**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. Puthussery 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. Tabsh et al. [2], Koenders et al. [3], and Silva 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. Bairagi et al. [5] and Oikonomou [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. Ying et al. [7] and Wang 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, Arezoumandi et al. [9] and Choi 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. Choi et al. [11] and Xiao 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. Wu 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, Chen et al. [15] and Dong 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. Fathifazl et al. [17] and Ma 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]. Xiao et al. [21] and Wang 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. Peng et al. [23] and Ma 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. The dynamic responses and failure modes of ordinary concrete and RAC specimens were compared and discussed by using the shaking table test on RAC frame-shear walls [25].

In general, recycled coarse aggregate is different from natural gallet and pebble aggregate, and its porosity is high, thereby resulting in high water absorption, large dry shrinkage and creep, and poor bond performance of the RAC. Considerable research on the mechanical properties, components, and structures of RAC have demonstrated that RAC shows similar mechanical properties to ordinary concrete and can be widely used in construction after rational design. The main load-bearing components of RAC structures include recycled coarse aggregates, which were popularized during the construction of towns after the Wenchuan earthquake in 2008.

Steel structure residences have the advantages of strong seismic resistance, high industrialization, recyclability, and reduced resource consumption and construction waste discharge. They are also among the residential structure systems preferred by developed countries at present. In addition to using steel to build steel structure residences, the material development and application technology of enclosure systems have also been assessed. This scenario is a technical problem for new building energy-saving materials and systems, and a social and economic development problem for factory construction of housing in terms of changing construction modes. Tong et al. [26] and Sun et al. [27] tested semi-rigid steel frame-filled concrete shear walls and determined that the energy dissipation of this composite structure mainly depends on the aggregate friction and occlusion between the cracks of filler walls and the yield of shear studs. The structure has multiple transmission paths of horizontal loads and a high safety reserve. Kurata et al. [28] and Guo et al. [29,30] conducted research on a steel-plate shear wall with a semi-rigid steel frame and found that the structure exhibits the advantages of semi-rigid joints with good rotation capability as well as energy absorption through the yield deformation of the steel plate. The hysteretic performance is stable and characterized by the simplified construction and efficient utilization of materials. Wu et al. [31] conducted an experimental study on steel frames with replaceable reinforced concrete walls. Other tests, such as tests on steel frames with fabricated autoclaved lightweight concrete panels, composite lightweight walls, and light-gauge steel stud walls, were conducted under horizontal low cyclic loading [32–34].

Prefabricated construction systems have the advantages of fast construction, stable quality, and energy saving, along with being an environmentally friendly and sustainable development technology. In this study, two aspects of improvements were considered on the basis of the structure of steel frames filled with concrete shear walls as proposed by the previous scholars. On the one hand, the RAC concrete was used to replace the ordinary concrete, and concrete shear walls made of recycled coarse aggregate were introduced into the steel frame structure. The walls bore the horizontal loads as the main lateral resisting components of the structure when they functioned as the enclosure, thereby providing a new idea for popularizing and applying RAC in steel structure residences. On the other hand, the prefabricated connection between steel frames and concrete shear walls was also considered to facilitate the rapid construction of the project and the timely replacement of the damaged walls. Therefore, the structure of steel frames with infilled RACSWs (SFIRACSWs) is proposed in this paper, and tests on SFIRACSWs were conducted under horizontal low cyclic loading. The effect of two composite forms of steel frames and RACSWs (namely, infilled cast-in-place and infilled prefabricated) on the mechanical behavior of SFIRACSWs is discussed. The failure modes and cooperative mechanism of steel frames and RACSWs is clarified, and the main seismic performance indexes of SFIRACSWs are evaluated comprehensively, providing a theoretical basis for popularizing and applying SFIRACSWs in practical engineering.

## **2. Experimental Program**

## *2.1. Specimen Design*

To analyze the influence of concrete type, the composite forms of steel frames and RACSWs, and the connecting stiffness of beam–column joints (BCJs) on the hysteretic behavior of SFIRACSWs, this study designed six specimens of one-story and one-bay, at a 1:3 scale, and divided them into three groups. The first and second groups were designed to analyze the effects of the composite forms of steel frames and RACSWs, and the third group was a pure steel frame that was used as a reference specimen.

The first group comprised specimens of infilled cast-in-place RACSWs, which were labeled SPE1, SPE2, and SPE3. Only the wall of SPE1 was made of ordinary concrete, to illustrate the effect of concrete type. The walls of the first group were poured at the construction site and connected with steel frames through shear stubs. The second group consisted of specimens of infilled prefabricated RACSWs, which were labeled SPE4 and SPE5. The walls were prefabricated in the factory and assembled rapidly with steel frames through T-shape connectors embedded in the walls. Specimen SPE6 belonged to the third group.

The span and height of the specimens were 1050 and 1200 mm, respectively; the beam section was HN 150 × 100 × 5 × 8, and the column section was HW 150 × 150 × 7 × 10. Two forms of BCJs, namely, welded–bolted rigid joint and flush end-plate semi-rigid joint, were adopted, as illustrated in Figure 1a,b, to investigate the influence of the connecting stiffness of BCJs on the seismic performance of the SFIRACSWs. Specimens SPE1, SPE2, SPE4, and SPE6 adopted rigid joints, while the other specimens used semi-rigid joints. The main parameters of the specimens are presented in Table 1, and the dimensions and details of the specimens are illustrated in Figure 1.

**Figure 1.** Dimensions and details of specimens.


**Table 1.** Main parameters of specimens.

The diameter of the recycled coarse aggregate was 10–30 mm, and the replacement rate of the coarse aggregate was 100%. The cast-in-place RACSWs with a thickness of 90 mm were connected with steel frames by M16 shear studs, which were welded on the steel frames at 110-mm intervals, as depicted in Figure 1d. The 925 × 860 × 90 mm prefabricated RACSWs were connected with steel frames by ear plates, T-shape connectors, and high-strength bolts. The spacing of the bolts was 100 mm, and the rapid assembly of the walls and steel frames could be achieved as illustrated in Figure 1e. The ear plate connectors were welded on the flange of steel beams and equipped with sti ffeners at 200-mm intervals. The T-shape connectors were embedded in the concrete wall and welded with the steel bars of the hidden beams (Figure 1f).

Hidden beams and columns were set around the walls. Four Φ8 steel bars were in the hidden beams and columns, and the diameter of the stirrup spacing of 50 mm was 6 mm. A double-layer steel mesh was arranged on the wall, and the diameter of the horizontal and vertical steel bars with a spacing of 120 mm was 6 mm. The reinforcement details of the infilled walls are presented in Figure 2.

**Figure 2.** Reinforcement details of infilled walls.
