Before we understand the working or operation of a concrete batch plant it is important to understand the basic components and structure of a batching plant. It is a machine that combines various ingredients like aggregates, sand, water, cement and additives. These components are first weighed individually and then combined to prepared ready mix concrete. This material prepared is widely used for several types of construction projects.
Since the mixing of individual components is by weight it is possible to alter and change weight of individual materials to prepare different grade or quality of concrete. Some of the grades of concrete for India are as 1:3:6, 1:2:4, 1:1:2 etc. Here we express the proportions of the ingredients of the concrete mix in terms of ratios or parts of cement, fine and coarse aggregates. When we talk about 1:3:6, it defines the ratio or parts of cement, fine and coarse aggregates. Meaning the mix material has one part of cement, three parts of fines and six parts of coarse aggregates. The proportion is by weight.
Types of concrete plants
There are two main types of concrete plants:
1. Dry mix
2. Wet mix type
As the name suggests dry mix plants make recipes that are dry before they dispatch the same into a transit mixer. All the required materials like aggregates, sand and cement are weighed and then dispatched into a transit mixer. Water is added into the transit mixer. On the way to the site, concrete is mixed inside the transit mixer.
In case of wet mix type machines, the materials are weighed individually and then added into a mixing unit the mixing unit will homogeneously mix the materials and then dispatch the same into a transit mixer or a pumping unit. Also known as central mix plants, they offer a much more consistent product as all the ingredients are mixed in a central location in a computer assisted environment which ensures uniformity of the product.
When we talk about the styles, there are two major styles that we can categorise the same: stationary and mobile. Stationary type is usually preferred by contractors who want to product from a single place, they do not have to change sites more often. The size of the stationary mixers is also bigger compared to the mobile type. Today, mobile batch plants are also reliable, productive, accurate and designed to perform for years to come.
Type of mixers: There are basically 5 types of mixing units: reversible drum type, single shaft, twin shaft type, planetary and pan type.
Reversible drum mixer as the name suggests is a drum that will move in both the directions. Its rotation in one direction will facilitate mixing and its rotation in the opposite direction will facilitate discharge of materials. Tilting and non tilting type of drum mixers are available.
Twin shaft and single shaft offer mixing using shafts driven by high horsepower motors. It is widely accepted in European countries. Planetary and pan type mixers are mostly used for pre cast applications.
Components of concrete batch mix plant
A concrete batch mix plant has a number of parts and accessories that combine and work together to produce quality concrete. Some of the major components are as below:
1. Aggregate feeders – Feeding unit is one of the important component. There are bins located side by side to facilitate feeding of coarse aggregates and sand. The different number of bins are there which will hold different sized aggregates / sand into different compartment. The reason behind putting each material (size wise) into separate bins is that each material will be weighed separately as per designated mix ratio.
2. Aggregate weigh conveyor – Weigh conveyor usually sits below the aggregate feeders. It is suspended and hanging on load cells. It is to weigh aggregates one by one and then transfer the same to charging conveyor of skip. Aggregates are discharged to the weigh conveyor by opening and closing of pneumatic cylinders.
3. Charging conveyor / Skip – Charging conveyor or a skip is to carry the weighed aggregates to the mixing unit.
4. Cement weigh scale – Cement weigh scale will hold the cement in small quantity that is transferred from the cement silo / hopper via a screw conveyor. Its task is to weigh the aggregates prior to dispatch into the mixing unit.
5. Water weigh scale – Water weigh scale receives water from water pump. It will hold water and weigh the water as per the recipe set in the control panel and then add the same into the mixing unit.
6. Additive weigh scales – Additive weigh scales are also suspended on load cells and they will weigh the additive material prior to dispatch into the mixer.
7. Cement silo / hopper with screw conveyor – Cement silo is to hold large volumes of cement that comes in bulk trucks. It is usually provided with safety accessories. A screw conveyor of suitable diameter and length is at the bottom enabling transfer of cement into the cement weigh hopper. Many small capacity machines are equipped with a cement hopper. The opening of the hopper is big and it facilitates adding of cement that comes in bags. Bags have to be placed on top of the hopper and once they are torn, cement will fall into the hopper. Cement hoppers are also provided with screw conveyor for transfer of cement into the weigh hopper.
Usually a weigh hopper has 2 openings on top for inlet of 2 screw conveyors.
8. Mixing unit – This is the most critical part and heart of the concrete batch mixing plant. The job of the mixer is to receive all the ingredients and mix them thoroughly for a fixed time. Then it will discharge the contents into a transit mixer or a concrete pump.
9. Air compressor – Air compressor is one of the important components here. It is used to control all the pneumatic operations of the concrete batch mixer like opening and closing of cylinders that control the feeder gates, opening and closing of butterfly valves fitted with weigh hoppers, opening and closing of the mixing unit gate, etc.
10. Control panel – Control panel will control the working of the machine. Latest machines are equipped with PLC panel for ease of usage. It is also possible to save and print mix material recipes in the latest equipment. Control panel assist in fast and accurate measurements of the ingredients. With growing demand for high quality and accurate concrete it becomes important to produce quality material without any wastage. This is where control panel helps to control all components of the batching plant.
Operation / working of concrete batching plant
Usually a stationary type concrete batching plant is bigger in size. Its operation is more or less similar to the mobile version. Stationary version is preferred by customers who want to product concrete at a single place and portable version is preferred by people who want to shift sites more often.
Working principle of concrete batching plant is dependent on five primary functions as below:
Aggregate feeding – Aggregates and sand have to be fed into the individual feeder bins.
Powder feeding – Powder here refers to cement, fly ash and additives. These are important components for binding of material.
Water – Water will help cement bind evenly with aggregates. Specific quantity of water is must for any ready mix.
Storage and conveyance – Storage and conveyance here refer to storage of cement, fly ash, etc. They have to hold up quantity in bulk and transfer small quantity by screw conveyor to the weighing hoppers.
Control system – Control system is very critical for any advanced machine. It makes operation fast and accurate.
Concrete batching plant process flow will start from the feeding of aggregates into individual bins. Size wise aggregates have to be fed into individual bins. These aggregates will be weighed individually as per the design set in the control panel. After weighing they will be transferred to the mixing unit. Above the mixing unit there are weigh hoppers for cement, additive and water. The job of the weigh hopper is to weigh and then transfer the contents into the mixing unit below. Cement is transferred to the weigh hopper by means of screw conveyor. Water is pumped up into the weigh hopper. After mixing for a specific time as set in the mixing unit, the mixer will discharge the contents into transit mixer or a concrete pump.
All the above processes are carried out carefully and systematically. Each process is carried out for a specific time frame. The time frame starting from the weighing of aggregates to the discharge of the ready mix material is known as a batch time.