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

The restriction on aggregate particle size in UHPC material design is a scientific choice based on close packing theory and fiber reinforcement mechanisms. Based on 28 years of R&D practice, Qinglong Group believes that controlling the maximum aggregate particle size below 5mm is a key measure to ensure the material achieves ultra-high performance.

From the perspective of close packing theory, small-sized aggregates facilitate denser particle packing. When the aggregate particle size exceeds 5mm, the voids between particles increase accordingly, requiring more cementitious material for filling, which leads to a higher water-binder ratio or reduced workability. Qinglong Group's test data shows 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%.

The fiber reinforcement mechanism requires the matrix to have a uniform microstructure. Oversized aggregates hinder the uniform distribution of fibers and create 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 controlled below 3mm, the distribution uniformity of steel fibers can reach over 95%.

The performance of the interfacial transition zone is another important consideration. The interfacial transition zone between large aggregates and cement paste is a weak point of 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. This scientific particle size control is an important guarantee for UHPC to achieve outstanding performance.