# Fire Behavior of U-shaped Steel Beams Filled with Demolished Concrete Lumps and Fresh Concrete
## 5 Conclusions  
Fire behavior of the U-shaped steel beams filled with DCLs and FC has been experimentally investigated in this paper and numerical models are developed using SAFIR to determine the thermal and structural responses of the specimens. From the experimental and simulation results, the following conclusions can be drawn:  
1. Fire behavior of the U-shaped steel beam filled with DCLs and FC including failure process, temperature distribution, mid-span deflection, and fire resistance is similar to that of the reference beam filled with FC alone, which implies that using DCLs has a very limited effect on the fire performance.
2. Satisfactory construction quality can be achieved using the concrete pouring method in this study (i.e., 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).
3. The interface thermal resistance between the U-shaped steel and the in-filled concrete near the mid-height of the beam is greater than that at the bottom. The thermal resistance induces a temperature drop of up to 280 \({}^{\circ}\)C at the steel-concrete interface. The temperature drop decreases when the thickness of the fire insulation increases.
4. When the load ratio and the thickness of the fire insulation are the same, the mid-span deflection of the beam with a longitudinal reinforcement ratio of 0.7% develops much more slowly than that of the beam without longitudinal reinforcements and the fire resistance of the former is about 50% longer than that of the latter.
5. When the actual load ratio is 0.45 and the insulation thickness is 10 mm, the beams without longitudinal reinforcements and with a longitudinal reinforcement ratio of 0.7% can meet the requirements of fire resistance classes R90 and R120, respectively. At the same time, the beam without longitudinal reinforcements and with an insulation thickness of 18 mm can satisfy the requirements of fire resistance class R120 at an actual load ratio of 0.625.
6. Neglecting the interface thermal resistance between the U-shaped steel and the in-filled concrete will greatly overestimate the fire resistance of the beams. The thermal resistance at the interface between the web of the U-shaped steel and the in-filled concrete is suggested to be 0.03 m\({}^{2}\) K/W and at the interface between the bottom flange of the U-shaped steel and the in-filled concrete is recommended to be 0.02 m\({}^{2}\) K/W.  
It should be noted that some findings mentioned above are only based on the fire tests of five beam specimens and corresponding numerical analysis. More experiments are needed in the future to further improve the aforementioned quantitative values (e.g., thermal resistance at the interface between U-shaped steel and in-filled concrete).  
[PERSON] and [PERSON] conceived and designed the experiments. [PERSON] performed the experiments. [PERSON] and [PERSON] analyzed the data and wrote the paper.  
The authors would like to gratefully acknowledge research grants from the National Key R&D Program of China (grant 2017 YFC0803300), the National Natural Science Foundation of China (grants 51438007, 51378225), the Key Project of Science (Technology) Research of Guangzhou (grant 201607020005), and the State Key Laboratory of Subtropical Building Science of China (grants 2018 ZC03, 2017 KC17, 2015 ZB21).  
The authors declare that there is no conflict of interest regarding the publication of this paper.