Research On The Axial And Eccentric Behavior Of Self-compacting Recycled Aggregate Concrete Filled Steel Tube Stub Columns

As a new type of composite structure, self-compacting recycled aggregate concrete filled steel tube(SCRACFST) combines self-compacting concrete(SCC),recycled aggregate concrete(RAC) and concrete-filled steel tube(CFST) together,which makes full use of their advantages and overcomes their shortcomings.Therefore, the SCRACFST has important engineering application value. In this dissertation, the method of mix design,the workability and the mechanical properties of high strength self-compacting recycled aggregate concrete(SCRAC)were studied, and the axial and eccentric behavior of SCRACFST stub columns were also studied. The main purpose of this dissertation is to provide the theoretical basis and scientific support for the practical application of the SCRACFST composite structure. The main contents of this dissertation are as follows:(1) Preparation of the high strength self-compacting recycled aggregate concreteBased on the fixed aggregate volume fraction method and the free water-cement ratio method, a mix design procedure for the high strength SCRAC was suggested. The initial mix proportion of high strength SCRAC was determined according to the mix design procedure suggested in this dissertation. According to the results of the workability test and the strength test, the final mix proportion was determined by making an alteration to the initial mix proportion.The high strength SCRAC was prepared using the recycled coarse aggregate instead of the natural coarse aggregate. The compressive strength of high strength SCRAC reaches strength grade C60, and the workability of high strength SCRAC meets the requirements of grade 2 self-compacting performance.(2) Research on the workability of the high strength self-compacting recycled aggregate concreteIn order to study the effect of recycled aggregate replacement ratio on the workability of high strength SCRAC, the slump flow test, the slump flow time T500 test and the U box test were carried out. The test results show that the flowability,filling ability and passing ability of high strength SCRAC all decrease with the increase of recycled aggregate replacement ratio. The test results also show that the viscosity increases with the increase of recycled aggregate replacement ratio,and the recycled aggregate has no significant effect on the segregation resistance.The workability of high strength SCRAC with different recycled aggregate replacement ratio can meet the requirements of grade 2 self-compacting performance, which can be used in normal reinforced concrete structures.(3) Research on the mechanical properties of the high strength self-compacting recycled aggregate concreteAn experimental study was conducted to investigate the effect of recycled aggregate replacement ratio on the mechanical properties of the high strength SCRAC, such as compressive strength, tensile strength and elastic modulus. The test results show that the failure mode of the high strength SCRAC under compression and tension are similar to those of ordinary concrete. The test results also show that the cubic compressive strength, the axial compressive strength, the splitting tensile strength and the elastic modulus of the high strength SCRAC decrease with the increase of recycled aggregate replacement ratio, the peak strain of the high strength SCRAC increase with the increase of recycled aggregate replacement ratio, and the recycled aggregate replacement ratio has no significant effect on the Poisson’s ratio of the high strength SCRAC. Based on the experimental data, the conversion formulas of the cubic compressive strength, the axial compressive strength,the splitting tensile strength,the elastic modulus and the peak strain of the high strength SCRAC were suggested.(4) Experimental investigation of the axial behavior of the self-compacting recycled aggregate concrete filled steel tube stub columnsAn experimental study was conducted to investigate the axial behavior of the SCRACFST stub columns. The effects of recycled aggregate replacement ratio and steel ratio on the mechanical behavior of the axially loaded SCRACFST stub columns were studied. The test results show that the failure mode of the axially loaded SCRACFST stub columns are plastic failure. The test results manifest that the mechanical properties of the SCRAC are improved as a result of the confinement provided by steel tube, and the confinement effect of circular steel tube is better than that of square steel tube. The test results also demonstrate that the recycled aggregate replacement ratio has no significant effect on the load-carrying capacity of the axially loaded SCRACFST stub column, but has significant effect on the peak strain of the axial loaded SCRACFST stub column.The test results also show that the steel ratio has significant effect on the load-carrying capacity and the peak strain of the axially loaded SCRACFST stub column. Based on the unified theory, the superposition method and the limit equilibrium theory, the empirical formulas for predicting the load-carrying capacity of the axially loaded SCRACFST stub columns were suggested respectively.(5) Experimental investigation of the eccentric behavior of the self-compacting recycled aggregate concrete filled steel tube stub columnsAn experimental study was conducted to investigate the eccentric behavior of the SCRACFST stub columns. The effects of recycled aggregate replacement ratio,eccentricity ratio and steel ratio on the mechanical behavior of the eccentrically loaded SCRACFST stub columns were studied. The test results show that the eccentrically loaded SCRACFST stub columns are failure due to lateral deflection.The test results indicate that the recycled aggregate replacement ratio has no significant effect on the load-carrying capacity of the eccentrically loaded SCRACFST stub columns, but has significant effect on the peak deflection of the eccentrically loaded SCRACFST stub columns. The test results also show that the eccentricity ratio and the steel ratio have significant effects on the load-carrying capacity and the peak deflection of the eccentrically loaded SCRACFST stub columns. Based on the unified theory and the empirical coefficient method, the empirical formulas for predicting the load-carrying capacity of the eccentrically loaded SCRACFST stub columns were suggested respectively.(6) Numerical investigation of the axial and eccentric behavior of the self-compacting recycled aggregate concrete filled steel tube stub columnsAn equivalent stress-strain relationship model of the SCRAC was proposed.In order to simulate the axial and eccentric behavior of the SCRACFST stub columns,a number of three-dimensional nonlinear finite element models were developed. A parametric modeling program was developed using the Abaqus scripting interface and Python language. The parametric modeling program can not only realize the parameterization of finite element model, but also proceed to the modeling automatically. The experimental work and the nonlinear finite element analysis reveal the mechanical behavior of the SCRACFST stub columns under axial compression and eccentric compression respectively. The axial behavior of the SCRACFST stub columns can be divided into elastic stage, elastis-plastic stage and destructive stage. The eccentric behavior of the SCRACFST stub columns can be divided into elastic stage, elastic-plastic stage and descending stage.