What Is the Dense Packing Design Principle of UHPC Base Materials?

The close-packing design principle of UHPC base materials is the physical cornerstone of their ultra-high strength and durability. Its core lies in achieving optimal spatial filling between particles through "optimized particle grading, maximized packing density, and minimized porosity." Drawing on 28 years of materials R&D experience, Qinglong Group has refined this principle into the coordinated application of the "continuous grading model" and the "particle interference model": cementitious materials (cement, silica fume) and fine aggregates (quartz sand) of different particle sizes are scientifically proportioned so that the voids between large particles are filled by medium particles, and the voids between medium particles are in turn filled by small particles and ultra-fine powders (such as silica fume, with particle sizes of approximately 0.1-1 μm), ultimately reducing porosity to below 3%. In formulating the roof UHPC for the Hainan International Convention and Exhibition Center, we introduced the Andressen packing model for computational optimization, bringing the packing density of the mix to over 80%, laying a dense foundation for compressive strength above 150 MPa.

Optimized particle grading is the prerequisite for close packing

Close packing is not simply about using fine particles; rather, it pursues a continuous distribution of particle sizes. In its mix design, Qinglong Group precisely controls the proportions of cement (D50≈15-20 μm), silica fume (D50≈0.1-0.5 μm), and quartz sand (D50≈0.1-0.6 mm) to form a continuous grading from the micron to the millimeter scale. This "coarse grains filled by fine grains" effect allows solid particles to achieve the densest possible arrangement in space, significantly reducing water demand and voids.

The key role of ultra-fine powders

Ultra-fine powders such as silica fume (particle size < 1 μm) play a dual role in close packing as "micro-ball bearings" and "reactive fillers." They not only effectively fill the nanoscale voids between cement particles, but their extremely high specific surface area also promotes hydration reactions, generating more hydration products that further seal micropores. Qinglong Group selects high-activity silica fume, typically dosed at 20-30% in UHPC, which is key to achieving low porosity.

Achieving minimum porosity and water demand

The ultimate goal of close packing is to achieve the lowest porosity and water demand. Through optimized grading, the water-binder ratio of UHPC can be reduced to below 0.20, far lower than that of ordinary concrete (0.4-0.6). A low water-binder ratio means fewer and finer capillary pores remain after hydration, greatly enhancing the material's strength and impermeability. Qinglong Group's production practice shows that well close-packed UHPC can have its total porosity after hardening controlled within 5%, with an extremely high proportion of harmless pores (pore size < 50 nm).

The close-packing design of UHPC is a precise materials science. Through advanced particle analysis software and extensive experimental data, Qinglong Group customizes the optimal mix proportion for each project, ensuring that material performance is maximized at the source.