# Fire Behavior of U-shaped Steel Beams Filled with Demolished Concrete Lumps and Fresh Concrete
## 3 Test Results and Discussions  
### Test Observation  
Generally speaking, the failure of the specimens containing DCLs and FC resembled that of the reference specimens containing FC alone. After 10 to 20 min of heating, steam was observed rising from the top of the specimens and from the interface between the U-shaped steel and the in-filled concrete at both ends of the specimens. The steam flow lasted until about 95 min. In most of the testing time, the mid-span deflection of the specimens increased slowly but it began to increase quickly as the heating time approached the fire resistance of the specimens. Figure 9 shows the specimens after the fire tests. It can be seen that all the tested specimens exhibited the same failure pattern (i.e., flexural failure).  
Figure 8: Locations of thermocouples (unit: mm).  
Figure 9: \(Cont\).  
After the fire test, no specimen showed any obvious slip at the interface between the U-shaped steel and the in-filled concrete at either end of the beam. This is quite different from some reported observations of simply-supported U-shaped steel beams filled with DCLs and FC subjected to two concentrated loadings at room temperature [19]. In those tests, a significant slip was observed at the interface between the U-shaped steel and in-filled concrete in the later stage of loading. This discrepancy probably arises because the modulus of elasticity of the steel decreases remarkably with a rising temperature, which facilitates its elongation when subjected to fire and loading.  
After the fire test, when the specimens were taken out from the furnace chamber, small cracks could be observed on the bottom and side surfaces of the concrete slabs, but almost no spalling of the concrete was observed due to normal-strength concrete being employed in this study.  
Although many cracks appeared on the surface of the fire-retardant coating, the insulation generally remained intact after the fire tests (see Figure 10). Fracture of the bottom flange and two webs of the U-shaped steel was observed in Specim B5 (see Figure 11), but it was not observed in any other specimen. The load ratio of Specim B5 was much higher than that of the others (see Table 1) and no longitudinal reinforcement was embedded in the lower part of the beam.  
Figure 10: Typical surface of fire insulation after the fire test.  
Figure 9: Photos of specimens after the fire tests: (**a**) Specim B1, (**b**) Specim B2, (**c**) Specim B3, (**d**) Specim B4, and (**e**) Specim B5.  
Figure 12 shows the surfaces of the in-filled concrete of Specimens B3 and B4 after the U-shaped steel has been removed. The cut sections of the in-filled concrete are shown in Figure 13. It can be seen that:  
1. The surface of the in-filled concrete is smooth and there are no obvious defects such as voids, holes, or pits on the surfaces or in the cut sections. This indicates that satisfactory construction quality can be achieved using the aforementioned concrete pouring method (all of the DCLs are put into the U-shaped steel at the same time and then the fresh concrete is poured with a continuous vibration).
2. The flexural cracks propagate from the bottom of the beam to the slab. The minimum crack spacing and the maximum crack width of Specim B3 are 180 mm and 15 mm, respectively. Specim B4 shows more and narrower cracks due to the presence of the longitudinal reinforcements in the lower part of the beam. Its minimum crack spacing is 80 mm and the maximum crack width of Specim B4 is 5 mm.
3. The cut sections of the in-filled concrete show that the DCLs and the FC bond well, which makes them work together.  
Figure 11: Fracture of U-shaped steel of Specim B5.  
Figure 12: Surface features of in-filled concrete after the fire test: (**a**) Specim B3 and (**b**) Specim B4.