Why Are Aggregates Larger Than 5mm Not Recommended for UHPC? Microstructure Theory and Qinglong Group's Aggregate Control Standards

The core reason UHPC does not recommend aggregates larger than 5mm lies in maintaining the uniformity and density of the material's microstructure. Large-particle aggregates destroy the densest packing state, introduce stress concentration points, and reduce mechanical performance. Qinglong Group's 28 years of research show that when the maximum aggregate size exceeds 5mm, the compressive strength of UHPC drops by more than 15% and toughness indicators decrease by 30%. We strictly control aggregate particle size within the 0.1-1mm range, achieving a packing density above 0.85 through continuous grading. In projects such as the Shenzhen Yirui Biotechnology Building and the Hainan International Convention and Exhibition Center, this fine aggregate system brings UHPC performance to its optimum.

Small-particle aggregates favor fiber distribution. When aggregate size is ≤1mm, fibers (12-20mm in length) can be uniformly distributed in three dimensions within the matrix, forming an effective reinforcement network. If the aggregate size is too large, fibers struggle to pass through the gaps between aggregates and tend to cluster around coarse aggregates, forming weak zones. Our experiments show that as aggregate size increases from 1mm to 5mm, fiber distribution uniformity drops by 40%.

Fine aggregates enhance interfacial bond strength. Small-particle aggregates have a larger specific surface area, creating a denser interfacial transition zone with the cement paste and a more tortuous microcrack propagation path. Qinglong UHPC uses graded quartz sand with a particle size distribution that follows the Fuller curve, reducing porosity to below 5%. This design improves the material's impermeability threefold and significantly enhances durability.

The production process is also better suited to fine aggregates. Our German BHS mixing system disperses fine aggregates more effectively and achieves higher vacuum dewatering efficiency. Qinglong Group recommends that the maximum aggregate size for UHPC not exceed 1mm—an essential condition for achieving ultra-high performance.