## 3 Results and Discussion  
### Influence of Stress Levels on Chloride Ion Penetration in BFRC  
To analyze the influence of stress levels on the chloride ion penetration in BFRC, the chloride ion content of different depth layers of the concrete specimens with respect to different compressive stress ratios were compared. All the selected specimens were soaked for 28 days after being submitted to elevated temperatures (20 \({}^{\circ}\)C, 200 \({}^{\circ}\)C, 400 \({}^{\circ}\)C, 600 \({}^{\circ}\)C). The comparisons among the chloride ion content of concrete penetrated under different compressive stress are shown in Figure 2a-d. Figure 2 shows that, as the compressive stress ratio increased from zero to 0.3, the chloride ion concentration in the same layer decreased continuously. For example, at depth 2.5 mm, the chloride ion content was decreased by 3.85%, 13.7% and 16.9% at stress ratios 0.1, 0.2, and 0.3, respectively. In general, the bigger the stress ratio, the more significant the decrease in chloride ion content, implying that a stress ratio of 0.3 and below, inhibit chloride ions' invasion. However, chloride ion concentration of BFRC under different compressive stress ratios after exposure to high temperatures are quite different. The difference in chloride ion content in concrete that was soaked under compression after 28 days can be clearly found. Illustrations for each figure is shown below:  
1. Figure 2b shows that, after being heated to 200 \({}^{\circ}\)C, the chloride ion content in BFRC followed almost the same changing rule as for that at the normal temperature. From the outside to the inside layers, the chloride ion concentration presented a fast diminishing trend. However, the chloride ion content after 200 \({}^{\circ}\)C was much higher than that at room temperature, because at higher temperatures, the chloride ions are more likely to enter the concrete matrix and the permeability resistance of concrete decreased. 2. Figure 2c shows that compared to the normal temperature and 200 \({}^{\circ}\)C, the content of chloride ions at 400 \({}^{\circ}\)C significantly increased. After 400 \({}^{\circ}\)C, calcium hydroxide and gel in concrete began to decompose. High-temperature cracks also provided more channels for chloride ion diffusion, and the permeability resistance of the concrete significantly reduced. From the outside to the inside layers, the chloride ion content first decreased before it went up again with increased stress. The chloride ion content was lowest when the stress ratio is 0.1. At stress ratio 0.2, it recovered to that of the no-stress condition. At stress ratio 0.3, it increased again and was higher than that of the no-stress condition. This indicates that the chloride ion concentration in the concrete surface layer suddenly increased when compressive stress was more significant than a certain level. This value is known as the chloride ion penetration compressive stress ratio threshold of concrete [21]. The current study shows that concrete's chloride ion penetration compressive stress ratio threshold at normal temperature is about 0.5. However, after being exposed to a high temperature of 400 \({}^{\circ}\)C, the chloride ion  
Figure 1: Loading system. penetration compressive stress ratio threshold is between 0.1 and 0.2. This result has significant implications for concrete structures after a fire disaster. 3. BFRC specimens which were heated to 600 \({}^{\circ}\)C, under different stress levels (Figure 2d) shows that the chloride ion content of BFRC as a whole was significantly higher compared with that of lower temperatures. After 600 \({}^{\circ}\)C, a large amount of calcium hydroxide and gel in the concrete decomposed, density decreased, and high-temperature cracks continued to expand and penetrate, providing more channels for chloride ion diffusion.