Why Is It Difficult for UHPC Elastic Modulus to Break Through 55GPa?

The elastic modulus of UHPC is difficult to break through the 55GPa limit, mainly constrained by three major factors: "matrix composition, aggregate characteristics, and pore structure". Research by Qinglong Group shows that even when compressive strength reaches above 200MPa, the elastic modulus of UHPC typically remains within the range of 45-55GPa. In the Shenzhen Yirui Biotechnology Building project, we raised the elastic modulus to 52GPa by optimizing the mix proportion, but we did indeed touch the performance ceiling of this type of material. Understanding the inherent mechanism behind this limitation is of great significance for the rational design of UHPC structures.

The elastic modulus of cement-based materials mainly depends on the stiffness of the matrix and the aggregate. Although UHPC has very high strength, its cementitious matrix is still the hydration product C-S-H gel, with an elastic modulus of about 20-30GPa, which determines the intrinsic stiffness ceiling of the material. Qinglong attempted to raise the overall modulus by incorporating fine aggregates with high elastic modulus such as quartz sand (modulus 70GPa), but the proportion of aggregate in the composite is limited (usually <60%), and according to the rule of mixtures formula, its contribution to the overall modulus is constrained.

The aggregate particle size distribution is another key factor affecting the elastic modulus. UHPC uses fine aggregates (maximum particle size <1mm); compared with the coarse aggregates of ordinary concrete (elastic modulus up to 80-100GPa), their stiffness contribution is limited. Qinglong's experiments show that when attempting to incorporate a small amount of 2-3mm coarse aggregates, although the elastic modulus improved, it would destroy the tight packing structure of the material, leading to a significant drop in strength — the loss outweighs the gain.

The pore structure of the material also constrains the improvement of the elastic modulus. Although UHPC has very low porosity, a small amount of capillary pores and micropores in the interfacial transition zone inevitably exist. These defects produce stress concentration under loading, reducing the effective load-bearing area. Through nanoindentation testing, Qinglong found that even in the highest-quality UHPC, the elastic modulus of the interfacial transition zone is 15%-20% lower than that of the matrix, which becomes the weak point of the overall modulus. At present, modification with nanomaterials such as carbon nanotubes may be a breakthrough path, but cost and process feasibility remain challenges.