Research On Bond Behavior Of Steel Tube Column Filled With Steel Fiber Reinforced Self-stressing Recycled Aggregate Concrete

As a type of environmentally-friendly building material,recycled aggregate concrete(RAC),with great application value,can reduce the pollution of construction waste landfill and can reduce the consumption of natural stone.In recent years,recycled aggregate concrete has been combined with steel tube(RCFST),to enhance its compressive behavior.As the foundation of concrete filled steel tubular structure,the bond behavior will be weakened by the dissatisfied mechanical properties of recycled aggregate,which limited the engineering application of recycled aggregate concrete.The author have made some efforts to enhance the bond behavior by using steel fibers and self-stressing cement.In this new structure,which is called steel fiber reinforced self-stressing recycled aggregate concrete-filled steel tube,steel fibers can resist the development of the cracks and the self-stressing cement,which can produce the selfexpansion behavior of concrete under the confinement of steel tube,can produce prestress to increase the contact area between recycled concrete and steel tube.Based on the quartic push-out tests of 3 CFST columns,16 steel fiber reinforced RCFST columns and 17 steel fiber reinforced self-stressing RCFST columns,this paper studies the bond behavior of RCFST column and the influence of the strength of concrete,the diameter-thickness ratio of steel tube,recycled aggregate replacement rate,steel fiber volume percentage and self-stress.The following conclusions are drawn:(1)Mix radio design and material properties research of steel fiber reinforced selfstressing recycled aggregate concreteBased on the theory of free water cement ratio,steel fiber reinforced recycled aggregate concrete and steel fiber reinforced self-stressing recycled aggregate concrete have been successfully compounded.The result of mechanical test shows that with the increasement of recycled aggregate replacement rate,the compressive strength decreases.With the dosage of steel fiber increases,the compressive strength of RAC increase gradually,but the increasement become not obvious when the dosage of steel fiber reached 1.8%.The self-stress increases with the drop of free water cement ratio or the diameter-thickness ratio of steel tube and drops with the increasement of the dosage of steel fiber.(2)The quartic push-out testsA total of 36 columns are tested in the quartic push-out tests,including 3 CFST columns,5 RCFST columns,11 steel fiber reinforced RCFST columns and 17 steel fiber reinforced self-stressing RCFST columns.The deterioration process of bond behavior showed by RCFST columns can be divided into three stages: chemical adhesion stage,interlocking stage and friction resistant stage.In the thirdly push-out test,,the load increases gradually with the increasement of slippage because of the vertebral body effect.The axial strain of steel tube has a triangular distribution along the longitudinal direction,which shows that bond strength has a uniform distribution in the quartic push-out test.(3)The influence of variables consideredThe variables considered in the test are the strength of concrete,the diameterthickness ratio of steel tube,recycled aggregate replacement rate,steel fiber volume percentage and self-stress.The increasement of the strength of concrete can enhance the bond strength,but has little influence on the degeneracy rate.When the recycled aggregate replacement rate the diameter-thickness ratio decrease,bond strength increases obviously and the degeneracy rate drop.Using the steel fibers can obviously enhance the bond behavior of RCFST columns but when the steel fiber volume percentage reaches 1.8%,the enhancement begins to stagnate.(4)The mathematical description of the bond strength in firstly push-out testBecause of the lack of relevant parameters,other formula cannot show the comprehensive influence of the strength of concrete,the diameter-thickness ratio of steel tube,recycled aggregate replacement rate,steel fiber volume percentage and selfstress.Base on the test result,the mathematical descriptions of the bond strength in firstly push-out test of steel fiber reinforced RCFST column and steel fiber reinforced self-stressing RCFST column have been made.The predictions of the formulas greed well with the test results and can reflect the influence of the variables considered.