Effect of Polypropylene Fiber Content on the High-Temperature Performance of Steel Slag UHPS

To solve the problems of explosive spalling and sharp deterioration of mechanical properties of ultra-high performance sprayed concrete (UHPS) under high-temperature conditions during tunnel fires, four groups of specimens with different volume contents (0%, 0.2%, 0.3%, 0.4%) of polypropylene fiber (PPF) were designed based on the optimal mix proportion at room temperature. Multi-gradient high-temperature tests at 20 °C, 200 °C, 400 °C, 600 °C and 800 °C were conducted to explore the effects of PPF on the high-temperature damage evolution and spalling resistance of UHPS. The test results show that no obvious spalling occurs in specimens exposed to temperatures of 400 °C and below. Surface peeling appears in the group without PPF addition at 400 °C, and severe explosive spalling happens in the 0% PPF group at 600 °C to 800 °C, while all PPF-incorporated groups maintain structural integrity. The mass loss rate increases with the rise in temperature and PPF content, reaching 15% in the 0.4% PPF group at 800 °C. In terms of mechanical properties, compressive strength, splitting tensile strength, and flexural strength all rise first and then decline with increasing temperature, and the 0.3% PPF group reaches peak values at 400 °C (compressive strength: 135.95 MPa, splitting tensile strength: 23.24 MPa, flexural strength: 27.42 MPa). Flexural toughness decreases continuously as temperature rises, and the 0.2% PPF group exhibits the best toughness retention. This study clarifies the high-temperature modification effect of PPF on UHPS and its optimal content range, providing important theoretical support and an experimental basis for the fire safety protection design of tunnel lining concrete.