# Study on Dynamic Splitting Properties of S-PP Hybrid Fiber Concrete after High Temperatures
###### Abstract  
To study the dynamic tensile mechanical properties of steel polypropylene hybrid fiber reinforced concrete (SP-HFRC) after high temperature, split Hopkinson pressure bar (SHPB) dynamic splitting tests were carried out, and the optimal fiber content combination was obtained. With the plain concrete (PC) as the control, the effects of fiber addition on energy dissipation and failure forms of concrete specimens after high temperatures were analyzed. LS-DYNA software was used to simulate the dynamic splitting test. The results show that the splitting strength of specimens increases first and then deteriorates with the increase of temperature. After high temperatures, HFRC has a positive and negative fiber hybrid effect. Among the studied fiber mixture combinations, S1 PP0.2 (1 vol% steel fiber + 0.2 vol% polypropylene fiber) concrete has the best splitting resistance. Compared with PC, the splitting strength increases by 106.8% at 25 \({}^{\circ}\)C and 128.2% at 800 \({}^{\circ}\)C. From the perspective of energy, we can conclude that adding hybrid fiber can significantly improve the dynamic splitting and tensile toughness of concrete after high temperatures, and defining damage variables can better characterize the damage degree of concrete. PC cracks seriously after high temperatures, while S1 PP0.2 concrete cracks but does not disperse at 800 \({}^{\circ}\)C, showing ductile failure characteristics. By modifying some parameters of the HJC model, the state of high-temperature  
point and high ductility fiber into concrete for performance research. The purpose is to optimize the combination of the excellent properties of the fibers with different constitutive properties to meet the specific requirements of engineering construction [7; 8]. Polypropylene fiber has a low melting point, which will melt and form pores under high temperatures. High-temperature water vapor in concrete can be discharged through the pores, which can effectively prevent the occurrence of the high-temperature burst phenomenon [9] to improve the fire resistance of concrete. At the same time, the anti-permeability is improved so that the mobility of water, chloride ions, and air in concrete is reduced. To delay the deterioration of steel material performance [10], the performance of the steel and concrete can play a better \"positive hybrid effect\". Some progress has been made in the modification of steel-polypropylene hybrid fiber concrete. [PERSON] [11] analyzed the influence law of tensile strength and strain rate of steel polypropylene hybrid fiber concrete (SP-HFRC) with different fiber content and length diameter ratio and believed that steel fiber increased the peak tensile strength, while polypropylene fiber mainly increased the residual strength after the peak. The damage degree of concrete could be significantly improved after mixing. [PERSON] [12] systematically analyzed the strengthening and toughening mechanism of steel polypropylene hybrid fibers from the macro- and micro-scales and believed that steel fibers and polypropylene fibers could play a role in cracking resistance at different structural levels and loading stages, produce positive synergistic effects, and significantly improve the splitting tensile strength and flexural strength of self-compacting lightweight aggregate concrete. Some researchers have also considered the impact of a high-temperature environment. Compared with plain concrete, they have studied the improvement of the mechanical properties of SP-HFRC after high temperatures. [PERSON] [13] believed that concrete with only steel fiber is prone to burst after high temperature treatment and adding an appropriate volume fraction of steel fiber and polypropylene fiber can significantly improve the fracture performance and burst resistance of the concrete after high temperatures.