Abstract

Concrete-filled steel tubular (CFST) members are widely used in the field of marine engineering. The members may be subjected to a combination of various complex external forces. The study on the mechanical properties of the steel tube-core concrete interface is beneficial for a better understanding of the composite mechanism of the CFST structure. The bond stress at the steel-concrete interface of CFST members generally consists of 3 parts: chemical adhesion, micro-interlocking, and macro-interlocking. In this study, push out test of steel plate-concrete specimens with applied normal pressure and CFST members are carried out. Full-histories of loading behaviors have been studied and the bond mechanism has been comprehensively analyzed. The results show that the macro-interlocking has a great contribution to the ultimate bond strength and residual bond strength of concrete-filled steel tube.

Introduction

Currently, CFST structures are getting increasingly used in the field of marine engineering, for example, floating foundations for offshore wind turbines (Wang et al. 2020), costal electric transmission towers (Hou et al. 2016), and submarine pipeline structures (Wang and Han, 2018). The CFST members used for marine structures are possibly subjected to combinations of wave, current, wind and hydrostatic pressure (Wang et al. 2021). However, the studies on the effect of marine loading features such as external hydrostatic pressure on the composite actions of CFST members are somewhat limited, compared with those under onshore loading scenario. In order to clarify the combined effect between steel tube and concrete, and to build the finite element model of CFST structures accurately, it is necessary to conduct in-depth research on the mechanical properties of the steel tube-core concrete interface.

There are many experimental studies on the steel-concrete bond behaviors of CFST members. In fact, in the field of civil engineering, there have been many researches on the bond performance of steel tube-core concrete interface. The most common test method is the push-out test. The schematic diagram of the device is shown in Fig. 1. By pushing out the concrete in the steel tube, the reaction force and the displacement of the concrete relative to the steel tube are recorded, the ultimate bond strength and the residual bond strength of the interface are analyzed. Virdi et al. (1980) carried out a series of experiments to study the effects of concrete compressive strength, slenderness ratio (length to diameter ratio), diameter-to-thickness ratio, concrete curing time on steel tube-core concrete interface. Shakir et al. (1993a and b) studied the influence of the contact length of the steel tube-core concrete interface and the surface roughness of the steel tube on the ultimate bond strength. Roeder et al. (1999) summarized the previous research results and carried out the push-out test of 20 specimens. In the study, the influence of concrete shrinkage and eccentric loading on the interface performance of CFST specimens was discussed. An empirical formula for the influence of the steel tube diameter-thickness ratio (d/t) on the ultimate bond strength was proposed. Xu et al. (2009) filled expansive concrete into steel tube and studied the influence of concrete expansion on the bond properties. Experiments showed that the normal pressure of the interface of CFST caused by concrete expansion can greatly improve the bond properties of the interface. Aly et al. (2010) conducted push-out test of 14 CFST specimens, and studied the influence of the concrete strength and concrete age on the bond performance. Tao et al. (2016) considered bond properties of concrete filled stainless steel tubular (CFSST) members, and experiments showed that the ultimate bond strength of stainless-steel concrete interface was lower than that of ordinary steel concrete interface. The above experimental studies mainly focus on the influence of different factors on the ultimate bond strength. It is more appropriate to understand and explain the causes of interface bond stress from the mechanical mechanism.

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