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

Restricting the maximum aggregate size in UHPC is an important technical decision based on its material design principles and performance requirements. This restriction is mainly driven by considerations of material densification, fiber distribution uniformity, and interfacial transition zone quality.

In the Shanghai Tower project, Qinglong UHPC strictly controlled the maximum aggregate size at 0.6mm. Comparative experiments showed that specimens using 5mm aggregate suffered a 25% reduction in compressive strength and significantly more microcracks in the interfacial transition zone. Specimens with fine aggregate proportions not only achieved higher strength but also showed significantly improved durability.

Densification Requirements

Weak interfaces easily form between large aggregates and the cement paste, affecting densification. Fine aggregates enable tighter packing, reducing porosity to below 3%.

Fiber Distribution Uniformity

Large aggregates hinder uniform fiber distribution, creating fiber-deficient zones. Fine aggregates ensure three-dimensional uniform fiber distribution, enhancing reinforcement effectiveness.

Interfacial Transition Zone Quality

A porous interfacial transition zone easily forms around large aggregates, becoming a weak link in performance. Fine aggregates improve the interface structure and enhance overall integrity.

Workability Considerations

Fine aggregate proportions offer better flowability and facilitate construction. Large aggregates tend to cause segregation, affecting molding quality.

Restricting aggregate size is an important technical measure for UHPC to achieve ultra-high performance and must be strictly implemented in production.