## 3 Results and Discussion
### Residual Compressive Strength  
The residual compressive strength of the concrete is the most important characteristic that should be considered in assessing the post-fire response of concrete structures. Therefore, the compressive strength tests were initially performed by loading the cubic specimens to failure after heating to various target temperatures. Figure 11 and Table 4 show that the residual compressive strength of the SCC specimens without and with fibers is compared with different target temperatures. It is observed that there is a key temperature of 400 \({}^{\circ}\)C for SCC without and with fibers, which divides the temperature effect on the compressive strength into two parts. Firstly, when the target temperature is lower than 400 \({}^{\circ}\)C, no significant influence of the temperature on the compressive strength is observed. Figure 11a shows that a slight increase in the compressive strength is observed in plain and steel fiber-reinforced SCC compared to specimens under room temperature. This phenomenon can be explained as follows: (i) Free water and chemically bound water disappear at about 200-300 \({}^{\circ}\)C [52], which makes the microstructure of the concrete more porous. (ii) High temperatures can promote the hydration progress of the unhydrated cement particles that exist in the concrete matrix because of the low water-to-binder ratio of SCC, which continues to hydrate and increase the density of the matrix. (iii) Calcium hydroxide (CH), a hydration production of cement, reacts with active silica in fly ash and produces more calcium silicate hydrate (C-S-H). This can help fill the pores and microcracks induced by increased vapor pressure build-up under high temperatures inside the concrete matrix. Considering the comprehensive effects of the abovementioned complex physical and chemical changes, a strengthened concrete matrix is obtained. It is worth noting that for SF mixtures, the compressive strength is enhanced with the inclusion of SF. The favorable impact of SF on the compressive strength of the concrete is obtained when the temperature is lower than 400 \({}^{\circ}\)C. This is because higher thermal conductivity allows  
Figure 10: Layered color of cubic specimens after heating to the target temperature of 1000 \({}^{\circ}\)C: (**a**) SF20 and (**b**) SF50. the steel fiber-reinforced SCC to quickly dissipate heat in a much larger volume than in plain SCC. In the case of PF, there is no clear effect of the cocktail fiber blended with SF and PF on the residual compressive strength. Figure 11b illustrates that the incorporation of a single PF shows a negative effect on the residual compressive strength of the concrete. This is because PF melts leave microscopic channels inside the concrete matrix, which causes a great increase in the concrete's porosity and induces a great decrease in the residual compressive strength. Secondly, Figure 11 demonstrates that when heated to temperatures above 400 \({}^{\circ}\)C, whenever a single fiber or fiber cocktail blended with PF and SF is incorporated into the concrete matrix, the residual compressive strength decreases significantly. This is because the temperature damage on the concrete matrix has begun, such as decomposition of C-S-H, and significant cracks propagate in specimens. In this circumstance, the fiber's favorable effect for enhancing the residual compressive strength is far less than the effect of the temperature damage on the heated SCC. For example, in comparison to the compressive strength at room temperature, only 24-30% of that is reserved after heating to the temperature of 800 \({}^{\circ}\)C. When the target temperature reaches 1000 \({}^{\circ}\)C, almost all of the compressive strength is lost and only approximately 10% of the compressive strength is retained. These results are confirmed by the findings of other researchers [12; 35]. Finally, Figure 11b illustrates that the cocktail fiber specimens can preserve higher residual compressive strength in comparison with PF specimens.