Effects of Steel Fiber Type and Volume Fraction on the Mechanical and Impact Performance of Ultra-High-Performance Concrete

Ultra-high-performance concrete (UHPC) exhibits exceptional strength and durability but remains inherently brittle without fiber reinforcement. This study investigates the effects of steel fiber type, geometry, surface condition, and volume fraction on the mechanical and impact performance of UHPC. Five steel fiber types with different diameters, lengths, end geometries, and surface treatments were evaluated through compressive, flexural, split tensile, direct tensile, and impact tests. Fine straight steel fibers were also incorporated into ultra-high-performance mortar to provide matrix-level insight into fiber–matrix interactions. Steel fiber reinforcement significantly improved the mechanical performance of UHPC. Fine, straight, brass-coated fibers (≈0.2 mm diameter) produced the greatest enhancement, increasing compressive strength from approximately 121 MPa for plain UHPC to over 200 MPa at an optimum fiber content of 1.5 vol.%. Flexural strength increased from about 18 MPa to over 30 MPa, while direct tensile strength increased from 3.7 MPa to 9.5 MPa. Impact resistance also improved markedly, with absorbed energy increasing from 10 J for plain UHPC to 33–47 J for fiber-reinforced mixtures. Although longer mechanically anchored fibers produced lower gains in static strength, they exhibited comparable impact resistance. Increasing fiber volume fraction reduced slump flow, from approximately 555 mm for plain UHPC to approximately 390 mm at 2 vol.% fiber content. Mortar testing provided matrix-level insight into fiber-induced strength enhancement, although the magnitude of strength enhancement differed from that observed in the aggregate-containing UHPC, with compressive strength increasing from approximately 50 MPa to 181 MPa at 1.5 vol.% fiber content. A confinement-based model using the Mohr–Coulomb failure criterion provided a first-order interpretation of the observed compressive strength enhancement. The results indicate that the observed strength enhancement is associated with the combined effects of fiber–matrix interaction, crack bridging, and possible fiber-induced restraint. Under the specific mixture proportions, fiber characteristics, and curing conditions investigated, compressive strengths exceeding 200 MPa were achieved using the conventional aggregates considered in this study.