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Ready Mix Concrete (R.M.C.)

Introduction

  • Ready-mix concrete (RMC) is a ready-to-use material, with predetermined mixture of Cement, sand, aggregates and water.
  • The idea of Ready Mix Concrete was first introduced by Architect Jurgen Heinrich Magens, he got his patent of RMC in Germany in 1903.
  • A plant in a central mixer or truck mixer, before delivery to the construction site in a condition ready for placing by the builder. Thus, ‘fresh’ concrete is manufactured in a plant away from the construction site and transported within the requisite journey time. The RMC supplier provides two services, firstly one of processing the materials for making fresh concrete and secondly, of transporting a product within a short time.
  • Between the years 1950 and 1980 considerable growth  of RMC took place in the United States.
  • In India RMC was first initially was used in 1950 during the construction sites of Dams like Bhakra Nangal, Koyna.
  • The increasing availability of special transport vehicles, supplied by the new and fast growing automobile industry, played a positive role in the development of RMC industry.

Objective of RMC

  • Better quality concrete is produced.
  • Elimination of storage space for basic materials at site.
  • Elimination of procurement / hiring of plant and machinery.
  • Wastage of basic materials is avoided.

Materials required for RMC

Aggregate

  • Aggregate are the important constituents in concrete. They give body to the concrete, reduce shrinkage and effect economy. The mere fact that the aggregates occupy 70-80% of the volume of concrete.
  • The size of aggregate bigger than 4.75 mm is considered as coarse aggregate and aggregate whose size is 4.75 mm and less is considered as fine aggregate.

Cement

  • Cement is a binder material which sets and hardens indepently, and can bind other materials together.
  • Cement is made up of four main compounds:

Dicalcium Silicate (2CaO SiO2),

Tricalcium Silicate (3CaO SiO2),

Tricalcium Acuminate (3CaO Al2O3),

Terta-calcium Aluminoferrite (4CaO Al2O3 Fe2O3).

  • These compounds are designated as C2S, C3S, C3A, and C4AF.

Admixture

  • Air-entraining admixtures (mainly used in concrete exposed to freezing).
  • Water-reducing admixtures, plasticizers (reduce the dosage of water while maintaining the workability).
  • Retarding admixtures (mainly used in hot weather to retard the reaction of hydration).
  • Accelerating admixtures (mainly used in cold weather to accelerate the reaction of hydration).
  • Super plasticizer or high range water-reducer (significantly reduce the dosage of water while maintaining the workability).

Fly Ash

  • Fly Ash is a by-product from coal-fired electricity generating power plants.
  • The fly ash is generally used in the concrete in the following ways.
  • As partial replace for cement.
  • As partial replacement for sand.
  • As simultaneous replacement for both cement and sand.

Water

  • The pH value of water should be in between 6.0 and 8.0 according to IS 456-2000.

Rice Husk / Hull Ash (RHA)

  • Today, rice is grown and harvested on every continent except Antarctica.
  • More than 550 million tons of rice is produced annually around the globe.
  • That rice husk are used in RMC to increase the strength.

Equipment Required

  • Storage of materials- Silos, containers and bins.
  • Batching arrangement
  • Measuring and recording equipment
  • Mixing equipment
  • Control systems
  • Electrical, hydraulic and pneumatic drives
  • Conveying systems (belt / screw / conveyors).

Mixing Process

  • Through mixing of the materials is essential for the production of uniform concrete. The mixing should ensure that the mass becomes homogeneous, uniform in color and consistency.
  • These are the types of mixing process of RMC: Transit Mixed concrete, Shrink mixed concrete, Central mixed concrete.
  • Transit Mixing: it is delivered to the worksite, often in transit mixers capable of mixing the ingredients of the concrete just before the delivery of batch. This results in a precise mixture, allowing specially concrete mixtures to be developed and implemented on construction sites. The second option available is to mix the concrete at the batching plant and deliver the ,mixed concrete to the site in an agitator truck, which keeps the mixed.
  • Centrally Mix: in the case of the centrally mixed type, the drum carrying the concrete revolves slowly so as to prevent the mixed concrete from “segregation” and prevent its stiddning due to initial set. However, in the case of the truck-mixed concrete, the batched

Standard ready mix concrete vs. site mix concrete

  • A centralized concrete batching plant can serve a wide area. Site-mix trucks can serve an even larger area including remote locations that standard trucks can not.
  • The plants are located in areas zoned for industrial use, and yet the delivery trucks can service residential districts or inner cities. Site-mix trucks have the same capabilities.
  • Better quality concrete is produced. Site mix can produce higher compression strength with less water than standard batching methods.

Scope of Ready Mix Concrete

  • Major concreting projects like dams, roads, bridges, tunnels, canals, etc.
  • For concreting in congested areas where storage of materials is not possible.
  • Sites where intensity of traffic makes problems.
  • When supervisor and labour staff is less.
  • To reduce the time required for construction, etc.
  • Huge industrial and residential projects.

Advantages of RMC

  • RMC is preferred to on-site concrete mixing because of the precision of the mixture and reduce worksite confusion. It facilitates speedy construction through programmed delivery at site and mechanized operation with consequent economy. It also decreases labor, site supervising cost and project time, resulting in savings. Proper control and economy in use of raw material results in saving of natural resources. It assures consistent quality through accurate computerized control.

Disadvantages of RMC

  • The materials are batched at a central plant, and the mixing begins at that plant, so the traveling time from the plant to site is critical over longer distances.
  • Access roads, and site access have to be able to carry the weight of the truck and load concrete is approx 2.5 tone.

Conclusion

  • Ready mix concrete is a modern technique of production of concrete in massive quantities away from the actual site of placing.
  • It is very useful where demand of concrete is very high and construction sites are in congested areas, where mixing on site is not possible due to lake of storage place.
  • RMC is ready to use material. It is widely adopted throughout the world.
  • It gives higher strength to the structure and it also provides higher Durability to the structure.
  • It is suitable for huge industrial and residential projects where time plays a vital role.

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