Coarse Aggregate Induced Fiber Dispersion and Its Role in UHPC Mechanics Across Flexural and Compressive Loading

Ultra-high-performance concrete (UHPC) exhibits exceptional mechanical properties and durability but faces challenges such as high heat of hydration and limited stiffness. Incorporation of coarse aggregates offers a potential solution; however, it alters the dispersion of steel fibers, thereby affecting the mechanical performance of UHPC under different loading conditions. This study systematically investigates the influence of coarse aggregates on UHPC performance under different loading conditions, including four-point bending, uniaxial compression, and triaxial compression tests. The spatial distribution of steel fibers was quantitatively analyzed via image analysis to elucidate changes induced by CA incorporation. Results reveal that with 20 vol% coarse aggregate (10 mm), UHPC's flexural strength is essentially unchanged (≈23 MPa), whereas flexural toughness decreases by about one-third. This toughness loss is linked to a slight increase in the fiber orientation angle (from 48.77° to 48.90°) and reduced continuity, which together weaken crack-bridging. Moreover, both flexural strength and toughness are governed primarily by the local steel-fiber content within the tensile zone. Under triaxial compression, confinement dominates: as confining pressure rises from 0 to 30 MPa, compressive strength increases by approximately 32.6%, 52.6%, and 71.3%. Due to crack-suppression by confinement overlapping with fiber bridging, the contribution of fibers to strength gains decreases with increasing confinement, and the competing and complementary interaction between coarse aggregate and steel fibers correspondingly weakens. These findings clarify the coupled effects of coarse aggregate and fibers in UHPC-CA, guide mix-design optimization for improved mechanical performance, and support broader practical adoption.