33533522-Ready-mix-concrete.pdf

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INTRODUCTION

1.1

READY MIX CONCRETE

RMC is a specialized material in which the cement aggregates and other ingredients are weigh-batched at 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. Sometimes Materials such as water and some varieties of admixtures can be transit-mixed (also known as Transit Mixture), that is they can be added to the concrete at the jobsite after it has been batched to ensure that the specified properties are attained before placement. Here materials are batched at a central plant and are completely mixed in the Batching Plant or partially mixed in transit. Transit-mixing keeps the water separate from the cement and aggregates and allows the concrete to be mixed immediately before placement at the construction site (Dry Concrete). This method avoids the problems of premature hardening and slump loss that result from potential delays in transportation or placement of central-mixed concrete. Additionally, transit-mixing allows concrete to be hauled to construction sites further away from the plant. There are several types of RMC plants varying in type of mixing and capacity of concrete production. These plants are generally available in capacities varying from 15 cum/hour to 200 cum/hour. A typical RMC plant is shown here.

Figure : 120 CuM/hr RMC plant functioning in Bandra-Kurla Commercial complex, Mumbai (Ambuja Cement Literature Vol. 90)

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1.2 HISTORY OF READY MIX CONCRETE The idea of Ready Mix Concrete was first introduce by Architect Jurgen Heinrich Magens, he get his patented of RMC in Germany in 1903. In 1907, he discovered that the available time for transportation could be prolonged not only by cooling fresh concrete but also by vibrating it during transportation. The first concrete mixed off site and delivered to a construction site was effectively done in Baltimore, United States in 1913 just before the First World War. 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. The first concept of transit mixer was also born in 1926 in the United States. In 1939, the first RMC plant was installed in United Kingdom and in 1933 first specifications on RMC was published in United Kingdom. Between the years 1950 and 1980 considerable growth of RMC took place in the United States with the maximum supply of 31 million cubic meters in the year 1974. However, on an average RMC supplies between 1974 to 1980 were 25 million cubic meters per year. By 1990, in the United Sates there were 3700 RMC producers existing and 75% of cement consumed by the construction industry was being utilised by RMC producers. In 1990 RMC plant in Japan were consuming nearly 70% of the total cement produced. In Malaysia, RMC plants utilised nearly 16% of the total cement consumed in the year 1990. In UK, 43% of the total cement consumed is being used by RMC plants.

Figure: Transit mixer in late 1940’s and Concrete pump in late 1940’s (Ambuja Cement Literature Vol. 90)

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In India RMC was first initially was used in 1950 during the construction sites of Dams like like Bhakra Nangal, Koyna. At the construction the transportation of concrete is done by either manually or mechanically using ropeways & buckets or conveyor systems. RMC at Pune in the year 1991. However, due to various pit falls and problems this plant did not survive for long and was closed. Within a couple of months in the year 1993, two RMC plants were set up in Mumbai to commercially sell RMC to the projects where they were installed. Unitech Construction set up one plant at Hiranandani Complex and Associated Cement Companies set up another plant at Bharat Diamond Bourse Commercial Complex. These plants were later allowed to sell RMC to other projects also. Thus RMC was successfully established sometime after 1994 in India. RMC producers from outside India soon became interested in the Indian market and therefore two very well known producers set their foot on the Indian soil i.e. Fletcher Challenge Ltd. from New Zealand and RMC Ready Mix of UK. As per the available record upto 2003, there are around 76 RMC plant in 17 cities with a total capacity of around 3875 CuM/hr, producing 3.8 million CuM of concrete per year.

RMC In India (2003) Number of RMC plants and their capacities in leading metros of India.

Metro

No. of Plants

Capacity (M3/hr)

Mumbai and

15

835

Bangalore

13

550

Delhi

11

660

Chennai

11

480

Hyderabad

7

350

Navi Mumbai

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1.3 USE OF RMC RMC is generally looked upon as a costly product rather than a facility to get an appropriate quality product on site as and when required. The first cost of RMC may seem higher. However, there are several hidden advantages which can cause considerable reduction in cost to the owner. Since they cannot be accurately determined, they are ignored while evaluating the cost of RMC over site mixed produced concrete. The following points answer the above question: • Generally speaking, the quality of concrete will be superior than site mixed concrete. However, it will greatly depend on the controls and checks exercised at site and at RMC producer's plant. • There is a considerable wastage of materials on site due to poor storage conditions and repeated shifting of the mixer location. This is prevented if RMC is used. • In most cities, the plot area is barely sufficient to store reinforcement steel, formwork, concrete and other construction materials. Using RMC can cause less congestion and better housekeeping on the site resulting in efficient working environment. • Obtaining RMC at site can reduce supervision and labour costs which would otherwise be required for batching and mixing of concrete at site. • Many sites in cities, house their work force on the site itself to reduce the time and cost of daily travel. This creates unsafe and unhygienic conditions on the site as well as for the surrounding areas. This will reduce to a certain extent if RMC is utilized. • Fluctuation of raw material prices and their availability has always caused delays and problems of inventory and storage for site producers of concrete. This is totally avoided when RMC is used. • Availability of labour gangs intermittently has always posed problems to concrete producers on site. This can now be avoided. Besides these labour gangs are difficult to supervise and control as they are only interested in completing the concreting operations as fast as possible. This results in addition of excess water and inadequacies in batching/mixing. • A problem of inspection, checking and testing of all concrete materials on site is avoided. However, to a certain extent these checks and tests may be required to be done at RMC producers' plant. • Concrete mix design and its control due to variations of material properties is avoided as RMC producers are responsible for the same and supply concrete as specified by the purchaser as per the requirements of the construction site. • In public places it creates less nuisance. Congested roads and footpaths are often blocked by carelessly stored concrete materials. RMC allows a much better flow of road traffic as well as pedestrian movement. • It .irnproves the environment and around the site. Nuisance due to stone dust and cement particles is reduced considerably. To a certain extent even noise pollution is reduced. • The modern RMC plants have an automatic arrangement to measure surface moisture on 4

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aggregates this greatly helps in controlling the water to cement ratio (w/c) which results In correct strength and durability. • RMC plants have proper facilities to store and accurately batch concrete admixtures (chemical and mineral). To improve properties of concrete both in plastic and in hardened stage this accuracy is useful. • In general, RMC plants have superior and accurate batching arrangements than the weigh batchers used on site. • RMC plants have superior mixers than the rotating drum mixers generally used for mixing concrete materials at site. • RMC plants have efficient batching and mixing, facilities which improve both quality and speed of concrete production. • Temperature control of concrete in extreme weather conditions can be exercised in a much better manner than done at site. • RMC helps encourage" mechanization and new technologies like pumped concrete bulk transportation of cement production of self-compacting concrete and high strength high performance concrete. • New materials like micro silica and fibers can be safely used in RMC which in conventional concrete may pose problems. • Introduction of RMC improves the rate of supply of concrete in the formwork and thereby automatically improves quality of formwork, layout of reinforcement steel and its detailing and safety / strength of scaffolding and staging.

1.4 DISADVANTAGES OF SITE MIXED CONCRETE Normally the concrete operation is carryout in India is of site mixed, which is having some disadvantages which is shown below: •

Quality Assurance not guaranteed.

•

Constant control on aggregates for size, shape & grading not exercised on site.

•

Arbitrary batching and mixing by volume. Strict water-cement ratio not exercised.

•

Wastage of materials.

•

Retarded speed.

•

Possible break down of mixers.

•

Concreting operations prolonged beyond day light without proper lighting.

•

Manual operation.

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Speed restricted depending on mixers.

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Restricted spaces.

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Storages of aggregates and cement. 5

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Blocking of roads / approaches

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Dust pollution

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Noise pollution

1.5 OBJECTIVE OF RMC The main objective to choose this topic is that an engineer should know the advantages of RMC and disadvantages of Site mixed concrete. As RMC are widely using in bigger and medium size of projects today, engineer should be aware the technicality of the RMC and the operational work to ensure the quality of work to be maintain. Site engineer to be know that what are the steps to be taken to check the concrete in RMC, what is required to be specified for RMC, what is the information required to be supplied by the RMC supplier, what checks are necessary by the consumer before ordering RMC, what are the checks needed at site prior and after to receipt of RMC.

1.6 SCOPE OF RMC IN INDIA Though delayed, but not very much, there a ready mixed concrete industry is developing and expanding at a fast pace in the country on a large scale. Over the period, due impetus to this development has been provided by various front-line construction and cement companies as well as technological bodies. The World Bank's “ India Cement industry Restructuring Project" under which a technical study report on the development of market for bulk cement in India was made in 1996, proved to be positive development towards modernization of cement distribution system in India, including setting up Ready mix concrete Plants. The objective of this technical study was to formulate an action plan for the development of market for bulk cement in large cement centres in India and for gradual shift. from the traditional mode of transportation in bags to bulk transportation through setting up of ready mixed concrete plants in different parts of the country. The recommendation of the action plan provided a useful guidance towards expanding bulk cement market thus paving a way for installation or ready mixed concrete plants in India. According to Cement Manufacturers Association, RMC is being increasingly recommended for all major public construction work such as highways, flyovers. In cities like Bangalore and Chennai, even small house builders have started displaying a marked preference for RMC instead of cement. According to the experts, there is lot of scope for the development and growth of RMC in India. It can grow to consume 40-45 percent of cement by 2015 through setting up of RMC plants in various consumption centers. For the healthy growth of industry, RMC industry in India has to fine-tune its own practices to following practices elsewhere in the advanced countries where RMC industry has been operating successfully. European Ready Mixed Concrete Organization (ERMCO) has defined the broad objectives to be achieved in design, management and operation of RMC which remain same as that of designing, and execution of concrete construction projects. The marketing of RMC should no more be in terms of strength grades 6

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only, but a combination of strength durability classification as per the Concrete Codes which improves the sellability of RMC in terms of the requirements of the projects. Appropriate environmental, safety and health regulations for the working force need to be kept in mind in the management and operation of RMC.

Figure : RMC Market Potential (CE&CR, June 2001, pg.no.30)

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2.0 TECHNICAL ASPECT

2.1 TYPES OF BATCHING PLANTS There are basically two principal categories of ready mix concrete. 1) Centrally Mixed Concrete or Wet batch mixer

In Centrally mixed concrete, the mixing is done at a central plant & the mixed concrete is then transported usually in the truck agitator truck which revolves slowly so as to prevent segregation and undue stiffening of the mix. 2) Truck mixed concerte or Dry batch concrete plant

In the truck mixed concrete, the materials is batched at a central plant but are mixed in a mixer truck either in transit to the site or immediately prior to the concrete is being discharged. Transit-mixing permits a longer haul and is less vulnerable in case of delay, but the capacity of a truck used as a mixer is only 63 per cent, or even less, of the drum while for central-mixed concrete is 80 percent.

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2.2 BATCHING AND MIXING PLANTS The principal functional elements of every stationary concrete production Plant comprises of the following: a)

Storage of materials - Silos, containers and bins

b)

Batching arrangement

c)

Measuring and recording equipment

d)

Mixing equipment

e)

Control systems

f)

Electrical, hydraulic and pneumatic drives

g)

Conveying systems (belt / screw conveyors)

a) Storage of Materials i) Cement Cement is generally stored in silos. The loading of cement is done with the help of pneumatic blower systems either installed on bulk carriers or a separate system available at the plant. If baggage cement is used then the cement is loaded using a compressed air loader and a splitter unit. Cement is weighed separately, and is transported from the silo into a mechanical or electro mechanical weigher by means of a screw conveyor.

Figure: Pneumatic blower system on a cement bulker unloading cement into silos. (Ambuja Cement Literature Vol. 12)

ii) Water Water is generally stored in tanks located close to the plant. It is accurately measured by a water gauge and microprocessor controlled system. The modern plants have new litronic MFM 85 moisture recorders (See Figure). These recorders actually measure the moisture present in sand while the entire batch flows past. A 9

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recording unit calculates the average moisture value of the sand and passes on the information to the batching control unit to allow corrective action to be taken. The system operates to an accuracy of as low as 0.2% relative moisture.

(a)

(b)

Figure: a) Compressed air cement loader and a splitter unit for loading cement packed in bags b) Electromechanical weighing system used on modern batching plant for cement and aggregates (Amubuja Cement Literature Vol.no.12)

Consistency of the mix is generally checked by visual observation later confirming it with a workability test like the slump test. However, in modern plants consistency of the concrete mix is checked by a remote recording system which is automatic, easy and more accurate. If concrete is very dry (stiff) the electrical resistance of the batch is measured and if the concrete is wet the motor output is measured. Accurate maintenance of the workability (consistency) of one cubic meter batch of concrete, may depend on as little as one litre or less of water. It is scarcely conceivable that such a production process could be controlled without actually measuring the workability and later correcting the consistency.

iii) Aggregates The storage of aggregates is done in various way depending on the type of plant. There are basically three types of plants generally in use.

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•

Vertical Production Plant In this the aggregates are stored above the batching and mixing elements, in one or more silos. These plants are not suitable for relocation at short intervals of time. As the aggregates are stored in silos it is relatively easy to protect the aggregates from very low temperature in winter period.

Figure: Schematic view of the Vertical Batching Mixing Plant with vertical aggregate and cement storage silos. ( Ambuja Cement Literature Vol.12)

•

Horizontal Production Plant They can be again broadly classified into four types i) Star pattern aggregate storage 11

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iii) Storage in pocket silo iv) Inline aggregate storage silos

Figure: Schematic view of the Horizontal Batching mixing plant with the star pattern aggregate storage bins at the rear & vertical storage silos. (Amubuja Cement Literature Vol.no.12)

The star bin storage of aggregates is most popular in India mainly because of climate conditions. The aggregates can be stored exposed to ambient temperature in different compartments forming a star type pattern. A storage capacity of upto 1500 CuM is possible in this type. The star pattern aggregates are stored in four to six compartments. They are bulked at a 45 degree flow angle against the batching plant's bulkhead and partition wall of the compartments using a boom type dragline loader. The drag-line operations are either fully manual, semi automatic or fully automatic. Fully automatic drag-line loader system operator. The star bin type plant requires more space and as the aggregates are stored in open they heat up at high ambient temperatures and freeze at very low temperatures. These types of plants are not suitable in extreme weather conditions. In silo type storage additional investment for loading equipment such as hopper, bucket elevator or conveyor belt plus rotary distribution are required. They have large active storage (upto 500 CuM) in a small areas. Loading is fully automatic, aggregates are well protected in extreme climatic conditions and storage is very clean. 12

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Figure: Horizontal Batching Mixing Plant with vertical aggregates & cement silos (Amubuja Cement Literature Vol.no.12)

Figure: Rear view of the batching mixing plant with star pattern having four aggregate storage bins (Amubuja Cement Literature Vol.no.12)

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b) MIXING ARRANGEMENTS There are various types of concrete mixers used on the concrete production plant. The two basic types are free fall mixers and power mixers. Most of our indigenously manufactured plants have free fall mixer. Free fall mixer consists of a rotating drum with blade fixed on the drum's interior. As the drum rotates, the material inside is lifted and dropped. The drum is loaded and emptied by changing the direction of rotation, dropping a flap or tipping it. Most of the imported plants have power mixer. The power mixer sets in motion the materials positively. The materials get thoroughly mixed by rotating arms. These mixers have shorter mixing time, give better homogeneity, consistency and strength to the concrete. Besides, they have better facility for inspection. The following are the most common designs.

Figure: Cross sectional view of the single shaft compulsory power mixer. (Amubuja Cement Literature Vol.no.12)

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Figure: Twin shaft compulsory power mixer (Amubuja Cement Literature Vol.no.12)

Power Mixer

Capacity CuM

Output CuM/hr

Max.

(Compacted concrete)

(Compacted Concrete)

Aggregate size (mm)

Mixing Time (Sec) 30

15

Single Shaft

3

120

--

150

Twin Shaft

3.5

120-150

--

190

Pan Type

3

120

--

64

Pan Type with additional agitator

3

--

140

64

If mixing is to be done on difficult concrete mixes, additional agitator is provided. The pan type mixer with additional agitator or two agitators is claimed to be far in advance of any if other mixer. Using additional agitators almost halves the mixing time. The additional agitator is driven by a separate hydraulic system and can be set to any speed between 0 to 200 revolutions per minute.

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c) CONTROL SYSTEMS Almost all imported production plants offer automatic systems for control functions. These are required for better quality control, higher economy and superior working conditions.Fully automatic plant control systems with multiple inputs for upto 120 mixes or template control system are usually housed in a container or control room of the plant. Micro processor controlled production plants represent the state of the art in the developed countries.

(a) (b) Figure: a) Fully automatic control panel of a modern mixing plant. b) Microprocess Control System on the modern mixing plant. (Amubuja Cement Literature Vol.no.12)

These controls are operated from main desk. It also has material availability monitor and printer plus an additional batching monitor. The entire plant can be operated by just one person. Microprocessor control besides having fully automatic running facility offers number of additional features like statistical data recording and processing, a printer unit, moisture adjustment arrangement, customer address, memory etc. All you need to do is push the required mix template into the slot and press the "start" button and the control system does the rest. Aggregate and cement weighment, moisture correction, overrun correction and additive weighment are done accurately to give the concrete mix of desired strength and workability. The built in trouble shooting programmes are most valuable and have a high reliability factor. Even upto 1500 mixes of different types can be stored along with names and addresses of the consumer and other data which is required to be stored in the computer for operation of the plant. The mix data with quantity can be if required printed by the printer which is very useful for invoicing the consumer for the concrete supplied to them.

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d) TRANSPORTATION EQUIPMENT: - TRANSIT MIXER There are developments taking place all over the world for different types of concrete equipments. However, the transit mixer is one of the most .popular equipments out of several modes available. In India too, a number of transit mixers are in use allover the country which are mainly mounted on Indian. Truck chassis. The mixer drum is either manufactured in India or is improved. However, in general, the hydraulic system is improved. There are several types and capacities of transit mixers available as given below: Normal capacity

4 to 12 CuM

Hydrulic Drive of Mixer

Separate engine or driven by truck engine

Water tank capacity

192 to 2000 litres

Mixer trucks

Twin axles for 4 CuM capacity Three axles for 6 to 7 CuM capacity, three/four axles for 8 to 10 CuM capacity, Semi trailer truck for 10 to 12 CuM.

In India 4 Cum. Truck mixers are popular while the 6 and 7 Cum. Truck mixers mounted on a 3 axle chassis enjoy a leading position on all world markets as it has a favorable cost- performance ratio, large selection of chassis, good maneuverability and is more suited to general batch size requirements.

Figure: Transit mixer 17

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3.0 QUALITY ASPECT 3.1 SAMPLING OF CONCRETE Critical decisions, often involving very high potential costs, are made on the basis of concrete test results. Correct sampling is paramount to the validity of these test results but is an aspect of testing that is frequently overlooked and often carried out by untrained people. It is therefore essential that the sampling is done correctly and is representative of the concrete delivered. According to the IS 4926-2003, “After the truck-mixer has re-mixed its delivery on site allow at least the first one-third of a m3 of concrete to be discharged prior to taking any samples. Take at least 4 incremental samples from the remainder of the load avoiding sampling the last cubic metre of concrete. Thoroughly re-mix this composite sample either on a mixing tray or in the sampling bucket and proceed with the required testing.” As per BS 1881: Part 101 & Part 102, describe the recommended sampling methods for ready mixed concrete in British code. Using a standard scoop, this can collect about 5kg of normal weight concrete. Each load of concrete to be tested should be nominally divided into a number of scoopfuls. BS 1881: Part 101: (The Standard method): To ensure that the concrete is representative of the whole load is standard sample consists of scoopfuls taken from at least four different parts of the load and collected in buckets. The scoopfuls should be taken at equally spaced intervals; the scoop being passed through the whole width and thickness of the stream in a single movement. The first and the last 1/6th portion of the discharge should be disregarded as unrepresentative. This is then thoroughly re-mixed on a non-absorbent surface before carrying out any individual test. This operation is necessary to even out any variation between individual scoopfuls and to counteract any segregation that may have occurred in transporting the sample from the sampling point to the testing area. BS 1881: Part 102 (The Alternative Method) : An alternative method of sampling concrete for slump testing from a truck-mixer before the majority of the load has been discharged is permitted. This enables the concrete to be tested before being placed. When this alternate method is used, an initial discharge of at least 0.3 m3 is made before a sample of six scoopfuls is collected from the moving stream; The sample is then re-mixed on a nonabsorbent surface and split into two equal parts. Each part is then tested or slump, with the average of the two tests recorded as the test result. This method of sampling is only applicable to the slump test. Concrete sampled by this method must not be used to make cubes for compliance testing, as it will produce erroneous results.

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(a)

(b)

Figure: (a) The Standard method of collecting sample (b) The Alternative method of collecting sample (CE&CR, June 2001, pg.no.35)

3.2 TESTING OF CONCRETE The results of concrete tests are used as the basis for deciding whether the delivered concrete is in accordance with the specification, if the reported results are below the compliance loyal, doubt is cast on the quality of the supplied concrete and the long term durability of the structure. If the investigation indicates that the sampling, preparation and or storage of the test specimens have not been in accordance with the required standard procedures, then the result will be invalid. Further valuable management time, better devoted to other management functions, will have been wasted. It is, therefore, in everyone's interest that sampling and testing is done correctly so that the results provide a valid basis for logical decision making. Where the contractor is required to perform on-site concrete testing, it is imperative that suitable test facilities are available, that the appropriate equipment is calibrated and that the staff has been fully trained in the relevant test techniques, one can look the factors that affect the price compliance test, (i.e.) the compressive strength test. As per IS 456 -2000, a result is the average of the compressive test specimens made for the same sample of concrete. Individual variation should not be more than 15 Percent of the average. If more. The results of the average. If more, the results of the sample are invalid. This is because differences of this magnitude indicate poor sampling, cube making, curing or crushing. It is, therefore essential that correct test facilities are provided and the people with responsibility for sampling and testing concrete are suitably trained. Slump test for predicting the workability of concrete is done at batching plant and working site. The main problem in the production of ready mixed concrete is maintaining the workability of the mix right uo to the time of placing. Concrete stiffens with time and the stiffening may also be aggravated by prolonged mixing and by high temperature. In case of transit mixing, water need not be added till nearer the commencement of mixing but, according to ASTM C94-94, the time during which the cement and moist aggregate are allowed to remain in contact is limited to 90 minutes; BS 5328: part 3: 1990 allows 2 hours. The 90 minutes limit can be relaxed by the purchaser. With the 19

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use of retarding admixtures, the time limit can be extended to 3 or even 4 hours, The United States Bureau of Reclamation provides for an extension of 3 to 6 hours in the time of contact between cement and wet aggregate in transport prior to mixing. In IS 4926-2003, The general requirement is that concrete shall be discharged from the truck-mixer within 2 h of the time of loading. However, a longer period may be permitted if retarding admixtures are used or in cool humid weather or when chilled concrete is produced.

3.3 ADMIXTURES RMC is generally transported to different construction sites and delivered with the help of revolving type transits mixers. These sites are located at long distances and the concrete delivered is workable, plastic and flowable. Experience shows that slump loss takes palce with certain types of cement and to prevent this superpplasticiser are used. RMC's often use admixtures formulated for special purposes like : (a) Improvement of screeds, renders, mortars and concrete for increased water resistance and less risk-from aggressive materials, (b) Integral waterproofers, (c) Foamed - for light weight insulation. void filling and semi structural support, (d) Water repelling for semi-dry block concrete, (e) Microsillica for use in high performance concrete. The introduction of microsillica was observed to improve the pore solution chemistry of HPC. For higher replacement of cement by silica fumes, the concentration of both K+ and OH- ions are substantially removed. However, upto 20% replacement will not cause a drop in pH below that of saturated CaOH solution, which is approx.12.5. Even at 30% replacement, pH does not drop below 11.5, which is considered to be a threshold value for maintaining a good passivity of embedded steel.

3.4 USE OF FLY ASH BASED IN CONCRETE To have a better performance characteristics in terms of durability of concrete, fly ash is been used successfully in concrete. The improvement in gel structure caused by pozzolanic action of fly ash leads to a very impervious concrete. These factors improve the resistance of concrete against external aggression. Since Fly ash is an environmental hazard, therefore by effectively using it in concrete it can be consumed constructively and thus contribute to the ecological balance. Many prestigious and critical structures have been built using either PPC or by blending fly ash directly in concrete. The names of the famous Petronus towers & Euro Tunnel can be definitely quoted in this regard. However , the Indian construction industry has offered some resistance in using PPC or fly ash in structural concrete. With advent of RMC in India the doubts regarding controls in using this material are slowly dispelled and engineers have become more open to the idea of using fly ash as pozzolan for partial replacement of cement in concrete. “Limestone is the raw material for the manufacture cement & limestone is limited resource.” It cannot last long it ever. We need to realize the importance of this fact. We need to use our cement rationally. Whereas we talk about huge figure describing development and building needs of our country, concrete still remains the most widely used and environmental friendly construction material to achieve this. 20

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4.0 OPERATIONAL ASPECT

4.1 NEED TO BE SPECIFIED FOR RMC The following need to be specified very clearly: • Characteristic strength or grade (N/mm2) • Target workability or slump in mm required at site • Exposure conditions for durability requirements
Maximum water to cement ratio
Minimum cement content
Maximum aggregate size
Type of cement
Mineral admixture and its proportion (Kg/m3) • Maximum aggregate size • Rate of gain of strength (for formwork removal or prestressing etc.) • Maximum temperature of concrete at the time of placing (in extreme climatic conditions or in case of massive concrete pours) • Type of surface finish desired. • Method of placing • Rate of supply desired to match the placing and compaction speed planned at site. • Quantity of concrete required. • Lift and lead of concrete transportation and placement at site. • Frequency of concrete testing • Details of materials and their required tests. • Permeability tests required (if any) • Placing of concrete in formwork to be under scope of RMC supplier (if required) • Permissible wastage • Mode of measurement.

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4.2 INFORMATION TO BE SUPPLIED BY THE PRODUCER The RMC supplier must provide the following information to the consumer if and when requested: • Nature and source of each constituent material including the name of the manufacturer in case of branded products like cement, admixtures etc. • Proportion of quantity of each constituent per CuM of fresh concrete. • Generic type of the active constituent of the chemical admixture and its solid content. • Chloride content in all constituent materials. • Compatibility of cement and chemical/mineral admixtures. • Compatibility of admixtures with one another when more than two types of admixtures are proposed. • Initial and final setting time of concrete when admixture is used. • Details of plant and machinery (capacity CuM/hr), storage (CuM) availability, type of facilities to dose admixtures, type of moisture measurement arrangement, type of mixer, rated capacity (CuM/min.) of the mixer. • Availability of number of transit mixers and their capacities. • Details of last calibrations done on various weighing /dosing equipments • Testing facilities available at RMC plant • Capacity and type of concrete pump and placing equipment available (if required).

4.3 CHECKS BY CONSUMER BEFORE ORDERING THE RMC The following need to be looked into by the consumer: •

Reliability of the plant and transit mixers for consistent and continuous concrete supply as per requirement.

•

Calibrations of all measuring devices and their accuracy.

•

Mode of operation of plant should preferably be fully automatic and not manual.

•

Quality of materials proposed to be used.

•

Adequacy of quantity of materials proposed to be used.

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Compliance of concrete specifications based on the mix parameters specified. 22

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Adequacy of testing facilities

•

Time likely to be taken by transit mixers from plant to site and back.

4.4 CHECKS NEEDED AT SITE PRIOR TO RECEIPT OF RMC

•

Reinforcement layout for proper concrete placement without segregation

•

Adequacy of formwork to take the hydrostatic pressure and adequacy of loading on propping system to match the speed of placing.

•

Openings and chutes provided, at predetermined locations, between reinforcement bars to lower the placing hose (if pumped concrete is planned) to avoid segregation of concrete

•

Adequacy of manpower and equipment for placing, compacting, finishing and curing of concrete.

•

Proper approach for transit mixers free from all encumbrances ego water logging, material stacking etc.

•

Proper platform to receive concrete.

•

Proper precautions required to be taken to ensure that concrete from the transit mixer is unloaded at the fastest possible speed does not take more than 30 minutes.

•

If pumping is proposed, the location of the pump should be approachable from both sides.

4.5 CHECKS NEEDED AT SITE DURING CONCRETING

•

Proper co-ordination between the RMC supply and placing and compacting gangs.

•

Proper signaling or communication at site is necessary.

•

Workability of concrete within accepted limits.

•

Adequacy of cohesiveness of concrete for pumpability.

•

Ensure that water addition or chemical admixtures are not added during transportation by RMC unauthorized persons and without the knowledge of the site in charge of the consumer.

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Temperature of concrete at the time of receipt at site (if specified).

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Continuous and steady supply at site and speedy unloading of the Monitor speed and progress of placing to avoid formation of cold joints transit mixers.

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Monitor proper placement without segregation. 23

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Monitor placement of concrete at the closest possible point to its final location.

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Arrange for curing as soon as finishing is completed. This is specially required in case of slabs, pathways and roads in hot/warm weather.

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Retempering should be prohibited as experiments shows the addition of water to RMC truck at the construction site may result in substantial reduction in strength. The reduction in strength was found to be proportional to the increase in slump. Large increase in slump means higher reduction in strength. When the amount of water added is not controlled, reduction of strength may be as high as 35%. In cases where controlled amount of water is added to restore the slump within the specification’s limits (100 ±25 mm), the reduction of strength may be below 10%.

4.6

THE UNNECESSARY RESTRICTIONS ON SUPPLIERS OF RMC BY PURCHASER

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Insistence on use of cement and admixtures of specific brands: This selection should be left to the RMC supplier as they have to decide this based on the comparability study.

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Inappropriately low water to cement ratio. This should be left to the RMC supplier or alternatively high strength of concrete specified.

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Restriction on use of water reducing admixtures. It is almost mandatory to use water reducing and / or slump retaining admixtures. Hence such restrictions can cause quality problems.

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Insisting on Indian Standard method of concrete mix design: It must be understood that IS 10262 (1982) only gives guidelines on design of concrete mixes. It does not cover high strength cements now available and does not cater to effects of admixtures. It also does not recommend changes necessary for RMC and pumpable concrete mixes. Concrete mixes designed by this method are generally found to be non-cohesive and require higher cement contents (uneconomical). The option of concrete mix design should be and must be left to the RMC supplier.

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Frequency of testing: This is often changed by the consumer than that specified in clause 6.3.2 of IS 4926 (2003). However, this needs to be mutually discussed and finalised prior to placement of order.

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Fixed slump insisted upon: Many a times fixed slump value is insisted upon by the consumer. This is practically not possible. Variations are likely to occur and should be within the limits say ± 25 mm as stated in clause 6.2.1 of IS 4926 (2003).

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Ambiguous specifications: Many consumers give ambiguous specifications. Both the specifier and the supplier need to resolve the ambiguity specially those dealing with specifications like durability as per IS 456 (2000) without mentioning exposure conditions or presence of chemicals in ground water and subsoil. Also specifying target mean strength 24

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instead of characteristic strength required without mentioning the accepted failure rate or standard deviation. •

4.7

Concrete Field strength should not be less than Target strength: Such a specification that the field strength should not be less than the target strength should not be less than the target strength belies the understanding of the definition of characteristics strength. If the requirement is for an M30 grade concrete, then the field strength of concrete should not be less 30 N/mm2 within a confidence limit of 95%. If the specification insists on target strength to be achieved in the field as well, then the concrete requested automatically becomes M39 or its equivalent. This makes the concrete unnecessarily expensive.

THE CONSTRAINTS FACED BY RMC PRODUCERS AT PRESENT

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RMC cost is likely to be slightly higher than site produced concrete of the same quality. This may be mainly due to sales taxes. However, to some extent if RMC consumer has no objection to addition of flyash or ground granulated blast furnace slag of required quality and consistency then perhaps the cost becomes more competitive with site produced concrete. RMC plants having accurate computerised batching and excellent mixing facilities can produce good quality RMC if they are careful in selecting the mineral admixture.

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Delayed payments and long credit period insisted upon by consumers affect their cash flow.

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RMC plants in cities are not permitted to be installed in residential zones. This results in their installation nearly 10-20 kms. away from their potential consumers located in residential zones.

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High cost of the plant and equipment results in high capital costs. However, many multinationals have started producing plant and equipment in our country. Hence costs have reduced. However, one has to be careful as quality of performance has dropped in comparison with equipment directly imported from countries like Germany.

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Bad quality of roads and traffic congestion and intermittent signals often delay the deliveries in metros.

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Availability of trained and skilled manpower for operations and maintenance of plant and equipment. As new plants come up, skilled workers keep changing jobs for better prospects.

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Price variations of all concrete ingredients specially cement.

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Non availability of consistent and good quality aggregates, mineral admixtures etc.

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Non-availability of bulk cement supply in most of the cities where RMC is marketed.

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Difficulty in immediate availability of spares or additional inventory carrying cost required to 25

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be kept in case of essential spares. •

Stipulations of pollution control board causing difficulty to obtain license for running RMC plant. Such clearances are not required if similar plants are installed on the construction site itself.

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Workability retention in hot weather.

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Site oriented problems at the consumer end such as the following:

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Delays in placing, compacting and finishing at consumer's end causes delays in unloading of transit mixer and stiffening of the concrete mix.

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Quality of formwork and its adequacy to take proper vertical loads and hydrostatic pressures, due to faster rate of supply and placing is often not taken care of at sites receiving concrete.

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Reinforcement layout and planning of placement, compaction and curing must be properly organised at site to suit the speed of supply and placement of RMC.

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In many countries, specialist agencies do pumping and placing of concrete. In our country, the onus of pumping and placing is either placed on the RMC supplier or on the construction site.

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Concrete cube failures and their acceptance criteria due to site inadequacies or sampling should not be attributed to RMC supplier.

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Plastic shrinkage cracks due to inadequate curing at site often results in blaming the RMC supplier.

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5.0 PROMOTING RMC IN INDIA RMC can be promoted in the right interest of the society is to ban any storage of materials at any construction site. It should be made mandatory that no public space shall be occupied by building materials during construction. Wherever multi-storied buildings are coming up, public buildings and buildings other than for the owners are constructed the government should come out and insist on RMC and quality. Government can play a very active role that would help the industry grow and give the people the benefits of a better technology that has been proved superior economically, environmentally and qualitatively world ever. There are several areas where the government could take bold initiative for the steady growth of the industry, such as taxation issue, the codes and specifications, allotment of land, uninterrupted power supply, transit mixers movements should be allowed through out 24 hrs. so that the speed of construction can be increased and customer can get uninterrupted supply of RMC. The concept of RMC itself is expensive as it involves high degree of mechanization that results in huge capital investment. Loading it further will only hamper the growth, because the industry is in a flux and is just taking off. Until the industry consolidates, and taxes burdens will be counter productivity. The government and local bodies can encourages the use of ready mixed concrete by suitably amending the tender clauses and providing facilities to the entrepreneurs to set up RMC plants. With the economic liberalization and emphasis on development in infrastructure in the country, the scope for RMC is tremendous.

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CONCLUSION Conventional approach to durable concrete structures, namely specifying maximum water cement ratio, minimum cement content and cement type, is now always satisfactory, especially under aggressive environmental condition. Site manufactured concrete cannot assure the same quality of concrete and that from controlled ready mix batching plant backed by advanced technology and project management. The advantages of RMC are particularly evident in construction projects with aggressive exposure conditions. Ready mix concrete has gained acceptance in Indian industry due to several advantages including quality control and overall economy. RMC plant are proliferating, especially in urban regions, not only because of the space restrictions around construction site but also due to the realisation of the advantages by engineers and construction industry. That concrete quality produced in RMC plant is highly consistent with low deviation order. It provides a high degree of overall strength of hardened concrete and the performance of the structure at a later date. RMC operations are highly mechanized and fully controlled through electronic controls and hence reduce the probability of errors in various operations. It is also environment friendly and brings down pollution due to dust at construction can also be accelerate with the use of RMC. The use RMC in civil construction is widely adopted through out the world. The beginning made in India is in tune with the developments outside and RMC uses provide numerous benefits to the consumers.

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REFRENCES •

“RMC in India” (June 2001), Civil Engineering & Construction Review.

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Kaushal Kishore (June 2001), “RMC: Some Technical Aspects”, Civil Engineering & Construction Review.

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Sanjay Bahadur (June 2001), “RMC- A Non-Negotiable Approach to Durable Urban Infrastructure”, Civil Engineering & Construction Review.

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Kshemendra Nath (June 2001), “RMC- The Need for Partnership”, Civil Engineering & Construction Review.

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Danish Rashid (June 2001), “RMC: The Indian Answer to World Class Construction”, Civil Engineering & Construction Review.

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IS 4926-2003, Standard on Ready mixed concrete – Code of Practice, BIS, New Delhi.

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“RMC on the move” (Oct. 2003), Ambuja Technical Literature, Vol. No. 90

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“Mechanisations of concreting, Part I- Batching, Mixing & Transporting” (Dec. 1996), Ambuja Technical Literature, Vol. No. 12

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Abdulrahman M. Alhozaimy and Abdulaziz I. Al-Negheimish (Dec 2002), “RETEMPERING OF READY-MIXED CONCRETE IN RIYADH, SAUDI ARABIA”, Civil Engineering Department, King Saud University.

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