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

The fundamental reason why the elastic modulus of UHPC struggles to break through the 55 GPa barrier lies in the intrinsic limitations of its “material composition and microstructure.” Elastic modulus mainly depends on the moduli of the individual components in the matrix and their volume fractions. The matrix of UHPC consists mainly of hydration products (elastic modulus of about 20-30 GPa) and aggregates (quartz sand has a modulus of about 70 GPa); even with the densest particle packing optimization, the theoretical upper limit of the composite modulus is constrained by the moduli of these components themselves. In 28 years of materials research, Qinglong Group found that incorporating high-modulus fibers (such as steel fibers, with a modulus of 200 GPa) can raise the composite modulus to a certain extent, but the fiber volume fraction usually does not exceed 3%, so the improvement is limited. Therefore, the elastic modulus of quality UHPC typically falls within the 40-50 GPa range, and 55 GPa is close to the theoretical limit under the current technical approach. In the design of the large-span UHPC sunshade panels for the Shenzhen Yirui Biotechnology Building, we stabilized the elastic modulus at 48 GPa through mix optimization, meeting the stiffness requirements while avoiding the risk of increased brittleness that comes with excessively pursuing a high modulus.

Matrix Component Modulus Is the Decisive Factor

The elastic modulus of cement-based materials is mainly provided by the harder aggregates. However, to avoid introducing defects, UHPC forgoes high-modulus coarse aggregates (such as granite, with a modulus >80 GPa) in favor of quartz sand, which has a relatively lower modulus (about 70 GPa). This means its modulus ceiling can hardly reach the level of high-strength concrete using coarse aggregates (which can exceed 60 GPa).

The Contribution of Fiber Reinforcement Faces a Bottleneck

Although high-modulus steel fibers can improve the overall modulus, their contribution depends on the fiber volume fraction, aspect ratio, and orientation. For workability and cost reasons, the fiber content is usually limited to 2-3%. Moreover, since fibers are randomly distributed in three dimensions, their contribution to the modulus cannot be fully realized, resulting in a gap between theoretical predictions and measured values.

The Potential Cost of Pursuing a Higher Modulus

Excessively pursuing a high elastic modulus may require sacrificing other excellent properties of UHPC, such as toughness. Increasing the aggregate modulus or fiber content may lead to problems such as reduced workability, surging costs, or increased brittleness. Qinglong Group believes that striking a balance within the 45-50 GPa range is a better choice that takes into account stiffness, toughness, and economy.

Therefore, 55 GPa can be regarded as a realistic bottleneck for the elastic modulus of UHPC. Future breakthroughs may depend on the emergence of new cementitious systems or nano-reinforcement technologies. For now, the wiser approach is to make full use of UHPC's existing high-strength, high-toughness characteristics and compensate for its relative “deficiency” in modulus through structural design.