger than that for six parameter regression results. The constant Poisson's ratio with time is obtained from four parameter analysis results and the constant value can be used in practice without serious error.

The majority of research studies and prediction models on the creep of concrete have been concerned with the uniaxial stress state. Research on creep under multiaxial stresses, however, has been relatively scarce even though concrete is subjected to multiaxial stresses in many structural members and structures. Although some experiments on the multiaxial creep of concrete have been performed, the results of these experiments, which mainly considered low-strength concrete, are controversial. Consequently, for today’s standards and practice—and for a better understanding of the properties of concrete—more experimental data are needed on the multiaxial creep of concrete with various strengths. In this study, multiaxial creep tests with 27 cubic specimens (200 mm 3 200 mm 3 200 mm) were carried out for three different strengths of concrete, namely 26, 44 and 54 MPa, and creep strains were measured in three principal directions. From the measured strains, Poisson’s ratio at initial loading was obtained, as was Poisson’s ratio resulting from creep strain and Poisson’s ratio resulting from the combined creep strain and elastic strain. These Poisson’s ratios were approximately equal for each concrete mix. The Poisson’s ratio at initial loading and the Poisson’s ratio for the combined strain increased slightly as the strength of the concrete increased. The volumetric creep strain and deviatoric creep strain were linearly proportional to volumetric stress and deviatoric stress, respectively.