Ultra-high-performance concrete (UHPC) is an advanced composite material known for its exceptional strength and durability, making it ideal for various civil engineering applications such as bridges, high-rise buildings, and offshore structures. However, the low water content and high solid volume fraction of UHPC present challenges in achieving the desired rheological properties for processing techniques like self-consolidating, pumping, spraying, and 3D printing. A recent review published in Engineering by Le Teng, Kamal H. Khayat, and Jiaping Liu provides critical insights into the rheological properties of UHPC and strategies for optimizing its performance.

The review highlights that the rheological properties of UHPC are influenced by a complex interplay of physical, physicochemical, and chemical factors. These include the water-to-binder ratio, the nature of cementitious materials, supplementary cementitious materials (SCMs), aggregate characteristics, fibers, and chemical admixtures. The authors discuss how these factors affect the yield stress, viscosity, thixotropy, and structural build-up of UHPC, which are crucial for its workability and processability.

One key finding is that the yield stress and viscosity of UHPC can increase significantly with a decrease in the water-to-binder ratio. For instance, the yield stress can increase by up to 100 times as the water-to-binder ratio decreases, but this increase can be mitigated through optimized particle size distribution and the selection of appropriate superplasticizers. The review also emphasizes the importance of understanding the relationship between mixture design and rheological parameters to balance contradictory requirements, such as low yield stress for ease of pumping and high viscosity for shape stability during 3D printing.

The authors delve into the effects of various constituents on UHPC’s rheological properties. For example, the use of fine cement particles and SCMs like silica fume and fly ash can enhance packing density and reduce yield stress and viscosity. The addition of fillers such as limestone powder can also improve packing density, while the incorporation of fibers affects the rheology based on their volume fraction, aspect ratio, and conformation. Chemical admixtures, particularly polycarboxylate ether superplasticizers (PCE) and viscosity-modifying admixtures (VMA), play a crucial role in modifying the rheological properties by influencing particle interactions and pore solution properties.

The review also addresses the challenges in characterizing the rheological properties of UHPC, such as the plug flow effect and the need for accurate measurement protocols. It suggests that the modified Bingham model is more appropriate than the Herschel–Bulkley model for characterizing shear-thickening UHPC. Additionally, the authors highlight the importance of considering the early-age hydration kinetics and the interaction between chemical admixtures and cementitious materials, which can significantly affect the structural build-up and thixotropy of UHPC.

The study concludes with strategies for tailoring the rheological properties of UHPC for specific applications, such as improving fiber distribution in self-consolidating UHPC, developing thin-bonded UHPC overlays for structural rehabilitation, and optimizing UHPC for 3D printing. The authors suggest that future research should focus on using novel binders, recycled materials, and active on-demand rheology control to enhance the sustainability and performance of UHPC.

This comprehensive review provides valuable guidance for researchers and practitioners aiming to optimize the rheological properties of UHPC for various applications, ultimately contributing to the advancement of sustainable and high-performance structural materials.

The paper “Recent Advances in the Rheological Properties of Ultra-High-Performance Concrete: A Critical Review,” is authored by Le Teng, Kamal H. Khayat, Jiaping Liu. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.04.011. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.