Ready Mixed Concrete - Definition and its Types
- Ready mixed concrete production is a process, whereby, mixing and transportation of concrete is combining together and the product is delivered to the site, at the right time in the mould straight away.
- IS-4926, the definition of ready mixed concrete is: “Concrete delivered at site or in to the purchaser’s vehicle in a plastic condition and requiring no further treatment before being placed in position in which it is to set and harden”.
- Concrete should be in uniformity and loss of slump are the major issues. Because this the process both the things like agitator and mixing are doing in same truck,
- Time of haul is restricted to 1+ to 2 hours, also depends upon the ambient temperature, for higher ambient temperature the time of haul is lesser.
- The maximum number of revolutions of the mixing drum is limited to 300 revolutions at agitating speed.
Types of Ready Mixed Concrete
Ready mixed concrete can be of three (3) kinds:
1. Truck mixer
In this case, at the site there is no need of a mixing plane. Because this truck can serve both the purpose mixing as well as transportation of concrete. It loads up dry ingredients and water is also added. Mixing process can start while traveling and appropriately can be discharged. But this is not a very popular method in the field.
2. Agitator trucks
In this type of ready mix concrete, concrete is mixed in mixing plane and then loaded on to the truck. If the prepared concrete is loaded on to a normal truck the mixed concrete will set, so agitator truck is used. It actually keeps the concrete agitating with slow speed, may be 3 or 4 rpm during travel. It also has a small water tank for cleaning purpose. It uses less power as compared to the previous type.
Various things can be used to maintain the required slump, like retarders, accelerators etc. Calcium chloride is also an accelerator. Retarders are materials which retarder the process itself and the hardening process. If too much amount of the retarder is used it will kill the concrete, meaning the concrete will never set. Some times it can be made useful like, to kill the concrete on conveyor belt which is stuck during the transport. Sugar can also be used as a retarder. There are many type of chemical retarders available in the market.
Strength Loss in Remixing
Re-agitating the concrete or remixing the concrete by adding a little bit of water causes loss of strength of the resulting concrete. That can be shown in the diagram below.
There are many other issues related to ready-made mix concrete, like while transporting concrete you may have to consider the traffic conditions. If the transporting vehicle is stuck in traffic then it can be problem for the resulting concrete.