Influence of Quartz Sand Gradation and Dosage on Workability and Strength of Ultra-High Performance Concrete

The particle size and dosage of quartz sand significantly affect the bleeding and segregation of UHPC mixtures, thereby influencing their strength and durability. However, the maximum particle size of quartz sand is a key focus of existing research, and the influence sensitivity of particle size gradation and dosage for UHPC performance are not clear. Based on this, this study systematically investigates the effects of particle size gradation and dosage of quartz sand on the slump flow and strength of UHPC, and the grey relational analysis method is employed to identify the sensitive particle size fractions. The results show that the compressive and flexural strength of UHPC with quartz sand mixing ratio of 40:40:20 and 40:50:10 is significantly improved at both 7 d and 28 d curing ages compared with those using coarse, medium, and fine quartz sand individually. The order of particle size affecting slump flow, flexural and compressive strength of UHPC is 0.6–1.18 mm > 0.3–0.6 mm > 1.18–2.36 mm > 0–0.075 mm > 0.075–0.15 mm > 0.15–0.3 mm. The key measure for enhancing strength and ensuring workability of UHPC lies in the proportion of quartz sand with particle size of 0.6–1.18 mm, which needs to be above 40% to serve a filler and framework function. When coarse, medium, and fine quartz sand mixing ratio equals to 40:40:20, the dosage increases lead to the decrease in UHPC slump flow, and as the quartz sand dosage varies from 900 kg/m3 to 1500 kg/m3, the 28 d compressive strength of UHPC first increases and then decreases. It is recommended to use a quartz sand dosage of 1050 kg/m3 ± 50 kg/m3 and mixing ratio of 0.6–1.18 mm quartz sand larger than 40% to produce UHPC exhibiting slump flow larger than 600 mm and compressive strength of 120–150 MPa. The findings provide important guidance for the preparation and performance regulation of UHPC.