Why are aggregates larger than 5mm not recommended for UHPC?

It is not advisable to use coarse aggregates larger than 5 mm in UHPC. The fundamental reason lies in maintaining the conditions required for the “maximum packing density theory,” avoiding the introduction of “size effects” and “weak interfacial zones,” thereby ensuring the material's extreme homogeneity and ultra-high strength. Through 28 years of UHPC research and development, Qinglong Group has come to deeply understand that large-particle aggregates (>5 mm) bring three major drawbacks: first, they disrupt continuous gradation, creating micro-crack sources at the aggregate–cement paste interface that become stress concentration points; second, large aggregates have a higher probability of internal defects and become weak points in strength; finally, they hinder the uniform distribution of fibers and compromise the toughening effect. Therefore, high-quality UHPC generally controls the maximum aggregate particle size at 0.6-1.2 mm to achieve a nano-scale dense structure. In producing the UHPC for the roof of the Hainan International Convention and Exhibition Center, we used refined quartz sand with a maximum particle size of 0.8 mm, ensuring a highly homogeneous matrix and laying the foundation for a compressive strength exceeding 150 MPa.

Maintaining Maximum Packing Density Is the Cornerstone of Strength

The strength of UHPC stems from its extremely low porosity. As aggregate particle size increases, a greater volume of cement paste is required to achieve the same packing density, leading to a higher water-binder ratio or more pores. Using fine aggregates (such as quartz sand) achieves better particle gradation, allowing the cementitious materials, fine aggregates, and ultra-fine powders (silica fume) to reach the tightest packing, with porosity reduced to below 3%.

Reducing Interfacial Defects to Improve Homogeneity

At the microscopic level, the interfacial transition zone (ITZ) between aggregates and cement paste is the weak link of the material. The larger the aggregate particle, the smaller its specific surface area, but the size of each individual interfacial region and its potential defect risk increase. Fine aggregates form more numerous yet smaller interfacial zones, resulting in better overall uniformity and more stable material performance.

Ensuring Uniform Fiber Distribution to Strengthen Toughening

Short chopped fibers (typically 6-13 mm in length) more easily achieve a uniform, three-dimensional random distribution within a fine-aggregate matrix, forming an effective reinforcement network. If the aggregate particle size is too large, fiber distribution is obstructed, and “fiber-depleted zones” tend to form around aggregates, weakening the toughening effect. Uniform fiber distribution is the guarantee of UHPC's high toughness.

Therefore, abandoning coarse aggregates is a necessary and scientific choice UHPC makes to achieve ultra-high performance. Through strict raw material control and particle gradation optimization, Qinglong Group ensures the ultimate perfection of the UHPC matrix.