## 5 Tensile Splitting Test Results and Analysis  
The splitting tensile strength results for plain concrete and PPF concrete at room temperature are shown in Figure 14, and the numbers in the bars represent the maximum splitting tensile strengths that the specimens could reach. It can be seen from the graph that the curve trend of tensile strength at lower temperatures was consistent with compressive strength. As the PPF content increased, the tensile strength initially increased, followed by a decrease. The tensile strength of plain concrete at room temperature was 3.11 MPa, and the peak tensile strengths of PPF, with lengths of 15 mm and 19 mm, were is achieved at a smaller dosage (0.5 kg\(\cdot\)m\({}^{-3}\)). The peak tensile strength of 3 mm and 9 mm PPF concrete occurred at a higher dosage (1.0 kg\(\cdot\)m\({}^{-3}\)), due to the shorter fiber length. Moreover, the curve shows that shorter fiber length resulted in a more significant improvement in tensile strength. The peak tensile strengths of PPF concrete at room temperature were PL3C1.0, PL9C1.0, PL15C0.5, and PL19C0.5, respectively. Compared to the compressive strength of ordinary concrete, they increased by 25.72%, 18.33%, 5.14%, and 0.64%, respectively. Figure 12: Compressive strength of PPF concrete at ambient temperature. Figure 13: Compressive strength of PPF concrete after elevated temperatures. The mechanism underlying the above phenomena needs clarification. The compactness of the internal structure of concrete, the tightness of the connection between various components inside the concrete, the number of defects, and the number of micro-cracks are all key factors affecting tensile strength. The three-dimensional mesh structure formed by PPF [61]. and the concrete colloid. can bridge the internal cracks of the concrete, effectively weaken the stress concentration at the crack tip, redistribute the stress, and change the crack direction, delaying the rate of micro-crack development [62; 63; 64] and effectively improving the splitting tensile strength of PPF concrete. The effect of fiber content on porosity is negligible. As dosage increases, the internal porosity increases, and the increase in fiber length also leads to a slight increase in porosity, providing more paths for crack propagation. Therefore, with an increase in fiber dosage, the compressive strength first increases and then decreases. However, the decrease in compressive strength is smaller than the effect of dosage. Short PPF is recommended for actual engineering applications under the same conditions. The splitting tensile strength test results for polypropylene fiber (PPF) reinforced concrete after exposure to high temperatures are shown in Figure 15. The tensile strengths of PPF concrete at 600 \({}^{\circ}\)C and 800 \({}^{\circ}\)C were approximately 1/66-1/60 and 1/70-1/90, respectively, of the residual compressive strength of ordinary concrete at room temperature. The curves for 3 mm and 9 mm length PPF concrete after high-temperature exposure show an initial increase followed by a decrease in tensile strength with increasing fiber dosage, which reached a maximum at a dosage of 0.5 kg\(\cdot\)m\({}^{-3}\). The fiber content had the largest effect on the 9 mm length specimens, with an increase in strength of 0.54 MPa and 0.42 MPa at 600 \({}^{\circ}\)C and 800 \({}^{\circ}\)C, respectively. For the 15 mm and 19 mm length specimens, the splitting tensile strength decreased with increasing fiber dosage, indicating that longer lengths had a more negative effect on tensile strength. The overall decrease in splitting tensile strength from 600 \({}^{\circ}\)C to 800 \({}^{\circ}\)C was approximately 20%, indicating further deterioration of the internal structure of the concrete and increased porosity, due to the decrease in the bonding ability of the C-S-H gel.