Axial Compression Mechanical Behavior Of Frp-steel Composite Tube Confined Concrete Stub Columns

A new type of Fiber Reinforced Polymer (FRP)-steel composite tube confined concrete (FSTCC) columns was proposed in the paper by combining corrosion-resistant FRP material with steel tube confined concrete (STCC) columns, whose steel tubes disconnect on both ends of the column and are not take axial load directly, according to the high durable requirements of new constructions of STCC columns (continuous rigid frame bridge piers), and the problems of poor ductility of FRP and the connection problems of FRP in engineering applications. Experimental study and theoretical analysis of the proposed FSTCC columns were conducted. The main works of this paper is follows as:(1) A total of21circular Carbon FRP (CFRP)-steel composite tube confined plain concrete stub columns,12circular CFRP-steel composite tube confined reinforced concrete stub columns,9circular CFRP-steel composite tube confined steel reinforced concrete stub columns and6circular Basalt FRP (BFRP)-steel composite tube confined plain concrete stub columns were test under monotonic axial compression, The parameters of the test are the type of fiber, the number of FRP layers, the diameter-to-thickness ratio of steel tube and the strength of concrete. The test results show that the axial load capacity of FSTCC columns increase almost linearly with the increase in the layers of FRP wraps; With the diameter to thickness ratio of steel tube increases, the ultimate bearing capacity of specimens decreased; The steel tube has yield before the stub column reaching the ultimate bearing capacity; With the increase of FRP layers, the shear failure mode of core concrete become less obvious.(2) According to the results of tests and the the theoretical analysis, the axial compression performances of FSTCC stub columns were studied, the interaction mechanism among FRP tube, steel tube and the core concrete was revealed. A stress-strain model of concrete has been proposed based on the regression analysis of experimental data in this pape, which applies to circular FRP-steel composite tube confined concrete. The predictions of the proposed model are in close agreement with existing test results. A practical bearing capacity formula for FSTCRC stub columns under axial compression was present.(3) A total of14square FRP-steel composite tube confined plain concrete stub columns,6square FRP-steel composite tube confined reinforced concrete stub columns and6square FRP-steel composite tube confined steel reinforced concrete stub columns were test under monotonic axial compression, The parameters of the test are the diameter-to-thickness ratio of steel tube, the number of FRP layers and the corner radius of square cross-section. The test results show that the axial load-deformation curves of all specimens show postpeak softening behavior followed by a stress plateau or bilinear ascending response, the more number of CFRP layers, the higher ascending curves after postepeak softening; The core concrete in CFRP-steel composite tube has large deformation ability than that in steel tube, but there are slight differences on load-capcity. When it comes to the same number of CFRP layers, the specimen which have small diameter-to-thickness of steel tube show higher load-capacity.