Experimental Research And Analysis On The Interface Conditon Of Concrete-Filled Steel Tubular

Concrete-Filled Steel Tubular can adapt to the progress of long-span, erect modern engineering structures and meet the need of withstand hash condition. It has become the forth structure come after the masonry structure、concrete structures、steel structure. It has broad prospects for development, and it is an important direction of development of the science of structural engineering.CFST structure is the composite structure of steel tube and concrete. Because of its particular material combining forms, CFST, on the one hand results in the improving effort with their mutual constrain and complementary, on the other side derives the complex interface properties between steel tube and concrete. The engineering practices prove that, under the complicated influence of external circumstances, the interface between steel tube and concrete in CFST structure may debond which absolutely affects the workability and structure safety of CFST. Therefore, it is important and necessary to investigate the interface bonding properties of CFST and its optimizing methods either in theoretical research or engineering practice.In this paper, theoretical analysis, experimental study and finite element method were used to analyze the status, properties and characteristics of interface conditions in CFST under the effect of thermal-mechanical coupling, including:experimental study on the interface characteristics of CFST under axial compression load, the qualitative analysis on the interface bonding condition of CFST measured by ultrasonic, the interface characteristics of CFST studied by homemade-stressing box, expanding properties of CFST and its interface performance optimization, axial compressive test of self-stressing CFST and the process and interface circumstance also analyzed by finite element software ANSYS, thermal shock test, thermal-mechanical coupling test, etc. The main works and results of the research are follows:1. The axial compressive test of CFST was carried out in this paper. The cross-section form of CFST, radius-thickness ratio (21～42), aspect ratio (3)、ferrule coefficients and other parameters were determined in order to investigate the interface characteristics and stressing states of CFST under different axial loads. The result shows that:the CFST specimens with the incorporation of expanding agent show better axial bearing capacity and distortion-resistant ability, and under axial compression the force condition of CFST can be obtained by measuring the strain changes, also with further optimizations the optimal compound of steel tube and concrete can be achieved.2. Ultrasonic, a kind of qualitative test method, was used in this research to study the interface characteristics of CFST.4CFST specimens (dense reference specimen, void specimen, debonding specimen and central hollow specimen) and27standard concrete bricks were made to test their related parameters:sound velocity, amplitude, frequency, and waveform of CFST and the concrete specimens in different curing stage:2d,3d,5d,7d,11d,14d,21d,28d,60d respectively, and the compressive strength of concrete specimens. According to sound wave propagation theory, the comprehensive evaluation method of ultrasonic on the interface characteristics of CFST are analyzed and proposed.3. Quantitative experimental study on the interface characteristics of CFST were completed by self-made stressing box. The restrained expansion of CFST and free expansion of concrete specimens were studied with different content of incorporating expanding agent (5%,8%,12%,16%and20%). Meanwhile, the rational restrained expanding scope of the core concrete was got by analytic method.4. Axial load test of self-stressing CFST, finite element analysis and interface characterization were carried out. The mechanical properties and interface characteristics during the loading process of CFST were studied, containing different contents of expanding agent:5%,8%,12%,16%and20%, respectively. The experimental and analysis results show that:in self-stressing CFST, at the initial set the core concrete is under the three-dimensional pressure, effectively confirming the bonding interface conditions of steel tube and concrete, compensating the shrinkages of core concrete that will result in disengaging, improving the ductility, ultimate bearing capacity and non-deformability of CFST. In this experiment, the optimal incorporation of expanding agent is12%, and the ultimate bearing capacity of CFST values improved by25%than theoretical calculated values.5. Temperature shock test, thermal-mechanical coupling test and finite element analysis were researched. The sectional temperature and interface stress distribution of CFST was received by thermal-mechanical coupling test of total5specimens with axial load process under5different temperatures of20℃,40℃,60℃and80℃, respectively, and its results also calculated by the heat conduction theory, theory of thermoelasticity, the nonlinear finite element method and the virtual temperature field, etc.In this paper, On the basis of theoretical studies, Series studyexperimental researches were carried out as well as finite element analysis and comparison,which are focused on the state of the composite structure interface, qualitative and quantitative testing techniques, reasonable domain of the core concrete expansion properties, thermal coupling test method.The interface characteristics of CFST under the complicated circumstances (thermal, force, etc) were systematic investigated, and the results has a certain theoretical direction and practical reference value to the interface optimization and the long-term performance in the harsh conditions of CFST.