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Mixing energy how to affect concrete material quality?

24May

This article mainly introducing mixing energy definition and how to affect concrete material quality? According to the engineers research and study reports, this article abstract the mixing energy for concrete mixer affecting power.

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Mixing energy definition
The mixing energy is defined as the product of the average power consumption during the whole mixing cycle and the duration of the mixing cycle. For reasons of economics, the mixing energy should be kept low but the quality of the concrete should be considered first.

Some test reports and papers
Johansson [The Relationship between mixing time and the type of concrete mixer] varied the mixing time and measured the homogeneity of the concrete discharged by measuring the variation of the composition of the concrete produced . He determined that a longer mixing time increased the homogeneity of the concrete discharged up to a point. The curve of aggregate distribution versus duration of mixing eventually reached a plateau, implying that any further mixing would not improve the homogeneity of the concrete produced. According to the measurements performed by Johansson [The Relationship between mixing time and the type of concrete mixer], the time at which the plateau is reached depended strongly on the type of mixer and has some dependence on the maximum coarse aggregate size. Of course, shorter mixing times that still obtain an acceptable homogeneity for a given mixture are desired. This could determine the best mixer for the application, if the loading method is kept constant. Therefore, the optimum mixing time should be determined for each concrete mixture before starting a large production.
The power consumption is often used to estimate the workability of the concrete. The theory behind this usage is based on principles of operation of a rheometer.
A rheometer is an instrument that measures the stress generated by the material tested while applying a strain. In this case the strain is the constant speed of the blades and the stress is measured by the energy consumption. If it were possible to rotate the blades at different speeds and measure the power consumption at each speed, the mixer could be used to characterize the concrete’s rheological behavior. Nevertheless, while the data obtained will not allow calculation of the rheological parameters of the concrete in fundamental units because the flow of concrete in a mixer is not linear and no equations are available for such a case, the measure of the energy
consumption at one speed can be used to compare concretes prepared with the same mixer [Measurement of the rheological properties of high performance concrete: state of the art report], or to monitor the workability of a concrete while it is mixed. For a given mixture composition, if the power consumption increases, it is an indication that the concrete workability is reduced. Therefore, the operator could determine the necessity of adding more water or HRWRA to obtain the workability desired. This methodology will avoid the necessity of discharging the mixer, measuring the workability using for instance a slump cone just to determine the amount of water, or determining the HRWRA dosage needed to obtain the desired workability. Therefore, the mixing energy is a very useful tool to determine variation in the workability of the concrete being produced. However, there is no strong evidence that mixing energy can be used to determine the efficiency of a mixer, unless the only performance requirement is the workability.
 


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