Investigating the influence of coarse aggregate characteristics and fiber dispersion on the flexural performance of ultra-high performance concrete (UHPC)

The flexural properties of ultra-high performance concrete containing coarse aggregate (UHPC-CA) strongly depend on the characteristics of coarse aggregates and the dispersion of fibers. However, the impact of the interaction between coarse aggregate and steel fibers on the bending performance of UHPC-CA is not well-understood. This study investigated the four-point bending properties of UHPC-CA to clarify the influence of coarse aggregates. Mechanical tests, laser scanning confocal microscopy (LSCM), scanning electron microscopy (SEM), and image analysis were performed to investigate the impact of coarse aggregate on flexural properties, considering coarse aggregate properties, and steel fiber dispersion. The results indicate that coarse aggregates affect both bending strength and toughness. The type of coarse aggregate primarily influences bending strength, while the quantity of coarse aggregate determines toughness. The peak strength is closely related to the strength of the parent rock, whereas toughness is significantly affected by fiber dispersion. Coarse aggregate can adversely impact toughness by their inherent brittleness and worsening the fiber dispersion within the matrix. Additionally, this paper provides parameters for selecting suitable coarse aggregate for UHPC. It is recommended that coarse aggregate added to UHPC possess a strength approximately 1.5 times that of the matrix, with a particle size not exceeding 10 mm and a content below 30 %.