The Residual mechanical performance of UHPC with the addition of carbon steel and polypropylene fibers after exposure to 600 °C

Ultra-high-performance concrete (UHPC) is a material that has revolutionized the construction industry due to its versatility of use and the reduction of the cross-sections of structural elements. However, when exposed to high temperatures, UHPC can suffer damage and structural impairment due to the spalling effect. Therefore, the objective of this study is to analyze the influence of the addition of polymeric fibers, associated with steel fibers, to combat the degradation effect at high temperatures characteristic of UHPC. The concretes were molded with the addition of polypropylene fibers, in the percentages of 0.0%, 0.2%, 0.6%, and 1.0%, in a mixture with 75 kg/m³ of steel fibers. The samples were subjected to mechanical compression and flexural tensile tests before and after exposure to a temperature of 600 °C. The results obtained for flexural tensile strength demonstrate that the samples with the addition of 0.2% and 0.6% of PP fibers presented at the first age of the test, an increase of 40.32% and 51.20%, respectively, compared to the composition without the addition of fibers, while the test specimens with 1.0% addition presented a decrease of 17.95%. During exposure to high temperatures, the lower percentage of polypropylene fiber addition was already shown to effectively reduce the chipping effect. In this scenario, the addition of 1.0% PP fibers proved to be the best composition for maintaining the residual mechanical properties of flexural and compressive traction in both proposed curing regimes. With the aid of scanning electron microscopy, it was possible to identify the interaction of the fibers with the cementitious matrix, in addition to analyzing the influence of exposure to high temperatures on UHPC.