| Concrete-filled-steel-tube(CFST)has excellent mechanical properties and has been widely used in major engineering construction.This is mainly due to the fact that the concrete can be in a multi-directional compression state under the constraint of steel tube,resulting in a significant increase in its bearing capacity and ductility.In addition,the steel tube can directly serve as the permanent formwork of concrete and avoid inward buckling under the internal support of core concrete.The two materials are combined in the form of CFST,and play a combined effect of 1+1>2.However,the shrinkage and creep of the core concrete during the curing period,as well as the difference in Poisson’s ratio between steel tube and concrete during the elastic stage,can lead to delayed confinement of the steel tube on the core concrete.In the plastic stage,when the steel tube buckles,the binding force of the steel tube will decrease.These problems are not conducive to the improvement of the bearing capacity,stiffness and ductility of CFST.An appropriate amount of expansive agent can effectively improve the shrinkage and interfacial bonding properties of core concrete,but it cannot change the characteristics of Poisson’s ratio.Moreover,there are few studies on the stress mechanism of expansive concrete-filled-steel-tube.In view of the above problems,this paper adopts the combination of expansive agent and CFRP to further give full play to the superior performance of core concrete and its constraints.The experimental parameters in this paper include MEA replacement rate,diameter-thickness ratio of steel tube,water-cement ratio and CFRP layers.A total of 20 groups of concrete specimens(12 groups of cube concrete specimens and 8 groups of cylinder concrete specimens)and 48 CFST specimens were made.The mechanical properties of MgO concrete and its volume stability under unconstrained and steel tube confined conditions are studied.The stress mechanism of expansive concrete-filled-steel-tube and the action mechanism of externally confined CFRP under axial pressure are explored,as well as the failure mode of the specimen.Finally,according to the data in this paper,considering the effect of prestress and CFRP restraint,the design formula of CFST is proposed.The main conclusions are as follows:The influence of MEA on the Poisson’s ratio of cylindrical concrete is not obvious,but it can reduce the Poisson’s ratio of CFST,and produce effective prestress to increase the rigidity of the specimen.Whether MEA can improve the mechanical properties of CFST is not only affected by pre-stress,but also related to the strength of core concrete and the diameter-thickness ratio of steel tube.The decrease of water-cement ratio and diameter-thickness ratio can effectively increase the bearing capacity and stiffness of the specimen,and the decrease of diameter-thickness ratio will also increase the ductility of the specimen.The influence of CFRP on the stiffness and Poisson’s ratio of CFST is not obvious,and its restraint effect mainly starts from the elastic-plastic stage.The bearing capacity of the specimen increases with the increase of CFRP layers.This increase is more obvious in the specimens with low water-cement ratio and large diameter-thickness ratio.The CFRP restrained specimen has a larger post-peak bearing capacity.The combination of proper expansion and CFRP can increase the bearing capacity of the specimen more significantly;It is difficult to accurately estimate the ultimate bearing capacity of CFST in the existing specifications.Compared with this,the design formula proposed in this paper can more accurately predict the bearing capacity of the specimen with expansion agent and CFRP constraint. |
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