Brajendra Singh, Chief Consultant, Cement Manufacturers’ Association, New Delhi.
BackgroundConcrete is one of the oldest construction materials known to man. Platforms made of lime concrete, found in Israel, have been dated to as far back as 7000 B.C. In India, a sunken city discovered off the coast of Gujarat a few years ago, and estimated to be 7000 years old, has many examples of concrete being used for construction purposes. In ancient Rome, instead of using marble like the Greeks were doing, the Romans used volcanic aggregates (formed out of hardened lava) to produce lightweight concrete, with which they plastered almost all their buildings.
Modern concrete construction only took off after the production of Ordinary Portland Cement (OPC) started in the 19th century. Then in 1903, Ready Mixed Concrete (RMC) was patented in Germany. Unfortunately, RMC could not be commercially exploited for another 10 years, since suitable transport for conveying it to construction sites within the required time frame, did not exist till then. It should be borne in mind that till 1910, several countries had a law that made it mandatory for a man with a red flag to walk in front of any motorized vehicle–mainly to warn horse–drawn transport to move aside. Finally, in 1913, the first dumper type of vehicle for conveying concrete made its appearance in USA, and RMC became a reality.
The next year, 1914, was a fairly significant one, both nationally and internationally as far as concrete was concerned. In India, the first cement plant (capacity 1,000 tonnes per annum) came up in the Mahatma’s birthplace Porbandar, and the country’s first concrete road was built in Madras. In Europe, in the same year, a study found that by mixing flyash with concrete one could obtain significant advantages, such as reduction in the heat of hydration resulting in fewer cracks, and improvement in durability.
What is RMC?
Advantages of RMC Uniform and Assured Quality of Concrete
Faster Construction SpeedIn view of the automated operations, construction activities can be sped up by continuous supply of concrete and mechanized placing. For big construction works like dams, raft foundations, bridge piers and superstructures, slabs of buildings and commercial complexes, and so on, large volumes of concrete are required on a continuous basis. With the use of ordinary site mixers and their labor-intensive techniques of producing concrete, it is just not possible to accomplish such jobs economically and in time. Therefore, in such cases it becomes essential to use RMC and adopt mechanized methods of handling and placing. Improved speed of construction thus achieved, invariably leads to lowering of the overall project cost.
Storage Needs at Construction Sites EliminatedWith the use of RMC, there is no need to store cement, sand, aggregates, water and admixtures at the workplace thus drastically reducing the space requirements at the construction site. In a metropolitan area, where there is a premium on space, this would lead to large cost savings. Further, the contractor/builder need not invest in temporary/permanent structures required for storing different concrete ingredients, nor worry about pilferage and re-supply problems.
Drastic Savings in Labor RequirementsIf concrete is produced at site, a large work force is required for handling the different concrete ingredients, as well as for batching, mixing, transporting and placing operations. By using RMC, this labor requirement is drastically reduced. Further, as the RMC manufacturer looks after the quality control needs, technical manpower of the contractor/builder required for quality assurance and quality control purposes, also gets minimized, leading overall to considerable saving in salaries and other allied costs.
Addition of Admixtures is EasierNow-a-days, the use of both mineral admixtures (flyash, blast-furnace slag, silica fume etc.) and chemical admixtures (plasticisers, retarders, superplasticisers, etc.) is growing. An RMC plant is well–equipped to handle these materials safely and efficiently. While a batching plant can easily be fitted with separate containers to store and use mineral admixtures, a well-designed dispensing arrangement can also be incorporated in it for adding chemical admixtures to the concrete. Further, in view of the high-efficiency of mixing in RMC plants, the fine powders of mineral admixtures are uniformly blended with other ingredients in the concrete; which is an essential requirement for them to impart the required characteristics to the mix.
Documentation of the Mix DesignWith the latest generation of RMC plants having micro-processor-based computer controls, it is possible to automatically document and keep a record of the mixes produced and delivered, as well as the quantities of the ingredients used. These records cannot only be helpful in keeping a control on cost, but also useful for pinpointing the problem area in case of any dispute or trouble at a later date.
Reduction in Wastage of MaterialsIn a labor-intensive job, wastage normally occurs in the handling of all materials, particularly cement. The losses of cement are generally of the order of 2 to 3 kg per 50–kg bag of cement. This wastage is drastically reduced in an RMC facility. Firstly, because RMC plants generally use bulk cement, which saves handling of individual bags. And secondly, because automatic handling is much more efficient than use of manual labor for the same purpose. Cement is a costly input in RMC and around 5% savings on this account could lead to substantial benefits overall.
RMC is Eco-friendlyAt a typical construction site in an urban area using site-mixed concrete, the stored ingredients of concrete, more often than not, spill on the footpaths and roads, obstructing pedestrian and vehicular traffic, besides clogging up manholes and drains. Such hazards do not exist on sites using RMC. A construction site using RMC would generally look quite neat and clean.
Further, as mentioned earlier, the use of RMC enables in reducing the wastage of materials, thus minimizing the uneconomical use of non-renewable raw materials.
At the RMC plant, relevant technology can be used to prevent or minimize dust emissions in accordance with local and national regulations. Plant pollution can be minimized through appropriate plant design, location and technology. Suitable action can be taken to improve effluent quality and reduce volumes of discharge.
RMC can thus be an eco-friendly product, an advantage that is certainly appreciated in the growing environmentally conscious world.
Situation in other CountriesDelivery of RMC, although started in 1913, invariably posed numerous problems, which were only sorted out by 1926, with the advent of the revolving-drum type transit mixer. RMC then came into its own and by the late 1920s and early 1930s, and was introduced in several European countries.
Some of the early RMC plants were of a very small capacity. In 1931, a Ready-Mixed Concrete plant set up in London, had a 1.52 m3 capacity central mixer, supplying six 1.33 m3 capacity agitators with an output of 31 m3 per hour. The cement was handled manually in bags. Till around 1940, there were only six firms producing RMC in the U.K. After the Second World War ended in 1945, there was a boost to the RMC industry in the whole of Europe, including the U.K., due to the large scale reconstruction required in cities which had been extensively damaged due to bombing, shelling or street fighting.
In Europe, the European Ready Mixed Concrete Organisation (ERMCO) was formed in 1967, which is a federation of the National Associations of the respective countries. As of 2004, there were over 6,000 companies represented by it, having a turnover in excess of 20 billion Euros and producing a total of 320 million m3 of RMC. Cement consumption for RMC ranged from 33–66% of total cement sales in different countries, and RMC consumption was of the order of 0.3 to 1.5 m3 per capita, per annum. By early 2004, there were as many as 1,100 RMC plants in the U.K. consuming about 45% of the cement produced in that country.
Coming to the USA, it may be noted that till 1933, only 5% of the cement produced in the country was utilized for making RMC. ASTM published the first specification for Ready-Mixed Concrete, in 1934. However, the industry in USA has progressed steadily since then. Between 1950 and 1975, the RMC industry’s consumption of cement increased from 33% to 67% of the total cement produced in the country; and by 1990, this consumption increased to 72.4 percent of the total cement used in that country. There were as many as 5,000 RMC companies in the country in 1978. However, due to consolidation in the industry, this number dropped to 3,700 in 1994, with only 6 to 7 percent of the companies controlling nearly 50% of the RMC market share. Since then the number of RMC companies in USA has more or less remained the same.
In Japan, the first RMC plant was set up in 1949. Initially, dump trucks were used to haul concrete of low consistency for road construction. In the early 1950s, rotating type truck mixers were introduced and since then there has been a phenomenal growth of the industry in that country. By 1973, there were 3,413 RMC plants in Japan and this number rose to 4,462 by the end of the 1970s. By 1992 Japan was the then largest producer of RMC, producing 181.96 million tonnes of such concrete per year. Even today, Japan is among the largest producers of RMC, though China, from where reliable figures are difficult to obtain, is fast catching up and may even have become No. 1.
In many other countries of the world, including some of the developing countries like Taiwan, Malaysia and Indonesia, as well as certain countries in the Gulf Region, the RMC industry is not only well–developed today, but is also expanding at a rapid rate.
Development in India
In 1974, a techno-economic feasibility study for setting up of RMC plants in India was conducted by the Central Building Research Institute (CBRI), Roorkee. This study recommended setting up of RMC plants in major metropolitan towns of the country. It also suggested the use of flyash as a partial replacement of cement to effect savings in cost.
In the late 1970s, the then Cement Research Institute of India (CRI) now the National Council for Cement and Building Materials (NCBM) carried out a techno-economic viability study of RMC, transported without agitation. In this study, it was observed that conventional RMC would be uneconomical under the then prevailing conditions, since only small volumes of concrete (1m3 or less) could be handled by the available transport at a time, thereby making the transportation cost per unit higher. To reduce the total cost, the study suggested that only a part of the mixing water (about 60%) be added at the central plant, and such a “semi-dry” mix be transported in non-agitating trucks to the construction site, where the mix could be discharged from the truck to the mixer and remixed with the addition of the balance required water. This study further recommended that once the demand for RMC goes up, conventional agitator trucks could be introduced, without any change in the central infrastructure. Based on this study, a feasibility report for setting up of an RMC plant at Delhi was jointly prepared by the NCB and the Central Public Works Department in 1988. The report, like many others gathered dust for a long time, before being implemented in the mid-1990s, only to have the plant close down after only a few years of operation.
It was during the 1970s, when the Indian construction industry went overseas, particularly to West Asia, that an awareness of Ready-Mixed Concrete was created among Indian engineers, contractors, builders etc. Indian contractors in their works abroad began to use RMC plants of 15-m3 /hr to 60-m3 /hr capacity, and some of these plants were brought back to India during the mid-1980s. In the meantime, Indian equipment manufacturers had also started manufacturing small RMC plants.
However, it was only after cement was fully decontrolled, and particularly since the early 1990s, that RMC has been talked about in our country on a commercial basis. The first plant belonging of Ready-Mixed Concrete Industries, Pune was set up in 1993. However, this plant was not a financial success due to several reasons. These included high taxes; a dispute as to whether RMC was a service (as claimed by the producer) or a manufactured product (as decided by the local authorities), the latter attracting much higher duties; hesitancy by builders to use the new product; and higher cost of RMC vis-à-vis site mixed concrete. Later in 1994, the Associated Cement Companies Ltd. (ACC) set up a commercial plant at Bandra in Mumbai. Since then, a large number of other players have set up RMC plants in the country, mainly in metropolitan areas. Currently, based on the information obtained from various RMC manufacturers, there are 198 RMC plants in existence in the country with a combined capacity of 9887 cubic mts per hour. More RMC plants are being planned to be set-up in the near future.
RMC for Road WorksConcrete roads, though initially somewhat costlier, have a large number of advantages over bituminous ones. It is due to this that a sizeable number of concrete roads are now coming up in our country in all sectors–rural, urban, inter-city, national highway and expressways. For highways, where slip-form pavers are used, single-lane pavers and 60 to 90 cubic metres per hour captive batching plants are often matched together for the work. Although these two machines suit each other, they cause delays on the job, thereby driving up costs. The reason for this is that most modern highways in our country, now have or will have, two or three lanes widths on each side. This road width forces single-lane pavers to go back and forth in order to concrete the entire breadth of the pavement. Extra-wide slip-form pavers that can concrete the entire road width in one go are available, or can be procured, but they would then need 150 to 250 cubic metres of concrete per hour, in order to operate at their optimum capacity. Batching plants that can supply such large quantities of concrete are generally too costly for use by construction contractors. However, these are well within the reach of RMC suppliers. Hence RMC is extremely suitable for concrete road construction, where it can save both time and money.
The Urban Construction Boom
With 6 mega cities, 23 metropolitan cities and almost 4,000 large and small towns, India is facing a construction boom. Cement consumption has crossed 120 million tonnes per annum. Unfortunately, most of this cement is used to make concrete in very small, primitive, on-site mixers; this results in large-scale pollution, wastage of cement and low quality output. The resultant construction is weak, sensitive to weather and has a short life. This is a criminal waste of our country’s limited resources. It is therefore essential that on-site mixing be banned and RMC made mandatory for all construction work. The reduction in pollution and improvement in quality that will take place as a result, will more than offset any problems such orders may cause.
Performance ParametersTill fairly recently, the strength of concrete was almost the sole criterion for its formulation. The other properties required for satisfactory performance of the completed construction, were generally assumed to be based on the strength factor. However, experiments, investigations and experience gradually proved that this was often a fallacious assumption. Today, by and large, performance criteria for concrete end products are formulated independently from strength. It is an accepted fact, by modern-day engineers; architects and builders, that properties for concrete can be predefined; and then obtained by suitably adapting the concrete mix design and the admixtures to be added to it. Concretes can thus be “tailor-made” to suit specifications and requirements laid down by customers but this can only be done in RMC plants.
Modern computer-controlled RMC plants can be used to literally design concrete mixes, thereby producing an almost unrestricted requirement of desirable end products; sleek, sophisticated, long-lasting, superior creations; whether they be buildings, roads dams, bridges or what-have-you. RMC means Concrete fit for posterity.