Mixing/ handling and compaction concrete describes the processes and equipment used for batching ingredients by mass and mixing them into a uniform concrete batch. Proper placing and compaction methods are outlined to ensure concrete is dense, strong, and durable by removing all entrapped air.

The mixing operation consists essentially of rotation. Stirring the objective of being to coat the surface of all aggregate particles with cement paste and to blend all ingredient of concrete into a uniform mass.

Usual type of mixture is known as batch mixer that means one bag of concrete is mixed or discharged before any more materials are put into the mixer.

There are four types of batch mixer.

1. Tilting drum mixer

One whose drum in which mixing take place is tilted for discharging. The drum is conical or bow shaped with internal veins and discharge is rapid and unsegregated so that this mixer are suitable. Mixes are of low workability.

• Non – tilting drum mixers

One which the axis of the mixer is always horizontal and discharge takes place and inserting a chute into the drum or buy reversing the direction of the drum. Because of slow rate of discharge, some aggregates may harden.

• A pan type mixer

A forced action mixer. Distinct veins from a drum and which rely on free flow of concrete inside the drum

A pan mixer, consist essentially of a circular pan tilting about the vertical axis

• Duo drum mixer

Sometime used in a highway construction. In this case there are two drum in series. The concrete being mixed in the first drum is then transferred to the other for the remaining mixing time before discharging. The drum is recharged so that initial mixing takes place without intermixing of batches

It is the time required to produce uniform concrete. The mixing time is recovered from the time all the solid materials have been put in the mixer.

The time varies with the types of mixer and depends on its size. Number of revolution of the mixer and time of mixing are independent of each other.

In high speed mixers the mixing time cab be as short as 35 seconds.

When light weight aggregate is used, the mixing time should not be less than 5 min. sometimes divided two minutes of mixing the aggregate followed by water after three minutes than you add the cement.

For mixtures of large capacity, the mixing time should be increased at rate of 20sec

The recommended minimum mixing time is show in the table below

Capacity of mixer (m3)	Mixing time in minutes
0.8	1.00
1.5	1.25
2.3	1.5
3.1	1.75
3.8	2.00
4.6	2.25
7.6	3.25

Order of feeding the ingredient into the mixture depends on the properties of the mix and those of the mixer.

Generally a small amount of water should be fed first followed by all solid material, preferably fed simultaneously into the mixture.

The choice of the mixer, depends upon the size, extend and nature of work.

It’s where concrete is mixed by hands, the uniformity is hard to achieve

The aggregates should be spread in uniform layer on hard, clean non-porous base.

Cement is then spread on the aggregate and the dry material is mixed to form one end of the heap to other and cutting with a shovel until the mix appears uniform. Turning three times is usually required.

Drum Mixer

The first type is the drum mixer, which can be classified as non-tilting, split drum, or titling drum mixers, as well as truck mixers or reversing drum mixers. 

Type of Mixer	Characteristics
Tilting drum mixer	They are most suitable in most project for concrete with large sized aggregate and medium since they have a rapid discharge rate, are suitable for low workability concrete. Internal blades lift and tumble the ingredients onto itself. Two primary types exist: It has single drum where materials are charged and discharged through a single opening
Non-tilting mixer	– It is a Single drum rotating about a horizontal axis. – Fixed blades works on concrete towards the discharge end of the mixer, in order to provide a rapid rate of discharge.
Reversing drum mixer	Rotate in one direction for mixing and in the reverse direction for discharge  One set of blades exists for each operation. Provide efficient mixing with very little build up within the mixer. Are suitable with dry concrete mixes.
Drum Truck Mixers	There are of Two types: rear discharge commonly used and front discharge. Both utilize fins to mix and discharge the concrete and are powered by engine driven-variable speed hydraulic systems.

A forced movement pan mixer has blades that are fixed to an assembly that agitates the concrete throughout the pan as the vertical shaft rotates. This mixer is most common where stiff or zero slump mixes are prevalent. “They have not gained acceptance in ready mix production because of the small size and reputation for high blade and liner wear Pan Mixers are most commonly found in precast concrete plants.

The final type is the continuous mixer. A free-falling continuous mixer is a gravity-fed mixer, classified by its continuous movement.

 In this process the charging of materials and discharging of the mixed concrete is released in one, uninterrupted process. Continuous mixers utilize a continuous weigh batching system as well.

 Materials are typically fed into the mixer by a conveyor system. This mixer is most suitable when large quantities of mass concrete are required (i.e. dams, foundations, retaining walls and mass concrete filling).

1. Output capacity
2. Efficiency
3. Availability
4. Performance /Cost ratio
5. Portability of machine
6.  Nature of Construction environment.