What Is a Concrete Batch-Type Plant?
A concrete batch-type plant is an equipment used to produce concrete by mixing aggregate, cement, water, and admixtures in a batch process.
Concrete can be produced in batch or continuous systems, but the batch system is more accurate and produces a better-quality product. Each material is measured and added from each feeder, and mixed in a mixer for a predetermined period of time to produce fresh concrete.
Uses of Concrete Batching Plants
The plant is generally installed near the site of demand, from where it is transported by mixer truck to the construction site. Fresh concrete hardens over time, so it is important for the plant to control time, and the plant is responsible for supplying the required quantity, at the required time, and with consistent quality. The plant makes meticulous production plans that can flexibly respond to demand.
For large-scale construction projects such as dams and tunnels, dedicated plants may be built near the construction site. This can be handled very efficiently. Mobile equipment is available for use in small-scale construction projects. One set of equipment can be mounted on a trailer, and the equipment can be moved to the construction site for each material to be supplied efficiently. Various sizes are available depending on the size of the project.
Principles of Concrete Batching Plants
The quality of concrete depends on the mixing ratio of materials, which is specified in JIS A 5308 Ready-mixed Concrete. Three types are available: ordinary, lightweight, and pavement. The roles of each material are shown below:
1. Cement
Cement is a setting and solidifying material. It hardens and bonds aggregates.
2. Water
Water assists the cement to harden and determines the softness of the fresh concrete. The ratio of water to cement affects the strength and workability of the concrete. A low water-cement ratio increases strength but reduces workability.
3. Aggregates
Aggregates, such as gravel and sand, account for 70-80% of concrete by weight. Aggregate is a filler, but its role is to reduce shrinkage and heat generation, and to impart stiffness and abrasion resistance. The size of the aggregate influences quality and other factors.
4. Admixture materials
Admixture materials include admixtures. Admixture is a material added in large quantities, such as an expander, to improve quality. Admixtures, such as AE water reducers, are added in small quantities to improve quality.
Structure of a Concrete Batching Plant
The plant consists of material feeders, storage facilities, weighing equipment, and mixers. Aggregate, cement, water, and admixture are weighed one batch at a time, fed into the mixer, and kneaded to produce a precise, uniform quality product.
1. Material feeder
Material feeders are installed at the top of the plant. Aggregate is mostly fed automatically using a turn head. Cement is generally fed automatically by a screen conveyor or chute.
2. Storage facilities
Storage facilities for storing materials are installed at the top of the plant. The material is automatically controlled and discharged by detection signals.
3. Weighing equipment
Weighing equipment weighs the input materials on an accurate scale. The accuracy of the scale is defined by JIS standards, and load cells with higher accuracy are generally used.
4. Mixer
A mixer is a device for mixing materials. There are various types of mixers. They include a tilting mixer shaped like the drum of a mixer truck, a tub-shaped pan mixer, and a forced dual-shaft mixer. Recently, forced twin-shaft mixers are widely used. This type of mixer is capable of mixing both hard and soft slumps in a short time.
Other Information on Concrete Batching Plants
1. Slump value of concrete
The slump value of fresh concrete is a measure of its softness before hardening. The higher the slump value, the softer the fresh concrete. Softer concrete has the advantage of being easier to work with, but its strength is reduced.
To measure the slump, place the fresh concrete in the specified slump cone, lay it down on a flat surface, and slowly pull out the cone. The height of the slump cone is the height of the concrete as it is lowered from its original height (300 mm). A large slump value causes adverse effects such as reduced watertightness, reduced durability, and increased drying shrinkage, so JIS has established a standard value.
2. Advantages of Concrete Batch Plants
- Homogeneous production in large quantities
Large-scale equipment can produce homogeneous fresh concrete in large quantities. Each batch allows for accurate weighing of materials and superior quality.
- Efficient production
Automation of weighing and in-plant transport is possible, enabling efficient production.
- Efficient transportation
Since the plant can be located near the construction site, there is no need to transport the material long distances by mixer truck. Required quantities can be quickly supplied.