| The function of concrete and steel tube can be fully exerted in concrete-filled steel tube (CFT) structure, CFT shows excellent mechanical performance. Whether concrete is fully, uniformly and compactly filled the whole steel tube will influence the mechanical performance of concrete-filled steel tube distinctly, and then influence the safety of the whole structure. However, it is difficult to guarantee the construction quality of concrete core while constructing, so that the accident happens frequently. In addition the creep and shrinkage deformation of concrete core will increase under sustained load, it can cause stress redistribution of concrete and steel tube, sometimes a local failure of steel tube occurs when the stress increases to a high level. The technology of self-stressing concrete and self-compacting concrete is applied to CFT in this paper. Self-compacting concrete with high flow ability and segregation resistance can guarantee concrete core uniform and compact, and overcome the shortcoming of arch and long column construction difficulty. Self-stressing concrete core produces volume expansion while hydrating. Concrete core restrained by steel tube is compressed in three directions. The lateral confinement of steel tube is fully utilized. Compared with conversional CFT, the mechanical performance of self-stressing concrete-filled steel tube is improved. Existing research indicates that expansion and self-stress produced by self-stressing concrete can remain stable for a long time, so the confinement degradation caused by creep and shrinkage in conversional CFT can be prevented using self-stressing concrete in CFT; Meanwhile, the creep deformation of self-stressing concrete core under accepted load is smaller than plain concrete, so the deformation of steel tube will keep together with self-stressing concrete in a long-term, the carrying capacity of self-stressing concrete filled steel tube will be stable in a long-term too. The mechanical performance of self-stressing concrete filled steel tube under axial load is researched through test and theoretical analysis. The major results are summarized as follows:1. The early age mechanical performance and expansive performance of self-stressing concrete is researched through test. The result of the test shows: self-stressing concrete with self-compacting ability can be obtained through adjusting mixture proportions. The strength of self-stressing concrete made with sulphoaluminate expansive cement develops very fast in early age. Compressive strength at 1 day can be up to more than 50% of compressive strength at 28 days; elastic modulus at 1 day can be up to more than 80% of elastic modulus at 28 days. Theself-stress of concrete restrained by steel tube can reach to 3~6MPa. It is found that the creep and elastic deformation can occupy about 2/3 of effective expansive deformation; these two parts of deformation can't be ignored in calculating. The calculating formula for effective expansive deformation varied with time is proposed in this paper, creep deformation is considered in the formula. On this basis of test results, the relationship of effective expansive deformation to restrained expansive deformation in different steel area to concrete area ratio is obtained through regression analysis. The results of calculation accord with the results of the test better. The calculating formula for restrained expansive deformation proposed in the paper can calculate restrained expansive deformation in different age. The initial self-stress can be obtained through internal force equilibrium condition.2^ The effect of initial stress on the mechanical performance of self-stressing concrete filled steel tube short column is researched through test. The factors that have effect on ultimate strength of short column, such as compressive strength of concrete, the area of steel tube to concrete core ratio, self-stress level, is discussed here. The result of the test shows: because of the effect of initial self-stress, the elastic working range of short column is extended to 90% of ultimate strength; the ultimate strength has improvement too. The concept of self-stress level is introduced in this paper, self-stress level is defined as initial self-stress to concrete strength ratio, how the self-stress level has effect on the ultimate strength is analyzed .The result of calculation indicates: the relationship of ultimate strength increment to self-stress level can be expressed by quadratic parabola. The summit of curve is optimal self-stress level, namely, the ultimate strength can be improved to the maximum extent when self-stress level reach to the optimal self-stress level. Based on the test result, the optimal self-stress level can be calculated, which is 0.116. With the deformation compatibility conditions and equilibrium conditions, the numerical analysis model is established using integral method. Load-deformation curves of self-stressing concrete filled steel tube short column is simulated; it is help to understand the working mechanism of self-stressing concrete filled steel tube short column.3. The effect of initial stress on the mechanical performance long column is researched through test. The factors that have effect on ultimate strength of long column, such as slenderness ratio, self-stress level, are disgussed. The result of the test shows: Because long column failed in different form, so initial stress has different influence on the mechanical performance of long column. The long column that happened stability disruption is still in elastic working range before failure. The function of initial stress is not fully exerted; initial stress has little influence on the mechanical performance of long column. The long column happened strength disruption, it is because concrete core is squish and the confinement of steeltube is lost, the function of initial stress is fully exerted. Compared with conventional CFT, the carrying capacity of self-stressing concrete filled steel tube can be improved about 31%. Slendemess ratio is still the main factor that influences the mechanical performance of self-stressing concrete filled steel tube. Based on test results, the relation of carrying capacity increment to slendemess ratio is obtained through regression analysis. The carting capacity of self-stressing concrete filled steel tube can be obtained on that.4.. Test on the long-term mechanical performance of self-stressing concrete filled steel tube under axial load is carried on. The effect of initial stress on the mechanical performance in long term is researched. The result of the test shows: Though the initial lateral stress can restrain the development of axial creep deformation, compare with conventional concrete core that bears the same load, the axial initial stress will increase stress ratio, the creep deformation of self-stressing concrete core will greater than conventional concrete core, but it is smaller than plain concrete. The ACI creep model and BP-KX creep model is applied in this paper. With deformation compatibility conditions and equilibrium conditions, the numerical model is established using age-adjusted effective modulus method, the calculating results agree with test results well. How self-stress level has effect on the mechanical performance of self-stressing concrete filled steel tube is analyzed on the basis of numerical model, so with concrete compressive strength and the area of steel tube to concrete core ratio. The coefficient of stress redistribution is deduced in the paper, the change of load that steel tube carried can be calculated through it, so the long-term carrying capacity can be calculated on the basis of the confficient. The formula reflects the factors that have effect on the long-term carrying capacity of self-stressing concrete filled steel tube.A new approach that self-stressing concrete can be applied is put forward in this paper. The expansive energy of expansive cement is fully utilized. The calculating theory of self-stressing concrete restrained by steel tube is solved. It makes a good foundation for self-stressing concrete applied in concrete-filled steel tube. The confinement degradation of steel tube cause by creep of concrete core is solved too, at the same time, the carrying capacity degradation of steel tube concrete in long-term is solved effectively too. Self-compacting concrete can guarantee the construction quality of concrete core, the difficult problem in construction can be easily solved. Test and theoretical analysis on this kind of CFT is carried on, it provides useful data for this new kind of CFT applied in actual proj ect.
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