Experimental study on confining-strengthening, confining-stiffening, and fractal cracking of circular concrete filled steel tubes under axial tension

Abstract

This paper studies the mechanical behavior of the circular concrete filled steel tubes (CCFTs) under axial tension. A group of three CCFTs with different steel ratios and in-filled concrete diameters are tested and compared with hollow steel tubes (HTs). Based on the experimental results in the present and previous studies, three key issues of CCFTs under tension, i.e. the confining-strengthening effect, the confining-stiffening effect, and the tension-stiffening effect, are studied at both the stress-resultant level (i.e., force-displacement relationship) and stress levels (i.e., stress-strain relationship). Due to the tendency of the shrinking and slipping of the steel tube in relation to the infilled concrete, the confining contact stress and bonding shear stress, in the vertical and tangential direction respectively, are distributed in the interface between the steel and concrete, which results in the confining and bonding effect of the CCFTs. At the stress-resultant level, the test results demonstrate that the strength and stiffness of CCFTs are average 10% and 29% larger than those of HTs, respectively. The strength increase is contributed by the confining-strengthening effect, and the stiffness enhancement is a combined result of the confining-stiffening and tension-stiffening effects. At the stress-strain level, the measured strains show that the steel tube of the CCFT is in the state of the bi-axial tensile stress state because of the difference of the Poisson's ratio between steel tube and infilled concrete, which contributes to the confining-strengthening and confining-stiffening effects of the CCFTs. In addition, a fractal cracking phenomenon of the core concrete of tensile CCFTs is observed and discussed.