Mechanical Properties Of Circular Steel Tubes Infilled With Post-pouring Concrete And Precast Segments Containing DCLs

Compound concrete made of fresh concrete(FC)and demolished concrete lumps(DCLs)is an effective way to recycle demolished concrete,and has been successfully applied in a series of practical projects.In order to further improve the construction simplicity of the structures using compound concrete,a new type of component is proposed in this paper-circular steel tubes infilled with post-pouring concrete(PPC)and precast segments(PSs)containing DCLs.The PSs containing DCLs are fabricated in the factory and no DCLs are handled on the site,so that the construction efficiency is significantly improved.The compression and shear properties,and the seismic performance of this proposed column are studied in this paper.The main work and conclusions are as follows:1.The axial compression tests are carried out on 10 proposed columns.The influence of the number of PSs,the cross-sectional size of PSs and the strength of PPC on the axial compression performance of the columns are investigated.The results show that:(1)even if the minimum distance between the PS and the inner wall of the steel tube is only 10 mm,the construction quality of the column can be ensured;(2)the strength of PPC has obvious influence on the axial bearing capacity and ductility of the columns;when using highstrength PPC,the number of PSs has obvious effect on the ductility of the columns,but the effect is limited when the strength of PPC is low;(3)the domestic and foreign specifications can be used to estimate the axial bearing capacity of the columns conservatively based on the combined compressive strength(the weighted average strength of the PSs and the PPC);(4)when the equivalent confinement index is less than 0.6,the proposed formula for the axial bearing capacity has better prediction accuracy than the domestic and foreign specifications.2.The eccentric compression tests are carried out on 14 proposed columns.The influence of the number of PSs,the cross-sectional size of PSs,the strength of PPC,the eccentricity and the eccentric direction on the eccentric compression performance of the columns are investigated.The results show that:(1)the eccentricity and the strength of PPC have a significant influence on the eccentric performance of the column,but the eccentric direction,the number and the cross-sectional size of PSs have limited effect on it;ignoring the eccentric direction will greatly facilitate the lifting operation of the PSs in the actual engineering;(2)the confinement effect of the steel tube on the concrete in the compression zone has obvious spatial distribution characteristics when the column is under eccentric load;(3)the calculated values of the equivalent bending stiffnesses based on AISC and EC4 are in good agreement with the measured values and are conservative;(4)The eccentric loading capacities predicted by domestic and foreign specifications(based on the combined compressive strength of the core concrete)are obviously conservative;as the strength of PPC increases,the degree of conservatism further expands.3.The shear tess are carried out on 34 proposed columns.The influence of the number of PSs,the cross-sectional size of PSs,the strength of PPC,the axial load ratio,the shear span ratio and the shear path on the shear performance of the columns are investigated.The results show that:(1)shear span ratio is the most important factor to control the failure mode,shear bearing capacity and ductility of the columns,followed by the strength of PPC and the axial load ratio;the shear path,the number and the cross-sectional size of PSs have limited influence on the shear performance of the columns so that these three factors can be neglected in actual engineering;(2)the proposed calculation method can well predict the shear bearing capacity of the columns with the shear span ratio less than 0.5,and has better prediction accuracy than other existing calculation methods.4.The cyclic tests are carried out on 16 proposed columns.The influence of the cross-sectional size of PSs,the strength of PPC,the location of the PPC spacer,the axial load ratio and the direction of the lateral force on the seismic performance of the columns are investigated.The results show that:(1)the failure mode of the proposed columns is similar to that of the ordinary compound concrete filled circular steel tubes;(2)the strength of PPC has a significant influence on the ultimate lateral capacity,initial stiffness and ductility of the columns,followed by the axial load ratio and the cross-sectional size of PSs;the location of the PPC spacer and the direction of the lateral force have little influence on the seismic performance of the columns,which can greatly facilitate the actual engineering construction;(3)the local buckling length is only related to the cross-sectional size of the steel tube,and has little to do with the variation of the core concrete;(4)The proposed calculation method can well predict the equivalent plastic hinge length of the columns.5.The numerical simulation and parameter analysis of the eccentric and seismic behavior of the proposed columns are carried out;the influence of the spatial distribution of confinement effect on the eccentric properties of the columns is preliminarily discussed.The results show that:(1)the OpenSEES fiber beam element model can be used to simulate the eccentric and seismic behavior of the columns based on the combined compressive strength of the PSs;(2)the eccentric behavior of the columns are simulated by two methods(based on constant confining pressure or variable confining pressure),respectively;the obtained eccentric loading capacities corresponding to the two methods differs by no more than 6.3%,but there are significant differences in the stress distribution of the core concrete;(3)As the strength of PPC increases,the effect of slenderness ratio on the eccentric loading capacity of the columns is reduced;(4)when the equivalent confinement index is less than 0.6 and the slenderness ratio is less than 50,the established M-N relationship agrees well with the test results,which can be used as reference for the design of the proposed columns.