Dual Steel-Carbon Confinement of HSC Columns Considering Fire Resistance

Over the past few decades, the use of high-strength concrete (HSC) has become increasingly popular in reinforced concrete (RC) structures. For HSC columns that are confined by steel reinforcement, to maintain ductility similar to normal-strength concrete columns, a large amount of transverse steel is required. This often leads to transverse reinforcement congestion in these columns and consequently to casting difficulties. External application of fiber-reinforced polymer (FRP) wrappings can provide the required confinement; however, it has a disadvantage of low fire resistance. This paper presents an innovative textile confinement system based on steel and carbon fibers. In addition to the steel ties, the HSC columns are reinforced with an internal mesh of polymer-free carbon fibers. Before the concrete is cast, a carbon-fiber mesh is wrapped around the rebar ties, thus providing concrete cover protection to the whole reinforcing system. Consequently, by combining the steel's ductility with the stiffness and the fire safety of the carbon mesh, hybrid confined HSC columns provide the required confinement level, as well as fire resistance for HSC columns. Initial results from an experimental examination are presented together with their analysis. To explain the role of each of the reinforcement components, a parametric study examines the effect of different amounts of steel and carbon fibers on the uniaxial load-displacement behavior of circular columns. It is shown that based on these initial experimental results, the hybrid confining system proves to be effective in high-strength concrete columns, improving not only their structural ductility but also their fire safety.