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

UHPC's core advantages stem from its dense microstructure and excellent comprehensive performance, and limiting aggregate particle size to ≤5mm is the key technical decision for achieving this goal. As a leading enterprise that participated in drafting the UHPC industry standard, Qinglong Group, drawing on 28 years of R&D and production practice, analyzes the deeper logic behind this specification from the dimensions of material principles, performance impact, and process compatibility.

Disrupting the close-packing structure, affecting density and strength. The strength and durability of UHPC rely on the closest-packing design of the matrix material, the core of which is to minimize porosity (≤1%) through multi-level filling with particles of different sizes. Aggregate particles larger than 5mm are bulky and have a small specific surface area, making it difficult to achieve effective filling with cementitious materials (nano-scale silica fume, micron-scale cement)—the gaps between large aggregates require large amounts of cementitious material to fill, which causes the cementitious material consumption to surge, not only increasing costs but also potentially forming voids due to insufficient cementitious material; meanwhile, the presence of large aggregates disrupts the packing order of fine particles, creating weak spots prone to stress concentration in the microstructure. Qinglong Group's test data shows that when 8mm aggregate is used, the porosity of UHPC rises from 0.8% to 3.5%, and the compressive strength drops from 150MPa to below 100MPa, failing to meet UHPC strength standards. In the Century Plaza project on Nanjing East Road in Shanghai, Qinglong strictly used quartz sand ≤5mm to ensure the density of the light-transmitting concrete components, providing a structural foundation for the uniform distribution of light guide rods and high strength.

Hindering fiber dispersion, weakening toughness and crack resistance. Fibers are the core component enabling UHPC to achieve high toughness, and their effectiveness depends on being uniformly dispersed in the matrix to form a three-dimensional support network. Aggregate particles larger than 5mm have a "blocking effect" on fibers: on the one hand, the rough surface of large aggregates tends to entangle and clump with fibers, resulting in uneven fiber dispersion—some areas have insufficient fiber concentration while others form fiber clusters, reducing the overall toughness of UHPC; on the other hand, the presence of large aggregates limits the effective length of fibers, weakening their bridging effect during crack propagation. Qinglong's tests show that when 6mm aggregate is used, the dispersion uniformity error of steel fibers rises from ±3% to ±15%, and the fracture energy of UHPC drops by more than 40%. In the sculpture project at the Shanghai Astronomy Museum, the use of ≤5mm aggregate ensured uniform distribution of steel fibers, guaranteeing that components over 3 meters have excellent impact resistance without localized toughness deficiencies. In addition, large aggregates may scratch the fiber surface (such as steel fibers), affecting the interfacial bond between fibers and the matrix and further weakening toughness.

Deteriorating workability, affecting the forming of complex shapes. UHPC needs good fluidity and cohesion to accommodate the casting and forming of complex shapes such as hyperbolic surfaces, hollow-out patterns, and thin walls. Aggregates larger than 5mm significantly increase the internal friction of the material and reduce fluidity—more water is required to ensure casting performance, but excessive water destroys the close-packing structure, leading to reduced strength and durability; meanwhile, large aggregates tend to accumulate and clog in mold corners and thin-wall sections, causing forming defects in components (such as voids and pitted surfaces). In the hyperbolic exterior wall panel project for the Guangxi New Media Center, Qinglong Group used fine aggregate ≤5mm, bringing the flow spread of UHPC to 220mm and smoothly filling the complex curved molds; the formed components have smooth and fine surfaces, with installation precision controlled within ±2mm. If large aggregates were used, not only would complex shapes be impossible to achieve, but later repair work would also increase, affecting project schedule and quality.

Increasing durability risks, shortening service life. The interfacial transition zone between large aggregates and cementitious materials is a weak link in UHPC; the larger the aggregate particle above 5mm, the larger the area of the interfacial transition zone, and a loose layer tends to form due to uneven hydration reactions. This loose layer becomes an invasion channel for harmful substances such as moisture and chloride ions, accelerating material aging: in freeze-thaw cycle environments, water freezing and expanding in the interfacial transition zone easily causes cracking; in corrosive environments, harmful substances invade the interior through the interfacial transition zone, triggering fiber corrosion (metal fibers) or matrix degradation. Qinglong's durability tests show that UHPC with 5mm aggregate has a strength loss rate of only 4% after 200 freeze-thaw cycles, while samples with 8mm aggregate have a strength loss rate of 18%. In the Panzhihua Third-Front Construction Culture and Tourism project, the application of ≤5mm aggregate enabled UHPC curtain wall panels to meet the design requirement of no strength decay for over 20 years.

The decision not to recommend aggregates larger than 5mm for UHPC is based on comprehensive consideration of material structure, performance, and process, and is a necessary condition for achieving its "three highs and one low" characteristics. Qinglong Group strictly follows this technical specification, using precisely screened ≤5mm quartz sand and scientific mix proportions to ensure stable performance in every batch of UHPC products. Customers are advised to pay attention to aggregate particle size as a key indicator when selecting UHPC products, to avoid performance defects caused by manufacturers improperly using large aggregates. Qinglong always takes industry standards as its guideline, controls product quality from the source of raw materials, and provides customers with reliable architectural decoration material solutions.