Multi-Scale Experimental Investigation of UHPC Rheology: From Cement Paste to Fiber-Reinforced Mortar Scale

Numerous studies have been published on various rheological aspects of conventional and high-performance concrete, some of which encompass multi-scale investigations. However, there is no published article that studies the rheology of ultra-high-performance concrete (UHPC) with a multi-scale approach. In this paper, a comprehensive investigation into the rheological properties of UHPC at three cementitious material scales was undertaken: the paste scale, the high-strength mortar scale, and the fiber-reinforced composite scale. The effect of cement type, supplementary cementitious materials (SCMs), and the water-to-binder ratio (w/b) on the rheology of UHPC at various material scales was evaluated using the appropriate rheometric apparatus. The results indicated that all of the UHPC mixtures in this study exhibited shear thickening behavior, and the degree of shear thickening increased as the w/b decreased. This phenomenon was systematically quantified at the paste, high-strength mortar, and fiber-reinforced composite scales, enabling direct comparison across material levels. Notably, the incorporation of silica fume suppressed the shear thickening behavior, as evidenced by the disappearance of the second-order term in the modified Bingham model, whereas slag had no such effect. The 28-day compressive strength of the investigated UHPC mixtures ranged between 100 and 150 MPa, and the mixture prepared with a combination of cement and silica fume (90C10SF) exhibited 35% higher compressive strength compared to the mixture prepared with cement and slag (90C10SL). Additionally, the UHPC mixture prepared with 90C10SF binder combination showed a 20% higher load-carrying capacity compared to the UHPC mixture made with 90C10SL and 80C10SL10SF binder combination.