## 1 Introduction  
Recycled aggregate concrete (RAC) can fundamentally solve concrete waste problems not only by reducing environmental pollution from waste concrete, but also preserving natural aggregates and reducing the consumption of natural resources and energy. RAC is one of the main approaches to developing a circular economy and promoting environmentally friendly buildings. The suitability of waste concrete as recycled aggregate for construction projects has been investigated. [PERSON] et al. [1] reported that recycled aggregate can be used as a building material for road construction, mass concrete engineering, and lightly reinforced sections, thereby providing ideas for recycling concrete waste. With regard to the mechanical properties of RAC, the compressive and tensile strengths of recycled coarse aggregate concrete with different sources and strength grades were studied. [PERSON] et al. [2], [PERSON] et al. [3], and [PERSON] et al. [4] found that the strength reduction of RAC is clearer with low-strength coarse aggregate than with high-strength aggregate, and the compressive and tensile strengths of RAC made of 50 MPa coarse aggregate are equal to those of natural aggregate concrete. [PERSON] et al. [5] and [PERSON] [6] proposed stress-strain curves of RAC with different aggregate replacement rates in which the constitutive relation of RAC with different aggregate replacement rates was similar, and only the decline stage was different. [PERSON] et al. [7] and [PERSON] et al. [8] studied the diversity of chloride ion diffusion in RAC and analyzed the influence of carbonation modification on the interface properties of RAC. Carbonation can improve the interface properties of RAC, especially when the water-cement ratios of new and old cement mortars are high; thus, the improvement effect is noticeable. With respect to the components of RAC, [PERSON] et al. [9] and [PERSON] et al. [10] tested the shear strength of RAC beams under short- and long-term loads and discussed the applicability of the design code, modifying compression field theory to the shear strength of the RAC beams. The axial compressive performance of RAC columns was also investigated. [PERSON] et al. [11] and [PERSON] et al. [12] found that the maximum axial compressive bearing capacity of RAC columns decreases slightly with an increase in the replacement rate of recycled coarse aggregate, and RAC columns can be used for the load-bearing member of the structure. [PERSON] et al. [13,14] proposed a concept for recycling mixed components based on the recycling technology of large-scale waste concrete blocks and systematically studied thin-walled steel tubular columns, U-shape steel beams, and thin steel-plate walls filled with RAC. By testing RAC-filled square steel tube (RACFST) columns and RACFST columns strengthened by carbon-fiber-reinforced polymer, [PERSON] et al. [15] and [PERSON] et al. [16] found that the RACFST columns exhibited good seismic performance under low axial compression; the aggregate replacement rate demonstrated a minimal influence on the RACFST columns. [PERSON] et al. [17] and [PERSON] et al. [18] studied steel-reinforced RAC (SRRAC) beams and columns and investigated the effect of the replacement rates of recycled coarse aggregate, axial compression ratios, and stirrup ratios on the seismic performance of the SRRAC columns. For the RAC structures, the seismic performance of two connections was compared and analyzed by testing the RAC beam-column joints (BCJs) [19,20]. [PERSON] et al. [21] and [PERSON] et al. [22] performed the shaking table test on RAC frames and reported the dynamic response of RAC frames. Tests on RAC shear walls (RACSWs) have been conducted. [PERSON] et al. [23] and [PERSON] et al. [24] found that the existing formulas cannot predict the peak load and failure modes of squat RACSWs and proposed a mixed flexural and diagonal compression mechanism.