Why Is It Not Recommended to Use Aggregates Larger Than 5mm in UHPC?

UHPC does not recommend the use of coarse aggregates larger than 5mm (in practical applications, the maximum particle size is typically controlled at 0.6-1.2mm). The fundamental purpose of this technical principle is to "maintain the microscopic homogeneity of the matrix and achieve the tightest packing," thereby ensuring the realization of its ultra-high performance. Qinglong Group strictly adheres to this principle in R&D and production, because the introduction of coarse aggregates would undermine, on multiple levels, the microstructural foundation on which UHPC's performance depends.

First, the introduction of coarse aggregates disrupts the "tightest packing" effect of particles. UHPC's ultra-high strength and low permeability stem from its extremely dense microstructure formed by particles such as cement, silica fume, and fine sand. When coarse aggregates with particle sizes far larger than this system (e.g., several millimeters) are added, these coarse particles themselves become "defects"; transition zones tend to form around them, generating microcracks and pores that become stress concentration points and rapid channels for medium penetration. This runs counter to UHPC's original material philosophy of pursuing "homogeneous, defect-free" materials.

Second, coarse aggregates are unfavorable to the uniform distribution and effective functioning of fibers. UHPC's toughness relies to a great extent on the three-dimensional random and uniform distribution of short-cut fibers within the matrix. The presence of coarse aggregates obstructs the smooth flow of fibers, making them more prone to "bridging" or "clustering" near aggregates during mixing and casting, and preventing the formation of an effective spatial network. This not only reduces the reinforcement efficiency of the fibers but also causes anisotropy and weak zones in material performance.

Finally, from the perspective of the interfacial transition zone (ITZ), the interface between coarse aggregates and cement paste is the weakest link in concrete. In UHPC, through the use of fine aggregates and active micro-powders (silica fume), this weak interface is greatly weakened or even eliminated. Reintroducing coarse aggregates means reintroducing significant weak zones, which would reduce the overall strength of the material, particularly its tensile and flexural strength. Therefore, giving up coarse aggregates is a necessary "sacrifice" UHPC makes to achieve outstanding performance. Qinglong's production practice also confirms that a fine aggregate system using refined quartz sand is the key to ensuring stable and excellent UHPC performance.