Spalling resistance and mechanical properties of strain-hardening ultra-high performance concrete at elevated temperature

This study aimed to investigate fire resistance of strain hardening ultra-high performance concrete (SHUHPC). A series of mechanical tests, spalling tests, thermal analysis, and microscopic observation were conducted. Polypropylene (PP) fibers of different dosages were adopted to mitigate spalling of SHUHPC. The finding showed that SHUHPC showed severe spalling even with 1.5 vol% polyethylene (PE) fibers. PE fibers were ineffective in spalling prevention. Creation of empty channels by PE fibers after exposure to 200 °C did not lead to a good spalling resistance of SHUHPC. To prevent spalling of SHUHPC, a dosage of 0.3 vol% of PP fibers was required. High thermal expansion of PP fibers before melting allowed the PP fibers to create microcracks in concrete and to enhance permeability at 150 °C, resulting in good spalling resistance of SHUHPC. Further, partially replacing PE fibers by PP fibers negatively affected tensile properties of SHUHPC at ambient temperature. Besides, the compressive of SHUHC was not affected by different proportions of PE and PP fibers at ambient temperature and high temperature.