Onsite Mixing
UHPC can be successfully mixed in the field using commonly available mortar mixers. One mixer that has worked well is the Mortarman 750 model by Imer. Onsite mixing has special challenges that go beyond plant mixing. We have devised various means to alleviate those challenges.
Field mixers have limited power. UHPC’s viscosity increases dramatically at turnover and the mixer’s engine can labor noticeably at higher mix loads and even stall. To address this limitation, a revised mix procedure can be used that has been shown through research to yield similar fresh and hardened properties as obtained from the original mix protocol. The new mixing process is shown schematically below and is as follows:
Dry mix cement, GGBS, silica fume, and a portion of the silica sands for 5 minutes.
Add water and superplasticizer till turnover and formation of thick slurry.
Incorporate remaining silica sands gradually and mix another 5 minutes.
Add fibers and continue to mix until fluidity is optimized (between 5 and 8 minutes).
Hot Weather Mixing: Mixing and casting UHPC on a warm day leads to two complications:
A reduction in the spread (flowability) because the excessive temperature compromises the
effectiveness of the HRWR, and
The potential for evaporation of water during mixing and placement.
To address the former, it is recommended that about 40% of the mix water should be replaced with ice. Substantially hotter days will require greater ice quantities, which can be ascertained by trial and error. The objective is to cool the mix to less than 85 °F to ensure effectiveness of the superplasticizer. The latter issue can only be resolved by speeding up the mixing and placing processes.
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
Figure above: Mortarman 750 mixer by Imer suitable for mixing UHPC onsite.
Figure above: Field mixing process.
Figure above: Revised mixing process to reduce burden on field mixer.
