The process of rolling, folding and spreading of particles is known as the mixing of concrete.
The materials of concrete should be mixed thoroughly so that there is uniform distribution of materials in the mass of concrete. The thorough mixing also ensures that cement water paste completely covers the surfaces of aggregates. The mixing of materials of concrete can be done either with hand or with the help of a cement mixer machine.
For hand mixing, the materials are stacked on a water-tight platform, which may be either of wood, brick or steel. The materials should be thoroughly mixed, at least three times, in dry condition before water is added. The prepared mix should be consumed in 30 minutes after adding water. The mixing by hand is allowed in case of small works of unimportant works where small quantity of concrete is required. For important works, if hand mixing is to be adopted, it is advisable to use 10 percent more cement than specified.
For machine mixing, all the materials of concrete, including water, are collected in a revolving drum and then the drum is rotated for a certain period. The resulting mix is then taken out of the drum. The features of machine mixing of concrete are as follows:
- It is found that mixing of concrete materials with the help of machines is more efficient and it produces concrete of better quality in a short time.
- The mixers of various types and capacities are available in the market. They may either be of tilting type or non-tilting type. They are generally provided with power-operated loading hoppers. For small works, a mixer capable of producing concrete of one bag of cement is used. For works such as roads, aerodromes, dams, etc. , special types of mixers are used.
- The water should enter the mixer at the same time or before other materials are placed. This ensures even distribution of water.
- The concrete mixer should be thoroughly washed and cleaned after use. If this precaution is not taken, the cakes of hardened concrete will be formed inside the mixer. These cakes are not only difficult to remove at a later stage, but they considerably affect the efficiency of the mixer.
- The inside portion of the mixer should be inspected carefully at regular intervals. The damaged or broken should be replaced.
- The time of mixing concrete materials in the mixer and the speed of mixer are very important factors in deciding the strength of concrete which is formed. The mixing time should be rotated at a speed as recommended by the manufactures of the mixer.
- The concrete discharged by the mixer, after thoroughly mixing concrete materials, should be consumed within 30 minutes.