The mixing process of batching plant operation includes the loading method, the mixing, the discharge method, the mixing time, and the mixing energy.
The loading method is composed of the loading constituents into mixer order and the loading period duration. The duration of this period depends on how long the constituents are mixed dry before the addition of water and how fast the constituents are loaded. The loading period is extended from the time when the first constituent is put into the mixer to when all the constituents are in the mixer. The loading period can be divided into two parts: dry mixing and wet mixing. Dry mixing is the mixing that occurs during loading but before water is added. Wet mixing is the mixing after or while water is being introduced, but still during loading. This means that materials are introduced any time during the loading period: all before the water, all after the water, partially before and partially after.The loading period is important because some of the concrete properties will depend on the order in which the constituents are introduced in the mixer.
Very often, the mixing time is defined as the time elapsed between the loading of the first constituent to the final discharge of the concrete. It should be noted that solid constituents can be added at various stages of the loading period: during dry mixing, after water is added, after a second period of mixing (third slope in Fig.). In any case, it is important that the mixing process be described fully for each batch of concrete.
The discharging process from the mixer should be arranged in order to increases the productivity (fast discharge), and it does not modify (slow discharge) the homogeneity of the concrete. For example, if the discharge involves a sudden change in velocity—as in falling a long distance onto a rigid surface—there could be a separation of the constituents by size or, in other words, segregation.
The energy needed to mix a concrete batch depends on the product of the power consumed during a mixing cycle and the duration of the cycle. It is often considered, inappropriately, a good indicator of the effectiveness of the mixer. The reason that it is not a good indicator is because of the high dependence of the power consumed on the type of mixture, the batch size and the loading method. For example, a mixer that has a powerful motor could be used to mix less workable or higher viscosity concretes. The mixing energy could be similar to that of a less powerful mixer but one filled with a more workable concrete.