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Component Selection

Nonproprietary UHPC is made of ordinary Portland Type I cement, ground granulated blast furnace slag (GGBS) cement (slag cement), silica fume, two types of silica sand and short steel fibers. To ensure workability, a superplasticizer is used. Optimum packing density of the particles is achieved as discussed in “How is High Packing Density Achieved?” The mix proportions by weight of a generic UHPC mix are shown in the table. Note that the weight of Portland Cement + Ground Granulated Blast Slag Cement = 1.0 and that all other components are scaled to this.

Table: Mixed proportions by weight of cement  (OPC+GGBS = 1.0)

White Portland Type I cement was used in the early development of generic UHPC due to its low tricalcium aluminate (C3A) and the high combination of di- and tricalcium silicate (C2S+C3S) resulting in exceptional performance in the fresh and hardened states. However, white cement is expensive (at $275 per ton [2019 price]). Research has shown that ordinary Portland cement Type I, which is much cheaper (at $150 per ton [2019 price]), can be successfully used. In general, the selected cement must have a tricalcium aluminate (C3A) content lower than 8% and a relatively low Blaine fineness to reduce water demand during the hydration. Many suppliers in the US can meet this requirement.

Silica fume is a by-product of the production of silicon alloys. The superfine spherical particles and pozzolanic reactivity densify the microstructure and significantly improve the compressive strength of UHPC. The median particle size is in the range of 0.1 to 10 microns. A lower carbon content is preferred because that decreases the water demand while promoting high flowability. Eliminating the coarse aggregate promotes high compressive strength. Instead of coarse aggregate, two types of quartz silica sand are used, with grain sizes of 70–200 μm and 400–800 μm. These grain sizes are optimized to enhance packing density.

Unlike regular concrete, UHPC uses a lot of cement, which increases costs and has environmental and ecological burden. It also has a negative impact on the hydration heat, which can lead to shrinkage problems. Therefore, slag cement (GGBS) is commonly added to make the mixes more environmentally friendly since GGBS is a byproduct of the steel making industry. GGBS is a beneficial mineral mixture for concrete because of its pozzolanic property and is known to positively affect the durability of concrete materials.

A polycarboxylate-based high range water reducers (HRWR), also known as superplasticizer, is used in UHPC mixture designs. Also, steel fibers with high strength (>2000MPa [>290 ksi] yield strength) are employed. Steel fibers are generally 0.2 – 0.3 mm in diameter and 13 mm – 25 mm in length.

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