User:Bob Evans Fibo/sandbox
Onsite concrete batching is the process of manufacturing concrete on a construction site rather than the concrete being batched off-site and transported to the construction site.
The benefits of on-site concrete batching are economical and environmental. There is less waste and transport is reduced.
Off-Site Batching Plant[edit]
Concrete is normally purchased from a Ready-mix concrete [1] supplier and is batched using a fixed concrete batching plant.
A batching plant is a machine that mixes the main ingredient of concrete. The ingredients are sand [2], aggregate [3], cement [4], water and admixtures.
The materials are stored in bins, hoppers or silos and are transferred to the mixer by augers and conveyors. The materials are dosed normally by weighing each ingredient to ensure the correct amount is mixed.
Off-site batching plant rage from 100 to 300 m3/hr production output
The concrete is normally transported to site in drum mixes.
On-Site Batch Plant[edit]
The mobile batching plant does exactly the same job as the fixed plant but is smaller and mobile. The plant can quickly be moved from site to site.
The sand and stone are stored in bulk on site in material bays. The cement is stored in a cement silo [5] and water is stored in a tank or comes from the local main water supply.
The materials are then loaded into the mobile batching plant hoppers and pre-weight cement silo for batching.
The concrete design mix is programmed into the mobile batching plant computer. The computer then batches the material by controlling the flow of material and weighs each material to ensure it complies with the design mix.
On-site batching plant rage from 5 to 100 m3/hr production output
There are two main types of onsite batching plant. Volumetric concrete mixer [6] and Weight metric pan mixers.
The volumetric machine doses the mix using the volume method. The materials pass through gates that measure the volume from the gate area.
The concrete is then mixed using a mixing auger.
The weight metric system batches the materials into a mixing pan. The mixing pan has a load cell [7] on three sides that weight each material in turn. The principle is the same as making a cake.
The weigh metric is more accurate than the volumetric system.
Mobile Pan Batching Plant Output Calculation[edit]
The formula of output is simple. Its how many cycles the plant produces per hour from the cycle volume.
For example, a batch cycle time of 4m will batch 15 times per hour. If the batch volume is 1 m3 the output is 15m3 per hour.
The complexity and confusion are created from calculating the cycle time.
Cycle Times[edit]
There are many variables to calculate the cycle time for a batch of concrete. They are:
The design mix The pan size The speed of dosing the aggregates and cement The water flow rate Separate stone and sand or a premix of stone and sand Pre-weight cement Number of admixes Mixing time Dispatch time
An Example[edit]
Let’s take a look at a 1800 litre size batching pan
Specification[edit]
1800 litre pan mixer with a wet concrete output of 1.2m3 per cycle. Dosing C25 concrete with a mix design of:
Water/Cement ratio of 0.7 Cement – 240 kg Water – 155 litres Sand – 800 kg Stone – 1080 kg
Load Cases[edit]
Below are four load cases using different dosing processes using the same mix design. In practice, the batching plant computer is programmed to batch the load case. The computer can program up to twenty different mixes. The steps are sequential the first step has to finish before the next step begins.
Load Case 1[edit]
Step 1 – Cement 22 sec Step 2 – Sand and water 27 sec Step 3 – Stone 41 sec Step 4 – Waiting time 6 sec Step 5 – Mixing 60 sec Step 6 – Emptying 30 sec Total cycle time 186sec Number of concrete batching pan mixer cycle times 19.3 Output 23.16 m3
Load Case 2[edit]
Step 1 – Cement 22 sec Step 2 – Premixed sand and stone with water 34 sec Step 4 – Waiting time 4 sec Step 5 – Mixing 60 sec Step 6 – Emptying 30 sec Total cycle time 150 sec Number of concrete batching pan mixer cycle times 24 Output 28.8m3
Load Case 3[edit]
Step 1 – Cement from a pre-weight silo, sand and water 27 sec Step 2 – Stone 41 sec Step 4 – Waiting time 4 sec Step 5 – Mixing 60 sec Step 6 – Emptying 30 sec Total cycle time 162 sec Number of concrete batching pan mixer cycle times 22.2 Output 22.15m3
Load Case 4[edit]
Step 1 – Cement from a pre-weight silo, Premixed sand and stone with water – both hoppers loading 34 sec Step 2 – Waiting time 2 sec Step 3 – Mixing 60 sec Step 4 – Emptying 30 sec Total cycle time 126 sec Number of concrete batching pan mixer cycle times 28.57 Output 34.28.8 m3
Conclusion[edit]
As you can see the same batching plant has an output from 22 m3 to 34 m3. The output varies due to the design mix and the dosing process.
When premix sand and stone are used with pre-weight cement the cycle time is reduced.
When the sand, stone, cement and additives are dosed in sequence the cycle time is much longer and the output is lower.
Other pan sizes are available that can produce a larger output.
On-Site Concrete Batching Benefits[edit]
Flexibility[edit]
You won’t have any more frustration and stress waiting for a promised delivery of ready-mix. Onsite batching delivers concrete on demand.
If there is an issue on site and the operation has to stop, no more waste ordered concrete and cost when you send the ready-mix back to the depot.
No more part load and waiting time charges.
Quality[edit]
A mobile onsite batching plant has a dosing accuracy of +- 3% as a standard and +- 1% when required. The onboard computer ensures every batch has traceability.
This means no more disputes with your ready-mix provider when the quality of the mix is under dispute.
The plant can store up to twenty mix designs. With standard quality assurance processes in place for standard concrete testing, you can be confident that your concrete meets the project specification.
No Wastage[edit]
Waste on site is one of the most significant issues we have today. Waste can represent 10% of a construction project cost. Your CEO will want you to reduce waste to support the environmental agenda and to minimise cost making more profit.
With onsite batching, you have more control of concrete production and can supply concrete on demand that is fresh with a longer workability time frame.
Cost-Effective[edit]
Onsite concrete batching is cost-effective as you are only paying for what you use. You can buy the raw materials in bulk storing them onsite.