Research On Flexural Behavior Of Steel Fiber Reinforced Self-stressing Recycled Aggregate Concrete Filled Steel Tube

The engineering application recycled aggregate concrete(RAC)is limited due to its dissatisfied mechanical properties.In this paper,steel fiber was used to improve its mechanical properties.Combined with the advantages of concrete-filled steel tube composite structure(CFST),RAC was poured into the steel tube.The mechanical properties of RAC filled steel tube(RACFST)were further optimized by using self-stress cement to make RAC expand and be restrained.Based on this,a new type of green,environment-friendly and energy-saving composite structure called steel fiber reinforced self-stressing RAC filled steel tube(SSRACFST)was proposed,and the experimental research and theoretical analysis of the flexural mechanical properties of SSRACFST were carries out.The main work contents and research conclusions are as follows:(1)Considering the characteristic of recycled coarse aggregate,combining the characteristic of the mix design method of free water cement ratio,fiber reinforced concrete and self-stressing concrete,and adding the proper amount of steel fiber,and using self-stressing cement,steel fiber reinforced self-stressing RAC(SSRAC)was successfully prepared with good working performance and mechanical properties.With the increase of the replacement rate of RAC and the decrease of the volume of steel fiber,the compressive strength of RAC decreases gradually.Under the constraint of the steel tube,the initial longitudinal self-stress value of SSRAC reaches5.43MPa～9.42 MPa,and the initial circumferential self-stress value reaches3.22MPa～5.62 MPa.With the decrease of free water-cement ratio,volume of steel fiber and diameter/thickness ratio of steel tube,the longitudinal self-stress of SSRAC increases.(2)The flexural experimental study of a total of 54 specimens were carried out.The failure mechanism of steel tubular steel fiber self-stressed recycled concrete members under the condition of pure bending is studied.The variables considered were the wall thickness of steel tube,the strength grade of concrete,the replacement rate of RAC,the volume content of steel fiber and the initial self-stress value.The results show that the flexural behavior of SSRACFSTs is better than that of steel fiber reinforced self-stressing recycled aggregate concrete filled steel tube.The flexural bearing capacity and flexural stiffness of specimens are significantly increased withsteel tube wall thickness increasing.The increase of concrete strength increases the flexural capacity of SSRACFSTs.The increase of the replacement rate of recycled coarse aggregate will reduce the flexural capacity of SSRACFSTs.With the increase of the volume content of steel fiber,the flexural capacity of flexural members increases to some degree.(3)By using existing steel tube concrete discipline formula,the flexural capacity and stiffness of specimen were calculated.Compared with other codes,the results of flexural capacity calculation using EC4 code are more consistent with the test values.The theoretical formula for calculating the flexural capacity of SSRACFST is put forward,and the calculated results are in good agreement with those obtained with the test results.Compared with other codes,the calculation results of flexural stiffness calculated by AIJ code in the service stage are in better agreement with the test results,and a formula for calculating the flexural stiffness of SSRACFST is proposed.The calculated results are in good agreement with the test results,and the calculated results are relatively safe.