Why Is It Difficult for UHPC's Elastic Modulus to Exceed 55GPa?

The elastic modulus of UHPC is an important parameter in its mechanical properties. Although it is significantly higher than that of ordinary concrete, it usually struggles to break through the upper limit of 55GPa. This limitation mainly stems from multiple factors such as the intrinsic properties of the material, component performance, and interfacial structure.

The intrinsic properties of cement-based materials determine the upper limit of the elastic modulus. The elastic modulus of cement hydration products is approximately 20-30GPa, and even with improved density through close packing, the matrix modulus can hardly exceed 40GPa. Test data from Qinglong Group shows that the elastic modulus of the UHPC cement paste matrix is typically around 35GPa, which is the fundamental limitation on the material's modulus.

The elastic modulus of aggregates is another limiting factor. Quartz sand, commonly used in UHPC, has an elastic modulus of 70-80GPa. Although higher than the cement matrix, it is still relatively low compared to metallic materials. According to the rule of mixtures for composite materials, the macroscopic elastic modulus of UHPC is constrained by the moduli of its components. Qinglong Group once attempted to use higher-modulus aggregates such as corundum, but found their compatibility with the cement matrix to be relatively poor.

The characteristics of the interfacial transition zone have a significant impact on the elastic modulus. Even when using the highest-quality aggregates, the interfacial transition zone between the cement matrix and the aggregates remains a weak link. Tests show that the elastic modulus of the interfacial zone is 20%-30% lower than that of the matrix, limiting the improvement of the composite's modulus. Qinglong Group has controlled the thickness of the interfacial zone to within 10μm by optimizing the mix proportion and curing regime, but it is difficult to completely eliminate its influence.