Study On Mechanical Behavior Of Ultra-high Performance Concrete With Coarse Aggregate Filled Circular Steel Tube Members

Ultra-high performance concrete(UHPC),with the highlights of ultra-high strength,superior toughness,excellent durability,good blast and impact resistance,is a novel type of cement-based composite material.The application of UHPC in engineering structures can reduce the size of member,raise the structural aseismic behavior,as well as reduce the natural resource consumption and environmental pollution.However,the utilization of UHPC in practical engineering is limited,due to the high preparation cost,complicated construction technology,serious auto-shrinkage,and worse ductility.One way to slove this problem is to incorporate moderate coarse aggregate(CA)in the UHPC system to decrease the consumption of cementitious materials,thus saving the preparation cost,and enhancing the autogenous shrinkage resistance.Filling UHPC into the steel tube makes the UHPC core under a three-dimensional compression state due to the confinement effect provided by steel tube,which can restrain the initiation of micro-crack and delay crack extension of UHPC,thus improving the ductility of UHPC.With the support of the key project of National Natural Science Foundation“Research on the mechanical properties and design calculation theory of UHPC filled steel tube under complex actions”(NO.51738011),using the combination of experimental study,numerical simulation and theoretical analysis,the effects of coarse aggregate characteristic parameters and steel fiber volume fraction on the basic mechanical properties of UHPC with coarse aggregate(CA-UHPC)are studied,as well as the mechanical properties of CA-UHPC filled circular steel tube columns are investigated systematically and deeply.The main works and results of this study are listed as follows:(1)A total of 21 batches of CA-UHPC mixtures,containing 504 samples,are designed and prepared,considering the effects of CA type and replacement rate,as well as the steel fiber volume fraction.The mechanical properties of CA-UHPC were experimentally investigated to obtain the cubic compressive strength,axial compressive strength,splitting tensile strength,elastic modulus,Poisson’s ratio,and axial compressive stress-strain curves.The effects of coarse aggregate characteristic parameters and steel fiber volume fraction on the basic mechanical properties of CA-UHPC are analyzed.The test results show that the 28d compressive strength of CA-UHPC with 2%steel fiber was increased by 31.5%,54.2%,30.0%by the addition of 15%,30%,40%CA in comparison to that the mixture without CA.The highest compressive strength was obtained by granite CA-UHPC,followed by basalt CA-UHPC,which the lowest for steel slag CA-UHPC.Besides,the incorporation of moderate steel fiber could increase the compressive strength of CA-UHPC,whilst it was not further enhanced as the increase of steel fiber volume fraction.The coefficient of lateral deformation of CA-UHPC remains between 0.20～0.25 prior to the peak stress,and then increases rapidly after the peak stress.The axial compressive stress-strain curve of CA-UHPC can be divided into five stages,including elastic,micro-crack extension,macro-crack formation,crack unstable propagation and stable propagation stages.The replacement rate and type of CA as well as the volume fraction of steel fiber have insignificant fluence on the ascending stage of full curves,nevertheless,the increase of CA replacement rate and steel fiber volume fraction results in a gentler descending stage.The conversion relation formulas between cubic compressive strength and axial compressive strength,splitting tensile strength and elastic modulus are proposed.Furthermore,the axial compressive stress-strain relationships of CA-UHPC with the consideration of CA replacement rate and steel fiber volume fraction are established.(2)Considering the influences of steel tube thickness,steel yield strength,compressive strength of UHPC core,steel fiber volume fraction,a total of 44 CA-UHPC filled circular steel tube stub columns are designed and tested under axial compression.The failure modes,load-deformation curves,load-strain relationships and the ultimate bearing capacity were obtained.The mechanical properties of CA-UHPC filled circular steel tube were deeply and systematically investigated based on the experimental results,and the working mechanism of short column under axial compression was also discussed through finite element analysis.The test results show that the failure mode of short column under axial compression depends on the confinement index(ξ=A_s f_y/A_c f_c _k)of specimen,which ranges from 0.5 to 0.9corresponding to the shear failure,and buckling failure for the confinement index greater than1.2,and combining the characteristics of shear and bucking failure for the confinement index between 0.9 and 1.2.Homologous,the load-deformation curves of short columns under axial compression can be divided into three categories,all curves contain elastic and elastic-plastic stages,specifically,typeⅠ(0.5≤ξ<0.9)includes descending and platform stages,typeⅡ(0.9≤ξ<1.2)contains descending and hardening stages,typeⅢ(ξ≥1.2)contains hardening stage.The increase of steel tube thickness,yield strength of steel,compressive strength of CA-UHPC can significantly improve the ultimate bearing capacities and ductility of specimens,while the increase of steel fiber volume fraction can slightly enhance the bearing capacities,and improve the ductility.There is no confinement effect on CA-UHPC core provided by steel tube at elastic stage and the initial phase of elastic-plastic stage,the steel tube produces confinement effect on CA-UHPC core when the load increases up to 90%of the ultimate carrying capacities.Finally,the formulas based on the test and numerical results are proposed to calculate the ultimate carrying capacity of CA-UHPC filled circular steel tube stub columns under axial compression.(3)A total of 42 CA-UHPC filled circular steel tube middle long columns are designed and tested under axial compression considering the parameters of steel tube thickness,steel yield strength,compressive strength of UHPC core,length-diameter ratio,to obtain the load-deformation curves,load-strain curves,and stability carrying capacity.The mechanical properties of CA-UHPC filled circular steel tube middle long columns were thoroughly investigated according to the axial compression test results,and the working mechanism of middle long columns under axial compression was discussed using finite element analysis.The experimental results show that the columns with the length-diameter ratio(L/D)ranging between 4 and 12 failed in inelastic buckling failure,and elastic buckling failure for the columns with the L/D greater than 12.The load-deformation curves contain elastic stage,elastic-plastic stage and descending stages,and the lateral load-deformation curves coincide with half-sine curve,the strain distributions at the mid-height of specimen satisfy plane-section assumption.The ultimate bearing capacities of middle long columns under axial compression were increased with the increase of steel tube thickness,yield strength of steel,compressive strength of CA-UHPC,however,the ultimate carrying capacity,initial stiffness and ductility were decreased as the length-diameter ratio increased.The confinement effect on CA-UHPC core provided by steel tube decreases with the increase of length-diameter ratio of columns,specially,the confinement effect at compression zone is greater than that at tensile zone.The calculated method of stability carrying capacity for CA-UHPC filled circular steel tube middle long column,referencing to the code CECS28-2012,is established.(4)Considering the parameters of steel tube thickness and the compressive strength of CA-UHPC core,7 CA-UHPC filled circular steel tube members are designed and fabricated.Based on the flexural tests,the moment-curvature curves,moment-strain relationships,flexural stiffness,and ultimate flexural carrying capacity were acquired.The flexural properties of members were deeply investigated according to the experimental results,and the working mechanism of CA-UHPC filled circular steel tube members under pure bending was revealed through finite element analysis.The test results show that the inclusion of steel fiber could decrease the number and width of crack at tensile zone,and alleviate the crushing degree of CA-UHPC core at compression zone.The moment-curvature curves include elastic stage,elastic-plastic stage and hardening stage,and the deflection along the span of specimen is consistent with the half-sine curve,besides,the distribution of longitudinal strains of steel tube at the mid-span along the height of section coincides the plane cross-section assumption.The increase of steel tube thickness can significantly increase the flexural bearing capacities and stiffness of CA-UHPC filled circular steel tube members,while it’s not same to the increase of compressive strength of CA-UHPC core,the incorporation of steel fiber can enhance the flexural capacities.During the loading process,the yielding of steel tube at tensile zone is earlier than that of compressive zone.When the load attains the ultimate flexural bearing capacities of specimens,both the tensile and compressive zones of steel tube at the mid-span have yielded,the cracking zone of CA-UHPC core exceeds 1/2 of the cross-sectional area,and the compressive zone edge of concrete core reaches the compressive strength of CA-UHPC.Based on the results of experimental research and finite element analysis,the formulas for flexural stiffness and the calculation method of the flexural carrying capacity of CA-UHPC filled circular steel tube member are proposed.(5)A total of 49 CA-UHPC filled circular steel tube columns are designed and fabricated considering the parameters of steel tube thickness,steel yield strength,CA-UHPC core compressive strength,length-diameter ratio and eccentricity.Based on the eccentric compression tests,the load-deformation curves,load-strain relationships,eccentric bearing capacities,and ductility were obtained.The mechanical properties of CA-UHPC filled circular steel tube columns under eccentric load were deeply investigated according the experimental results,and the working mechanism of specimens under eccentric compression was discussed through finite element analysis.The test results indicate that the specimens were failed in buckling failure due to the large lateral deformation at the mid-height of specimen.The load-deformation curves are similar to these of middle long columns under axial compression,including elastic,elastic-plastic and descending stages.The lateral deformation along the height of specimen coincides with the half-sine curve,and the distributions of longitudinal strains of steel tube at the mid-height of specimen along the height of section satisfy the plan-section assumption.The ultimate bearing capacities and ductility of CA-UHPC filled circular steel tube columns under eccentric load were increased with the increase of steel tube thickness,yield strength of steel,compressive strength of CA-UHPC core,while decreased with the increase of length to diameter ratio and eccentricity.During the loading process,the yielding of steel tube at compressive zone was earlier than that of tensile zone.When the load attained the ultimate bearing capacities,the tensile zone edge of steel tube may be under tensile or compressive state depending on the eccentricity,and the confinement effect provided by steel tube for CA-UHPC core at the compression zone was higher that of tensile zone,the confinement effect was decreased with the increase of eccentricity and L/D.The N-M correlation equation for CA-UHPC filled circular steel tube columns under eccentric compression is proposed based on that of normal concrete filled steel tube columns proposed by Zhong,as well,a calculation formula of ultimate carrying capacity of the column under eccentric compression is put forward using experimental coefficient method.Finally,on the basis of the summaries of this paper,the further research suggestions in this filed are proposed.