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Ready mix concrete is manufactured by the following two methods of mixing.
5.5.1 Centrally-Mixed Concrete: Concrete produced by completely mixing cement, aggregates, admixtures, if any and water at a stationary central mixing plant and delivered in containers fitted with agitating devices, except that when so agreed to between the purchaser and the manufacturer, the concrete may be transported without being agitated.
5.5.2 Truck-Mixed Concrete: Concrete produced by placing cement, aggregates and admixtures in a truck mixer at the batching plant, the addition of water and admixtures to be added along with mixing water, and the mixing being carried out entirely in the truck-mixer either during the journey or on arrival at the site of delivery. No water shall be added to the aggregate and cement until the mixing of concrete commences. A typical RMC plant consists of silos and bins for the storage of cement and aggregates respectively, weigh batchers for proportioning different ingredients of concrete, high efficiency mixer for thorough mixing of ingredients, and a computerized system controlling the entire production process. The quality of the resulting concrete is much superior to site-mixed concrete.

5.5.3 Upkeep of Production Facility: It is necessary that the production facility possess capabilities to produce quality concrete. The producer of RMC should carry out routine maintenance of storage, handling, calibration, batching, mixing and transporting equipment as well as weighing equipment as per the recommended frequencies. While ordering the RMC, it is recommended that the engineer in charge must visit the plant and ensure that the facility is in order. Fig. 5.4 gives the suggested frequencies of inspection, maintenance/calibration as per IS 4926-2003.

The benefits of RMC in terms of quality, speed, life-cycle cost and environmental friendliness are superior to those of site mixed concrete. Following brief comparison illustrates this vividly:

Quality of concrete: RMC Plants use sophisticated plant and equipment, which ensures quality concrete. RMC Plants exercise strict control on the quality of all ingredients through rigorous testing, stringent controls on process parameters, meticulously monitors key properties of concrete in the fresh and hardened state. All these efforts result in providing uniform and assured quality of concrete. In contrast, in a typical site-mixed concrete there is poor control on the quality of input materials, batching of ingredients and mixing of concrete, thus the resultant quality of concrete is poor, non-uniform and inconsistent.
Speed of construction: Mechanized operations in RMC plants ensure that construction activities are speeded up. While the production output from a typical site-mixed concrete operation using 8/12 m3/hr mixer is around 4 to 5 m3 /hr, the output form a 60 m3 /hr RMC plant is around 45 m3 / hr. Thus there is nearly a 10 fold increase in the output from RMC plant, which translates into direct savings.
Elimination of material procurement requirements and storage hassles: With the use of RMC, customers are not required to procure and store raw materials like cement, aggregates, sand, water and admixtures at site. This reduces the space requirements at construction sites and also minimizes efforts to procure different materials, ensure their proper storage and check their quality parameters from time to time. Hence, RMC enthuses good housekeeping at sites.
Saving in labour requirement: Site-mixed concrete is a highly labour-intensive operation and with the use of RMC the labour requirements is minimized. Further, as RMC plants look after the entire QA & QC needs, the manpower requirement for QA & QC operations at site is minimized.
Reduction in wastage: In site-mixed concrete job, wastage occurs in handling of all materials, including cement. The latter is generally of the order of about 2-3 kg per 50 kg bag of cement. All such wastages are considerably minimized in RMC facility.
Improved life cycle cost: Increased speed of construction coupled with reduction in labour cost and wastage results in considerable savings. Further, the improved quality of concrete translates into enhanced long-term durability of concrete, thus minimizing the maintenance and repair costs. Overall, when one considers the life cycle costs, the use of RMC becomes cost-effective in the long run. .
The use of RMC is beneficial in all angles; better quality, higher speed, better durability, savings in labour, reduction in wastages, reduction in life cycle cost, etc.

Will be Continued in Issue 4..