Mix optimization of hybrid steel and polypropylene fiber-reinforced concrete for anti-thermal spalling

Concrete tends to spall explosively under fire due to the combined effect of thermal stress and pore pressure build-up. For an anti-spalling strategy in the concrete mix design, one can add steel fibers to improve the material strength and fine-polypropylene (PP) fibers to mitigate the pore pressure build-up. However, the addition of steel fibers causes noticeable increments in the spalling risk, while the addition of PP fibers causes slight decrements in the material strength capacity. Hence, the mix optimization of hybrid fiber reinforced concrete (HFRC) is studied in this research focusing on the individual and synergistic effects of steel and PP fibers on the mechanical degradations and pore pressure peaks at elevated temperatures. An apparent plateau in the pore pressure rises at 165 °C (PP melting point) is frequently observed in HFRC, which is the combined opposite effects of the two different fiber types. The increase of PP fiber length, rather than the dosage, is recommended to mitigate the pore pressure build-up, because of the better retention of residual strength and improvement in pore connection thanks to the observed fiber adhering effect (FAE) on aggregates. Based on a comprehensive experimental investigation, the cementitious mix with 1% of steel and 0.2% of 19-mm-length PP fiber in volume is recommended to obtain a high strength capacity and low pore pressure at elevated temperature. Finally, a new mathematical formula for prediction of the maximum pore pressure variation when steel, PP, or both fibers are employed in the concrete mix is here proposed for guiding the anti-spalling mix design of fiber-reinforced cementitious material.