| At present,the combined use of Fiber Reinforced Polymer（FRP）,concrete and steel tube,three traditional building materials,has attracted the attention of various researchers.Previous studies have shown that the new structural forms of FRP,concrete and steel tube have the characteristics of high performance.Among these new structures,typical structural forms include FRP-confined concrete-filled steel tube columns（FCCFST）and FRP-concrete-steel tube double-skin tubular columns（DSTC）.Among them,DSTC has two forms: Hollow-core FRP-concrete-steel（HC-FCS）columns and Double-tube concrete columns（DTCC）.The main difference between them is whether concrete is filled in the steel tube.Double-skin hollow columns composed of FRP,concrete and steel tubes are convenient to construct and have good seismic performance.Compared with ordinary concrete columns exposed directly in the air,double-skin hollow columns benefit from the restraint wrapping of outer FRP jacket,which makes double-skin hollow columns have better corrosion resistance,lighter self-weight and higher bearing capacity and ductility than FRP confined concrete columns.Up to now,many scholars have done a lot of research on these new structural forms,and have also made their own durability research on various structural forms of FRP-confined concrete,concrete-filled steel tube,plain concrete,etc.However,there is little research on the durability of GFRP tube-concrete-steel tube double-skin hollow columns.Based on this,the durability of GFRP-concrete-steel tube double-skin hollow columns with different GFRP tube thicknesses in laboratory simulated marine environment is studied in this thesis.In the simulated marine environment of the laboratory,samples were taken out at different temperatures（room temperature,40℃,60℃）at regular intervals（0[ control samples without soaking],30,90,180,270,360 days,and 360 days of dry-wet cycle）for comparative analysis.This experiment includes 68 specimens,including 34 specimens with3 mm and 6mm GFRP tube thickness,and 2 specimens with 3mm and 6mm GFRP tube thickness in the control group,totally 4 specimens.There are 10 specimens with 3mm and6 mm GFRP tube thickness at soaking temperature respectively,that is,there are 20 specimens at each temperature and 60 specimens in total;And two dry-wet cycle specimens with GFRP tube thickness of 3mm and 6mm respectively,totally four.In addition,30 mm wide rings were cut from the same batch of GFRP tubes and soaked in simulated seawater to analyze the change of mechanical properties（tensile elastic modulus）of GFRP tubes.Through the axial compression test of the specimen,the phenomenon and failure mode of the specimen in the loading process are described in detail,and the data obtained from the test are compared with the existing DSTC model.After a year of soaking and drying-wetting cycle,the test results are as follows:（1）The use of thick GFRP tube can effectively improve the ultimate load and ultimate axial strain of GFRP tube-concrete-steel tube double-skin hollow column,greatly improve the axial deformation capacity of double-skin hollow column specimens,and also slow down the expansion speed of sandwich concrete,making full use of the advantages of thick GFRP tube with higher circumferential binding force.Changing the thickness of GFRP tube,soaking temperature and soaking time has no great influence on the hoop strain of double-skin hollow column specimen,because the thickness of sandwich concrete of sandwich specimen is constant,so the expansion amount of sandwich concrete under compression is basically the same.（2）In this test,the ultimate axial strain of thick pipe string after soaking in 270 days and 360 days increases,which is attributed to the resin falling off inside GFRP pipe.Through scanning electron microscope observation of GFRP tubes of control group and experimental group,it is found that the GFRP of control group and 180-day or shorter soaking time has a large amount of resin,and most of them do not debond with the fiber,which makes the fiber and resin as a whole provide lateral restraint.However,in the 270-day and 360-day GFRP tubes,the resin retention is very low,and there are gaps between the fibers.When the sandwich concrete expands,the innermost fiber works first,and the outermost fiber works later,which leads to the delay of the moment when the whole GFRP tube is activated. |
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