Why Are Aggregates Larger Than 5mm Not Recommended for UHPC?

Aggregate particle size control is a key technical requirement in UHPC preparation, and the industry generally advises against using aggregates larger than 5mm. This restriction stems from the material properties and performance targets of UHPC, directly affecting its strength, density, and durability. Understanding the core reasons for aggregate size limitations helps standardize raw material selection and ensure UHPC product quality.

I. Affects Close Packing Effect, Reducing Microscopic Density

The core advantages of UHPC rely on the close packing design principle, achieving low porosity (≤1%) through multi-level particle combination. Aggregate particles larger than 5mm are oversized and will disrupt the particle gradation balance, increasing voids between coarse aggregates and making it difficult for fine particles and active admixtures to fill them adequately, causing porosity to rise significantly. Qinglong's test data shows that when 10mm aggregates are used, UHPC porosity rises from 0.8% to over 2.5%, with density decreasing substantially. Reduced density not only directly affects compressive strength but also forms permeation channels for moisture and harmful media, weakening durability. In contrast, quartz sand with a particle size of 0.15-1.2mm can form a perfect gradation with cement, silica fume, and other components, achieving optimal packing and laying the foundation for high performance.

II. Causes Stress Concentration, Reducing Mechanical Performance and Toughness

The interfacial transition zone between large-sized aggregates and the UHPC matrix tends to become a weak link: the larger the particle, the smaller the interface area and the lower the bond strength, making stress concentration more likely under load and leading to crack initiation and propagation. With aggregates larger than 5mm, UHPC flexural strength drops by 15%-20%, fracture energy decreases by more than 30%, and toughness deteriorates significantly, failing to meet the “high toughness” characteristic requirement. In Qinglong's Shanghai Astronomy Museum project, comparative tests showed that UHPC sculptures using 5mm aggregates had 25% lower impact resistance than products using 2mm aggregates, making them unable to support large, complex forms. In addition, large-sized aggregates tend to cause uneven mixing and localized strength fluctuations, affecting product performance stability and increasing usage risks.

III. Affects Construction Performance and Surface Quality

Aggregates larger than 5mm increase the viscosity of UHPC slurry and reduce flowability, creating difficulties in mixing, pouring, and molding. This is especially problematic for thin-walled components, hollow-out designs, and fine decorative surfaces, where problems such as insufficient vibration compaction and surface defects are likely to occur. Qinglong's UHPC wood-grain translucent panels require fine surfaces and precise light transmission; using large-sized aggregates would result in an uneven surface and reduced light transmission uniformity. Meanwhile, large-sized aggregates may damage molds and increase production costs. For complex molds such as the tree-texture molds for Century Plaza on Nanjing East Road in Shanghai, large aggregates easily cause mold wear, affecting form precision. Small-sized aggregates, on the other hand, improve slurry flowability, ensure molding quality, and guarantee smooth, fine component surfaces.

IV. Qinglong's Aggregate Control and Practical Verification

Qinglong strictly controls aggregate particle size, selecting high-purity quartz sand of 0.15-1.2mm and removing large particle impurities through multiple screening processes. Each batch of incoming aggregates must be tested for particle size distribution to ensure that oversized particle content is ≤0.5%. During production, automated aggregate screening equipment is used to monitor particle size indicators in real time, preventing substandard aggregates from being put into use. Verified through extensive project practice, UHPC products made with small-sized aggregates achieve compressive strength above 150MPa and surface flatness deviation of ≤±2mm, perfectly suited to architectural decoration, sculptures, curtain walls, and various other application scenarios. Strict aggregate particle size control is a critical guarantee of Qinglong's leading UHPC product quality and also confirms the scientific basis and necessity of not using aggregates larger than 5mm.