UHPC is an ultra-high performance concrete. Ultra High Performance Concrete (UHPC), also known as Reactive Powder Concrete, is an innovative cement-based engineering material that achieves a significant leap in engineering material performance.
The basic preparation principle of "ultra-high performance concrete" is to improve the fineness and activity of the components, without using coarse aggregates, to minimize the internal defects (pores and microcracks) of the material, in order to achieve ultra-high strength and high durability.
The design theory of ultra-high performance concrete is the maximum packing density theory, which forms the closest packing of particles with different particle sizes in the best proportion. That is, the gaps between millimeter sized particles (aggregates) are filled by micrometer sized particles (cement, fly ash, mineral powder), and the gaps between micrometer sized particles are filled by submicron sized particles (silica fume). As early as 1931, Andressen established a mathematical model for the maximum packing density theory. However, it was not until the late 1970s, based on the significant improvement of high-efficiency water reducing agent technology and product performance, that the first generation of ultra-high performance concrete designed and formulated using this model was born in the Cement og Beton Laboratory in Aalborg, Denmark, called CRC (Compact Reinforced Composite). CRC has basically the same mechanical properties as the current UHPC, with a maximum compressive strength exceeding 400MPa. It uses sintered alumina as aggregate and steel fibers to improve the toughness of the material, hence it is called a "composite material". Due to the limitations of the performance of high-efficiency water reducing agents at that time, CRC or early UHPC were relatively sticky, difficult to compact by vibration, and not suitable for cast-in-place application. In the 1990s, Europe launched collaborative research projects, and related research was widely conducted around the world. This material was given a new name "Reactive Powder Concrete, abbreviated as RPC". The name "Ultra High Performance Concrete (UHPC)" was coined in this century because, compared with early CRC or RPC, with the improvement of design theory, the emergence of super efficient water reducing agents (polycarboxylate series), and the advancement of formulation technology, this material has already possessed the construction performance of ordinary concrete, and can even achieve self compaction and room temperature curing, which has the conditions for widespread application.
The differences between UHPC and ordinary concrete or high-performance concrete include: no use of coarse aggregates, the use of silica fume and fibers (steel fibers or composite organic fibers), a large amount of cement, and a low water cement ratio.
UHPC can be regarded as the most durable engineering material, and the mechanical properties of appropriately reinforced UHPC are similar to those of steel structures. At the same time, UHPC has excellent wear resistance and explosion resistance. Therefore, UHPC is particularly suitable for use in large-span bridges, blast resistant structures, thin-walled structures, as well as in high wear and corrosion environments.
