Plant Mixing

Conventional concrete is generally easy to mix using commonly available mixers and can be conveniently adapted to most construction conditions. However, mixing UHPC requires equipment that provides more energy and shear than regular concrete due to the low water content and high powder content. In general, the expected performance (including fresh and hard-solid properties) of the selected mixture cannot be achieved when low-mix energy mixers are used to mix UHPC. Moreover, use of a low-energy mixer will also increase the turnover time of the mixture, causing the temperature of the mix to rise, which is detrimental to the UHPC mixing process (high temperature delays mix turnover).

UHPC can be mixed in plants that have large capacity pan mixers with multiple mixing paddles. The paddles should have bottom and side scrapers to ensure a good mix result. The mixing process is as follows:

• The silica sand and silica fume are first dry-mixed for about five minutes. Cement and GGBS are added to the mixture and dry-mixed for another five minutes.

• Water and HRWR are first mixed together and then added gradually to the dry material. Premixing the HRWR and water aids in more uniformly distributing the HRWR during mixing. The UHPC mixture shows appropriate workability (turn over) approximately five to seven minutes after the addition of water and HRWR.

•  Once an adequate mixture consistency is achieved, the steel fibers are added into the mixer and allowed to mix until the fibers are dispersed

WARNING: Dry mixing silica fume, fine sand and cement can cause dust which when inhaled can lead to silicosis. Appropriate protection equipment must be worn at all times during mixing. A N95 mask and full Tyvek suit are recommended.

Once mixing has concluded, a spread test should be carried out to ensure that the material has mixed appropriately. The spread should be between 175 mm (7 in) and 300 mm (12 in).

Figure above: Precasting plant mixer with multiple paddles. Note the bottom scrapers. The mixer also has side scrapers (not in view)

See video below showing how to mix generic UHPC