Buckling Behavior Of Steel Rebars In LRS FRP-Confined Rectangular RC Columns

Fiber reinforced polymer(FRP)composites have the advantages of high strength-to-weight ratio,corrosion resistance and fatigue resistance,which are widely used in the seismic strengthening of concrete structures.Large rupture strain FRP(LRS FRP)is an economical and environmentally friendly newly emerging material made of recyclable plastic with a large rupture strain,usually more than 5%,which is more than twice that of traditional FRP(including CFRP,GFRP,and AFRP).In the seismic strengthening of concrete structures,the large strain characteristic of LRS FRP may provide better ductility for the structures.Buckling of longitudinal steel bars is an important failure model of reinforced concrete columns.Bars buckling would reduce the load-bearing capacity of columns,which may lead to failure of columns or even the collapse of the whole structure.Especially for the concrete structures designed according to the old codes before 1980 s,the stirrup spacing is generally larger and thus the problem of buckling deformation of longitudinal steel rebars is more prominent.The external FRP jackets can prevent the peeling off of concrete cover layers and inhibit or even postpone the buckling deformation of longitudinal bars in RC columns.Due to the relatively small rupture strain of conventional FRPs,the brittle fracture damage of FRP jackets frequently occurs at the corner of RC columns under seismic load for the common rectangular columns in concrete engineering,where the buckling deformation of longitudinal steel rebars is generally small.While for the LRS FRP with relatively large strain,the FRP jackets still retrain from rupture in the case of large deformation in the plastic hinge zone,where the interaction between the steel rebars and FRP jackets is more protrude.Therefore,this thesis studied the buckling behavior of longitudinal steel bars in rectangular columns confined with LRS FRP and mainly composed of four parts as follows:(1)A total of 27 PET FRP-confined rectangular columns were experimented under monotonic axial compression,comprising 18 reinforced concrete columns and9 plain concrete columns.The test parameters included longitudinal reinforcement position,the layers of external FRP jackets,stirrup spacing,and section aspect ratio.During the tests,it was observed that the failure models of FRP-confined columns was caused by the rupture of external FRP jackets in the corner of the nonoverlapping zone.By cutting outer the external FRP jackets and concrete after tests,it was found that longitudinal steel bars buckled obviously before the rupture of FRPs.(2)From the load-displacement curves of FRP-confined columns,the study made a further analysis about the effects of the layers of external FRP,stirrup spacing,longitudinal reinforcement position and section aspect ratio on the anti-seismic properties of FRP-confined rectangular reinforced concrete columns.Then,from the experimental results,the study separated the average axial stress-strain curves of a single longitudinal bar from every FRP-confined reinforced concrete column,and the test curves were compared with the theoretical curves obtained from the standard tensile test of longitudinal steel rebars.And further analyzed the influence of various parameters on the average stress-strain curves of longitudinal steel rebars.(3)The “composite beam” model and the “tensile-flexural beam” model were established to calculate the lateral support stiffness of the longitudinal steel rebars in the corner and the middle section of columns in the FRP-confined rectangular reinforced concrete columns respectively.Then,the theoretical stress-strain curves of longitudinal steel rebars considering the influence of lateral support from FRPs were compared with average stress-strain curves of a single longitudinal bar separated from test results,and further analyzed the effects of test parameters on theoretical curves.(4)Combined with the theoretical model of FRP-confined rectangular concrete,the theoretical way of predicting the load-displacement curve of FRP-confined reinforced concrete columns is proposed.The results of theoretical prediction were compared with the load-displacement curves from tests,which further verifies the accuracy of the theoretical model of longitudinal reinforce bas considering buckling.