Research On Mechanical Properties Of Double GFRP Tube Heterostrength Concrete Composite Columns After Freeze-thaw Cycles

Combination structures composed of steel tubes,steel shapes,and concrete have been widely used in building and bridge structures.However,the problem of steel corrosion has not been completely resolved in steel-concrete composite structures.In recent years,fiber-reinforced polymer（FRP）materials have gradually replaced steel in newly constructed concrete structures due to their light weight,high strength,ease of construction,good corrosion resistance,and good fatigue resistance.GFRP materials have been widely used in various fields in recent years.When used in the outer tube of composite columns,GFRP materials can protect the concrete structure.Currently,the research on the mechanical properties of double-layer GFRP pipe concrete combined columns is relatively scarce.This paper investigates the mechanical properties of concrete-filled double-GFRP tube（CFDGT）combined columns under freeze-thaw cycles.The axial compression and seismic performance of CFDGT combined columns under freeze-thaw cycles are deeply studied by finite element simulation in this paper.The specific contents are as follows:In recent years,glass fiber-reinforced polymer（GFRP）materials have been widely used in various fields.The use of GFRP in composite column outer tubes provides protection for concrete structures.Currently,there is rich research content on GFRP-concrete-steel tube composite columns,but there is little research on the mechanical properties of double-layer GFRP tube-concrete composite columns.This study considers the influence of freeze-thaw conditions on the double-layer GFRP tube-concrete structure and conducts research on the mechanical properties of double GFRP tube high-strength concrete composite columns under freeze-thaw cycling.The axial compression,eccentric compression,and low-cycle fatigue properties of the composite columns are thoroughly studied using finite element simulation under freeze-thaw cycling.The specific contents are as follows:1.Finite element analysis was carried out on 12 glass fiber-reinforced polymer（GFRP）tube-concrete composite short columns under axial compression,using ABAQUS software to obtain the load-vertical displacement curve of the specimens.The rationality of the selected material constitutive model and modeling method was verified by comparing with the existing test data.2.The axial compression performance of full-scale concrete-filled double GFRP tube（CFDGT）composite columns under freeze-thaw cycling was studied.Using finite element software,50 CFDGT composite columns under axial compression were analyzed for their expanding parameters under freeze-thaw cycling,and the effects of freeze-thaw cycle frequency（N）,slenderness ratio（λ）,compressive strength of concrete in the core and sandwich layer(f_cki、f_cko),and thickness of the outer GFRP tube（T）on the axial compression load-carrying capacity of the specimens were examined.Based on the numerical simulation results,the calculation formulas for the axial compression load-carrying capacity of full-scale CFDGT composite columns under freeze-thaw and non-freeze-thaw conditions were established.3.Based on the CFDGT specimen axial compression finite element model,34 CFDGT composite columns were simulated for their seismic performance（10 non-freeze-thaw CFDGT specimens and 24 freeze-thaw cycling CFDGT specimens）,with ratio of shear span to depth（λ’）,compressive strength of concrete in the core and sandwich layer(f_cki、f_cko),freeze-thaw cycle frequency（N）,thickness of the outer GFRP tube（T）,and axial compression ratio（n₀）as control parameters.The failure mode characteristics of the CFDGT composite columns under horizontal low-cycle reciprocal loading,the influence of different parameters on the hysteretic performance,skeleton curve,energy dissipation capacity,resistance attenuation,and stiffness degradation of the CFDGT composite columns were analyzed,and the recovery force model of such columns was proposed.It provides theoretical support for the popularization and application of this kind of composite column in severe cold area.