Research On Mechanical Behavior Of Self-stressing And Self-compacting Concrete Filled Steel Tube

In the recent decades, the concrete filled steel tube (CFST) is more and more widely used in construction of projects. The function of concrete and steel tube can be fully exerted in concrete-filled steel tube structure, which shows excellent mechanical performance. But the structure of CFST still has some drawbacks. For example, it is difficult to tamp after pouring, shrinkage and creep in concrete hardening will decrease the restraining force. The technology of self-stressing concrete and the technology self-compacting concrete are applied to CFST that can make concrete core uniform and compact. In the same time, Self-stressing concrete core produces volume expansion while hydrating. Concrete core restrained by steel tube is compressed in three directions. Compared with ordinary CFST, the mechanical performance of self-stressing concrete-filled steel tube is improved.As a part of the project of theoretical research on self-stressing concrete and self-compacting concrete filled steel tube (NO.50578027) funded by National Natural Science Foundation, the specific investigation are carried out as follow:(1) Different mixture proportion of self-stressing and self-compacting concrete is successfully compounded. The test results show that the strength of self-stressing concrete made of sulphoaluminate expansive cement develops very fast in early age and release much heat. The compactness and uniformity of the concrete are good. The performance of self-stress concrete under free expansion, confined expansion is tested and to find their influencing factors. The results indicate that the expansion of self-stress concrete has a close connection with the mixture proportion and the proportion of cement is the main factor. Mixture proportion of cement, steel ratio and slender ratio affect self-stress greatly.(2) The push-out tests of seventeen self-stressing concrete filled circular steel tubes and three normal concrete filled circular steel tubes show that the strength of bond between self-stressing concrete and steel tube is much higher than the ordinary concrete and steel tube. Self-stress can improve the condition of the interface between concrete and steel tube, and with the self-stress increasing, the bond strength become higher.(3) 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 are carried out. The results indicate that the flexural capacity of the specimens is influenced by the initial stress. With the initial stress increasing, the flexural capacities are enhanced; Initial stress has no apparent influence on the flexural stiffness of the specimens.(4) The pure bending test specimens are numerically simulated by ANSYS program. It simulates the expansion of the concrete by heating concrete in steel tube and gets the same results with the test about the self-stress distribution. But the ratio of longitudinal stress and latitudinal stress has a little difference with the test results. The calculating results indicate that self-stress has little influence on the flexural behavior of concrete filled steel tube. Compared with the test results of chapter 4, we can draw a conclusion that: self-stress can improve the material performance of the concrete in the steel tube and then improve the flexural behavior of the whole specimen.