There are three principal categories of RMC. They are as follows
- Transit Mix Method or Truck - Mixed Method
- Concrete Mixed at the Job site
- Concrete Mixed in the Yard/Central Batching Plant
- Concrete Mixed in Transit
- Shrink Mixed Concrete
- Central-Mixed Concrete
Transit - Mixed Method
In transit mixed method, all the raw materials are charged directly in the truck mixer. Most of the water is usually batched at the plant. The mixer drum is turned at charging (fast) speed during the loading of the materials. There are three options for truck-mixed concrete as mentioned above:
- Concrete Mixed at the site: While traveling to the job site, the drum is turned at an agitating speed (slow speed). After arriving at the job site, the concrete is completely mixed. The drum is then turned for 70 to 100 revolutions or for about 5 minutes of mixing speed.
- Concrete Mixed in Yard: The drum is turned at high speed or 12-15rpm for 50 revolutions. This allows a quick check of the batch. The concrete is then agitated slowly while driving to the job site.
- Concrete- Mixed in Transit: The drum is turned at medium speed or about 8rpm for 70 revolutions while driving to the job site. The drum is then slowed to the agitating speed
Shrink Mixed Concrete
Concrete that is partially mixed in a stationary plant mounted mixer and then discharged into the drum of the truck mixer for completion of the mixing is called as shrink-mixed concrete. Generally, about 2 months of mixing in the truck drum at mixing speed, is sufficient to completely mix. Shrink-mixed concrete. The amount of mixing in transit mixer depends on the extent of mixing done in the central mixing plant. Tests should be conducted to establish the requirement of mixing in the drum mixer.
It is also called as the central batching plant where the concrete is thoroughly mixed before loading into the truck mixer. Sometimes the plant is also referred as wet-batch or pre-mix plants. While transporting the concrete, the truck mixer acts as agitator only. Sometimes when workability requirement is low or the lead is less, non -agitating units or dump trucks can also be used.