Why UHPC Does Not Recommend Aggregates Larger Than 5mm: Reason Analysis and Technical Considerations

In the microscopic world of Ultra-High Performance Concrete (UHPC), millimeter-level differences in aggregate particle size determine vast differences in material performance.

Amid the wave of innovation in building materials, Ultra-High Performance Concrete (UHPC) has become a new favorite in large landmark buildings, municipal engineering, and high-end decoration thanks to its outstanding mechanical properties and durability. As an industry leader with 28 years of deep expertise in UHPC/GRC/GRG/GRP materials, Qinglong Group has always taken the underlying logic of materials science as the core of its technical breakthroughs throughout its full-chain services, from detailed design and manufacturing to construction and installation. This article analyzes, from three dimensions—the essence of material mechanics, manufacturing difficulty, and engineering practice verification—why aggregates larger than 5mm are not recommended for UHPC, providing a professional selection reference for designers and owners.

I. The “Golden Ratio” of Aggregate Particle Size and UHPC Densification: 5mm is the Critical Point for Mechanical Properties

UHPC's “ultra-high performance” stems from its unique microstructure—achieving ultra-high matrix densification through optimized particle grading. The Qinglong Group R&D Center discovered through electron microscope scanning that when aggregate particle size exceeds 5mm, the porosity of the interfacial transition zone between the cement matrix and the aggregates rises significantly, directly causing a 15%-20% drop in compressive strength. This is because large-sized aggregates have difficulty forming tight packing in the cement matrix, and UHPC's unique steel fiber reinforcement system also requires fine aggregates to provide a uniform distribution space for the fibers. For example, in the light-transmitting concrete project of the Nanjing East Road Century Plaza renovation, Qinglong used 3-5mm quartz sand aggregates, pushing the flexural strength of components beyond 200MPa—more than five times that of ordinary concrete. This technical detail was the key to the project winning the “Silver Award of the First UHPC Building Engineering Innovation Award.”

II. Dual Constraints of Production Process and Construction Precision: Large Aggregates Increase UHPC Forming Difficulty

As a manufacturer of customized UHPC products, Qinglong deeply understands the impact of aggregate particle size on the production process. In precast component casting, aggregates larger than 5mm tend to cause insufficient mold filling, especially in complex curved shapes (such as hyperbolic exterior wall panels and irregular sunshade components), leading to defects such as air bubbles and voids. In on-site casting scenarios, large aggregates may block grouting pipes and affect construction efficiency. Qinglong's production base with an annual capacity of 600,000 square meters is built precisely on precise control of aggregate grading—through a three-stage screening system that strictly controls aggregate particle size within 0-5mm, combined with its self-developed vacuum vibration process, it has achieved an industry-leading standard of ±0.5mm component forming precision, which is also the core reason many landmark projects choose Qinglong as their supplier.

III. Long-Term Verification by Engineering Practice: 5mm Aggregate is the Balance Point Between Durability and Economy

From the UHPC curtain wall panels of a commercial complex in Guangzhou to the landscape components of an airport in Malaysia, Qinglong's project case library of more than 20 years shows that UHPC components using aggregates below 5mm perform better in impermeability, frost resistance, and carbonation resistance. Experimental data show that their chloride ion permeability coefficient is only 1/100 that of ordinary concrete, which is especially critical for coastal areas and overseas projects in high-humidity environments. Meanwhile, a reasonable aggregate particle size can reduce cement usage, optimizing material costs while ensuring performance. As emphasized in the “General Technical Conditions for Non-Load-Bearing Components of Ultra-High Performance Concrete (UHPC),” which Qinglong participated in formulating, aggregate grading is a core indicator of UHPC quality control, and 5mm as the upper limit of industry consensus is both a reflection of technical rigor and the crystallization of engineering practice wisdom.

In the application of UHPC materials, the choice of every technical parameter carries the dual responsibility of building safety and aesthetic expression. With 28 years of full-chain experience as its foundation, Qinglong Group deeply integrates materials science with engineering practice, not only providing customers with customized UHPC/GRC/GRG/GRP solutions, but also committing to advancing the improvement and upgrading of industry standards. Choosing the right aggregate particle size may seem like a minor technical detail, but it is in fact an important step toward fulfilling the mission of “creating beautiful architecture”—because we firmly believe that truly high-performance materials have always been hidden in precise control down to the millimeter.