# Fire Behavior of U-shaped Steel Beams Filled with Demolished Concrete Lumps and Fresh Concrete
## 2 Experimental Program  
### Test Specimens  
Five beam specimens including three beams filled with DCLs and FC and two reference beams filled with FC alone were fabricated. All the specimens were 5300 mm long and mounted with the same clear span of 4400 mm. Steel plate with a nominal thickness of 3 mm (the measured value was 2.4 mm) was processed by a bending machine to form the U-shaped steel. L 50 \(\times\) 50 \(\times\) 3 mm steel angles with a length of 280 mm were welded to the top flanges of the U-shaped steel to transfer the longitudinal shear force between the steel and the in-filled concrete. The steel angles were positioned 400 mm apart along the clear span of the specimens. The measured yield strength and ultimate strength of the steel angles were, respectively, 286.4 MPa and 421.8 MPa. For each specimen, the thickness of the concrete slab was 90 mm and the slab was 300 mm wide at both ends and 700 mm wide in the middle to match the size of the furnace chamber. Double-layer bidirectional steel mesh was incorporated in the slab and steel bars 6 mm in diameter were employed as the longitudinal reinforcements (8\(\upphi\)6) and transverse reinforcements (\(\upphi\)6\(\upphi\)200). The measured yield strength of the steel bars was 358 MPa. Schematic diagrams of the specimens are given in Figure 2.  
The parameters considered in the tests included the replacement ratio of DCLs \(\eta\) (0% or 33%), the longitudinal reinforcements located at the lower part of the beam (none or 2\(\upphi\)18), the thickness of fire insulation \(t\) (10 mm or 18 mm), and the load ratio \(n\) (0.45 or 0.625). In this study, the replacement ratio \(\eta\) is a ratio of the weight of DCLs in the U-shaped steel to the total weight of the in-filled concrete  
Figure 2: Schematic diagrams of specimens (unit: mm): (**a**) top view, (**b**) A-A section, and (**c**) U-shaped steel and steel angle. and \(n\) denotes a ratio of the applied bending moment at the mid-span section to the flexural capacity of the specimen at room temperature calculated according to the formulas proposed by [PERSON] and [PERSON] [19]. Those formulas are derived from plasticity theory and have been validated in the test of U-shaped steel beams filled with DCLs and FC at room temperature [19]. Table 1 gives the details of the specimens. In this table, \(\rho_{\text{s}}\) is the longitudinal reinforcement ratio. \(N\) is the load applied by the hydraulic jack and \(R\) is the measured fire resistance of the specimen. The effect of the replacement ratio on fire behavior of the beams can be revealed by comparing the test results of Specim B1 (B2) with those of Specim B3 (B4). The influence of the longitudinal reinforcements on fire performance of the beams can be examined by comparing the test results of Specim B1 (B3) with those of Specim B2 (B4). For Specimens B4 and B5 subjected to the same applied load, the beneficial effect of increasing fire insulation thickness on the beam's fire resistance is compared with that of embedding longitudinal reinforcements. The fresh concrete for all the beams came from one batch of ready-mix. The demolished concrete was obtained from RC beams, which had served as a retaining and protection structure during excavation of the foundations for 16 months. Fifteen core samples (100 mm in diameter by 100 mm in height) were drilled from the waste beams and the compressive strength of these samples was measured. The measurements were used to estimate the 150 mm cubic compressive strength of the demolished concrete (i.e., \(f_{\text{cu, old}}\) in Table 1), according to the Chinese standard JGJ/T 384-2016 [28]. The proportions of fresh and demolished concrete are shown in Table 2. Both the fresh and the demolished concrete were made from Portland blast furnace-slag cement (P.S.A 32.5), natural crushed limestone (coarse aggregate) with a maximum size of about 20 mm, and local river sand (fine aggregate, 0-4 mm).