Mechanical combing-induced fiber alignment in UHPC: Specimen-thickness effect, fiber distribution, and mechanical performance

The low utilization efficiency of randomly distributed steel fibers in ultra-high-performance concrete (UHPC) remains a major obstacle to its cost-effectiveness and structural optimization. Existing fiber alignment methods suffer from a strong coupling between specimen thickness and fiber distribution characteristics, which hinders the scalability and transferability of their reinforcing effects across different scales. This study employed a mechanical combing method to align steel fibers and systematically evaluated the mechanical performance and alignment characteristics of UHPC specimens with different thicknesses. Mechanical combing effectively narrowed the fiber alignment distribution, with most fibers exhibiting inclination angles between 30° and 40°, and achieved a maximum zone-specific orientation efficiency factor of 0.81 after excluding the regions affected by the wall effect. The orientation efficiency factor remained essentially independent of specimen thickness, substantially weakening the coupling between specimen thickness and fiber distribution. Consequently, the mechanical properties were markedly enhanced. The strengthening effect of fiber alignment became more pronounced with increasing specimen thickness. The flexural-strength enhancement ratio increased from 26.6% to 57.2%, while the tensile-strength enhancement ratio increased from 17.2% to 33.1%. Fiber alignment transformed the compressive failure mode from shear-dominated to splitting-dominated failure. A semi-empirical strength model revealed that aligned fibers mitigate thickness sensitivity by reducing the thickness-effect exponent.