8.2 : Mixing Time

The concept of mixing time is significant in producing a uniform concrete mix with the required strength. The mixing period starts once all components are in the mixer. Initially, the mixer is charged with 10% of the water, followed by the consistent addition of solids and then 80% of the water. The remaining water is added later, within the first quarter of the mixing period. The minimum mixing time varies according to the mixer's capacity; for example, mixers with up to 1 cubic yard capacity require at least 1 minute, with an increment of 15 seconds for each additional cubic yard. It is also crucial to consider the mixer type, its rotation speed, and the quality of blending during charging when determining optimal mixing times. While standard mixers often require a mixing time of not less than 1 to 1.5 minutes, modern mixers are effective with shorter periods. If lightweight aggregate is used, the mixing duration should not fall below 5 minutes, and for air-entrained concrete, it should be at least 2 or 3 minutes. Prolonged mixing can lead to water evaporation, changes in aggregate grading, workability reduction, increased temperature, and, for air-entrained concrete, a decrease in air content.