| GFRP-Concrete-Steel Hybrid Double Skin Tubular Columns(DSTCs)is a new type of column structure composed of three materials:external GFRP tube,internal steel tube and concrete poured between two tubes.It has good mechanical properties,light weight and strong corrosion resistance.It has broad application prospects in corrosive areas such as marine engineering and saline-alkali land.Geopolymer concrete(GPC)is a new type of green building material with industrial by-products as cementitious materials.It has the advantages of simple preparation process,high strength,good corrosion resistance and low CO2 emissions during production.It is expected to replace ordinary cement concrete as a new green building material.Geopolymer concrete has the characteristics of high brittleness and poor ductility of normal strength concrete(NSC).Therefore,GPC is applied to DSTC components in this paper,and the two-way constraint of GFRP tube and steel tube is used to improve its strength and ductility.On this basis,through experimental research,numerical calculation and active monitoring,the axial compressive mechanical properties of GFRP tube-GPC-steel tube composite columns and the damage monitoring and identification of composite columns under normal working conditions are studied.The main research contents are as follows:(1)Experimental study on axial compression of GFRP-geopolymer concrete-steel hybrid doubleskin tubular columnsA total of 11 DSTC specimens were tested under axial compression.The failure mode,loaddisplacement curve,and the influence of different GFRP tube thickness,concrete type,waste stone powder content,steel tube thickness,hollow ratio and other parameters on its mechanical properties were studied.Based on the existing bearing capacity calculation formula,the correction coefficient is proposed.The results show that the failure modes of the hollow sandwich GFRP tube-GPC-steel tube composite column mainly include shear failure,splitting failure and local compression failure.The loaddisplacement curve and load-GFRP tube strain of the specimen are bilinear.The load-steel tube strain curve of the hollow sandwich GFRP tube-GPC-steel tube composite column has an obvious curved yield platform compared with the hollow sandwich GFRP tube-concrete-steel tube specimen.The larger the thickness of GFRP tube is,the more obvious the ductile failure characteristics are,and the greater the circumferential restraint force is,which effectively improves the bearing capacity and deformation capacity of the specimen.The ultimate bearing capacity and ultimate displacement of the specimen gradually increase with the increase of the thickness of the steel tube.The appropriate hollow ratio can effectively improve the mechanical properties of the specimen.When the content of waste stone powder is 50%,the mechanical properties of the specimen are the best.Changing the type of concrete has little effect on the bearing capacity and deformation performance of the specimen.Based on the calculation formula of axial compression bearing capacity of FRP-concrete-steel double-walled hollow tube short column proposed by Zhang Juan,combined with the influence of various parameters on the bearing capacity,the correction coefficient is proposed.The results of the modified bearing capacity calculation formula are closer to the experimental values.The average ratio of the calculated bearing capacity N3 to the experimental value Pu is about 0.997,which is 5.6%higher than that before the correction.(2)Numerical analysis of GFRP-geopolymer concrete-steel hybrid double-skin tubular columnsThe finite element analysis of two GFRP-geopolymer concrete-steel hybrid double-skin tubular columns was carried out.The accuracy of the model was verified by comparing the load-displacement curve obtained by the test and the finite element,and the stress cloud diagram of each material.On this basis,the influence of various parameters on the load-displacement curve and ultimate bearing capacity of the specimen was studied.The results show that the influence of GFRP tube thickness and steel tube thickness on the mechanical properties of the specimen Is consistent with the test results.The ultimate bearing capacity increases with the increase of tube thickness.The thickness of GFRP tube has a significant effect on the performance of the specimen,which significantly improves the overall stiffness of the specimen.The higher the elastic modulus of GFRP tube,the better the bearing performance.When the fiber winding angle of GFRP tube is 15°,the mechanical properties of DSTC specimen are the best.(3)Real-time damage monitoring of GFRP-geopolymer concrete-steel hybrid double-skin tubular columnsIn this test,a total of two DSTC specimens were subjected to damage monitoring tests.By analyzing the time-domain signal comparison diagram,time-domain signal detail,wavelet packet energy,damage index,etc.,the damage changes of sandwich concrete of DSTC specimens with the increase of load were judged.The results show that the damage change trend of concrete inside the specimen is roughly the same,and the damage situation can be roughly divided into three stages.The first stage:before 7%of the ultimate load,the signal amplitude and wavelet packet energy increase with the increase of load,and the damage index decreases,approaching 0.The ultimate load of 7%～60%,sandwich concrete damage slow growth,the indicators tend to be stable.The third stage:60%～100%of the ultimate load,the signal amplitude and wavelet packet energy drop suddenly,the signal amplitude and wavelet packet energy are small,and the damage index is equal to 1,indicating that the internal concrete damage is more serious at this time.There are many internal cracks,and the cracks develop rapidly.The specimen enters the strengthening stage.At this time,the GFRP tube has a strong restraining effect on the concrete.The results of this study are basically consistent with the change trend of the specimen presented by the load displacement curve. |
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