Ultra High Performance Concrete
Ultra-high performance concrete (UHPC) belongs to a special class of cementitious materials that show very high mechanical properties and enhanced durability. Compressive strengths in excess of 120-150 MPa are generally reported for UHPC mixtures. When reinforced with high volumes of steel fibers (in the order of 2-3% by volume), these composites exhibit high tensile strengths and strain hardening. Some of the applications for which UHPC is well suited for are bridge piers, decks and deck-level connections between modular precast components, blast protection elements, and tunnels. The use of non-standard cement replacement materials such as quartz (silica) flour, rice husk ash, and nanoparticles (nanosilica, nano-metakaolin) to achieve high strengths, in additional to common high-performance replacement materials such as silica fume and metakaolin, has been reported for developing UHPC.
First Principles Based Design of Ultra-High Performance Concrete (UHPC)
A multi-scale design strategy has been devised to design UHPC mixtures by coupling the design of paste phase at the microstructure level and the aggregate phase at the meso-structure level. The design of UHP binder phase is based on microstructural packing-based and rheology-based criteria. Packing is a function of particle sizes and their distribution, while sizes and distribution along with particle surface characteristics influence the rheology. A microstructural packing algorithm is used to determine parameters relevant to particle packing. Yield stress, plastic viscosity, and mini-slump spread are used as the rheological parameters. This proposed method relies on the premise that, for low w/b concretes where only a fraction of the cement hydrates, (i) improved packing through the use of cement replacement materials and fine fillers is a better means of strength enhancement than increasing cement content, and (ii) better rheology helps better dispersion of the grains, aiding in mixture placement as well as hydration in the presence of low amounts of water, and consequently better mechanical properties.
The selection of aggregates is governed by aggregate packing at the meso-scale since average aggregate sizes are about 100 times larger than the average sizes of the powders in the pastes. The use of a proper size distribution of aggregate particles can help increase the overall packing density of the concrete mixture in combination with the use of a properly chosen paste phase.