Equipment India | April 2009

Guest Column

The RMC Story
As the use of RMC can ensure better quality of structures, its production should be promoted by the Indian government, opines Jagvir Goyal.

Ready mix concrete is no more a stranger to the Indian construction world. The construction scene has undergone a major change during the last 10 years. During 1998, even an engineer needed to be explained the full form of RMC. Now, RMC plants have sprung up in all major cities of India, thanks to the cement manufacturing companies who have taken initiative to set up most of these plants and utilise part quantity of their cement production for RMC. The venture has proved profitable for them.

RMC equipment suppliers are also not left behind. Standard equipment companies of the world have developed their network of branches in India to supply RMC plant equipment. Since long, it was being felt that RMC is one such item that needs to be brought to the centre of Indian construction scene if the desired parameters
of quality, progress and durability are to be achieved. The dream is being realised now, though RMC, a house-hold term abroad, is yet to see a six to seven fold increase in India.

What exactly is RMC?
To be precise, RMC is concrete that is prepared and supplied to construction sites in a plastic, unhardened and ready to use state. The consumer avoids all sorts of burden of procurement of various aggregates, cement, plant and machinery. Above all, a strict quality control can be ensured at the RMC station. The batching and mixing of different ingredients is done at a central batching and mixing plant. Final mixing is done in the truck-mounted transit mixers which carry the concrete to the site of its pouring. In a way, RMC is the concrete produced as per specifications laid by the buyer and delivered to him under strict quality controlled conditions.

RMC equipment
RMC plants are designed for different concrete production capacities and batch sizes. Capacities of individual components obviously depend upon total production capacity. In addition, the batch size of concrete also matters. Components of a 30 cu m per hour plant with batch size of 1 cu m shall be different from that of a
30 cu m per hour plant with batch size of 0.5 cu m. Therefore, before choosing a RMC plant, concrete production required per day, number of working hours and batch size should be fixed.

For a requirement of 100 cu m concrete per day, with 10 working hours per day, a RMC plant of 15 cu m per hour capacity with batch size of 0.5 cu m per hour shall be sufficient. As it can produce the required quantity in just 7 hours, it will take care of any unforeseen stoppages of work.

An RMC plant can set up its own aggregate crushing plant to save on the cost of readily available crusher if financial resources for initial investment are available. Otherwise it may buy ready aggregate from the market. If the RMC plant is required to be at aggregate quarry, aggregate crushing and screening plant with conveyors, crushers and screens shall be required.

Features
Automatic batching of concrete ingredients is preferred for better efficiency of the equipment and stricter quality control of concrete. Aggregates, cement and water must be added by weight and not by volume. In addition, an admixture dosing system should inject required quantity of admixture if the concrete mix design asks for it. All batching process should preferably be controlled by PLC. Automatic batching system should produce an alarm after every production. Bulkage of sand should be accounted by it through a moisture correction. Unit control board should have SCADA based controls, display monitors, moisture control display, data printing arrangements, computerised mix proportions and choices of both manual as well as automatic controls. Number of batches produced, to be produced; value of concrete produced, batch record output should be given by the control systems. In fact, a host of control parameters and data is generated by the control panel and operator should be able to have a complete control of the RMC equipment through the mouse and keyboard.

Mixing options
RMC is prepared by mixing the ingredients at the centrally installed plant itself and later in the transit mixers while being carried to the site. Revolving speed of transit mixers is controlled to avoid segregation of concrete and to keep the concrete in plastic and unhardened state. This mixing option is exercised when the distance of site from the RMC plant is smaller. Otherwise, when the transportation distances large, concrete ingredients are are batched and added to transit mixer for mixing them during transportation.

The batching and mixing plant of an RMC equipment has a number of attachments such as loading and unloading screws, inclined belts, extraction belt, load cells or scales for weighing aggregates in batches – all provided to simplify the operations. Mixers are lined with wearing plates or special tiles to save them from wear and tear as the mixers keep running constantly to meet the concrete demand.

Some desirable features
The capacity of RMC equipment may vary any where from 15 to 150 cu m of concrete per hour. Manufacturers are not sitting silent and working on producing even higher capacity equipment. A builder can choose one depending upon the demand of concrete. A RMC station needs to be supported by a fleet of transit mixers required to transport ready mixed concrete to various sites. Again, the capacity of each of these mixers may vary somewhere from 4 to 7 cu m per hour. Concrete can be delivered to places as far as 40 km from the RMC station. RMC equipment must provide the user with the following results:

• Minimum breakdowns despite continuous use. Equipment should be strong enough to produce concrete continuously.
• Least power and oil consumptions should be there.
• Least maintenance of parts of equipment should be required.
• There should be no decrease in output of concrete with the passage of time.
• Spares and service should be easily available.
• Computerised controls for maximum automation should be available.
• Investment cost should be least or competitive.
• All components of the equipment should be easily accessible.

Basic needs of RMC equipment
Land: First requirement is availability of sufficient land. Depending upon the capacity of the plant, especially the storage capacity, area of land needs to be worked out and acquired. A 30 cu m per hour plant may require around 500 sq m area including that for bins. Only the plant structure may need hardly 10 m X 10 m area i.e. only 100 sq m for setting up.

Foundation: The equipment supplier will ask for foundations for the equipment components. And these need to be created at site as per the drawings provided by the equipment supplier. If a mobile RMC plant is to be used, no foundations shall be required.

Assembly cranes: In order to erect the RMC plant equipment, cranes shall be demanded by the equipment supplier. A manufacturer of RMC equipment normally has his equipment production unit at one place and transports the equipment components to site where the user provides him with crane facilities for erection of plant and commissioning it.

Water supply: The user has to create a source of water supply at site of RMC station. For this, he has to make arrangements, preferably by boring a tubewell or from other sources, if available and reliable. Generally, an outside source may be available but not reliable. Further, the user has to get the water tested and check that it is potable and suitable for concrete.
Electric power supply: Electric power supply shall also be required for operation of RMC equipment. Necessary connection of required load shall be required to be obtained from the electricity department. In addition, standby diesel generator set arrangements shall be required.

Earthing arrangement: RMC equipment essentially needs to be earthed in view of abundant use of metal. Necessary earthing facility needs to be created by the owner or buyer.

Air conditioning: Control unit of the RMC plant needs to be kept air-conditioned for trouble free running of computer systems and to provide good environment to the operators and staff.

Plumbing and drainage work: Water supply network needs to be laid at site for availability of water at different locations including testing laboratory. This arrangement has to be made by the user. Similarly, site drainage for rain water or spillages need to be provided to keep it workable.

Fly ash connection: A highly promising advantage of RMC is large scale utilisation of fly ash because of its best blending with other materials in an RMC plant when fly ash concrete is to be produced. Fly ash is increasingly becoming an environmental hazard and its utilisation in concrete is essential to maintain the ecological balance. Economy achieved due to saving of cement is an additional advantage. The problem that engineers have been facing earlier was that fly ash didn’t get fully mixed up with concrete ingredients. At RMC stations, dispersion of fly ash within concrete is not at all a problem even in concrete with high slump values. To make this use, RMC producer has to arrange his supply of fly ash from nearby sources like thermal power stations and tell the RMC equipment supplier that fly ash shall be used in concrete and some concrete mix designs with fly ash as an ingredient should be available in the mix options made available.

Different from batching plants
The concept of RMC plant is different from that of batching plants used at
most of the project sites. The batching plants are established at project sites on temporary basis in order to cater to the needs of the project with respect to certain defined mix-designs. Many times, these plants are manually operated or semi-mechanised. A RMC station is established permanently in a city or town where the consumers lodge their concrete requirements along-with the required mix-designs and concrete is supplied to different locations as per given requirement and mix. An RMC station has a computerised and fully automatic arrangement for batching and mixing of ingredients and as many as 100 mix-designs can be stored in the RMC control panel to produce concrete for any of them. The consumers – may be a Government department, a private firm, a building contractor or even an individual – can exercise their option among the available mix designs or can handover the concrete mix design evolved by them.

Cost factor
Reputed foreign companies have gauged the untapped potential that India holds in RMC field and have established their set ups here. A plant producing about 30 cu m /hour concrete costs about Rs 35 to 40 lakh while a plant of 18 cu m per hour capacity costs Rs 25 to 30 lakh. Taking all costs and taxes into account including the cost of land, transit mixers, site facilities like water supply, power supply, offices, stores, testing laboratory, utilities, roads and approaches, lighting arrangements etc., the cost of setting up a 30 cu m per hour capacity RMC plant should be below Rs 40 million if land has been allotted by the government in the industrial zone at low rates to promote the industry. The concrete prepared at site and supplied by RMC plant should be around Rs 2500 per cu m for M20 concrete after taking in to account the present cost of cement.

Promoting RMC in India
As the use of RMC can ensure better quality of structures, its production should be promoted by the Indian government. To promote its use in India, special tax concessions and holidays should be announced by the government for RMC industry. Land should be made available at concessional rates More emphasis should be laid on setting up of RMC stations for its commercial supply like any other commodity. Use of RMC can be made mandatory by the Government departments for major power, irrigation and other infrastructure projects. Though Indian construction industry is highly labor-intensive, the scene is changing now as most of the skilled labour has shifted to Middle-East, looking for better avenues and labor is no longer cheaper in India. Under the present scenario, an industrialist can rest assured that the future belongs to RMC and can diversify or invest in this field for rich gains.

Indian scene
Scores of RMC plants supplied by various players are presently operating in India. A survey of construction scene across the globe shows a complete transformation of concrete production methods from conventionally followed manual and semi-mechanised methods to fully automatic and computerised ready-mixed concrete production. USA is using 75 per cent of its annual cement production in ready-mixed concrete through its 3700 RMC plants. Sri Lanka has shown real initiative by making RMC mandatory for all high rise buildings. In India, L&T and ACC are two major concerns that have planned installation of RMC plants all over the country. These two firms have the capacity to implement their decisions. Present infrastructure and housing boom is going to augment their plans. At present, L&T has more than 40 plants in India, producing about 2 million cu m of RMC and there is no stopping. ACC, Birla and Ahlcon are also engaging themselves whole-heartedly in production of RMC. Ahlcon has been planning to set up 10 to 12 more plants in Gurgaon, Noida, Ahmedabad and Kolkata. Indian RMC scene is picking up and RMC equipment suppliers can encash the opportunity by supplying the equipment at discounted rates.
* Dy. Chief Engineer Civil, Author, Technical
books and columns, can be reached at
jagvirgoyal@yahoo.co.in

Components of an RMC plant
• Basic structure of the plant.
• Cement silos.
• Aggregate bins.
• Aggregate batching gates.
• Aggregate weighing system.
• Cement weighing system.
• Water weighing system.
• Compressor.
• Turbo mixers.
• Mixer discharge hopper.
• Conveyor for cement.
• Cement feeding hopper.
• Admixture weighing system (if required)
• Unit control board.
• Micro computer control system.
• Transit mixers.
• Water tank.
• Ladders for operator cabin and mixer platform.

RMC Advantages
• Total automation of material-batching and weighing eliminates under/over-batching or weighing of materials.
• Water cement ratio, the most crucial factor gets maintained.
• Admixtures can be added to concrete with high accuracy.
• A cement saving of 15 to 25 kg per cum of concrete is certain due to better control.
• Accurate measurements result in a reduction in standard deviation thus reducing
cost factor.
• Pilferage of material, so much associated with construction works, gets eliminated.
• There is a significant reduction in wastage of materials and labor.
• Degree of quality control can be very high.
• Fly ash can be successfully used in production of concrete.




 

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