Ready mixed concrete Ready mixed concrete (RMC) is a specialized material in which cement, aggregate, and other ingredients are weigh batched at a plant in a central or truck mixer before delivery to the construction site in a condition ready for placing by the customer. RMC is manufactured manufactured at a place away from the construction site, the two locations being linked by a transport operation. IS: 4296-2003 defines ready mix concrete as ‘concrete mixed in a stationary mixer in a central batching and mixing plant or in a truck mixer and supplied in a fresh condition to the purchaser either at site or into purchaser’s vehicle’. The short life of fresh concrete, with only 2-3 hrs before it must be placed, results in ready mixed concrete being a very much local delivery service, with rarely more than 30-60 min journey to the construction site. The need for supply of ready mixed concrete to fit in with the customer’s construction program means that RMC has to be both a product and a delivery service. This means that the ready mixed supplier is in two separate businesses – firstly, processing materials and secondly, transporting product with a very short life. Historical growth of ready mixed concrete Read Ready y mixed mixed concr concret ete e was was first first paten patented ted in germa germany ny in 1903, 1903, but but mean means s of transporting was not sufficiently developed by then to enable the concept to be utilized commerc commercially ially.. The first commerc commercial ial delivery delivery of ready ready mixed mixed concrete concrete was made made in baltimaore, USA in 1913 and the first revolving – drum type transit mixer, of a much smaller capacity than those available today, was born in 1926. In 1920 and 1930, ready mix concrete was introduced in some European countries. Some early plants were of very small capacities. In 1931, a ready mix concrete plant set up at what is now Heathrow airport, London, had 1.52 m3 capacity central mixer, supplying six 1.33m3 capacity agitators with an output of 30.58 m3/h. aggregate were stored in four compartments, each of 76.45m3 capacity. Cement was handled manually in bags. Till the beginning of world war II,
Advantages of RMC
Uniform and assured quality of concrete: since RMC is factory produced, the raw material and production quality is better than conventional site mixed concrete.
Durability of concrete: RMC can ensure correct w/c ratio to be maintained. Hence the durability of RMC is consistent and better.
Faste Fasterr constr construct uctio ion n speed: speed: in site site mixed mixed concr concrete ete,, the the contra contracto ctorr need needs s to mobilize labour for mixing as well as placing. In RMC, fresh concrete is supplied in a peaceable condition and can directly be placed by pumping. Hence a faster construction speed can be achieved.
Elimi Eliminat nation ion of stora storage ge need needs s at the the cons constru tructi ction on site: site: in case case of site site mixe mixed d concrete, all raw materials such as aggregates, sand and cement have to be
stored at the site. In urban situations and when the work is progressing close to the highways, there is a problem of storage of raw materials affecting smooth flow of traffic. In case of RMC, this problem is completely avoided as the storage of materials takes place at the central plant.
Easier admixture addition: In RMC, admixtures can be added in controlled methods of releasing exact quantities needed. This is not possible in normal concreting.
Documentation of mix design: The contractor purchases fresh concrete from the supplier of RMC, who is responsible not only for documentation but also for maintaining records.
Reduction in wastage of material: In RMC plants materials are stored in bulk and used in bulk. Hence wastage that occurs in loose handling of cement, etc. Is completely avoided.
RMC is eco friendly: the production of RMC is done in an environmentally assessed and licenced central plant. Hence, dust and noise pollution which is envitable in concrete is avoided.
Disadvantages of Ready-Mix Concrete •
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The materials are batched at a central plant, and the mixing begins at that plant, so the traveling time from the plant to the site is critical over longer distances. Some sites are just too far away, though this is usually a commercial rather than technical issue. Generation of additional road traffic; furthermore, access roads, and site access have to be able to carry the weight of the truck and load. Concrete is approx. 2.5tonne per m³. This problem can be overcome by utilizing so-called 'minimix' companies, using smaller 4m³ capacity mixers able to access more restricted sites. Concrete's limited time span between mixing and going-off means that readymix should be placed within 90 minutes of batching at the plant.
Components of RMC plant
The RMC plants consists of
I)
The RMC plant with auxiliary/supporting equipment ii) Transit mixers and iii) Site equipment for handling concrete.
Ready mixed concrete plant Generally an RMC plant of capacity 55-60 m3/h would meet the requirement of a small town. Initially the plant could work on 8-10 hrs shift: with a growing market,
16-18h is feasible. Expanding operations can be made by an additional 55-60m3/h RMC plant at the same site. The typical layout of an RMC plant is shown in fig 1 Process adopted for central ready mixed concrete plants Centrally batched concrete is manufactured in plants and transported to various sites. The cement silos feeds cement directly into the weigh hopper. This in turn feeds the cement directly into the transit mix truck. Generally, the plant is capable of manufacturing all types of concretes used by today’s construction industry. The aggregates are accurately weighed by the front end loader into the weigh hopper and then transferred directly into the truck by means of conveyor system. Bulk admixtures drums are generally kept adjacent to the conveyor system. Another option is to ‘live’ overhead storage. Storage can range from 100 to 1000 tonnes of four to six different materials. This is used when a higher production rate of ready mix concrete is required. With the load-cell type weigh batch the digital scale display can easily be read by the front-end operator. This is to ensure that a high degree of accuracy of the order of ± 10 kg is achieved during batching. A similar type of weigh batcher using the traditional knife-edge lever scale system and an analogue scale read-out is also effective, and can be easily maintained. This type of plant can be erected and operated within 1-2h of it arriving on the site. The cement weigh hopper valves and installed aerators ensure that an accurate, even flow of cement is weighed before discharging it into the mixing bowl of the truck. There are various admixtures that can enhance the performance of concrete. These admixtures must be carefully measured for each batch. This is achieved by using precision measuring devices. A full range of air entrainers, superplastizers, retraders, water reducers and accelerators can easily handled with such devices attached to the equipment. The controls for these activities are located in the batching control room. The amount of instrumentation will depend upon the size, desired performance, production rate and the degree of automation of the plant. Whether the plants are situated at temporary sites for large civil construction projects such as dams, airport runways, bridges, etc.. or loaded in permanent city sites, great care should be taken to ensure that dust does not pollute the atmosphere. This is important from enviormental protection consideration the plant with cement capacity of 225 tonne and live aggregate storage of 300 tonne and a production rate of 60-70 m3/h using an automatic batching system is used normally in a city ste up. Trucks are washed and cleaned prior to loading each time. The yard is generally designed to conserve all water by directing spillover to ponds so that water can be reused. Ready mix trucks coming back after delivery may have small concrete remaining in their bowls. This is washed out and stored in specially designed containment ponds. These ponds allow all solids to settle while water circulates through the ponding system. Solid material cam be removed from the site and reused as a base course or a fill. The water is used as mixing water for concrete production through the plant after through testing. The water storage plant can have a capacity ranging from 40 to 80 m3, depending on plant size. The depth of ponds range from 1.5 to 2.5m.
Classification of ready mixed concrete: There are three principal categories of RMC. 1. Transit mixed or truck mixed concrete: in transit-mixed concrete, also called truck-mixed or dry batched concrete, all of the raw ingredients are charged directly in the truck mixer. Most of the water is usually batched at the plant. The mixer drum is turned at charging (fast) speed during the loading of the materials. There are three options for truck mixed concrete. I) concrete mixed at the job site. ii) concrete mixed in the yard or central central batching plant. iii) concrete mixed in transit. 2. shrink mixed concrete : concrete that is partially mixed in a stationary plant – mounted mixer and then discharged into the drum of the truck mixer for completion of the mixing is called shrink mixed concrete. Generally about two minutes of mixing in truck drum at mixing speed, is sufficient to completely mix shrink-mixed conrete.
Central mixed concrete: central mixed concrete batch plant include a stationary, plantmounted mixer that mixes the concrete before it is discharged into a truck mixer. Central-mix plants are sometimes referred to as wet-batched or pre-mix plants. The truck mixer is used primarily as an agitating haul unit at a central mix operation
Fig 1: schematic diagram of central ready mix plant
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F H
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A Aggregate delivery C Aggregate storage E Cementitious material storage G Cement delivery I Admixtures K Recycled water M Pump O Concrete loaded in ready-mix truck
B Aggregate receiving hopper D Conveyor belt F Weigh hopper H Mixer J Ready mix truck with returned concrete L Reclaimed aggregates N Water storage P Control Room
Batching, Mixing, Transporting, and Handling Concrete
The specification, production, and delivery of concrete are achieved in different ways. The basic processes and common techniques are explained here. ASTM C 94 provides standard specifications for the manufacture and delivery of freshly mixed concrete. Three options for ordering or specifying concrete are described in ASTM C 94: 1. Option A is performance based. It requires the purchaser to specify the compressive strength only, while the concrete producer selects the mixture proportions needed to obtain the required compressive strength. 2. Option B is prescription based. The purchaser specifies mixture proportions, including cement, water and admixture contents. 3. Option C is a mixed option. It requires the concrete producer to select the mix proportions with the minimum allowable cement content and compressive strength specified by the purchaser . BATCHING Batching is the process of measuring concrete mix ingredients by either mass or volume and introducing them into the mixer. To produce concrete of uniform quality, the ingredients must be measured accurately for each batch. Most specifications require that batching be done by mass rather than by volume (ASTM C 94 or AASHTO M 157). Water and liquid admixtures can be measured accurately by either volume or mass. Volumetric batching (ASTM C 685 or AASHTO M 241) is used for concrete mixed in continuous mixers. Specifications generally require that materials be measured for individual batches within the following percentages of accuracy: cementitious material ±1%, aggregates ± 2%, water ±1%, and admixtures ± 3%. Equipment should be capable of measuring quantities within these tolerances for the smallest batch regularly used as well as for larger batches. The accuracy of scales and batching equipment should be checked periodically and adjusted when necessary. Liquid chemical admixtures should be charged into the mixture as aqueous solutions. The volume of liquid, if significant, should be subtracted from the batched quantity of mixing water. Admixtures that cannot be added in solution can be either batched by mass or volume as directed by the manufacturer. Admixture dispensers should
be checked frequently since errors in dispensing admixtures, particularly overdoses, can lead to serious problems in both fresh and hardened concrete.
MIXING CONCRETE All concrete should be mixed thoroughly until it is uniform in appearance, with all ingredients evenly distributed. Mixers should not be loaded above their rated capacities and should be operated at the mixing speed recommended by the manufacturer. Increased output should be obtained by using a larger mixer or additional mixers, rather than by speeding up or overloading the equipment on hand. If the blades of a mixer become worn or coated with hardened concrete, mixing action will be less efficient. These conditions should be corrected. If concrete has been adequately mixed, samples taken from different portions of a batch will have essentially the same density, air content, slump, and coarse aggregate content. Maximum allowable differences to evaluate mixing uniformity within a batch of ready mixed concrete are given in ASTM C 94 (AASHTO M 157). Structural low-density concrete can be mixed the same way as normal-density concrete when the aggregates have less than 10% total absorption by mass or when the absorption is less than 2% by mass during the first hour after immersion in water. For aggregates not meeting these limits, mixing procedures are described in PCA (1986). STATIONARY MIXING Concrete is sometimes mixed at the jobsite in a stationary mixer or a paving mixer. Stationary mixers include both onsite mixers and central mixers in ready mix plants. They are available in sizes up to 9.0 m3 (12 yd3) and can be of the tilting or nontilting type or the open-top revolving blade or paddle type. All types may be equipped with loading skips and some are equipped with a
swinging discharge chute. Many stationary mixers have timing devices, some of which can be set for a given mixing time and locked so that the batch cannot be discharged until the designated mixing time has elapsed. Careful attention should be paid to the required mixing time. Many specifications require a minimum mixing time of one minute plus 15 seconds for every cubic meter (yard), unless mixer performance tests demonstrate that shorter periods are acceptable and will provide a uniform concrete mixture. Short mixing times can result in non homogenous mixtures, poor distribution of air voids (resulting in poor frost resistance), poor strength gain, and early stiffening problems. The mixing period should be measured from the time all cement and aggregates are in the mixer drum, provided all the water is added before one-fourth of the mixing time has elapsed. Under usual conditions, up to about 10% of the mixing water should be placed in the drum before the solid materials are added. Water then should be added uniformly with the solid materials; leaving about 10% to be added after all other materials are in the drum. When heated water is used in cold weather; this order of charging may require some modification to prevent possible rapid stiffening when hot water contacts the cement. In this case, addition of the cementitious materials should be delayed until most of the aggregate and water have intermingled in the drum. Where the mixer is charged directly from a batch plant, the materials should be added simultaneously at such rates that the charging time is about the same for all materials. If supplementary cementing materials are used, they should be added after the cement. If retarding or water-reducing admixtures are used, they
should be added in the same sequence in the charging cycle each time. If not, significant variations in the time of initial setting and percentage of entrained air may result. Addition of the admixture should be completed not later than one minute after addition of water to the cement has been completed or prior to the start of the last three-fourths of the mixing cycle, whichever occurs first. If two or more admixtures are used in the same batch of concrete, they should be added separately; this is intended to avoid any interaction that might interfere with the efficiency of any of the admixtures and adversely affect the concrete properties. In addition, the sequence in which they are added to the mix can be important too.
TRANSPORTING AND HANDLING CONCRETE Good advanced planning can help choose the appropriate handling method for an application. Consider the following three occurrences that, should they occur during handling and placing, could seriously affect the quality of the finished work: Delays. The objective in planning any work schedule is to produce the fastest work with the best labor force and the proper equipment for the work at hand. Machines for transporting and handling concrete are being improved all the time. The greatest productivity will be achieved if the work is planned to get the most out of personnel and equipment and if the equipment is selected to reduce the delay time during concrete placement. Early Stiffening and Drying Out. Concrete begins to stiffen as soon as the cementitious materials and water are mixed, but the degree of stiffening that occurs in the first 30 minutes is not usually a problem; concrete that is kept agitated generally can be placed and compacted within 1 1⁄2 hours after mixing unless hot concrete temperatures or high cement contents speed up hydration excessively. Planning should eliminate or minimize any variables that would allow the concrete to stiffen to the extent that full consolidation is not achieved and finishing becomes difficult. Less time is available during conditions that hasten the stiffening process, such as hot and dry weather, use of accelerators, and use of heated concrete. Segregation. Segregation is the tendency for coarse aggregate to separate from the sand cement mortar. This results in part of the batch having too little coarse aggregate and the remainder having too much. The former is likely to shrink more and crack and have poor resistance to abrasion. The latter may be too harsh for full consolidation and finishing and is a frequent cause of honeycombing. The method and equipment used to transport and handle the concrete must not result in segregation of the concrete materials.
Shot Crete concreting or guinting Shotcrete is mortar or very fine concrete deposited by jetting it with high velocity (pneumatically projected or sprayed) on to a prepared surface. The system has different properietory name sin different countries such as blast concrete, blowconcrete, gunconcrete, jetcrete, nucrete, pneukrete etc. shotcrete offers advantages over conventional concrete in a variety of new construction and repair works. Shotcrete is frequently more economical than conventional concrete because of less form work requirements, requiring only a small portable plant for manufacture and placement. It is capable of excellent bonding with a number of materials and this may be an important consideration. Shotcrete has wide application in different constructions, such as thin over head vertical or horizontal surfaces, particularly the curved or folded sections: canal, reservoir and tunnel lining; swimming pools and other water retaining structures and prestressed tanks. Shotcrete concrete is verey useful for the restortation and repair repair of concrete structures, fire ddamaged structures and water proofing of walls. Shotcrete has been successfully used in the stabilization of rock slopes and temporary protection of freshly excavated rock surfaces. Its utility has been proved for protection against long-term corrosion of piling, coal bunkers, oil tanks, steel building frames and other structures, as well as in encasing structural steel for fire proofing.
Shotcrete was invented in the early 1900s by American taxidermist Carl Ackley, used to fill plaster model of animals. He used the method of blowing dry material out of a hose with compressed air , wetting it as it was released. This was later used to patch weak parts in old buildings. In 1911, he was granted a patent for his inventions, the "cement gun", the equipment used, and "gunite", the material that was produced. Until the 1950s when the wet-mix process was devised, only the dry-mix process was used. In the 1960s, the alternative method for gunning by the dry method was devised with the development of the rotary gun, with an open hopper that could be fed continuously. Shotcrete is also a viable means and method for placing structural concrete. The nozzleman is the person controlling the nozzle that delivers the concrete to the surface. The nozzle is controlled by hand on small jobs, for example the construction of small swimming pools. On larger work the nozzle is held by mechanical arms and the nozzleman controls the operation by a hand-held remote control.
Types of shotcreting concrete There are two basic types of shotcreting process:
1.
The dry mix method involves placing the dry ingredients into a hopper and then conveying them pneumatically through a hose to the nozzle. The nozzleman controls the addition of water at the nozzle. The water and the dry mixture is not completely mixed, but is completed as the mixture hits the receiving surface. This requires a skilled nozzle man, especially in the case of thick or heavily reinforced sections. Advantages of the dry mix process are that the water content can be adjusted instantaneously by the nozzleman, allowing more effective placement in overhead and vertical applications without using accelerators. The dry mix process is useful in repair applications when it is necessary to stop frequently, as the dry material is easily discharged from the hose.
2.
Wet-mix shotcrete involves pumping of a previously prepared concrete, typically ready-mixed concrete, to the nozzle. Compressed air is introduced at the nozzle to impel the mixture onto the receiving surface. The wet-gun procedure generally produces less rebound, waste (when material falls to the floor), and dust compared to the dry-mix procedure. The greatest advantage of the wet-mix process is that larger volumes can be placed in less time.
Properties of shotcrete concrete The properties of shotcrete are essentially the same as for conventional concrete of same materials, proportions and void system. However, the following points should be borne in mind. 1. 2.
3.
in shotcrete concrete, generally, a small-maximum-size aggregate is ussee and cement content is high. These should enhance durability in most cases. whereas conventional concrete is consolidated by vibration, shotcrete is consolidated by the impact of a high velocity jet impacting on the surface. This process not only increases the cement content due to rebound but also brings about different air void system affecting the durability of shotcrete. The application procedures have a great effect on the in place properties of shotcrete than the mix proportions.
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A Aggregate delivery B Aggregate receiving hopper C Aggregate storage D Conveyor belt E Cementitious material storage
F Weigh hopper G Cement delivery H Mixer I Admixtures J Ready mix truck with returned concrete K Recycled water L Reclaimed aggregates M Pump N Water storage O Concrete loaded in ready-mix truck P Control Room
