Concrete mixing is the process of properly mixing the materials needed to form concrete, such as cement, sand, aggregate, water, and admixtures (if any).
The primary goal of concrete mixing is to make the concrete mass homogeneous and uniform in colour while maintaining the required consistency.
All of the aggregate particles should be coated with cement paste, and all of the concrete’s constituents should blend together into a homogenous mass.
This process is so crucial that if the concrete-making ingredients aren’t mixed properly, it can lead to weak concrete sections, which can lead to concrete failure under compression load.
As a result, sufficient precautions should be followed while mixing concrete.
Methods of Mixing of Concrete
Generally there are two methods for concrete mixing. The selection of the method is totally dependent on the size of the construction work. The two methods of concrete mixing are mentioned below.
- Hand Mixing
- Machine Mixing
Hand Mixing of Concrete
Small construction or maintenance works typically require hand mixing of concrete. Because the concrete need for small construction works is minimal and employing a machine mixing method is not cost effective, so the hand mixing method of concrete is used.
Hand mixing can be done on an impermeable floor or a large flat iron tray (concrete mixing tubs), depending on the situation.
To begin, alternate layers of coarse and fine aggregate are distributed over the floor or tray in prescribed proportions.
The cement is then poured on top, and the elements are mixed dry using a shovel until the colour is consistent.
The components are combined dry first since it is more efficient and requires less human strength. The process of mixing concrete becomes more difficult after water is added.
This mixture is then spread out to a thickness of 200 mm and sprinkled with water. The mixture is continually turned over until a consistent colour is produced.
Because hand mixing cannot be comprehensive, it is preferable to add some additional cement (10%) to compensate for the perhaps inferior concrete generated by this method.
Once the proper consistency of the concrete mixture has been obtained, it is deployed using steel pans.
Also Read : Methods of Batching of Concrete
Also Read : Types of Concrete
Machine Mixing of Concrete
The process of mixing concrete materials in a concrete mixer is known as machine mixing. This sort of concrete mixing technique is commonly used for medium- large construction projects where a huge amount of concrete is required and hand mixing is ineffective.
These machine mixers are classified into two types mainly :
- Batch Mixer
- Continuous Mixer
The concrete is mixed batch by batch in a batch mixer at predetermined intervals. Batch mixers are commonly used in small to medium-sized construction projects. Batch mixers are further divided into two types:
- Drum Type Mixer
- Pan Type Mixer
There are three different types of drum type mixers dependent on how the mixed concrete is discharged: tilting, non-tilting, and reversing mixers.
Non-Tilting Mixer :
It consists of a cylindrical non-tilting drum with blades within and two circular openings at both ends. The drum revolves around a horizontal axis.
The concrete-making ingredients are fed via one entrance, and the mix is discharged by an angled chute through the other opening.
The only disadvantage of this mixer is the segregation that happens as a result of the slow discharge rate.
Tilting Mixer :
Tilting mixers come in two varieties. One is hand fed, in which all of the concrete-making materials are physically supplied into the mixer. The components are dumped into the revolving drum using steel pans.
The second method is loader fed, which involves attaching a loader to the rotating drum. When all of the concrete components are placed into this loader and the lever is raised, the contents are automatically dumped into the drum.
Both types often have the same overall structure. The mixer is usually formed like a bowl or a double conical frustum. It can be tilted for concrete discharge.
The mixing blades on the revolving drum lift and free fall the mix, combs it, and agitates it. By tilting the drum downwards, the mixed concrete is poured from the open top.
Reversing Drum Mixer :
These mixers, which are also known as forced action type mixers, are utilised for large-scale projects. It is made up of a horizontal, non-tilting drum.
There are two sets of blades on it. The concrete mix is mixed by one set of blades while the drum rotates in one direction. When the drum is reversed, the second set of blades discharges the concrete mixture.
Continuous mixers, on the other hand, keep producing concrete till the plant is operational. Continuous mixers are utilised for large-scale projects like dam construction and bridge construction.
Pan Type Mixer :
These mixers are classified as non-mobile mixers. It means they can’t be transferred from one location to another.
This is one of the reasons why it is less popular than other types of concrete mixers. They’re frequently employed as a central mixing plant or in precast concrete factories.
The mixer’s construction is made up of rollers and blades in a rolling pan. They not only mix the components, but they actually knead and crush them.
Dual drum mixers are used to mix concrete for roads and pavements. There is no segregation in the concrete generated by these mixers.
What is Transit Mixer?
A transit mixer is a movable concrete mixer that can mix and transport fresh concrete from one location to another.
The mixing drum in a transit mixer is directly installed on the truck. It is a type of truck that is typically used for concrete mixing.
The concrete components are thoroughly combined in the transport mixer to produce concrete. It is also used to deliver fresh concrete from the batching plant to the construction site.
The transit mixer’s capacity ranges from 4 to 16 cu.m.
Earlier transit mixers were inefficient, requiring extended mixing times depending on the degree of dryness in the mix.
Their primary purpose was to keep the mix stirred, but with the advancement of the twin fin process mixer, transit mixers have become more efficient.
These mixers have two opposed interior spirals in addition to the outside spirals. The mix components are conveyed to the bottom of the drum by the outer spirals, while the opposing mixing spirals drive the mix towards the feed aperture.
Within the mixer drum, the counter current mixing operation is repeated. A number of unique nozzles on the lower side of the inner mixing spirals spray water on the mix under pressure along the whole length of the drum, precisely and equally.
As a result, the entire drum contents are thoroughly mixed in a short period of time. As a result, high-quality structural concrete can be produced at a faster pace in the transit mixer.