Analysis-oriented stress-strain model for concrete under combined FRP-steel confinement

Fiber reinforced polymer (FRP) jacketing of reinforced concrete (RC) columns is now a widely accepted strengthening/retrofit technique. The extensive existing research conducted on concrete in FRP-confined RC columns has led to a large number of stress-strain models, but most of these models are applicable only to FRP-confined RC columns with limited transverse steel reinforcement (i.e., steel hoops or spirals). In many RC columns such as those designed following modern design standards, the transverse steel reinforcement may provide significant confinement which needs to be accounted for in design. This paper presents an analysis-oriented stress-strain model for concrete in such columns where the concrete is under combined confinement from the FRP jacket and the transverse steel reinforcement (i.e., FRP-steel-confined concrete). Two alternative versions of the proposed model are proposed to cater for different considerations. Both versions of the proposed model are shown to provide accurate predictions of existing test results and to be superior to the existing models of the same type.