This study investigates the influence of metallic fibers on the workability and compressive strength of Ultra-High Performance Concrete (UHPC), a material increasingly used in advanced structural applications due to its superior mechanical properties and durability. Understanding the interaction between fiber geometry, length, and volumetric content with mixture fluidity remains a critical challenge in UHPC mix design and optimization. Six UHPC mixtures were developed with varying proportions of cementitious materials and aggregates, incorporating three types of steel fibers with different geometries and lengths at three volumetric contents. Workability was assessed through slump flow tests, while compressive strength was evaluated at 28 and 56 days. The results indicate that fiber characteristics significantly influence both fresh and hardened properties. Short and straight fibers exhibited a limited effect on compressive strength, whereas hooked-end fibers notably improved mechanical performance due to enhanced anchorage within the matrix. In contrast, the use of long and thick fibers at high volumetric contents negatively affected mechanical efficiency and reduced workability. Furthermore, it was observed that both fiber dosage and initial mixture fluidity play a decisive role in fiber dispersion, directly impacting the overall performance of UHPC. These findings contribute to a better understanding of fiber–matrix interactions and provide practical guidelines for optimizing UHPC mixtures. Potential limitations of this study include the controlled laboratory conditions and the specific range of materials used, which may influence the generalizability of the results.
M. et al. (Mon,) studied this question.