Influence of synthetic fibers on the performance of ultra-high performance concrete (UHPC) at elevated temperatures

Adding synthetic fibers to ultra-high performance concrete (UHPC) is an effective method for improving its resistance to high temperatures. This study examined the impact of different synthetic fibers on high-temperature spalling and corresponding residual mechanical performance of UHPC, including compressive strength, elastic modulus, and flexural strength. This study conducted high-temperature tests on UHPC with different synthetic fibers (PET, PP, NY, PVA, PAN fibers) and analyzed their microscopic morphology after exposure to 200 °C using scanning electron microscopy (SEM). The findings revealed that conventional steel fiber UHPC (without synthetic fibers) experienced severe spalling only after 400 °C. To some degree, synthetic fibers enhanced the resistance of UHPC to high-temperature spalling, but significant differences were observed among the different fibers. PP fibers provided the most significant improvement in high-temperature resistance, followed by NY fibers, while other fibers were less effective. Although the contributions of different synthetic fibers to high-temperature resistance varied, the differences in compressive strength and elastic modulus of UHPC with different synthetic fibers after high-temperature exposure were not substantial. NY fibers, despite being slightly less effective than PP fibers, provided superior flexural performance at both room and high temperatures compared to PP fibers. Both PP fibers and NY fibers inhibited high-temperature spalling of UHPC and maintained its mechanical performance. Even at 1050 °C, UHPC that did not spall retained residual mechanical performance, with compressive strength, elastic modulus, and flexural strength after exposure to 1050 °C being 24.4 %, 29 %, and 26.8 % of their respective values at room temperature.