Steel-free hybrid reinforcing bars for concrete structures

Extensive research has been conducted on the replacement of steel rebars with FRP rebars to eliminate the steel corrosion problem in conventional steel bar-reinforced concrete structures. However, as the performance of FRP rebars is substantially inferior in compression (due to issues such as fibre micro-buckling) than in tension, their use in concrete columns is generally not recommended, which poses a significant challenge when a steel-free structure is needed. This paper presents the novel steel-free hybrid rebar developed at the Hong Kong Polytechnic University that overcomes the above-mentioned problem: such a hybrid rebar typically consist of a central FRP rebar, a thin external FRP confining tube and an annular layer of high strength cementitious material such as ultra-high performance concrete. To demonstrate the performance of these hybrid rebars, results from a series of preliminary tests and associated modelling work are presented in the paper. These results indicate that: (1) the FRP rebar at the centre is well supported against bar buckling and fibre micro-buckling; (2) the compressive strength of the FRP material can be fully mobilized; (3) the stress-strain response of hybrid rebars can be designed to resemble an elastic-plastic response with some post-yielding hardening.