Introduction.
The object of mixing is to make the concrete mass homogeneous and uniform in colour and consistency. All the aggregate particles should have a coat of cement paste and all the ingredients of the concrete should blend into a uniform mass. The mixing is done either by hand or by machine called mixer.
1. Hand mixing.
It is used for small jobs. Hand mixing is done over an impervious floor. Measured quantities of coarse aggregate and fine aggregate are spread over the floor in alternate layers.
Then cement is poured over it and the ingredients are mixed dry with shovel until uniformity in colour is achieved. This mix is spread out in thickness of 200 mm and water is sprinkled. The mix is kept on turning over till a uniform colour is achieved. As the hand mixing cannot be thorough, it is desirable to add some more cement (10%) to cater for the possible inferior concrete produced by this method.
2. Machine mixing.
For quality works mixing is carried out by mixer. Mixers can be broadly classified as batch mixers and continuous mixers.
The batch mixers produce concrete batch by batch with time interval, whereas continuous mixers produce concrete continuously till plant is working. Batch mixers are used for small and medium size works. Continuous mixers are used for large size works, e.g., dams.
Batch mixer may be of pan type or drum type.
The drum type may be further classified as tilting (T), non-tilting (NT), or reversing (R) type, based on the technique of discharging the mixed concrete. These are designated by a number representing its nominal mixed batch capacity in litres. The standard size of the mixers available are as follows: Tilting: 85T, 100T, 140T, 200T Non-tilting: 200NT, 280NT, 340NT, 400NT, 800NT Reversing: 200R, 280R, 340R, 400R. Sometimes the mixers are specified by two quantities, the total volume of ingredients added and the volume of concrete produced. For example 285/200-litres mixer takes 285 litres of ingredients and yields 200 liters of concrete.
Tilting mixers.
The tilting mixers may be hand fed or loader fed. The mixer is generally bowl shaped or double conical frustum type. It can be tilted for discharging concrete. Blades are fixed inside the drum. The revolving drum with the mixing blades gives lifting and free fall to the mix, combes and agitates it. The mixed concrete is discharged from the open top of the drum by tilting it downwards. The concrete tips out rapidly under gravity in an unsegregated mass as soon as the drum is tilted. These mixers may be used to their advantage for mixes of low workability and containing large size aggregate. The disadvantage of these mixers lies in the fact that some mortar adheres to the drum and is left out in the drum during discharging. To check this some amount of mortar is mixed in the mixer before the first batching called buttering.
Non tilting mixers.
It consists of a Non-tilting cylindrical drum with blades inside and two circular openings at the two ends. The drum rotates about a horizontal axis.
The ingredients are fed from one opening and the mix discharged from the other opening at the other end by at inclined chute. The drawback is the segregation that occurs owing to slow rate of discharge.
Reversing drum mixers.
These are also known as forced action type mixers and are used for large size works. It consist of a horizontal non-tilting type drum. It has two sets of blades. One set of blades mixes the mix while drum is rotated in one direction. The second set of blades discharges the mix when the drum is reversed.
Pan type or stirring mixers.
These are non-mobile mixers and are used either as a central mixing plat or at precast concrete factory. Primarily these are used for making mortar but are also used efficiently for stiff and cohesive mixes. The rollers and blades rotate in a rolling pan.
Apart from mixing the ingredients, they also produce kneading and crushing action. A pan type mixer is shown in Fig. 10.6. These are also known as dual drum mixers and are used for mixing concrete for roads or pavements. The drum comprises of two hemispherical halves, rotating on a horizontal shaft. One half of the drum is attached to the end of the shaft and the other half is free to slide over the shaft allowing the drum to open at the centre. A rubber joint and adjustable closing pressure ensures a perfect sealing when the drum is closed. Due to difference in circumferential speeds because of the design of blades cross flow is created which forces the concrete towards the centre causing turbulence and produces a perfect homogeneous mix. The rotating sliding drum-half can be retracted for discharge of concrete in few seconds. The concrete produced by this type of mixers show no segregation.
Transit mixers.
Truck mounted mixers also know as transit mixers are very popular and have replaced the dumpers and agitator cars used earlier to transport fresh concrete from the batching plant to the site. Transit mixers of capacity 4 to 12 cum mounted on truck chesis are available. These mixers are not very efficient and need long mixing duration depending on the extent of dryness in the mix. Their function is mainly to keep the mix in an agitated condition. With the development of twin fin process mixer (, the transit mixers have become more efficient. These mixers in addition to the outer spirals have two opposed inner spirals. The outer spirals convey the mix materials towards the bottom of the drum while the opposed mixing spirals push the mix towards the feed opening. The repeated counter current mixing process is carried out within the mixer drum. A number of special nozzles provided on the lower side of inner mixing spirals, precisely and uniformly spray water on the mix under pressure along the entire length of the drum.
This results in through mixing of the entire drum contents within a short time.
Hence, production of high quality structural concrete in the transit mixer is possible at a faster rate.
Charging the mixers and mixing time.
The order of feeding the ingredients into the mixer is as follows: About 25 per cent of water required for mixing is first introduced into the mixer drum to prevent any sticking of cement on the blades and bottom of the drum.
Then the ingredients are discharged through the skip. In the skip the sequence of loading should be to add first half the coarse aggregate then half the fine aggregate and over this total cement and then the balance aggregates.
After discharging the ingredients into the drum the balance water is introduced. The mixing time is counted from the instant complete water is fed into the mixer. The speed of the mixers is generally 15 to 20 rpm.
For proper mixing, the number of revolutions per minute required by the drum are 25 to 30
A poor quality of concrete is obtained if the mixing time is reduced. On the other hand if the mixing time is increased it is uneconomical.
However, it is found that if the mixing time is increased to 2 minutes the compressive strength of concrete produced is enhanced and beyond this time the improvement in compressive strength is insignificant.
A prolonged mixing may cause segregation. Also, due to longer mixing periods the water may get absorbed by the aggregates or evaporate resulting in loss of workability and strength.
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