Mixing of Concrete

Thorough mixing of the materials is necessary for the production of uniform concrete. The mixing should ensure that the mass becomes homogeneous, uniform in colour and consistency. There are two methods adopted for mixing of concrete.

1. Hand mixing
2. Machine mixing

Hand mixing

Hand mixing is practised for small-scale unimportant concrete works. As the mixing cannot be thorough and efficient, it is desirable to add 10 per cent more cement to cater for the inferior concrete produced by this method.

Hand mixing should be done on an impervious concrete or brick floor of sufficiently large size to take one bag of cement. Spread out the measured quantity of coarse and fine aggregate in alternate layers. Pour the cement on top of it and mix them dry by shovel, turning the mixture repeatedly until uniformity of colour is achieved. Water is then sprinkled over the mixture and simultaneously turned over. This operation is continued until a good, uniform and homogeneous concrete is obtained. It is of particular importance to see that the water is not poured but only sprinkled. Water in small quantity should be added towards the end of the mixing to get just the required consistency. At this stage, even a small quantity of water makes a difference.

Machine mixing

Mixing of concrete is invariably carried out by machine for reinforced concrete work and for medium- or large-scale concrete work. Machine mixing is not only efficient and fast but also economical when the quantity of concrete to be produced is large.

The mixers for mixing concrete can be classified as batch mixers and continuous mixers. Batch mixers produce concrete batch by batch with intervals whereas continuous mixers produce concrete without stoppage. Continuous mixers are used in large works such as dams. In normal concrete work, the batch mixers are used. Batch mixers may be of pan type or drum type. The drum type may be further classified as tilting, non-tilting and reversing or forced action type.

To get better efficiency, the sequence of charging the loading skip is as described below. Firstly, about half the quantity of coarse aggregate is placed on the skip over which about half the quantity of fine aggregate is poured. On that the full quantity of cement is poured, over which the remaining portion of coarse and fine aggregate is deposited in sequence. This prevents spilling of aggregate while discharging into the drum and also the blowing away of cement in windy weather.

Before the loading skip is discharged into the drum, about 25 per cent of the total water required for mixing is introduced into the mixer drum to wet the drum and to prevent any cement from sticking to the blades or the bottom of the drum. Immediately on discharging the dry materials, the remaining 75 per cent of water is added to the drum.

Concrete mixers are generally designed to run at a speed of 15–20 revolutions per minute. On the site, the normal tendency is to reduce the mixing time to increase the outturn. This results in poor quality of concrete. On the other hand, if the concrete is mixed for a comparatively longer time, it is uneconomical from the point of view of rate of production of concrete and fuel consumption. Therefore, it is of importance to mix the concrete for such a duration that will give optimum benefit. It is seen that the quality of concrete in terms of compressive strengths will increase with increase in the time of mixing, but for mixing time beyond 2 minutes the increase in compressive strength is not very significant.

Concrete mixers are often used continuously without stopping for several hours for mixing and placing. It is of utmost importance that a mixer should not stop in between the concreting operation. For this, the mixer must be well maintained. The mixer is placed on the site at a firm and levelled platform. The drum and blades must be kept clean at the end of the concreting operation. The drum must be kept in tilting position or kept covered when not in use to prevent the collection of rainwater. The skip is operated carefully and it must be provided with proper cushion such as sand bags.