TYPES OF CONCRETE MIXERS
Based on the type of concrete job, you can use different types of concrete mixer.
Concrete is a mixture made up of aggregate materials that have been combined with cement. These substances create a slurry when combined with water, which can then be poured and shaped into any shape. These ingredients must be mixed properly and in a specific proportion to produce high-quality concrete. Poor quality concrete can result if it is not properly mixed.
What is a Concrete Mixer?
A concrete mixer is a machine that mixes cement, aggregates, and water to create a paste or slurry of concrete. In addition to mixing the ingredients consistently, a good concrete mixture should be discharged without affecting the consistency of the mixture. This can be determined by the slurry’s uniform consistency and color.
Concrete can also be mixed manually if there are no concrete mixers available. However, this only applies to small amounts of concrete. Furthermore, workers have a tendency to mix concrete incorrectly and, in many cases, over-mix it. Therefore, it is recommended to use machine mixers to mix the components for 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 concrete mixing methods are discussed below.
- Hand Mixing
- Machine Mixing
Hand Mixing of Concrete
Hand mixing of concrete is typically required for small construction or maintenance projects. The hand mixing method of concrete is used because the amount of concrete required for small construction projects is minimal and using a machine mixing method would not be cost-effective.
Depending on the circumstances, hand mixing can be done on a solid surface or a sizable flat iron tray (concrete mixing tubs).
On the floor or tray, alternate layers of coarse and fine aggregate are first applied in the specified ratios.
After that, the cement is poured on top, and the elements are dry mixed with a shovel until the color is consistent.
The components are combined dry first because it is more effective and requires less human strength. When water is added, the process of mixing concrete becomes more difficult.
Water is then sprayed on top of the mixture after it has been spread out to a 200-mm thickness. The mixture is constantly turned over until a consistent color is produced.
Due to the limitations of hand mixing, it is preferable to add some additional cement (10%) to compensate for the possibly inferior concrete produced by this method.
Once the concrete mixture has reached the proper consistency, it is placed using steel pans.
Machine Mixing of Concrete
Machine mixing refers to the process of mixing concrete materials in a concrete mixer. This type of concrete mixing technique is commonly used for medium-large construction projects that require a large amount of concrete and hand mixing is ineffective.
These machine mixers are primarily classified into two types:
- Continuous Mixer
- Batch Mixers or Drum Mixer
A. Continuous Mixers
Continuous mixers, as the name implies, feed the ingredients into the drum, mix them, and discharge the slurry all at the same time and continuously until a break is called or the work is completed. Screw feeders are used for continuous material loading.
These mixers are made up of on-tilting drums with screw-type blades spinning in the middle of the drum in constant motion. The drum is tilted downwards, usually at a 15-degree angle towards the opening, and the tilt determines the mixing time.
Continuous mixers are used in large-scale projects that require a continuous flow of concrete, such as bridges, dams, or skyscrapers. These mixers are also used for low-slump concrete applications such as pavements. Since the mixing time is limited and a continuous discharge of concrete is required, the air content and bubbles are difficult to control, especially with air in the admixtures.
B. Batch Concrete Mixers
The most popular kinds of mixers used to make concrete are batch mixers. Batch mixers, as opposed to continuous mixers, mix and discharge concrete one batch at a time.
The batch mixers consist of rotating drums with internal blades. All of the ingredients are fed into the drum or pan in the desired proportion in this mixer, and the drum is spun at a constant speed. The resulting mixture is discharged and used. Once the first batch is finished, the drum must be refilled. After being combined, the ingredients are released to create a new batch. This process should be repeated until the required quantity of concrete is produced.
Batch mixers are classified into two types.
- Drum type mixers
- Pan type mixers
Drum Type Mixer
In drum type mixers, the ingredients for the concrete are mixed in a drum that is actually shaped like a double conical frustum. Drum mixers are divided into three types:
- Tilting drum mixers
- Non-tilting drum mixers
- Reversing drum mixers
Tilting Drum Mixers
Tilting mixers have vanes inside a conical or bowl-shaped drum. These mixers are best suited for low-workability mixes and those containing large aggregates. The mixture is discharged by tilting the mixing chamber, also known as the drum.
Advantages of Tilting Concrete Mixer
- Larger aggregates can be effectively mixed in tilting concrete mixers.
- A tilting concrete mixer is used to mix low-workability concrete.
- Tilting mixers are simple to install and appropriate for large-scale projects.
Disadvantages of Tilting Concrete Mixer
- These concrete mixers are not suitable for producing high-workability concrete.
Non- tilting Drum Mixers
Non-tilting mixers are incapable of tilting and have a horizontal axis. The discharge is accomplished by either inserting a chute into the drum, rotating the drum counterclockwise, or occasionally splitting the drum (which is done rarely).
Advantages of Non-Tilting Concrete Mixer
- There are openings on both sides of non-tilting mixtures.
- Non-tilting concrete mixers are best suited for small construction sites.
Disadvantages of Non-Tilting Concrete Mixer
- Non-tilting concrete mixtures are not suitable for large aggregates.
- Cleaning the non-tilting concrete mixer is extremely difficult.
Reversing Drum Mixers
The Reversing Drum concrete mixer has a reversing arrangement. The ingredients for the concrete in the reversing drum mixer will rotate only in one direction and discharge in the opposite direction.
Reversing drum concrete mixers are similar to non-tilting concrete mixers. For mixing the concrete, different sets of blades are arranged in reversing drum concrete mixers. This type of concrete mixer is ideal for mixing dry concrete.
Advantages of Reversing Drum Concrete Mixer
- Reversing drum concrete mixers are used to mix dry concrete and have blades that aid in the proper mixing of the concrete.
Disadvantages of Reversing Drum Concrete Mixer
- The reverse drum concrete mixer’s drum will only rotate in one direction.
- In this type of mixer, the concrete is discharged in the opposite direction.
Pan Type Mixer
These mixers are known as “non-mobile mixers. This means they cannot be moved from one location to another.
This is one of the reasons that it is less popular than the other kinds of concrete mixers. They are commonly used as central mixing plants or in precast concrete factories.
The mixer is made up of rollers and blades in a rolling pan. They do not just mix the ingredients, but also knead and crush them.
Concrete for roads and pavements is mixed using dual drum mixers. The concrete produced by these mixers contains no segregation.
Advantages of Pan Type Concrete Mixer
- A pan-type concrete mixer has a large cylindrical pan that allows a huge amount of concrete to be mixed.
- In the case of a pan-type concrete mixer, the concrete can be discharged from the pan’s bottom.
Disadvantages of Pan Type Concrete Mixer
- When compared to other types of concrete mixers, pan-type concrete mixers are more expensive.
What is Transit Mixer?
A transit mixer is a mobile concrete mixer that is capable of mixing and transporting fresh concrete from one location to another.
In a transit mixer, the mixing drum is mounted directly to the truck. It is a type of truck that is commonly used for concrete mixing.
Concrete is made by thoroughly combining the concrete’s component parts in the transport mixer. Additionally, it is used to transport freshly mixed concrete from the batching plant to the construction site. The capacity of the transit mixer ranges from 4 to 16 cu.m.
Depending on how dry the mix was, earlier transit mixers were ineffective and necessitated lengthy mixing times.
Transit mixers were originally designed to keep the mix stirred, but with the advancement of the twin fin process mixer, they have become more efficient.
In addition to the outside spirals, these mixers have two opposed interior spirals. The outer spirals transport the mixed components to the drum’s bottom, while the opposing mixing spirals propel the mix towards the feed aperture.
The counter current mixing operation is repeated within the mixer drum. On the lower side of the inner mixing spirals, a variety of special nozzles evenly and precisely spray water under pressure on the mix along the length of the drum.
Thus, the entire contents of the drum are thoroughly mixed in a brief amount of time. As a result, in the transit mixer, high-quality structural concrete can be produced at a fast pace.