What Are the Dense Packing Design Principles for UHPC Substrate Materials?

The close packing design principle of UHPC base materials is the physical foundation for achieving ultra-high performance. Its core lies in optimizing the gradation of particles of different sizes to achieve the densest possible spatial arrangement of solid materials, thereby minimizing porosity to the greatest extent. R&D work by Qinglong Group's CNAS laboratory has shown that achieving the closest packing is a prerequisite for lowering the water-binder ratio and improving matrix density and strength.

The application of the close packing principle in UHPC mainly follows the "densest packing curve" theory: particles of different sizes (such as cement, silica fume, quartz sand, and quartz powder) are combined in specific proportions so that fine particles fully fill the voids between coarse particles, even finer particles then fill the voids of the previous level of particles, and so on. Ideally, "zero voids" or "infinite-stage filling" should be achieved between particles. Qinglong's UHPC formulations typically employ cementitious materials with a wide particle size distribution (such as cement), micro fine admixtures (such as silica fume, whose particle size is two orders of magnitude smaller than that of cement), and rigorously graded quartz sand to achieve continuous gradation from the micron to the millimeter scale.

The most direct effect of this close packing is that the amount of water required to prepare a mix with the same flowability is reduced to an extremely low level. The water-binder ratio of Qinglong UHPC can typically be as low as 0.18-0.22, far below that of ordinary concrete (0.4-0.6). The reduction of excess water means a dramatic decrease in capillary pores and voids after hardening, resulting in an extremely dense microstructure. This structure not only endows UHPC with ultra-high strength, but is also the fundamental reason for its excellent durability (low permeability and high freeze-thaw resistance).

In actual production, Qinglong determines the optimal particle gradation through extensive testing, and uses high-range water reducers (polycarboxylate-based) to overcome the high viscosity of the mix caused by the low water-binder ratio, ensuring workability. The close packing principle is the "soul" of UHPC material design; it builds the cornerstone of high performance at the physical level, and subsequent fiber reinforcement further enhances the material's toughness and ductility on this basis.