# Fire Behavior of U-shaped Steel Beams Filled with Demolished Concrete Lumps and Fresh Concrete
## 1 Introduction  
Recycling waste concrete contributes to environmental protection and reduces the depletion of this natural resource [1]. It has been extensively investigated in recent decades. Ongoing research is mainly related to recycled aggregate concrete (RAC), which is produced by substituting all or part of natural aggregates with recycled aggregates (i.e., crushed and sieved waste concrete debris). A series of studies on the mechanical properties [2; 3], thermal performance [4], and durability [5] of RAC and on the performance of structural members with RAC (including slabs [6], beams [7; 8], columns [9], wallboard [10], and frames [11]) have been carried out worldwide. Positive findings from these studies support and encourage the application of RAC in real buildings and structures. Guidelines and specifications for RAC have been documented by some researchers and organizations [12; 13; 14]. Although RAC provides an attractive means of recycling waste concrete, manufacturing high-quality recycled aggregate is still complicated and time-consuming and energy-consuming in actual practice since fine crushing, screening, and purification are needed. At present, a largeamount of waste concrete still ends up at waste disposal sites. In order to reuse waste concrete in a more straightforward and simplified way, a new kind of structural member containing both fresh concrete (FC) and demolished concrete lumps (DCLs) with a distinctly larger size than conventional recycled aggregates (e.g., 60 mm to 300 mm for DCLs versus \(\leq\)31.5 mm for recycled aggregates, according to the Chinese standard) has been proposed by the authors [15]. In this way, waste concrete only needs to be coarsely crushed into large pieces and both the old aggregates and the old mortar in the waste concrete can be reused. The unit energy consumption and the cost of producing DCLs are, therefore, significantly reduced [16]. A series of studies on structural members containing DCLs has been carried out over the past decade. They have involved flexural and shear tests on reinforced concrete beams containing DCLs [15], compressive and cyclic loading tests of steel tubes filled with DCLs and FC [17; 18], flexural tests of U-shaped steel beams filled with DCLs and FC [19], cyclic loading tests of composite shear walls filled with DCLs and FC [20], tests of composite slabs filled with DCLs, and FC at room temperature and in fire [21; 22], and compressive tests of FRP-confined compound concrete containing DCLs [23]. It is found that, when the compressive strength of demolished concrete is close to that of FC, the mechanical behavior, seismic performance, and fire resistance of structural members containing DCLs are similar to or only slightly inferior to those of conventional members made of FC alone. In recent years, structural members (including columns, beams, and slabs) containing DCLs have been successfully applied in real building structures [24; 25]. Figure 1 shows the application of the U-shaped steel beams filled with DCLs and FC in a real office building. Hollow U-shaped steel can serve as formwork and reinforcement for in-filled concrete. In addition, putting DCLs into hollow U-shaped steel is more convenient than placing DCLs into steel reinforcement cages composed of longitudinal bars and stirrups, which makes it an effective way to reuse DCLs in concrete filled U-shaped steel beams. Tests have shown that U-shaped steel beams filled with DCLs and FC have comparatively satisfactory flexural behavior at room temperature [19], but further research on the fire behavior of such beams is necessary especially considering that the U-shaped steel is not embedded in the concrete.