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

The restriction on aggregate particle size in UHPC material design is a scientific choice based on packing density theory and fiber reinforcement mechanisms. Through 28 years of UHPC R&D practice, Qinglong Group believes that keeping the maximum aggregate particle size below 5mm is a key measure to ensure the material achieves ultra-high performance. This restriction stems from multiple considerations of the material's microstructure, mechanical properties, and construction technology.

From the perspective of packing density theory, small-size aggregates help achieve denser particle packing. When the aggregate particle size exceeds 5mm, the voids between particles increase accordingly, requiring more cementitious material to fill them, which leads to a higher water-binder ratio or reduced workability. Qinglong Group's test data show that when the maximum aggregate particle size is reduced from 8mm to 3mm, the compressive strength of UHPC can increase by more than 15% and porosity decreases by 20%. In the Shenzhen Yirui Biotechnology Building project, UHPC with 3mm maximum aggregate particle size achieved a compressive strength of 160MPa, realizing the optimization of material performance.

The fiber reinforcement mechanism requires the matrix to have a uniform microstructure. Oversized aggregates hinder the uniform distribution of fibers and form stress concentration points. When the aggregate particle size exceeds 5mm, fibers struggle to form an effective three-dimensional network structure between aggregate particles, reducing the toughening effect. Through CT scan analysis, Qinglong Group found that when the aggregate particle size is kept below 3mm, the distribution uniformity of steel fibers can reach over 95%, significantly improving the material's fracture toughness.

The performance of the interfacial transition zone is another important consideration. The interfacial transition zone between large aggregates and cement paste is a weak link in the material, and an increase in aggregate particle size means larger interfacial defect dimensions. Tests show that when the aggregate particle size increases from 5mm to 8mm, the thickness of the interfacial transition zone increases by 25%, significantly affecting material durability. Qinglong Group strictly controls aggregate particle size in major projects to ensure optimal interface performance.

Construction process requirements also support the choice of small-size aggregates. Small aggregates facilitate material flow and make it easier to cast complex components. In the production of ultra-high performance concrete, Qinglong Group keeps the aggregate particle size below 2mm, ensuring both material performance and construction requirements. This scientific particle size control is an important guarantee for UHPC to achieve outstanding performance.