Ultra-high-performance concrete (UHPC) has gained worldwide popularity due to its high mechanical performance. This research studied the influence of adding a mixture of two fibers (steel and PVA) on the compressive strength, modulus of elasticity, and flexural tensile strength of UHPC. The mixtures were prepared by adding steel fibers and PVA fibers using a standard procedure defined in the research, which is the time to mix the dry materials and the time to mix the admixture and water. The Central Composite Rotational Design (CCRD) methodology was used for the experimental design of the compressive strength and longitudinal deformation modulus tests. The results were analyzed using statistical software to investigate the influence of fibers on these two mechanical properties of UHPC. With this technique, an optimized design for the study of flexural tensile strength was arrived at. It was found that the standardized equations for the modulus of elasticity, directed to conventional concrete and high-strength concrete, are inadequate for estimating the modulus of UHPC in this research. Statistical analysis indicated that the range of fiber amounts analyzed did not significantly affect the compressive strength and modulus of elasticity. Regarding the optimized mixture, its flexural tensile strength indicated that the fiber content should be higher for UHPC to be suitable for structural use.
Keywords: CCRD; UHPC; compression strength; elasticity modulus; fibers; tensile strength.