Concrete Cylinders Confined by Prestressed CFRP Filament Winding under Axial Load

During last decades, most R&D and field application projects have concentrated on concrete columns repair and reinforcement using composite materials, which result in considerable enhancement in strength and ductility of axially in compression loaded concrete. The use of composite materials could upgrade civil engineering structures and be the most effective steps to introduce composites into widespread civil engineering use. Experimental study of concrete cylinders confined by CFRP sheet displays a distinct bilinear stress-strain response with transition zone around the ultimate strength of unconfined concrete. The slope of the branches after transition zone and confining pressure depend on the volumetric ratio of the confining device and has lower modulus than the initial branch. The use of wrapped concrete columns above the transition zone is questionable because of internal damage of concrete and low tangential slope of loaddisplacement curve. The lower modulus may create problems to take favor of increased strength by confinement due to reduced Euler stability load. Therefore it would be favorable to raise the load level of the transition zone to be able to use the higher modulus of this initial branch to achieve better stability at least up to this load level. This function could be achieved by prestressing the confinement.The investigations were executed loading concrete cylinders with diameter 150mm and height 300mm with compressive load. Five different concrete series with cube strengths 34.2; 60.5; 76.2; 81.4 and 104.1MPa, that cover nearly the whole range in practice, were investigated. The parameters taken under consideration for the confinement were three levels of prestress enabled by the prestressing device 300, 600 and 800 N on yarn, which resulted in prestress in carbon filaments equal to 80; 160 and 210MPa (Carbon filaments had the tensile strength 3800 MPa and modulus 228 GPa). The confining reinforcement ratio was 2.48%. The friction between steel loading plates and concrete surface was reduced by three layers of teflon sheet. The specimens were fully instrumented and apart from load, axial and lateral strains were measured with LVDTs and strain gauges.The confining action of carbon filaments with prestress was effective enough to obtain a longer first elastic branch of stress-strain relation, showing stiff load-unload behaviors. The confining action was engaged from this early loading stage by the prestress of confinement, which delayed the formation of internal cracks. More restricting action by the prestressed confinements makes it difficult for the internal crack to propagate so the large crack was prevented to form. Theoretical prediction showed that prestress of confinement could elevate the transition zone with 20 to 30 percents. The higher the concrete strength, the less effective the confinement action was. However, a higher elastic limit in concrete behavior was obtained.Cyclic load caused no degradation in confining action. On the contrary, the ultimate strength could be higher, when load was increased after cycling. Stress-strain relationships of the monotonic load act like an envelope of that of cyclic load since they have similar loading paths. For the satisfactory prediction of strength, the stress analysis of crack growth inside concrete after transition zone should be accompanied by the analysis of confinement and its interaction with the damaged concrete