Seismic Performance Of Recycled Aggregate Concrete Filled Steel Tubular Columns Based On Visual Measurement Methods

It is urgent and imperative to promote the application of recycled aggregate concrete(RAC)for the purpose of environmental pollution reduction from waste concrete.Recycled aggregate concrete-filled steel tubular columns(RACSTC)has been proposed as an approach to compensate for the performance defects of recycled aggregate concrete.Steel tubes significantly improve the strength and durability of recycled aggregate concrete by means of restriction and inclusion;recycled aggregate concrete strengthens the anti-buckling performance of the tubes by internally supporting them.This novel type of composite component has excellent mechanical and seismic performance,lending it substantial scientific significance as well as attractive market prospects.When the RACSTC is under the nonlinear loads such as earthquake and vibration which leads to the acceleration of component damage and deformation,the seismic performance and its damage assessment of the components would be the key and essential content to structure safety.However,this novel component still demands much more efforts on deep understanding of its theoretical analysis,especially in the main influencing parameters such as RAC replacement ratio,axial ratio,the accuracy of traditional strain measurement of certain points.Strain gauges play an important role in civil engineering and structural performance analyses.The traditional contact measurement technique is neither cost-effective nor reusable,however,its accuracy is subject to environmental uncertainties.Moreover,it is difficult for a limited number of strain gauges or sensors to record the deformation and destruction of the whole component.A monocular vision method for the static measurement of specimen deformation and a binocular vision method for the dynamic measurement of the specimen deformation during loading test,are proposed respectively with target-less technique.The vision system,including monocular vision and laser range finder,detects the deformation of the damaged specimens.An improved Hough-transform algorithm is put forward to calculate the edge information of the deformation target,and the mean measured relative error is 2.95%.This monocular vision method is comparatively economic,but inferior to binocular vision method in dynamic detection and three-dimensional reconstruction.Therefore,a binocular vision method is built to follow the three-dimension deformation of the specimens dynamically during test.The image processing algorithm and the principle of three-dimensional measurement is elaborated,the least-square-based circle fitting algorithm is proposed,and the authors constructs a binocular vision system for the static and dynamic collection of full-field three-dimensional deformation data.Three-dimensional reconstruction and comparison against traditional contact measurement is performed.The mean relative error of the maximum convex curved diameter in visual static and dynamic measurements are 0.19%and 0.33%,respectively.The mean relative error of full-field strain in visual measurement is 0.20%(h = 50 mm)and 0.50%(h = 55 mm)respectively.The proposed visual method comprehensively reflects the deformation process and full-field strain values of the components under engineering IT7 accuracy requirements.Against this background,a large-scale tubular RACSTC is designed,and the low cyclic loading test is carried out,hysteresis curve,skeleton curve and strain curve are sketched to analyze the test results.A finite element model is built to further investigate the seismic behavior of the RACSTC,and a hysteresis model is proposed based on the simulation result database,the theoretical model is then compared and verified by the test results.In an attempt to study the characteristics of deformation and failure,the real-time visual measurement method is introduced for accuracy and convenience.This non-contact,target-less,full-field method depicts the structure of a vision system and the principle of 3D deformation measurement;and to some extent,it records completely the strain and deformation data of the specimens under low cyclic loading test,realizing the full-automation measurement of 3D deformation via the self-developed vision system.In comparison of the strain gauge results and the vision method results,seismic performance of the RACSTC,including the hysteresis ability,ductility,degradation behavior,energy dissipation ability and full-field 3D deformation,is illustrated quantitively.The specific innovation and study content are elaborated as below.Fifteen RACSTC and two normal concrete-filled steel tubular columns(NCFSTC)are designed to investigate the seismic performance of recycled aggregate concrete filled steel tubular columns through low cycle loading test.The impact factors of seismic behavior are systematically studied,including axial compression ratio(0.2,0.4 and 0.6),thickness of steel tube(6 mm and 8 mm),steel strength(Q235 and Q345)and RAC strength(C40 and C50).The hysteresis curve,skeleton curve,ductility factor,energy-dissipation capacity,equivalent viscosity coefficient is analyzed.Specifically,the hysteresis curve is full-shaped without obvious "pinch" phenomenon,indicating that the RACSTC performs extraordinary hysteresis behavior as well as the NCFSTC.The test results demonstrate that recycled aggregate concrete has little impact on the initial stiffness,ductility,deformation ability and energy dissipation capacity of the specimens.Convex deformation without crack is observed at the bottom of the steel tubular columns.Under the same axial compression ratio,the hoop coefficient is increased,the ductility is increased,and the energy consumption is improved.The elastic strength of the specimen with low strength of the core recycled concrete is better,the ductility coefficient is lower,the limit displacement angle is larger,and the yield displacement angle is larger.Smaller,the descending section of the skeleton curve is more gradual,and the energy consumption of the displacement cycle of each stage is smaller.Under the same core recycled concrete strength,as the axial compression ratio increases,the ductility coefficient of the specimen decreases,which will reduce the ductility.The strength of the steel tube has little effect on the ductility,and the yield displacement angle and the ultimate displacement angle are both reduced.small.When the hoop coefficient and material are the same,the axial compression ratio is increased,the fullness of the hysteresis curve is reduced,the energy consumption is reduced,the lateral stiffness is increased,the ductility is reduced,and the load capacity is greatly reduced after the peak load is over.When the ferrule coefficient is close,the test piece with high strength of the steel pipe has better energy consuming ability.The strength of the steel pipe has little effect on the ductility,and the recycled concrete has little effect on the initial stiffness,deformation capacity,ductility and energy dissipation performance of the test piece.Finite element method is adopted to simulate the seismic model under combining constant axial force and flexural cyclic force,thus examining the impact of slenderness ratio,thickness of steel tube and the axial compression ratio on RACSTC seismic performance.A database of the skeleton curve is therefore established,characteristic points including yield,peak and failure points were determined and calculated.An optimized threefold linear model for the hysteresis curve of RACSTC is proposed using twenty-seven numerical simulation specimens.This theoretical equation fits well with the experimental results.Also,the simulation results exhibit that with the increase of the thickness of steel tube,the yield displacement would increase,but the energy dissipation coefficient and the ductility coefficient would reduce;with the increase of the slenderness ratio,the yield displacement would increase,the ductility coefficient would decrease,the energy dissipation coefficient first increased and then decreased with an insignificant decline amplitude.These results could contribute calculation preference for the research on structural earthquake response and elastic-plastic time interval analysis.Combining the experiment,numerical simulation analysis and full-field deformation vision measurement,the conclusion would be drawn that,the RACSTC could meet the seismic structural design specification.Compared with the NCFSTC,the RACSTC have desirable seismic performance and deformation ability.Thus,it is feasible and applicable to promote the utilization of recycled aggregate concrete in the load-bearing structure in seismic regions as a green alternative to concrete.For the non-contact measurement,two visual methods both overcome the defects such as inaccuracy caused by limited gauge points.Using the binocular method,the three-dimensional deformation process can be reflected intuitively,the parameters of the three-dimensional deformation curved surface can be calculated,and the full-field strain value can be obtained.The measurement accuracy of two vision methods both meet the engineering IT7 requirement.Vision method can be used as a reasonable alternative to traditional contact measurement methods such as strain gauges.