Concrete Mixers and Concrete Mixing Methods:

Introduction

As for all materials, the performance of concrete is determined by its microstructure. Its microstructure is determined by its composition, its curing conditions, and also by the mixing method and mixer conditions used to process the concrete. The mixing procedure includes the type of mixer, the order of introduction of the materials into the mixer, and the energy of mixing (duration and power). To control the workability or rheology of the fresh concrete, for example, it is important to control how the concrete is processed during manufacture. In this overview, the different mixers commercially available will be presented together with a review of the mixing methods. Further, the advantages and disadvantages of the different mixers and mixing methods and their application will be examined. A review of mixing methods in regards to the quality of the concrete produced and some procedures used to determine the effectiveness. of mixing methods will also be given. To determine the mixing method best suited for a specific application, factors to be considered include location

MBW concrete mixer with 6 cu. ft. steel drum

The Mixers
Batch mixers
Mixers that produces concrete one batch at a time, and needs to be emptied completely after each mixing cycle, cleaned (if possible), and reloaded with the materials for the next batch of concrete. In the second type, the constituents are continuously entered at one end as the fresh concrete exits the other end. The various designs of each type of mixer will now be discussed. The two main types of batch mixer can be distinguished by the orientation of the axis of rotation: horizontal or inclined (drum mixers) or vertical (pan mixers). The drum mixers have a drum, with fixed blades, rotating around its axis, while the pan mixers may have either the blades or the pan rotating around the axis.

Drum Mixers
All the drum mixers have a container with a cross section. The blades are attached to the inside of the movable drum. Their main purpose is to lift the materials as the drum rotates. In each rotation, the lifted material drops back into the mixer at the bottom of the drum and the cycle starts again. Parameters that can be controlled are the rotation speed of the drum and, and in certain mixers, the angle of inclination of the rotation axis.

Mixing Method
In describing the mixing process, the mixer hardware is only one of several components. The mixing process also includes the loading method, the discharge method, the mixing time, and the mixing energy.

Loading, Mixing, and Discharging
The loading method includes the order of loading the constituents into the mixer and also the duration of the loading period. 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 introduced in the mixer to when all the constituents are in the mixer. RILEM (Reunion Internationale des Laboratoires d´Essais et de Recherches sur les Materiaux et les constructions) divides the loading period into two parts: dry mixing and wet mixing. Dry mixing is the mixing that occurs during loading but before water is introduced. 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. It is well known that the delayed addition of high range water reducer admixture (HRWRA) leads to a better dispersion of the cement. The same workability can be thus be achieved with a lower dosage of HRWRA. Unfortunately, there is no systematic study, to our knowledge, that has examined the influence of the order of constituent loading on concrete properties. Most operators rely on experience and trial and error to determine the loading order of their 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. RILEM took another approach defining mixing time as the time between the loading of all constituents and the beginning of concrete discharge. 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. Both definitions are acceptable. In any case, it is important that the mixing process be described fully for each batch of concrete. The discharge from the mixer should be arranged so that it increases productivity (fast discharge), and it does not modify (slow discharge) the homogeneity of the concrete. For instance, if the discharge involves a sudden change in velocity as in falling a long distance onto a rigid surfacethere could be a separation of the constituents by size or, in other words, segregation.

Mixing Energy
The energy needed to mix a concrete batch is determined by 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.

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