How Long Should UHPC Be Mixed?

The mixing time of UHPC is a key process parameter for ensuring uniform fiber distribution and thorough homogenization of the paste, thereby achieving the designed performance. Insufficient mixing time leads to fiber clumping and uneven strength, while excessive mixing may cause loss of workability and temperature rise. For UHPC premix products supplied by Qinglong Group, the optimal mixing time must be determined through trials based on the specific formulation, batch size, mixer power, and mixer type, but it generally follows a well-defined range and a strict sequence.

For standard procedures using planetary or twin-shaft forced-action mixers, the total mixing time is typically controlled at 5 to 8 minutes. This time can be broken down into two stages: 1) Dry mixing stage: charge the UHPC premix dry materials (containing cement, supplementary cementitious materials, aggregates, fibers, etc.) into the mixer and dry-mix for 1–2 minutes first, so that all components are initially mixed evenly. 2) Wet mixing stage: gradually add the accurately measured mixing water (and liquid admixtures, if any) while the mixer is running, and continue mixing for 4–6 minutes. The mix must be observed to become uniform and glossy, with fibers fully dispersed and no dry material lumps or fiber balls. The entire mixing process should be carried out continuously, without interruption.

The mixing time is dynamically adjusted. The actual mixing time must be determined through on-site trial mixing. Qinglong's technical personnel will guide trial mixing before project commencement, taking the observed condition of the mix as the standard. The mixer model and efficiency are critical; drum-type mixers are strictly prohibited. The batch size should not exceed 60% of the mixer's nominal capacity to ensure sufficient mixing space. When the ambient temperature is high, consider using ice water for mixing to control the mix temperature. The mixed UHPC should be placed within 30 to 45 minutes to prevent a decline in fluidity from affecting construction quality. Strictly following a verified mixing regime is the first step in producing qualified UHPC mix.