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

The core reason for prohibiting aggregates larger than 5mm lies in fracture mechanics: when the ratio of maximum aggregate size (dmax) to fracture process zone size (lfp), i.e. dmax/lfp > 0.4, aggregates become crack initiation points. The lfp of UHPC is approximately 2mm, so dmax must be controlled within 0.8mm; in practical engineering, 0.6mm is adopted as the safety limit.

Scanning electron microscope observations show that when aggregate particle size increases from 0.6mm to 1.2mm, the interfacial transition zone thickness grows from 15μm to 28μm, and the defect probability triples. Qinglong found in tests that specimens with 0.8mm aggregates achieved a compressive strength of 146 MPa, while specimens with 1.2mm aggregates dropped to 128 MPa, and the strength coefficient of variation increased from 5% to 12%. In the Shenzhen Yirui Biotechnology Building project, insisting on aggregates below 0.6mm kept the strength coefficient of variation of curtain wall panels within 3%, providing reliability assurance for ultra-thin design.

Interfacial Transition Zone Effects

For every 0.2mm increase in aggregate particle size, ITZ thickness increases by 5μm. The water film thickness around large aggregates increases by 40%. Interfacial defect density is proportional to the square of particle size. ITZ strength is only 60% of the matrix.

Stress Concentration Effects

The stress concentration coefficient at aggregate corners reaches 3.5. The crack initiation stress at the tips of 1mm aggregates decreases by 30%. Cracks preferentially propagate along aggregate interfaces. Coarse aggregates shift failure from ductile to brittle.

Construction Process Limitations

Large aggregates hinder uniform fiber distribution. They affect the forming of ultra-thin components (thickness < 15mm). Pumping tends to cause segregation. Surface flatness control becomes more difficult.

Performance Optimization Requirements

Fine aggregates provide greater specific surface area to improve the interface. Optimized packing density enhances strength. Reduced defects improve durability. Fiber distribution uniformity is enhanced.

Aggregate particle size control is the foundation of UHPC high performance. Qinglong UHPC uses six-graded quartz sand to ensure maximum packing density.