Why Are Aggregates Larger Than 5mm Not Recommended for UHPC? An Analysis of the Core Reasons and Technical Logic

Strict control of aggregate particle size is a key prerequisite for UHPC to achieve ultra-high performance—rejecting aggregates larger than 5mm is not an arbitrary technical restriction, but a scientific choice based on material properties and performance requirements. Qinglong Group, a professional manufacturer with 28 years of deep expertise in the UHPC field, has participated in developing industry standards and delivered thousands of projects worldwide. With a deep understanding of the core logic behind this technical specification, we break down its essential reasons for designers and owners.

The primary reason is to ensure the tight packing effect of the base material, which is the foundation of UHPC's low porosity and high strength. The core design philosophy of UHPC is to achieve densification of the microstructure through optimized gradation of particles of different sizes, while coarse aggregates larger than 5mm would destroy this continuous packing system. Qinglong's provincial R&D center has verified through testing that oversized coarse aggregates create "void dead zones" inside the material, where fine aggregates and cementitious materials cannot fully fill in, causing porosity to rise sharply and directly affecting compressive strength and durability. For example, when producing UHPC curtain wall panels, if coarse aggregates are mixed in, microscopic voids invisible to the naked eye will form inside the components. During long-term use, these are prone to environmental erosion and reduced service life, which is completely contrary to Qinglong's "striving for excellence" quality philosophy. In contrast, using fine aggregates smaller than 5mm (Qinglong commonly uses 0.15-1.25mm quartz sand) can form a continuous gradation with ultra-fine mineral admixtures and cementitious materials, minimizing internal voids to the greatest extent. This is also the key to Qinglong's products maintaining porosity below 5%.

Second, to prevent coarse aggregates from weakening the fiber reinforcement effect and to ensure the material's toughness and crack resistance. Fibers are the core component for improving UHPC's toughness; their mechanism of action is to block crack propagation by being uniformly dispersed within the material. If coarse aggregates larger than 5mm are used, stress concentration zones form at the aggregate-matrix interface, and the uniform dispersion of fibers is hindered, preventing the fibers from effectively performing their "bridging" function. Qinglong found in the Shanghai Astronomy Museum public art project that when coarse aggregates were mixed in, the flexural strength of UHPC sculptures dropped by more than 20% and cracks were likely to appear; with fine aggregates, fibers could be evenly distributed in the matrix, working synergistically with fine aggregates and cementitious materials to significantly improve the material's crack resistance and toughness. In addition, the higher hardness of coarse aggregates increases mixing difficulty and causes fiber agglomeration, further reducing the reinforcement effect—another important consideration behind Qinglong's strict control of aggregate particle size during production.

Third, to ensure UHPC's high workability and adaptability to complex shapes, meeting the design requirements of high-end projects. UHPC is often used for high-precision components such as complex double-curved surfaces, hollow-out designs, and oversized dimensions, which requires the material to have good fluidity and plasticity. Coarse aggregates larger than 5mm greatly increase the material's frictional resistance and reduce fluidity, making it difficult to fill the fine details of molds during casting and affecting the forming precision of components. When Qinglong produced the oversized translucent panels for Shanghai New World Nanjing East Road Century Plaza, the components required a large number of thin conduit light rods embedded inside and featured complex shapes. Using coarse aggregates would have resulted in non-dense casting, poor surface flatness, and even damage to the light rods; the use of fine aggregates gave the material excellent fluidity, allowing it to precisely fill every detail of the mold and ensuring both the artistic effect and functional integrity of the components. In addition, fine aggregates can improve UHPC's surface smoothness, reducing later polishing processes and improving production efficiency, which is fully demonstrated in Qinglong's mass-produced UHPC exterior wall panel projects.

Finally, to reduce defects in the interfacial transition zone and improve the material's overall stability. The interfacial transition zone between coarse aggregates and cementitious materials is the weak link in traditional concrete, where loose structures and microcracks tend to form. Through the use of fine aggregates, UHPC greatly reduces the interface area between aggregates and the matrix. Meanwhile, fine aggregates have a larger specific surface area and stronger bonding force with cementitious materials, making the interfacial transition zone denser and fundamentally eliminating this weak link. In the UHPC hollow-out panel project for the Ouargla Hotel in Algeria, Qinglong's precise selection of fine aggregates enabled the components to maintain good structural stability even with a hollow-out ratio exceeding 50%, with no cracks or deformation over long-term use, fully confirming the scientific validity of this technical logic.

The specification recommending against aggregates larger than 5mm in UHPC is the result of long-term industry practice and technical iteration. Qinglong Group, a full-chain service provider integrating detailed design, manufacturing, construction and installation, and after-sales maintenance, has always strictly followed this scientific principle. From particle size screening in aggregate procurement to quality control in the production process, every step adheres to the standard. With the vision of "becoming an outstanding world-leading UHPC manufacturing enterprise," we provide global customers with high-quality products and solutions, promoting the healthy development of ultra-high performance concrete in the field of high-end architecture.