| In a combination of recycled aggregate concrete(RAC)and hybrid FRP-concrete-steel double-skin tubular columns(DSTCs),a novel kind of fiber-reinforced polymer(FRP)composites,hollow-core FRP-RAC-steel tubular columns(FRSTCs),was proposed in this research.This novel composite consists of an outer FRP tube,an inner steel tube and the annular area between two tubes filled with RAC.Since DSTCs have high bearing capacity and good ductility,FRSTCs are expected to inherit most merits of DSTCs which fully exploit the advanced mechanical properties of every component of these composites.Furthermore,FRSTCs help alleviate the environmental pollution caused by demolished concrete,realizing the sustainable development of natural resources.FRSTCs also help upgrade the application level and develop the application area of RAC to provide an alternatively environmentally friendly structural composite.In order to understand the main mechanical properties of FRSTCs,to testify the feasibility of RAC applying in DSTCs and to help promote this new type of composites in practical projects,the axial compressive behavior and seismic performance of FRSTCs were studied in this research.In order to understand the monotonic axial compressive behavior of FRSTCs,21 small-sized FRSTCs and three FRP confined RAC tubular columns(with a diameter of 150 mm)were tested under monotonic axial compression.The influence of two parameters,the recycled coarse aggregate(RCA)replacement ratio and the thickness of FRP tube,were analyzed on the load-axial shortening curves,load-FRP strain curves and curves of hoop strain-axial strain in FRP tubes for FRSTCs.The monotonic axial compression test results have shown that:(1)the monotonic axial compressive behavior of FRSTCs was similar with that of DSTCs.The ultimate bearing capacity of FRSTCs decreased with an increase in the RCA replacement ratio,while their ultimate axial shortening increased with an increase in RCA replacement ratio.Both the ultimate bearing capacity and the axial shortening of FRSTCs improved with an increase in the thickness of FRP tubes.The FRSTCs with an RCA replacement ratio of 30%tended to have better performance in bearing capacity and ductility.(2)The load-axial strain curves and load-FRP strain curves of FRSTCs showed a tendency of two-linear sections,while the load-steel strain curves showed a tendency of three-linear sections.(3)The deviation of calculated ultimate bearing capacity of FRSTCs by the model based on the stress-strain relationship was less than 7%from the test values,which means the bearing capacity model for DSTCs is still applicable to FRSTC.In order to understand the cyclic axial compressive behavior of FRP confined RAC tubular columns,8 relatively large-sized basalt FRP confined RAC tubular columns(with a diameter of 300 mm)were tested subjected to cyclic axial compression.The influence of three parameters,the RCA replacement ratio,RAC strength and loading rate were analyzed on the load-axial shortening curves,axial stress-strain envelope curves and curves of hoop strain in FRP tubes for specimens.The relationship of residual strain and unloading strain was established as well as the relationship of stress deterioration rate and the unloading strain in the envelop curve.The cyclic axial compression test results have shown that:(1)the cyclic axial compressive behavior of FRP confined RAC tubular columns was similar with that of FRP confined ordinary concrete tubular columns.The axial stress-stain curves for FRP confined normal-strength RAC tubular columns had an obvious enhanced section,while those for FRP confined high-strength RAC tubular columns had a slight drop at the beginning of the enhanced section.The RCA replacement ratio had little influence on the ultimate bearing capacity of specimens.Increasing the loading rate had a negative influence on the ultimate bearing capacity of specimens but a slightly positive influence on their ductility.(2)Based on the constructive models for FRP confined original concrete columns,after applying the new unloading strain and the first stress deterioration rate,as well as considering the influence of concrete strength,the modified model of reloading/unloading path for FRP confined normal concrete under cyclic axial compression was applicable to predict the cyclic axial comprssive properties of FRP confined RAC.In order to understand the seismic performance of FRSTCs,quasi-static tests of 16 full-scale FRSTCs(with a diameter of 400mm and an effective height of 1650mm)were conducted under the constant axial compression and cyclic lateral loading.The influence of nine parameters,including the RCA replacement ratio,the axial loading method,the existence of shear connectors on the steel tube,the fiber type of the FRP,the thickness of the steel tube,the void ratio of the column,the thickness of the FRP tube,the axial compression ratio and the RAC strength,were analyzed on the following aspects to make a comprehensive understanding of the seismic performance of FRSTCs,including the hysteretic loop,the skeleton curve,the deterioration coefficient of bearing capacity,the ductility index,the displacement rotation,the curvature,the length of the equivalent plastic hinge region,the stiffness deterioration coefficient,the energy dissipation,the equivalent viscous damping coefficient and the cumulative damage coefficient.The quasi-static test results show that:(1)the hysteretic loops were fully rounded,indicating favorable deformation capacity and energy dissipation of FRSTCs.(2)Three parameters,including the increasing axial compression ratio,the existence of shear connectors on the steel tube and the axial loading method of cored area loading,can increase the ultimate lateral load of FRSTCs but apparently decrease the ductility of specimens,the curvature at the bottom end of the column,as well as increasing their cumulative damage.Increasing axial compression ratio and the existence of shear connectors on the steel tube apparently decreased the capacity of energy dissipation.(3)Increasing the thickness of the steel tube can improve the lateral load and the energy dissipation of FRSTCs but barely increased their ultimate lateral displacements,and their ductility was slightly decreased.Increasing the diameter of the steel tube can improve the ultimate lateral displacement of FRSTCs but enhanced the pinch effect of their hysteretic curves.(4)RCA replacement ratio had little influence on the seismic performance of FRSTCs,while with an increase in RCA replacement ratios,the pinch effect of the hysteretic curves was slightly weaken,and their total energy dissipation and their equivalent viscous damping coefficients were slightly increased.(5)The confinement effect of the FRP tube with two mixed fibers was generally between that of the two tubes with an individual fiber.Increasing the thickness of the FRP tube can decrease the bearing capacity deterioration of FRSTCs,as well as slightly decreasing their ductility and the total energy dissipation.The strains in the FRP tube on the tensile and compressive sides developed symmetrically with an increase of lateral loading displacement in the early loading stage.The FRP tube in FRSTCs made a partial contribution on bearing the axial compression,while the tension was mainly borne by the steel tube.(6)The proposed restoring force model can well predict the hysteresis performance of FRSTCs. |
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