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

With the increasing popularity of bridges with large span and high buildings, the concrete filled steel tube (CFT) is more and more widely used in constructions of projects. But in many practical engineering applications, the structure of CFTs still has some drawbacks. For example, difficulty in tamping after pouting, decrease of restraining force caused by shrinkage and creep in concrete hardening, and etc. In order to improve the mechanical performance of CFT, this paper aims to advance a new structure model, called self-stressing and self-compacting concrete filled steel tube. It takes advantage of the techniques of concrete filled steel tube, self-stressing concrete and self-compacting concrete altogether. On the one hand, in this new composite structure, the self-expanding characteristic of self-stressing concrete is utilized to overcome the shrinkage and creep of concrete core, enhance structural stability. As the expansion grows big enough, an initial self-stress can begenerated to improve mechanical properties of CFTs. On the other hand, the self-compacting characteristic of concrete is utilized to avoid vibration and keep structural compactness. In cooperation with the project on Theoretical research on self-stressing concrete and self-compacting coneretefilled steel tube (No.50078008) funded by National Natural Science Foundation, the influences of initial self-stress on specimens subjected to axial compression load, eccentric compression load trod cyclic load are discussed. The specific investigations are carried out as follow:(1) A total of six batches of self-stressing and self-compacting concrete with different mixture proportions are successfully compounded. Through tests, it is found that the fluidity of these concrete compounded meets casting condition, whose compactness and uniformity of concrete core are also good.(2) Four batches of self-stressing concrete with different mixture proportions are tested under free expansion and another sixteen self-stressing CFTs with different mixture proportions, steel ratios and sizes are tested under confined expansion. The aim is to discuss the different performances of self-stressing concrete under free expansion, confined expansion, effective expansion and to outline their influencing factors. The hydration peculiarity of self-stressing concrete is analyzed from microscopic view by scanning electron. The experimental results show that free expansion of self-stressing concrete has a close connection with mixture proportion. The change of cement proportion and water-cement ratio can affect the result of free expansion. In CFTs, some factors that include mixture proportion of concrete, steel ratio, sectional size and slender ratio will affect self-stress and con_fined expansion considerably. The microstructure of self-stressing concrete is analyzed under different restrictive conditions. It is shown that the hydrated products are hardly different on free or restrictive conditions; but 90 days later, the structure is more concentrated and major ionic distributions are more homogeneous under a restrictive condition. Under a free condition, the ettringite is found along interracial zone between cement paste and limestone aggregate. With the improvement of restrictive condition, this phenomenon disappears gradually.(3) A total of eight self-stressing concrete filled steel tube short columns and one plain concrete filled steel tube short column subjected to axial compression are tested to discuss the whole process under axial load. Based on the results, the effect of the initial self-stress on mechanical properties of CFT is drawn. The test results show that in the whole process, concrete core is always under compression in all direction because of initial self-stress, which will lengthen elastic range, increase ultimate beating capacity evidently and reducedisplacement ductility factor of short column. In this paper, the concept of self-stress level is introduced. And the effect of initial self-stress is considered as the enhancement of concrete strength. The amplification coefficient method of concrete core is adopted in regression analysis, and there is a good agreement between simulation results and test results.(4) A total of twelve self-stressing CFT columns and two CFT columns are tested under eccentric compression loads to analyze the influence of the main parameters on mechanical properties. The main parameters include initial self-stress, concrete strength, confining parameter, eccentricity ratio and slenderness ratio. The results show that the initial self-stress changes the influence law of concrete strength and confining coefficient on load-deformation curves of CFTs. But the effect decreases with the increase in eccentricity ratio. The influence laws of eccentricity ratio and slenderness ratio on self-stressing CFT are the same as those onplain CFT. However, the enhancement on ultimate bearing capacity generated by initial self-stress is obvious. The values of which are increased by 10% from plain CFTs and also exceed calculated values subjected to standards. Calculating the self-stressing CFTs under eccentric compression load by means of the amplification coefficient method, it can be found that calculated results is closer to experimental results, which validate this method.(5) Based on the amplification coefficient method of concrete core, the model of the self-stressing CFT is established. Its aim is to analyze load-deformation curves of specimens under axial compression and eccentric compression. By combining test data and the simulation result, the feasibility of this method are evaluated. The results show that the load-deformation curves simulated is closer to the test curves. The curves simulated can reflect displacement characteristics of specimens on the whole and the ultimate beating capacity can be calculated accurately. (6) A total of eight self-stressing CFT columns and three plain CFT columns are tested under cyclic load. The test results show that the self-stressing CFT column and the plain CFT column all express a good seismic-resistant behavior. And the failure form is not altered by initial self-stress. Many main factors that include axial load ratio, initial self-stress, concrete strength, steel ratio and different loading ways have significant effect on skeleton curve, stiffness degeneration, energy dissipation and ductility. Among the main factors, the change of the initial self-stress can induce enhancement of ultimate horizontal load, initial rigidity, and ductility. But it will improve the rate of rigidity degeneration and decrease energy consumption and equivalence damping ratio.