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Consider the following statements in respect of ‘mixers’

1. Mass batch mixing of ingredients is the most desirable method.

2. Charging all materials into a drum mixer is done ‘at once’.

3. The quantity of material fed into a mixer should be not more than the quantity that can be used in less than 30 minutes after completion of mixing.

4. Reversing mixers have less capacity than tilting mixers.

5. In large mixers, additional time of mixing is allowed.

Which of the above statements are correct?

1. 1, 2 and 3
2. 1, 3 and 5
3. 2, 3 and 4
4. 2, 4 and 5

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Correct Answer - Option 2 : 1, 3 and 5

Statement 1: Correct

Weigh batching: As per IS 456: 2000, Mass or weigh batching is most preferable. It is the correct method of measuring the materials. For important concrete, invariably, the weigh batching system should be adopted.

The use of a mass system in batching facilitates accuracy, flexibility, and simplicity. Different types of weighing batches are available, the particular type to be used, depends upon the nature of the job. Large weigh batching plants have automatic weighing equipment.

Statement 2: Incorrect

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. 

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.

Drum is continuously charged by materials up to the desirable mix.

Statement 3: Incorrect

The quantity of the material fed into a mixer should be not more than the quantity that can be used in less than 30 minutes after completion of mixing.

Statement 3: Incorrect

The reversing drum is similar to the non-tilting mixer except that the same opening is used to add the constituents and to discharge concrete. The drum rotates in one direction for mixing and in the opposite direction for discharging the concrete. There are two types of blades attached to the inner walls of the drum. One set drags the concrete upwards and toward the center of the mixer when the drum rotates in one direction; the second set of blades pushes the concrete toward the opening when the drum rotates in the other direction. The blades have a spiral arrangement to obtain the desired effect for discharge and mixing. Reversing drum mixers has more capacity than tilting mixers. 


Statement 3: Incorrect

In large mixers, additional time of mixing is allowed for proper and adequate mixing.

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