# Study on Dynamic Splitting Properties of S-PP Hybrid Fiber Concrete after High Temperatures
## 3 Experimental Results and Analysis
### Dynamic Splitting Strength  
Dynamic splitting strength is the change of dynamic splitting strength of concrete with different fiber content combinations with temperature shown in Figure 7. It can be seen from the figure that under the same incident wave energy change rate, the splitting strength of SP-HFRC and PC increases first and then decreases with the increase of temperature. The temperature strengthening threshold temperature of S1 PP0.4 (1 vol% steel fiber + 0.4 vol% polypropylene fiber) and S1.5 PP0.4 concrete is around 100 \({}^{\circ}\)C, and the temperature strengthening threshold temperature of concrete with other fiber content is around 200 \({}^{\circ}\)C. At the same time, it is found that SP-HFRC shows a positive hybrid effect at room temperature, and its dynamic splitting strength is higher than that of PC. After 200 \({}^{\circ}\)C, the splitting strength of S1 PP0.4 and S1.5 PP0.4 concrete is gradually lower than that of PC, showing a negative hybrid effect. S1 PP0.2 concrete has the best splitting performance among the fiber content combinations studied. Compared with PC, the splitting strength at 25 \({}^{\circ}\)C is increased by 106.8%, and the splitting strength at 800 \({}^{\circ}\)C is increased by 128.2%. At 200 \({}^{\circ}\)C, the dynamic splitting strength of S1 PP0.2 concrete reaches a maximum of 16.93 MPa. Cause analysis: the mixed steel fiber and polypropylene fiber are randomly distributed in the micro-cracks in the concrete to prevent the occurrence of connected cracks, which significantly weakens the tensile stress causing damage to enhance the impact resistance and splitting tensile resistance of concrete. For steel fibers, the bond strength between the hydration products of cement and steel fibers gradually increases before 200 \({}^{\circ}\)C. The splitting performance is improved, the temperature continues to rise, the hydration products of concrete have different degrees of thermal decomposition, the concrete structure becomes loose, and the splitting performance gradually decreases. The melting point of polypropylene fiber is 165 \({}^{\circ}\)C. Before reaching the melting point temperature, polypropylene fiber is closely connected with the internal aggregate, giving full play to the role of crack resistance and toughening. It has an apparent strengthening effect on the splitting strength of concrete. After reaching the melting point temperature, the polypropylene fiber melts, fills the pores between the aggregates, and the compactness of the matrix is practical. The pores formed by melting are conducive to the discharge of high-temperature water vapor in the concrete, effectively improving the internal pressure environment, thereby improving the splitting performance and burst resistance of the material. When the volume content of polypropylene fiber is 0.4%, more pores are produced after melting. After being subjected to impact load, more cracks are generated through pore connectivity, and the splitting performance is seriously degraded, thus showing a negative hybrid effect.