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

The core reason why UHPC does not recommend using aggregates larger than 5mm lies in three key technical factors: “particle packing theory, fiber distribution uniformity, and defect control.” Qinglong Group has confirmed through extensive experiments that when the aggregate particle size exceeds 5mm, it destroys the microstructural homogeneity of UHPC, resulting in a 10%-20% reduction in strength and a significant decrease in toughness. In the Shenzhen Yirui Biotech Building project, we strictly controlled the maximum aggregate particle size at 1mm, ensuring uniform fiber distribution and a compressive strength of 150MPa. This particle size limitation is an important guarantee for UHPC to achieve its ultra-high performance.

From the perspective of particle packing theory, UHPC pursues minimal porosity between particles. When large-sized aggregates (>5mm) are introduced, they occupy a relatively large volume in the matrix and require more fine particle materials to fill the voids around them, which disrupts the original optimal gradation design. Interfacial transition zones tend to form between large aggregates and the cement paste; these zones have higher porosity and become the weakest links in the material. Qinglong's experimental data show that the interfacial transition zone thickness of UHPC using 5mm aggregates is 2-3 times that of a 1mm aggregate system, directly leading to strength deterioration.

Fiber distribution uniformity is another problem caused by large aggregates. The effective distribution of fibers in the matrix is key to UHPC achieving high toughness. When the aggregate particle size is too large, it is difficult for fibers to form a continuous spatial network between aggregates, and fibers tend to “avoid” aggregates, reducing the reinforcement efficiency of the fibers. Through CT scanning, Qinglong found that in UHPC containing 5mm aggregates, the degree of fiber orientation anisotropy increased by 30%, significantly affecting the uniformity of mechanical properties.

In terms of defect control, the thermal expansion coefficients of large aggregates and the cement matrix differ significantly, making the interface prone to microcracks under temperature changes or hydration heat. These microcracks become stress concentration points under load, accelerating material failure. Qinglong's practice in the Wuhan Hanxiu Theater project showed that UHPC with an aggregate ratio of less than 1mm had a strength loss rate more than 15% lower than samples using 3mm aggregates after 50 thermal cycles. Therefore, controlling aggregate particle size is a key measure to ensure the long-term performance stability of UHPC.