| Reinforced concrete（RC）bridge piers which have been built and served for several decades were usually inevitably subjected to corrosion,collisions or natural disasters,resulting in damage or functional degradation.In order to ensure the safety of bridge operations,this type of RC piers urgently need to be strengthened.Steel jacketing is one of the universal strengthening methods,which can effectively improve the mechanical behavior of RC bridge piers.Using this strengthening method to strengthen the RC pier,that is,the steel tube is sheathed on the outside of the original RC pier,and the filling concrete is cast in the narrow gap between them to form the strengthened structure.Predecessors have carried out some researches on the axial compressive behavior of steel jacketing strengthened columns.However,the preloading of the original RC column was neglected by most researches,and it is unreasonable to use the test method of the original RC column,the steel tube and the filling concrete under full-section compression.In addition,to make full utilization of the excellent mechanical behavior of steel jacketing strengthened column,it is very necessary to find more excellent grouting materials of filling layer.Therefore,a new strengthening method called self-compacting and micro-expanding concrete filled steel tube（SMCFST）confinement method was proposed in this thesis,and a study on the axial compressive behavior of SMCFST confined columns was conducted.Especially,the preloading of the original RC columns and the local compression of SMCFST confind columns were focused,which reflected the actual situation after the RC bridge piers have been strengthened.The main works were carried out in this thesis as following:1)Fifteen test columns were designed and fabricated,and an axial compressive test was conducted.The test parameters included the steel tube thickness,the SMC strength,the filling concrete type,the cross-section type of RC column,the confinement method,the initial axial preloading and the eccentricity.The axial compressive behavior of the test columns was investiaged through six aspects including failure mode,load-vertical deformation curve,load-strain curve,ultimate bearing capacity,ductility and initial stiffness.The research results showed that the ultimate bearing capacity,ductility and initial stiffness of the RC columns were significantly improved after being strengthened by SMCFST confinement method.The ultimate bearing capacity and ductility of the SMCFST confined columns were substantially increased with the increase of the steel tube thickness.The increase of the SMC strength led to the slight enhancement of the ultimate bearing capacity and initial stiffness of the SMCFST confined columns.The type of filling concrete,the cross-section type of RC column and the initial axial preloading had little effect on the axial compressive behavior of the SMCFST confined columns.The ultimate bearing capacity the SMCFST confined columns were significantly decreased with the increase of the loading eccentric distance.2)The finite-element model of the SMCFST confined column was established using finite-element software ABAQUS to further explrore the axial and eccentric compressive behavior of SMCFST confined columns.The validity and accuracy of the finite-element model were verified through three aspects including failure mode,ultimate bearing capacity and load-vertical deformation curve.The parameter analysis was carried out with the expansion parameters including the steel tube thickness,the steel tube yield strength and the initial axial preloading.The research results showed that the increase of the steel tube thickness and the steel tube yield strength significantly improved the ultimate bearing capacity of the SMCFST confined columns.The increase of the initial axial preloading could improve the initial stiffness of the SMCFST confined columns.3)The simplified calculation formulas for the axial compressive ultimate bearing capacity of the SMCFST confined columns were derived or modified referring to the calculation theories of concrete-filled steel tube and related researches results.The feasibility and accuracy of the calculation formulas were compared and analyzed.The research results showed that the modified formula based on GB 50936-2014 "Technical code for concrete filled steel tubular structures" gave an accurate and stable prediction of the axial compressive ultimate bearing capacity of SMCFST confined column,so it was recommended to choose.4)The finite-element full-scale models of unconfined RC full-scale pier and SMCFST confined full-scale pier were established using finite-element software ABAQUS to explrore the axial and eccentric compressive behavior of SMCFST confined full-scale piers.The research results showed that the axial and eccentric compressive behavior of RC full-scale pier was significantly optimized after being strengthened by SMCFST confinement method.The axial compressive behavior of the SMCFST confined full-scale pier was similar to that of SMCFST confined columns.However,the increase of loading eccentricity would decrease the ultimate bearing capacity of the SMCFST confined full-scale piers.Additionally,the modified formula based on GB 50936-2014 "Technical code for concrete filled steel tubular structures" could accurately and stably predict the axial compressive ultimate bearing capacity of SMCFST confined full-scale pier,but it could only be used to conservatively predict the eccentric compressive ultimate bearing capacity of SMCFST confined full-scale pier with a loading eccentricity less that 0.917. |
