Benefits of synthetic fibers on the residual mechanical performance of UHPFRC after exposure to ISO standard fire

This study investigated the effects of synthetic fiber type, replacing ratio of steel and polyethylene (PE) fibers, and hybrid use of polypropylene (PP) and nylon (Ny) fibers on the mechanical properties of ultra-high-performance fiber-reinforced concrete (UHPFRC) before and after exposure to an ISO 834 fire. For this, four types of synthetic fibers, i.e., PP, polyvinyl alcohol (PVA), PE, and Ny, and single straight steel fiber were considered. Scanning electron microscope (SEM) images were used to evaluate the status of synthetic and steel fibers after exposure to the fire, and mercury intrusion porosimetry (MIP) was used to analyze changes in the porosity of cement matrix. The test results revealed that the PVA fiber was most effective in enhancing the fire resistance and residual compressive, tensile, and flexural performances of UHPFRC. In particular, the PE fibers were ineffective in enhancing the fire resistance and preventing explosive spalling of UHPFRC. Increasing the steel fiber content up to 2% improved the mechanical properties of ultra-high-performance concrete (UHPC) under normal conditions; however, the steel fiber alone could not prevent the occurrence of explosive spalling under fire conditions. There was no clear effect of the hybrid use of PP and Ny fibers on the residual compressive, tensile, and flexural strengths of UHPFRC; however, it obviously improved the residual toughness due to a gradual decrease in the post-peak stress after the fire. It is thus recommend the use of single PVA or hybrid PP and Ny fibers to enhance the fire resistance of UHPFRC.