Concrete-filled steel tubular structural members can be recognized as an effective mean to improve the mechanical behavior in terms of strength, stiffness, ductility, and energy dissipation for the initial recycle aggregate concrete deficiencies compared with natural aggregate concrete. A small-scale model of square concrete-filled steel tubular column–reinforced concrete beam frame realized employing 100% recycled coarse aggregates was tested under combined axial loads and cyclic reversed lateral flexure. The failure modes, plastic hinges sequence, hysteresis loop, skeleton curve, stiffness degeneration, energy dissipation capacity, and ductility of the frame were presented and analyzed in detail. The structural behavior of square concrete-filled steel tubular column–reinforced concrete beam frame with 100% recycled coarse aggregates was compared with circular concrete-filled steel tubular column–reinforced concrete beam frame made with 100% recycled coarse aggregates. A fiber-based program model for the nonlinear analysis of concrete-filled steel tubular column–reinforced concrete beam frames incorporating recycled coarse aggregates was developed using SeismoStruct, to highlight the effect of recycled coarse aggregate content on mechanical behavior of recycled aggregate concrete and the confinement effect provided by outer tubes on core concrete. The analysis results show that the numerical model can well simulate and predict the seismic behavior of concrete-filled steel tubular column–reinforced concrete beam frames with 100% recycled coarse aggregate content. Both experimental and numerical results demonstrate that concrete-filled steel tubular column–reinforced concrete beam frames with large content of recycled coarse aggregates have a receivable seismic performance, and it is feasible to apply and popularize recycled aggregate concrete into concrete-filled steel tubular structures in seismic regions.

This paper presents the results of an experimental study on the residual properties of recycled concrete after exposure to high temperatures. Three parameters were considered in this test: (a) high temperatures (i.e. 20, 200, 300, 400, 500, 600, 700 and 800°C); (b) recycled coarse aggregate replacement percentages (i.e. 0, 30, 50, 70 and 100%); (c) recycled coarse aggregate types (gravel and pebble). A total of 144 standard prism specimens (150 mm × 150 mm × 300 mm) of recycled concrete were prepared and subjected to different high temperatures for 1 h; after cooling down to ambient temperature, the following basic physical and mechanical properties were then tested and discussed: (a) surface change and mass loss ratio; (b) compressive strength; (c) compressive stress–strain curve; (d) elasticity modulus. Based on the mass loss ratio, a residual strength assessment method for recycled concrete after exposure to high temperatures was put forward.