|DISCHARGE OF READY MIXED CONCRETE FROM THE TRANSIT MIXER|
Proportioning of the Ready Mixed Concrete :-
The Ready Mixed Concrete proportioning aims at obtaining an economical and practical combination materials to produce concrete with required properties for its intended use, such as workability, strength, durability and appearance. The following basics of a good concrete mix should be considered :-
- Concrete aggregates are required to meet appropriate specification.
- Fly Ash and other supplementary cementing materials which enhances the properties of concrete are normally added to the RMC. Least amount of water which can produce required workability must be used.
- Admixtures are commonly used in small quantities varying from 0.5% to 2% for improving properties of both fresh and hardened concrete such as setting time, workability, rate of strength development etc. mainly Water Reducers are used like Naptha Base, PCE or Polyvinyl Carboxylic Ether.
Production Of Ready Mixed Concrete :-
First of all Concrete Mix Design is done to obtain the required mix proportions for the required strength and workability. Ready Mixed Concrete can be produced using any efficient concrete mixer including paddle mixer, free-fall mixers and truck mixers, but forced action mixers are generally preferred. Ready-made self-compacting concrete may take longer time to obtain complete mixing than for normal concrete due to reduction of frictional force and to fully activate the superplasticizer.
Admixtures should not be added directly to the dry constituent materials but dispensed together with or in the mixing water. Different Admixtures should not be blended together prior to adding.
The preferable procedure is to load the aggregates first together with cement, and this immediately followed by the main mixing water and superplasticizer.
Classification of Ready Mixed Concrete :-
- Concrete mixed at job site while travelling to the job site the drum is turned at agitating speed that is slowly. After arriving at the job site, the concrete is completely mixed. The Drum of the Transit Mixer is turned for 70 to 100 revolutions, or about five minutes at mixing speed.
- Concrete mixed in the yard or central batching plant the drum is turned at high speed or 12-15 rpm 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 the medium speed or about 8 rpm for 70 revolutions while driving to the job site. The drum is then slowed to agitating speed.
- Concrete that is partially mixed at the central batching plant and then discharged into the drum of the truck mixer for completion of the mixing is called Shrink-mixed Concrete.
- In this case concrete is fully mixed and prepared in the Central Batching plant and then carried to the job site by the truck mixers.