## 1 Introduction  
The use of recycled aggregate is one way to potentially extend the life of natural resources by supplementing their supply and reducing the environmental impacts of material extraction as well as the impact of construction demolition in landfills [1]. As a green building material, recycled aggregate concrete (RAC) can effectively deal with construction waste and realize the sustainable development of construction resources [2]. However, due to the accumulated internal damage caused by the secondary crushing of recycled coarse aggregate (RCA), the mechanical properties of RAC are weaker than those of natural aggregate concrete (NAC). Through a large number of experimental studies, scholars had confirmed that recycled concrete has a lower strength, elastic modulus, energy dissipation and durability but a larger peak strain, Poisson's ratio, shrinkage and creep, compared with NAC [3; 4; 5; 6; 7; 8; 9; 10; 11].  
Therefore, improving the inherent defects of RAC to promote the research and application of RAC structure has become a hot issue in the field of concrete research.  
In recent years, research on the combination of RAC and steel has been carried out gradually. Through the constraint effect of steel on RAC, the strength and deformation performance of RAC materials are improved, so as to realize the optimal combination of the two materials, and at the same time, construction waste is turned into treasure. Based on the mature research into concrete filled steel tube (CFST) structure, relevant scholars [12] have put forward the concept of a recycled aggregate concrete filled steel tube (RACFST) structure, that is, a new composite structure formed by RAC placed inside a steel tube. On the one hand, the core RAC is in the state of three-dimensional compression so the defects in the RAC's mechanical properties will be improved. On the other hand, the RAC can provide effective support to increase the stability of the external steel tube. Based on this theory, many scholars [13; 14; 15; 16; 17; 18; 19; 20; 21] have carried out experimental research on the mechanical properties of RACFST under different forces (e.g., axial compression, eccentric compression and cyclic loading). The results show that the bearing capacity of a RACFST column is similar to that of an ordinary CFST column, and that the static and seismic performance indices of a RACFST column (including a square steel tube and circular steel tube) were slightly different from those of an ordinary CFST column. Therefore, it is feasible to use RAC with reasonable preparation in a CFST, and a RACFST is an effective way to recycle waste concrete.  
With the development of research and the urgent needs of engineering applications, engineers pay more attention to the seismic behavior of the RACFST frame in terms of whether it can meet the requirements of existing seismic design codes, which is an urgent question to be answered through research. However, at present, research on the RAC filled circular steel tube (RACFCT) frame structure is still scarce. Therefore, it is necessary to carry out the corresponding research on the RACFCT frame to reveal its real internal mechanisms and seismic behavior index. This paper firstly presents a test of a small scale model of a circular CFST column-RC beam frame with RACs under low cyclic loading in order to investigate its seismic behavior. Furthermore, the seismic behavior indices (e.g., load-displacement curves, failure behavior, stiffness degeneration, energy dissipation capacity and ductility) of the RACFCT frame were analyzed quantitatively to enrich the research regarding the RACFCT structure and provide a reference for the research, design and application of the RACFCT structure. Finally, the general behavior of the RACFCT frame was compared with that of the reinforced concrete (RC) frame proposed by [PERSON] et al [22].