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What is Ready Mixed Concrete?
As the name indicates, Ready Mixed Concrete (RMC) is the concrete which is delivered in the ready-to-use manner. RMC is defined by the American Concrete Institute’s Committee 116R-90 as:
“Concrete that is manufactured for delivery to a purchaser in a plastic and unhardened state”.
The Indian Standard Specification IS 4926:2003 defines RMC as:
“Concrete mixed in a stationary mixer in a central batching and mixing plant or in a truck-mixer and supplied in fresh condition to the purchaser either at the site or into the purchaser’s vehicles”.
In India, concrete has traditionally been produced on site with the primitive equipments and use of large labour force. Ready mixed concrete is an advanced technology, involving a high degree of mechanization and automation. 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.
Site Mixed Versus Ready Mixed Concrete
Technologically speaking, ready mixed concrete is certainly an advancement over the age-old site mixed concrete. The benefits of RMC in terms of quality, speed, life-cycle cost and environmental friendliness are overwhelmingly superior to those of site mixed concrete. Following brief comparison illustrates this vividly:
Quality of concrete : RMC-India uses sophisticated plant and equipment, which enables it to produce quality concrete. The Company exercises strict control on the quality of all ingredients through rigorous testing, applies stringent controls on process parameters, meticulously monitors key properties of concrete in the fresh and hardened state and applies the well known Cusum technique to quickly detect any changes in the properties of concrete. All these efforts result in providing uniform and assured quality of concrete to customers. 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 at RMC-India’s plants ensure that construction activities are speeded up. While the production output from a typical site-mixed concrete operation using 8/12 mixer is around 4-5 m3/hour, the output form a 60-m3/hour RMC plant is around 45 m3/hour. Thus there is nearly 10-fold increase in the output from RMC plant, which translates into direct savings to the customer!
Elimination of material procurement requirements and storage hassles : With the use of RMC, customers are not required to procure and store cement, aggregates, sand, water and admixtures at site. This not only drastically reduces the space requirements at construction sites but also minimizes efforts on the part of customers to procure different materials, ensure their proper storage and check their quality parameters from time to time.
Saving in labour requirement : Site-mixed concrete is an labour-intensive operation and managing lage labour force is a big hassle for the customer. With the use of RMC the labour requirements are minimized considerably, thus benefiting customers. Further, as RMC-India looks after the entire QA & QC needs, the customer’s manpower requirement for QA & QC operations is minimized. This is a saving for the customers.
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 at RMC facility.
Improved life cycle cost : Increased speed of construction coupled with reduction in labour cost and wastage results in considerable savings to customers. 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 become cost-effective in the long run. The benefits directly accrue to the customers.
RMC is eco-friendly : All plants of RMC-India pass the pollution control norms and are duly certified by the state pollution control authorities. As mentioned earlier, wastages are reduced drastically with the use of RMC. Further, RMC-India optimizes the mix proportions using the maximum possible potential from each material ingredient. All these improve the environmental performance of concrete produced by the Company. Finally, with the approval of customers/consultants, RMC-India uses a variety of supplementary cementitious materials like fly ash, blast-furnace slag, silica fume, etc. in concrete, thus conserving cement and helping in reduction in emissions of green house gases like CO2. Thus, concrete produced by RMC-India can certainly be considered to be eco-friendly.
Overall, the use of RMC is beneficial to users from all angles: better quality, higher speed, better durability, savings in labour, reduction in wastages, reduction in life cycle cost, etc.
Frequently Asked Questions (FAQs)
Q. What are the advantages of using ready-mixed concrete over site-mixed concrete?
How much is the difference between the cost of ready-mixed and site-mixed concrete?
A
A detailed answer to the first part of the question is enumerated in the write-up on “What is ready-mixed concrete?” under the heading Customer Support. Briefly, the two main advantages of ready-mixed concrete are: vast improvement in the quality and uniformity of concrete and enhanced speed of construction. Besides, there are many other advantages which include savings in labour, reduction in wastage, elimination of material procurement requirements and storage hassles, etc. All these advantages clearly establish the technical superiority of ready-mixed concrete over site-mixed concrete. As regards cost, ready-mixed is slightly costlier than site-mixed concrete. This increased cost is mainly on account of government taxation. It is indeed unfortunate that a quality product like ready-mixed concrete is taxed in our country, whereas there is no tax on site-mixed concrete! If tax component is removed, the cost of ready-mixed would be quite close to that of the site-mixed concrete. However, if due consideration is given to the higher speed of construction (which is possible due to use of ready-mixed concrete) the savings on account of early completion of the project would far outweigh the increased cost of ready-mixed concrete. Further, the improved quality of RMC would go a long way in enhancing the long-term durability, thus reducing the maintenance and repair expenses and hence the life-cycle cost of your structures. In addition, the savings in labour and wastage would also be available to you. Thus, if you look at the overall advantages and savings, the increased cost of ready-mixed concrete would appear to be a paltry sum.
 
   
Q. Is ready mixed concrete a well established technology? How long this technology is being used in practice in India?
A.
Yes, ready-mixed concrete is a well-established technology. It was first patented in Germany, way back in 1903. By 1950s, the use of ready mixed concrete picked up in most of the advanced countries in Europe and America. Presently, nearly 75 % of the cement used in the USA is routed through ready-mixed concrete outlets. The corresponding average figure in Europe was 46.7% in the year 2005. Technologically speaking, the production process of ready mixed concrete has been continuously been upgraded, leading to improvement in product quality and uniformity. Today’s plants are highly automated and consist of state-of-the-art equipment having computerized controls on the entire production process.
Compared to the advanced countries, India was a late starter in ready-mixed concrete. RMC-India is one of the pioneers in bringing this technology to India in 1996. During the past more than a decade, RMC-India and many other leading manufacturers have expanded their operations to various metropolitan and other big cities in India. Sufficient experience and expertise have been now obtained in the production and use of ready-mixed concrete. You can certainly bank on this experience..
 
   
Q. How do we arrive at the exact quantity of concrete required at our site?
A.
Your architect/engineers need to work out the grade-wise concrete requirements and communicate the same to our plant. They are the right people and are in a better position to decide the quantities required. Concrete is generally specified in terms of the 28-day compressive strength and workability (usually slump) at placement point. These two are basic minimum requirements for us to design concrete mixes for you. We will ensure that concretes in accurate quantity and appropriate quality are delivered to your satisfaction.
 
   
Q. Can we base the measurements of concrete on the simple L (length) x B (breadth) x H (height) multiplication?
A.
All over the world, the basis of purchase and hence measurement of ready-mixed concrete is the volume of freshly mixed concrete. The Indian Standard on ready-mixed concrete, IS 4926:2003 states, “All concrete will be supplied and invoiced in terms of cubic meters (full or part) of compacted fresh concrete”.
 
If the measurement is based on the simple L x D x H multiplication, it would be unfair to the RMC producer as it would not include the wastage and spillage of concrete, miscalculations in form volume, deflection or distortions of forms, settlement of wet mixes, over excavation, loss of entrained air, etc. In most of these cases, the responsibility does not lie with the RMC producer.
 
Improper excavation is a common site problem. Deflection or distortion of forms is also quite common. It is observed that if the centre of 150-mm thick formwork of a slab gets deflected by 5 mm, an increased volume of 3 % would be required at the centre, decreasing towards the edges. Therefore ready-mixed concrete should only be measured based on the actual volume of fresh concrete supplied through transit mixers.
 
   
Q. How can we ensure that your transit mixer actually carries the correct quantity as claimed in your delivery docket?
A.
We are a professionally managed ready-mixed concrete company, operating a fully automated and computerized batching system with the help of highly motivated personnel trained to control all process parameters carefully. Production data is automatically recorded and stored electronically in plant-based computers and can be retrieved and verified whenever you wish to do so.
 
Our plant has a weighbridge and all incoming materials are accurately weighed before being stored. Similarly, transit mixers carrying concrete are weighed from time to time to verify the accuracy of the material being supplied to various sites. You may visit our plant with prior appointment and verify the weights of the outgoing trucks on random basis.
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Further, we measure the plastic density of fresh concrete from time to time and match the same with the theoretical density. In case of any noticeable discrepancy in the two values, immediate action is taken to rectify errors, if any. Last, but not the least, our Company has an unblemished reputation of adhering to ethical practices. Hence, you may safely trust us for your quantities.
 
   
Q. How do you guarantee the quality of your concrete delivered at site?
A.
Our Company attaches immense importance to provide highest level of quality assurance to our customers. The basic features of the quality systems practiced by the Company are summarized as below :
  Strict control on the quality of all input materials through rigorous in-house or third-party testing
  Stringent process control measures involving thorough inspection, good upkeep of equipment, monthly scale calibration of weighing devices, etc.
  Continuous monitoring of the key properties of concrete in the fresh and hardened states, including their statistical analysis
  Alignment of Company’s QA & QC practices with the world-renowned Quality Scheme for Ready-Mixed Concrete (QSRMC) from U.K. This involves continuous monitoring on all stages of production and supply i.e. order processing, purchase/control on input materials, concrete mix design, process control, sales, delivery and statistical quality control.
  Modern in-house central laboratory at different locations for testing of concrete and its ingredients
  Active involvement of well-qualified and experienced technical team trained in QA & QC practices.
 
Another key feature of the QA system practiced by the Company includes the use of CUSUM (cumulative summation) technique. The system assists in quick detection of changes in the properties, and indicates when action should be taken to increase the probability of meeting the specifications. This early-warning system has helped us meeting clients’ requirements meticulously. RMC-India is probably the only producer in the country to implement and use CUSUM system to ensure batch-wise consistency in concrete quality.
 
   
Q. How can you assure us about the long-term durability of our structures with your concrete?
A.
Long-term durability of your structures is dependent upon a variety of factors. Important amongst them are the exposure conditions to which the structure will be subjected to during service life, structural design and detailing, quality of concrete and precautions taken during placement, compaction and curing of concrete. A plethora of research conducted on the topic of durability indicates that permeability of concrete is the crux of durability and that it is governed by numerous factors such as water-to-cementitious ratio, mix proportions, properties and replacement levels of supplementary cementitious materials such as fly ash, silica fume, ground granulated blast-furnace slag, and the levels of controls exercised during placement, compaction and curing.
 
Our strict QA & QC practices will ensure good quality concrete, which will provide some assurance to you about its long-term durability. In addition, with the consent of owner/architect/consultant, we do incorporate supplementary cementitious cementitious material in our concretes to improve long-term durability. However, it well known that durability is also governed by field practices, and you would appreciate that these are beyond our control. Correct placement of concrete, its proper compaction and adequate curing go a long way in ensuring the long-term durability. However, the responsibilities on these accounts lie within the scope of the contractor.
 
   
Q. How the use of fly ash enhances quality of concrete? How to ensure good quality fly ash?
A.
Fly ash, obtained from thermal power plant in itself does not possess cementitious properties. However, in its fine form, its particles react with the calcium hydroxide produced in the hydration process of cement, leading to generation of beneficial products of hydration. Voluminous research work done throughout the world on the use of fly ash clearly reveals a number of advantages, leading to multi-level improvements in the properties of concrete, both in its fresh and hardened states. These mainly include improvement in workability and pumpability, reduction in bleeding, improved later-age strength gain and enhancement in long-term durability of structures.
 
These improvements would happen, provided fly ash used in concrete possesses specific physical properties and adequate curing is carried out by the contractor. On our part, we take utmost care to ensure that only good quality of fly ash is used in production. The physical properties of fly ash as well as its uniformity requirements are checked from time to time to ensure compliance with the requirements of IS 3812:2003. As a quick and reliable check, each consignment of fly ash is checked by us for the % retained on 45 micron sieve. If the requirements are not satisfactory, we reject the consignment. You would thus observe that on our part, we take utmost care to provide you a better quality fly ash concrete that is better in terms of later-age strengths and long-term durability.
 
   
Q. During concrete placement operation, how detrimental it is to detain a transit mixer beyond 30-45 minutes after arrival at site? Why?
A.
One of the important hallmarks of ready-mixed concrete is higher speed of construction. If this objective is to be realized, it is essential for your contractor to align his operations to the mutually-agreed schedule of concrete placement. If speedy production and bulk transportation are not matched by equally speedy placement, the basic advantage of using RMC would be lost. Placing concrete with the help of pump would be an ideal solution. In fact, we also provide this service and you can surely entrust this responsibility on us.
 
However, pumping is not suitable for all applications. You may have to use other means of placement. While doing so, kindly ensure that there is least involvement of manual labour and that our transit mixers are emptied within a span ranging between 30-45 minutes. The IS 4926: 2003 clearly states, “The purchaser should plan his arrangements so as to enable a full load of concrete to be discharged within 30 minutes of arrival on site”.
 
   
Q. How much batch-to-batch variation is allowed in the slump of concrete that we receive from your plant?
A.
We carry out sufficient number of lab and field trials for various concrete mixes, which are designed for client-specified slump and compressive strengths. On approval of the mixes the recipes are finalized and incorporated in the computer-controlled automated production systems of our plants. Once this is done, there is less likelihood of any manual error on our part. However, slight variation in slump occurs due to a host of factors such as variation in the ambient temperature and humidity, possible delay in transport due to traffic jams, variation in aggregate grading, errors in sampling and in carrying out slump test, etc. With a view to account for these factors, the BIS code allows a ± 25 mm variation in slump. Thus, if the specified slump is 100mm, variation from 75 to 125 mm is allowable by code. You should therefore be satisfied if the slump at your site is within ± 25 mm of the specified limit.
 
   
Q. How much strength do you guarantee in 7 days?
A.
When the 28-day strength is specified, we are responsible and would provide guarantee for the same. As far as 7-day strength is concerned, we can provide you only the indicative range of strengths. However, contractually speaking you would appreciate that we cannot provide guarantee for the 7-day strength. In case you specify early-age strength, (say strength at 3 or 7 days), we have the capability to satisfy your requirements; however our quotation for the early-age strength would usually be higher than that for the 28-day strength.
 
   
Q. What is the normal frequency of sampling of fresh concrete?
A.
The IS 4926:2003 recommends that one sample should be taken for every 50 m3 of production or every 50 batches, whichever is the greater frequency. For an automated production process having computerized controls, established in all our plants, manual errors are drastically minimized leading to reduction in batch-to-batch variations; hence it is not essential to take samples frequently. Many specifications in other countries specify a much larger frequency of sampling; e.g. the ACI 318:08 specifies one sample to be drawn for each 150 yd3 (115 m3) of concrete, while the European code EN 206 specifies one sample for every 200 m3 of concrete with production control certification. In our day-to-day practice we follow the IS norms, that is one sample for every 50 m3 of production or every 50 batches, whichever is the greater frequency.
 
In addition to having automated and computer-controlled production facility, we also practice a meticulous QA and QC practice. As a result, variations in properties of concrete are further minimized. We feel that once you are able to develop certain level of confidence within our systems, it may be possible for you to specify larger frequency of sampling. We would welcome this step and extend the necessary co-operation to you.
 
   
Q. Why is it essential to employ trained personnel for sampling and preparation of test cubes?
A.
Although sampling and test cube preparation are simple methods, the procedures are operator-sensitive; hence it is essential to employ trained personnel. Take for example, extraction of a “representative” sample. You know that correct procedure involves taking four incremental samples from the middle 80% of the load, ignoring the first and the last 10% portion coming out from the truck mixer and then thoroughly mixing the incremental samples. Unless workmen are trained in this operation, they tend to violate the correct procedures. Similarly, standard procedures for making cube test specimens (filling cubes in layers, tamping each layer 25 times, adequate protection and curing of test specimens, etc.) tend to get violated on many occasions. This is mainly because the workmen at site are not trained adequately to do these jobs. If sampling and cube preparation work are done incorrectly, test results could be erroneous and the producer may get punished for no fault his own. Therefore, it is our considered opinion that sampling and cube preparation should be done only by the trained and experienced personnel.
 
   
Q. Can we arrange for transportation of concrete from your plant to our site in our transit mixer/dumper?
A.
Yes. However, you would appreciate that our responsibility for the quality of concrete (slump, density and compressive strength) will only be valid till we pour concrete in your vehicles.
   
Q. Do we have to start curing earlier for ready-mixed concrete after placement?
A.
Irrespective of the type of concrete being used, the curing process for concrete should commence immediately once the concrete surface looses its sheen, which is normally within 2 hours after finishing compaction of concrete. Else, it will lead to plastic shrinkage cracks. For more details, you may refer to our booklet on “Placing, Compaction and Curing of Concrete: Quick Tips”, a pdf version of which is available on our web site.
 
   
Q. What is the significance of RMC plant being certified by the Ready Mixed Concrete Manufacturers’ Association (RMCMA)?
A.
The RMCMA is an organization of some of the leading RMC manufacturers from India. This professional body, with the help of eminent National Experts has evolved a “Quality Scheme for RMC” and the same is being implemented vigorously. The scheme is based on the best practices followed in some of the advanced countries and strict observance of various provisions in the concrete-specific codes of the Bureau of Indian Standards. The scheme has laid down a framework for quality which is contained in two documents – QC Manual Part I and II. The essence of the scheme lies in the audit of production facility carried out by an external auditor. Once the production facility is successfully audited, it clearly indicates that the facility is capable of producing quality concrete. To bring in transparency and improve customer confidence, the scheme allows customers to visit RMC facility and cross-check any provisions with the provisions in the certified check list. Thus, the certificate from RMCMA for a particular facility provides you with certain level of assurance about the quality of concrete.
 
   
Q. How many of your plants certified by RMCMA?
A.
As of July 31st, 2009, more than 70% of RMC plants of the Company have been certified by the RMCMA and some more are due for audit. In case you wish to check as to which of our plants have been certified by the RMCMA, kindly get in touch with us.