## 4 Residual Properties Analysis
### Bearing Capacity Degradation  
Figure 13 shows the degradation on bearing capacity of specimens after exposure to different elevated temperature. As seen from Figure 13a-d, compressive strength of test blocks after high temperature is deeply downtrend as the temperature ascend. The bearing capacity decreases by about 18% in the scope from 200 degrees centigrade to 400 degrees centigrade. And bearing capacity decreases by 48% when exposure to elevated temperature is at 600 degrees centigrade. Meanwhile, bearing capacity decreased by 74% when exposure to elevated temperature is at 800 degrees centigrade.  
After exposure to elevated temperature, the evaporation of free water and bound water in concrete leads to internal cracks. And then, the structure of hardened cement paste is crisp pine. The thermal performance of the coarse aggregate and concrete is inconsistent. Therefore, internal cracks continue to develop due to uncoordinated thermal expansion and syneresis micro-deformation. And then, the interface between aggregate and cement paste is further loosened when the temperature is above 700 degrees centigrade. Then, the pore size of cement slurry increases further. The crack between aggregate and slurry expands rapidly, the crack width increases. Therefore, the strength of concrete decreases. It can be proved by the scanning electron microscope (SEM) test in the literature [35].  
As seen from Figure 13e, the bearing capacity of SRRAC components decreases significantly, especially with higher exposure to elevated temperature. However, the amplitude of reduction is slower than the test block. The law of bearing capacity degradation of the bending beam and the shear beam is basically the same. The bearing capacity is degraded slowly when \(T\) is less than 400 degrees centigrade. The degradation rate speeds up when \(T\) is greater than 400 degrees centigrade. Bearing capacity degradation rate is about 30% when \(T\) is 600 degrees centigrade. Its value is equaled to 50% of the rectangular prism specimens. The law of bearing capacity degradation of SRRAC columns under compression is similar to the prism block. And the degradation rate of SRRAC column is slower than that of rectangular prisms specimens, when \(T\) is greater than 400 degrees centigrade. The bearing capacity degradation of compression components is more serious than that of flexural components after exposure to the same elevated temperature. The \(\beta_{\text{N}}\) of compressive components is about 11%  
Figure 13: Bearing capacity degradation. (**a**) rectangular prisms specimens; (**b**) columns; (**c**) bending beams; (**d**) shear beams; (**e**) The influence of temperature; (**f**) The influence of replacement percentage.  
to 22% smaller than that of flexural components in the scope from 200 degrees centigrade to 600 degrees centigrade.  
As seen from Figure 13f, the replacement percentage of recycled coarse aggregate has an insignificant effect on RAC test blocks and SRRAC components after exposure to elevated temperature. As the increase of replacement percentage, the variation ranges of the bearing capacity of prism test block, compression column, bending beam and shear beam are 1-16%, 1-4%, 2-1% and 6-10%, respectively. Generally speaking, the bearing capacity of specimens increases slightly with the increase in the replacement percentage. Free water in concrete evaporates at elevated temperature action to form pore. And then, the bearing capacity of concrete falls. However, the surface of recycled coarse aggregate is rough and porous, which can hold water firmly and reduce the evaporation of free water. So bearing capacity of RAC degenerates slower than ordinary concrete. In conclusion, after exposure to elevated temperature, the bearing capacity of SRRAC components is slightly better than that of ordinary steel reinforced concrete components.