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

The elastic modulus of UHPC is an important parameter among its mechanical properties. Although it is significantly higher than that of ordinary concrete, it usually struggles to break through the upper limit of 55GPa. Based on materials science research, Qinglong Group believes that this phenomenon mainly results from the combined effect of multiple factors, including the intrinsic properties of cement-based materials, aggregate modulus limitations, interfacial transition zone characteristics, and fiber reinforcement mechanisms.

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 hardly exceeds 40GPa. Qinglong Group's test data show that the elastic modulus of the UHPC cement paste matrix is typically around 35GPa, which is the fundamental limitation on the material's modulus. By incorporating active admixtures such as silica fume, it can be slightly raised to 38GPa, but the room for improvement is limited.

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 with 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 tried using higher-modulus aggregates such as corundum, but found that their compatibility with the cement matrix was poor, which instead compromised overall performance.

The characteristics of the interfacial transition zone have a significant influence on the elastic modulus. Even with 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. By optimizing the mix proportion and curing regime, Qinglong Group has controlled the thickness of the interfacial zone to within 10μm, but it is difficult to completely eliminate its influence.

Fiber reinforcement offers only limited improvement in elastic modulus. Although steel fibers have an elastic modulus of up to 200GPa, their contribution to the macroscopic modulus of the composite is small because the dosage is only 2%-3%. More importantly, fibers primarily improve the material's toughness and post-cracking performance, with no significant effect on raising the elastic modulus. Through 28 years of practice, Qinglong Group has recognized that 55GPa can be regarded as a reasonable upper limit for the elastic modulus of UHPC, and that excessively pursuing a high modulus may sacrifice other important properties.