Size Effect on Confining Force of Circular Concrete Filled in Steel Tube under Axial Compression

Abstract

Size effect is one of the principal mechanical properties of concrete under uniaxial compression. Size effect on compressive strength of concrete has been investigated and provided the corresponding rules of the size effect. However, size effect on confining action on concrete subjected from steel tube has been not sufficiently studied. To fill the gap, size effect on the confining stress of concrete filled in steel tubular under axial compression was discussed. 4 groups of circular short columns of concrete filled in steel tube were conducted under monotonic axial compression. The diameters of the specimens were 219mm, 426mm, 630mm, and 820mm respectively. The ratios of diameter to thickness of the specimens were 88. The grade of concrete was C30. Damage processes of the specimens with different sizes were described. The final failure modes of all specimens were shear failure. Based on the calculation method of steel tube stress derived by Gu’s, horizontal and longitudinal strain of steel tube obtained from each measuring point on the specimen during the test was transformed into the corresponding horizontal and longitudinal stress on the surface of steel tube respectively. Then, the average of the steel stress of all measuring points arranged on one specimen was taken to behalf the stress of overall steel pipe in longitudinal and lateral directions. Therefore, the confining stress when the peak load was achieved was obtained based on the average lateral stress of the steel tube. The analysis results indicated that when the peak load was achieved, the ratio of the vertical stress to the yield strength of the steel tube was enlarged with the diameter of the specimen increased; the ratio of the transverse stress to the yield strength of the steel pipe decreased with increment of the diameter of the specimen; the confining force provided by steel tube to the core concrete also reduced with the enlargement of the size of the column. All the variation degree gradually slowed down when the size of the column enlarged. The confinement action to the concrete existed size effect. According to the test results, calculation model of confining force considering the DOI 10.21012/FC9.103 Z. B. Li, W. J. Wang and Z. Y. Tang 2 size effect was built, and results calculated by the model were in good agreement with experimental results.