## 3 Test Results and Analysis
### Mechanical Properties
#### 3.4.1 Degradation of Bearing Capacity  
Figure 13 shows the degradation of bearing capacity of each specimen after exposure to different high temperatures. It can be seen from Figure 13a-g that with the increase of high temperatures, the compressive strength of prism specimens shows a significant downward trend. For maximum temperatures of 200-400 \({}^{\circ}\)C, the compressive strength of prism specimens decreases by about 18%, that decreases by about 48% when the maximum temperature is 600 \({}^{\circ}\)C and the compressive strength is only 26% at normal temperature when the maximum temperature is 800 \({}^{\circ}\)C. The strength of prism specimens can directly reflect the material strength of the RCA. The degradation of the compressivestrength of the RCA after exposure to elevated temperatures can be attributed to the fact that after the high temperature, the evaporation of free water and bound water in the concrete form the internal interface crack. The cement stone structure is damaged, and the thermal properties of the coarse aggregate and concrete are inconsistent. The micro-deformation caused by thermal expansion and synoresis is uncoordinated, resulting in the continuous development of interfacial cracks. In addition, due to the rapid cooling of the concrete during the cooling process, the central temperature drops slowly, and a non-uniform temperature stress field is formed between the two to generate additional cracks. The higher the temperature experienced, the more significant the damage of these cracks is, and the greater reduction of strength is. It can be observed in Figure 13h that the bearing capacity of different RAC member decreases significantly with the increase of the high temperature experienced, but the decrease is slower than that of prism specimens. Deformation laws of the bending and shear capacity of SRRAC beams are basically the same. When the temperature is lower than 400 \({}^{\circ}\)C, the bearing capacity degrades slowly, when the temperature is larger than 400 \({}^{\circ}\)C, the degradation accelerates, for the maximum temperature of 600 \({}^{\circ}\)C, the degradation amplitude is about 30%, only 50% of prism specimens. However, the law of bending and shear bearing capacity of RRAC beams is different. Bearing capacity of shear beams decreases with the increase of temperature, while the peak load of the bending beams shows a decreasing-increasing-decreasing tendency, fluctuating between \(-\)19% and \(-\)8%. The degradation of the compressive bearing capacity of RAC columns is similar to that of prism specimens. For exposure temperature greater than 400 \({}^{\circ}\)C, the degradation of columns is slower than that of prism specimens. From results of degradation, for SRRAC members subjected to the same high temperature, the bearing capacity of the compressive members is degraded more severely than that of the flexural members. For maximum temperatures of 200-600 \({}^{\circ}\)C, for SRRAC members, the degradation coefficient (\(\beta_{N}\)) of compressive members is 11%-22% less than that of flexural members, and for RRAC members, the degradation coefficient (\(\beta_{N}\)) of compressive members is 16%-20% less than that of flexural members. Figure 13: Influence of variation parameters on bearing capacity degradation. (**a**) Prism; (**b**) SRRAC column; (**c**) RRAC column; (**d**) SRRAC bending beam; (**e**) SRRAC shear beam; (**f**) RRAC bending beam; (**g**) RRAC shear beam; (**h**) Temperature; (**i**) Replacement percentage. It can be seen from Figure 13i that the replacement percentage of RCA has no significant effect on the compressive strength of RAC after exposure to high temperatures and the bearing capacity of SRRAC members subjected to high temperatures. With the increase of replacement percentage, the strength of prism specimens varies from 1% to 16%. For SRRAC members, the compressive bearing capacity of columns varies from \(-\)1% to 4%, the bending capacity of beams varies from \(-\)2% to 1%, and the shear capacity of beams varies from 6% to 10%.