## 3 Test Results and Analysis
### Mass Loss  
After specimens are subjected to high temperatures, its mass is reduced. In order to reflect this physical change, the mass loss ratio \(\beta_{m}=(M-M_{T})/M\times 100\%\) is defined, where \(M\) is the original quality of the specimen, and \(M_{T}\) is the mass of the specimen after exposure to high temperatures.  
The trend of mass loss ratio of specimens with the temperature experienced is given in Figure 7. It can be observed in the figure that the mass loss ratio \(\beta_{\rm m}\) of specimens increases with the increase of temperature. For maximum temperatures of 200-400 \({}^{\circ}\)C, the \(\beta_{\rm m}\) value increases the fastest, the reason is that after the temperature reaches 200 \({}^{\circ}\)C, the moisture retained in the concrete evaporates and the most combustibles in the concrete burn as they reach the ignition point. During the high temperature action of specimens in the electric furnace, when the temperature rises to 200-400 \({}^{\circ}\)C, a large amount of white mist emerges from the electric furnace. This phenomenon reflects that the internal moisture of the RAC is most significantly evaporated in the range of 200-400 \({}^{\circ}\)C. When the temperature is larger than 600 \({}^{\circ}\)C, the growth rate of mass loss ratio of specimens slows down, indicating that after the temperature increases to a certain extent, the mass loss ratio of specimens tends to be stable due to the exhaustion of moisture and combustibles.  
Figure 6: Apparent morphology of specimen after high temperatures. (a) 20 \({}^{\circ}\)C; (b) 200 \({}^{\circ}\)C; (c) 400 \({}^{\circ}\)C; (d) 600 \({}^{\circ}\)C; (e) 800 \({}^{\circ}\)C.  
Figure 7: The mass loss. (a) Prism; (b) SRRAC column; (c) RRAC column; (d) SRRAC beam; (e) RRAC beam; (f) Influence of temperature; (g) Influence of replacement percentage.  
For each type of specimens, the mass loss ratio of all specimens subjected to the same temperature is averaged, and Figure 7f is drawn. It can be seen from the graph that the increase of mass loss ratio of prism specimens is the most significant, and that of beams is the smallest, which may be due to the smallest volume of prism specimens and the easiest influence of high temperature field. The data in Figure 7g has averaged the mass loss ratio of all specimens under the same replacement percentage. It can be seen from the figure that the mass loss ratio increases with the increase of the replacement percentage. This is explained by the large amount of old cement slurry is attached to the surface of the RCA, and more free water can be absorbed during the mixing process. The more the RCA is contained, the more free water absorbs in the RAC, the more water will evaporate after exposure to high temperature, and the mass loss ratio will be greater.  
Comparing with SRRAC members, it is found that the mass loss ratio of RRAC members varies roughly the same with the increase of temperature. but the mass loss ratio of RRAC members is slightly larger than that of SRRAC members, which may be due to the fact that SRRAC members are equipped with more steel (the mass of steel is almost unchanged after high temperatures).