Why is it not recommended to use aggregates larger than 5mm in UHPC?

UHPC is not recommended for use with aggregates larger than 5mm. The core reason is that large-particle aggregates destroy its dense packing structure and reduce mechanical performance and workability. This rule is a scientific conclusion based on material characteristics and production practice, and Qinglong strictly follows this standard in all UHPC projects.

1. Destroys the Dense Packing Structure and Reduces Density

Dense packing is the foundation of UHPC's high performance, and large aggregates break the particle gradation balance. Increased porosity: aggregate particles larger than 5mm are coarse, and the particle size gap with the cementitious materials and fine aggregates is too large, forming large pores between particles that are difficult to fill, causing UHPC's total porosity to rise from ≤3% to over 8%; Reduced packing efficiency: the maximum dense packing theory relies on "complementary filling of large and small particles"; large aggregates cannot be fully enveloped by fine particles, forming "skeletal voids". Qinglong's experimental data show that when 10mm aggregates are used, density drops by 20%, directly affecting strength; Deteriorated surface quality: large aggregates easily cause pitted and exposed-aggregate surfaces on components, affecting the decorative effect. If Qinglong's light-transmitting components for Shanghai New World used large aggregates, the uniformity of light transmission would drop significantly.

2. Reduces Mechanical Performance and Increases Cracking Risk

Large aggregates cause stress concentration and weaken UHPC's strength and toughness. Strength degradation: the interfacial transition zone between large aggregates and the cementitious materials is prone to microcracks, with compressive strength dropping by 30%-40% and flexural strength dropping even more significantly (40%-60%). Qinglong's tests show that the compressive strength of UHPC with 8mm aggregates is only 65% of that of fine aggregate mixes; Insufficient toughness: large aggregates hinder uniform fiber distribution, so fibers cannot effectively bridge cracks, and UHPC is prone to brittle fracture instead of the ideal "cracking without collapse"; Reduced durability: large pores provide channels for corrosive media (water, chloride ions), reducing freeze-thaw and corrosion resistance. Qinglong's simulation experiments for coastal projects show that large-aggregate UHPC loses 15% of its strength after 200 freeze-thaw cycles, far exceeding the 5% loss of fine aggregate mixes.

3. Impairs Workability and Cannot Accommodate Complex Shapes

UHPC requires high fluidity to accommodate complex forming, and large aggregates severely restrict workability. Reduced flowability: large aggregate particles have high inter-particle friction, and the spread of fresh UHPC drops from ≥750mm to below 500mm, making self-compaction impossible; vibration is needed to fill the molds, which easily produces air bubbles; Poor adaptability to complex shapes: for complex components such as double-curved, openwork, and thin-walled pieces, large aggregates easily clog mold gaps and cause forming failure. Qinglong's double-curved UHPC components for the Guangxi New Media Center could never achieve precise filling if large aggregates were used; Reduced production efficiency: large aggregates easily cause uneven mixing and blockages in delivery pipelines, extending production cycles and increasing construction costs.