Research On Self-stressing And Self-compacting Concrete Filled Steel Tube Structures

Self-stressing concrete filled steel tube member (SCFST) is a new type concrete filled steel tube (CFST) structure. For SCFST members, the expansive concrete mixes are used to cast in the steel tube instead of the ordinary concrete. Comparing with the CFST members, SCFST ones have some new characteristics. It guarantees the construction quality of concrete core due to he high work ability of the concrete mixes. Microstructure of concrete core is more compact because the expansive behavior is confined by steel tube. Such expansion can result in good bond at concrete-steel interface, which can ensure the concrete and steel to work together. Because of high expansive energy, higher self-stress can generate in the steel tube, which can offer confining pressure for concrete core. The SCFST members improve the mechanical performance of CFST structure. It is a self-development and self-perfection of CFST structure.Based on the National Natural Science Foundation of China (No 50578027), following work has been carried out in this paper(1)Sulfoaluminate-type expansive cement is used in this experiment. The expansive behavior of expansive concrete is systemically investigated. The influencing factors, such as concrete mixes, dimensions of steel tubes, external confined style and so on are discussed. This paper plays an emphasis on the expansive behavior of SCFST members. Based on the experimental observation, this paper established an three dimensions finite element model to investigate the expansive process of SCFST member. The calculated results have a good agreement with the test results.(2)Based on the our past experimental work, mechanical performances of 12 SCFST columns with higher confined coefficient are tested. An analysis-separation method is used to analyses the test data, and then a constitutive relationship for the concrete core of SCFST column under axial load is proposed.(3) Based on a bounding surface model of the concrete and elastic-plasticity constitutive relationship of steel, this paper establishes the finite element model of the self-stressing concrete filled steel tube short column under axial load and eccentric load. the mechanical performances are analyzed. The calculated results indicate that the self-stressing concrete-filled steel tube short column has higher carrying capacity than common concrete-filled steel tube short column duo to self-stress. The carrying capacity of self-stressing concrete-filled steel tube short column increases with increasing self-stress. Steel ratio of the specimen is the principal factor which affects the shape of the load-strain curve.(4) The expansion behaviors and the bond carrying capacities of self-stressing concrete filled short steel tube (SSCFST) columns are experimentally investigated. For comparing purpose, conventional concrete filled steel tube (CFST) specimens are also used in this experimental programme. The results indicate that both concrete mixes and dimensions of the steel tube have important influence on self-stress in the concrete core, which is a sensitive parameter to the bond strength as well as the load-slip relationship. Present studies also indicate that SSCFST columns have much higher bond strength than do CFST columns and this recommends a new method to improve the bond strength of composite structures. Finally, an empirical equation for predicting the bond strengths of SSCFST columns is proposed.(5) In addition, the tests of twelve self-stressing concrete filled circular steel tubes and three normal concrete filled circular steel tubes subjected to pure bending were carried out. The results indicate that the flexural capacity of the specimens is influenced by initial stress. With the initial stress increasing, the flexural capacities were enhanced; Initial stress has no apparent influence on the initial flexural stiffness of the specimens. A three dimension finite element analysis is also made. The test results have a good agreement with the calculated results.(6) The results of shear tests on 33 SCFST specimens compared with 6 CFST specimens are also presented in this paper. The effects of self-stress, shear span to depth ratio and end plate on the shear performance of SSCFST short beam are studied. The experimental results indicate that SSCFST specimens have higher shear capacities than CFST specimens when compressive strength of the concrete core, yield strength and geometrical dimensions of the steel tube and shear span to depth ratio are kept same. Shear does not seem to be critical for a SSCFST/CFST specimen with a shear span to depth ratio of 0.5 or higher. The bond between concrete core and steel tube affects the shear capacities of SCFST/CFST specimens. Finally, an empirical equation for predicting the shear capacity of SCFST short beam is proposed(7) Influence of long-term load on mechanical performance of SCFST members are experimental investigated by 10 SCFST specimens. The ACI creep model is also used to analysis the long-term behaviors of these specimens. The test and calculated results indicate that long-term deformation of SCFST members are higher than CFST ones.