Steel-free hybrid reinforcing bars for concrete structures

Extensive research has been conducted on the replacement of steel rebars with fibre-reinforced polymer rebars to eliminate the steel corrosion problem in conventional steel bar-reinforced concrete structures. However, as the performance of fibre-reinforced polymer 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; this poses a significant challenge when a steel-free structure is needed. This article presents a novel steel-free hybrid rebar developed at The Hong Kong Polytechnic University that overcomes the above-mentioned problem. Such a hybrid rebar typically consists of a central fibre-reinforced polymer rebar, an external fibre-reinforced polymer confining tube and an annular layer of high-strength cementitious material such as ultrahigh-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 article. These results indicate that (1) the fibre-reinforced polymer rebar at the centre is well supported against bar buckling and fibre micro-buckling, (2) the compressive strength of the fibre-reinforced polymer material can be fully mobilized and (3) the stress-strain response of hybrid rebars can be designed to resemble an elastic-plastic response with some post-yielding hardening.