Influence of Fine Aggregate Characteristics on the Fresh and Mechanical Properties of Ultrahigh-Performance Concrete

Abstract The present research examines two key challenges in ultrahigh-performance concrete (UHPC): reliance on proprietary mix designs; and the effects of fine aggregate morphology on the workability and mechanical properties of UHPC. This study aims to address scalability challenges in UHPC by developing mixes with locally available materials. A systematic mixing procedure was established to produce UHPC mixes achieving compressive strengths in the range of 100–175 MPa. These mixes incorporate supplementary cementitious materials such as ground granulated blast furnace slag and ultrafine slag, which were found to significantly improve mechanical performance and workability. Further, a comprehensive investigation was conducted to evaluate the role of sand morphology in UHPC using crushed sand (CS), quartz sand (QS), and river sand (RS) on packing density, hydration kinetics, flowability, and mechanical properties of UHPC. This study examined their effects on packing density, hydration kinetics, flowability, and mechanical properties. The results revealed that QS, characterized by smooth, spherical grains, significantly enhanced workability and achieved the highest 28-day compressive strength of 106 MPa, despite a lower packing density. In contrast, CS, with angular and rough-textured particles, exhibited higher packing density and improved interlocking; however, it led to lower strength (99 MPa). RS, composed of subangular grains with moderate angularity, showed limited interlocking capacity and achieved the lowest strength (91 MPa), though it provided acceptable workability. Micrographs from scanning electron microscopy revealed this paradox by showing crack propagation through microcracked CS grains, whereas the monolithic structure of QS exhibited resistance to fracture. These findings establish a practical framework for the design of UHPC mixtures using regionally sourced materials, while highlighting the critical role of fine aggregate morphology in governing the fresh and mechanical performance of UHPC.