Tuesday, 8 April 2014

Ready Mix Concrete [RMC]




INTRODUCTION


               Ready-mix concrete is made at a batching plant and delivered by a cement truck to a work site. This type of concrete speeds up the construction process by having the concrete already mixed and ready to pour once it reaches the site. Ready-mix concrete saves builders time and money.




History:-


        Ready-mix concrete was first mixed in a factory in the 1930s. It wasn't until the 1960s that ready-mix concrete became more in demand. Since that time, the ready-mix concrete business has continuously grown and is now a major industry.



APPLICATIONS:-

·                                This technique is useful in congested site or diverse work Places and saves the consumer from the botheration of procurement, storage and handling of concrete materials.
·                           Ready mix concrete is produced under factory conditions and permits a close control of all operations of manufacture and transportation of fresh concrete.

SPECIFICATIONS

        Quality of ready mix concrete is generally specified in terms of two systems

·        Performance parameters
·        Prescriptive specifications

1. Performance parameters:

·        It is the best way to order ready mixed concrete because RMC producer, who is expert in the field, would design an economical mix with the desired properties
·        The RMC producer accepts the responsibility for the design of the mixture for the desired performance

2. Prescriptive specifications:

·        Purchaser specifies aggregate size, slump, air content, cement content, or weight of cement per cubic meter of concrete, maximum water content and admixtures required.

            The purchaser accepts the responsibility for concrete strength and its performance.


How Ready-Mix Concrete is made

        Ready-mix concrete is made out of 10 to 15 percent cement, approximately 60 to 75 percent aggregates, and about 20 percent water with admixture it depends on distance from RMC plant to working site. When distance is more percentage of admixture can be increase.

Function:-
        Ready-mix concrete is used for small and large construction projects. It is used to pave driveways and for foundations on homes and large commercial buildings. It is also used for bridges, roads and sidewalks.

Benefits:-
        There are several benefits to the use of ready-mix concrete. Ready-mix concrete helps speed up the building process. Construction companies save time and money by using concrete that is already mixed. It eliminates the need for a crew to mix concrete at the work site. Ready-mix concrete also helps to reduce construction-site pollution. A lot of dust is created when concrete is mixed on site.

Limitations:-
        Ready-mix concrete cannot be transported over long distances. When concrete is mixed at a batching plant, it must be poured within two to three hours to hold up to industry standards. This creates the need for central mixing plants to serve the construction industry.

Speculation:-
        The advantages of ready-mix concrete far outweigh all limitations. The building and construction industry would be greatly inhibited without it. What used to take several man-hours to do can now be accomplished in a very short period of time






MAKING OF RMC

        Its simplest form, concrete is a mixture of paste and aggregates (sand & rock). The paste, composed of cement and water, coats the surface of the fine (sand) and coarse aggregates (rocks) and binds them together into a rock-like mass known as concrete.
        Within this process lies the key to a remarkable trait of concrete: it’s plastic and can be molded or formed into any shape when newly mixed, strong and durable when hardened. These qualities explain why one material, concrete, can build skyscrapers, bridges, sidewalks, and superhighways, houses and dams.




Proportioning:-                                 
        The key to achieving a strong, durable concrete rests on the careful proportioning and mixing of the ingredients. A concrete mixture that does not have enough paste to fill all the voids between the aggregates will be difficult to place and will produce rough, honeycombed surfaces and porous concrete. A mixture with an excess of cement paste will be easy to place and will produce a smooth surface; however, the resulting concrete will be more likely to crack and be uneconomical.
        A properly proportioned concrete mixture will possess the desired workability for the fresh concrete and the required durability and strength for the hardened concrete. Typically, a mixture is by volume about 10 to 15 percent cement, 60 to 75 percent aggregates and 15 to 20 percent water. Entrained air bubbles in many concrete mixtures may also take up another 5 to 8 percent
        The character of concrete is determined by the quality of the paste. The strength of the paste, in turn, depends on the ratio of water to cement. The water-cement ratio is the weight of the mixing water divided by the weight of the cement. High-quality concrete is produced by lowering the water-cement ratio as much as possible without sacrificing the workability of fresh concrete. Generally, using less water produces a higher quality concrete provided the concrete is properly placed, consolidated and cured


Other Ingredients:
        Although most drinking water is suitable for use in concrete, aggregates are chosen carefully. Aggregates comprise 60 to 75 percent of the total volume of concrete. The type and size of the aggregate mixture depends on the thickness and purpose of the final concrete product.
        Relatively thin building sections call for small coarse aggregate, while aggregates up to six inches in diameter have been used in large dams. A continuous gradation of particle sizes is desirable for efficient use of the paste. In addition, aggregates should be clean and free from any matter that might affect the quality of the concrete.


Hydration Begins:-
        Soon after the aggregates, water, and the cement are combined, the mixture starts to harden. All Portland cements are hydraulic cements that set and harden through a chemical reaction with water. During this reaction, called hydration, a node forms on the surface of each cement particle. The node grows and expands until it links up with nodes from other cement particles or adheres to adjacent aggregates.
        The building up process results in progressive stiffening, hardening, and strength development. Once the concrete is thoroughly mixed and workable it should be placed in forms before the mixture becomes too stiff.
        During placement, the concrete is consolidated to compact it within the forms and to eliminate potential flaws, such as honeycombs and air pockets. For slabs, concrete is left to stand until the surface moisture film disappears. After the film disappears from the surface, a wood or metal hand float is used to smooth off the concrete. Floating produces a relatively even, but slightly rough, texture that has good slip resistance and is frequently used as a final finish for exterior slabs. If a smooth, hard, dense surface is required, floating is followed by steel toweling.
        Curing begins after the exposed surfaces of the concrete have hardened sufficiently to resist marring. Curing ensures the continued hydration of the cement and the strength gain of the concrete. Concrete surfaces are cured by sprinkling with water fog, or by using moisture-retaining fabrics such as burlap or cotton mats. Other curing methods prevent evaporation of the water by sealing the surface with plastic or special sprays (curing compounds).
        Special techniques are used for curing concrete during extremely cold or hot weather to protect the concrete. The longer the concrete is kept moist, the stronger and more durable it will become. The rate of hardening depends upon the composition and fineness of the cement, the mix proportions, and the moisture and temperature conditions. Most of the hydration and strength gain take place within the first month of concrete's life cycle, but hydration continues at a slower rate for many years. Concrete continues to get stronger as it gets older.



READY MIX CONCRETE MUST BE PREPARED CORRECTLY

        Ready mix concrete is a pre-mixed building material containing Portland cement, sand and gravel aggregate. It is popular with do-it-yourselfers because the only ingredient that must be measured is water. When using ready mix concrete, it is important to properly mix and pour to obtain the best results.

Estimating the Amount:-
        It is important to know how much concrete you will need before the project begins to avoid purchasing too much or too little. Multiply the width, length and depth of the area that you need to fill to obtain the cubic meter. For example, a space that is 1-by-2-by-3 feet requires six cubic feet of concrete. Bags of ready mix concrete will tell you how many cubic feet of concrete they contain. Use this information and your calculations to determine how much to purchase.

Mixing by Hand:-
        Empty the contents of the bag into a wheelbarrow or mixing trough. Measure the amount of water instructed on the bag in a 5-gallon bucket and add it to the concrete. Mix the concrete with a hoe or shovel until you reach the desired consistency. It should be thick enough to hold its shape but not so thick that it cannot be mixed. This option works well for small amounts of concrete, but larger amounts should be mixed with a power mixer.

Mixing by Machine:-
        Check the mixer to make sure there is no dirt, dried concrete or other foreign matter before you begin. Pour half the water into a mixer and turn it on. Let the mixer run for a few minutes and pour in the concrete and the rest of the water. Let the concrete mix for about three minutes and transfer it to a wheelbarrow. Wash out the mixer with a garden hose before the concrete can set or the mixer will be severely damaged.

Pouring and Curing:-
        Ready mix concrete can be shoveled or poured into a form. These forms are typically constructed of lumber and hold the concrete within a certain shape while it cures. Spread the concrete evenly, being careful not to trap air pockets. Use a screed to level the concrete and use a float to push the aggregate to the bottom and bring excess water to the surface. Ready mix concrete must be allowed to cure until it has completely hardened and cooled. The time required will depend on the thickness of the concrete, but it is typically no less than five to seven days.


39 Important Technical Points about Ready Mix Concrete

Ø Dusting Concrete Surfaces
Ø Scaling Concrete Surfaces
Ø Crazing Concrete Surfaces
Ø Cracking Concrete Surfaces
Ø Plastic Shrinkage Cracking
Ø Joints in Concrete Slabs on Grade
Ø Cracks in Concrete Basement Walls
Ø Discrepancies in Yield
Ø Low Concrete Cylinder Strength
Ø Strength of In-Place Concrete
Ø Curing In-Place Concrete
Ø Hot Weather Concreting
Ø Concrete Blisters
Ø Finishing Concrete Flatwork
Ø Chemical Admixtures for Concrete
Ø Flexural Strength of Concrete
Ø Flow able Fill Materials
Ø Radon Resistant Buildings
Ø Curling of Concrete Slabs
Ø Delimitation of Toweled Concrete Surfaces
Ø Loss of Air Content in Pumped Concrete
Ø Grout
Ø Discoloration
Ø Synthetic Fibers for Concrete
Ø Corrosion of Steel in Concrete
Ø Jobsite Addition of Water
Ø Cold Weather Concreting
Ø Concrete Slab Moisture
Ø Vapor Barriers under Slabs on Grade
Ø Supplementary Cementations Materials
Ø Ordering Ready Mixed Concrete
Ø Concrete Pre-Construction Conference
Ø High Strength Concrete
Ø Making Concrete Cylinders in the Field
Ø Testing Compressive Strength of Concrete
Ø Structural Lightweight Concrete
Ø Self Consolidating Concrete
Ø Pervious Concrete
Ø Concrete Maturity



ADMIXTURES OF READY MIX CONCRETE


Hydration Begins:-

        After the aggregates, water, and the cement are combined, the mixture remains in a fluid conditi0on for about four to six hours which permits transporting, placing and finishing in its final location, then the mixture starts to harden. All Portland cements are hydraulic cements that set and harden through a chemical reaction with water. During this reaction, called hydration, crystals radiate outwards from cement grains and mesh with other adjacent crystals or adheres to adjacent aggregates. The building up process results in progressive stiffening, hardening, and strength development. Once the concrete is thoroughly mixed and workable it should be placed in forms before the mixture becomes to stiff. During placement, the concrete is consolidated to compact it within the forms and to eliminate potential flaws, such as honeycomb and air voids. 




Proportioning:-
        The proportioning of a concrete mix design should result in an economical and practical combination of materials to produce concrete with the properties desired for its intended use, such as workability, strength, durability and appearance.
       The ready mixed concrete producer may independently select the material proportions to provide the performance you need or may receive instructions through the job specifications, such as minimum cement content, air content, slump, maximum size of aggregate, strength, and others.
        Regardless of the source of instructions, there are established methods for selecting the proportions for concrete for each batch. The Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete (ACI 211.1-91) published by the American Concrete Institute Committee 211 is often referenced for selecting concrete proportions.


Here are the basics of a good concrete mix:-
        Cement and water combine chemically to bind the sand and aggregate together. Fly ash or other cementations materials, which enhance concrete properties, may supplement some of the cement. The key to quality concrete is to use the least amount of water that can result in a mixture that can be easily placed, consolidated and finished.
        Fine and coarse aggregates make up about 70% of the concrete volume and impart volume stability to the concrete. Concrete aggregates are required to meet appropriate specifications and in general should be clean, strong and durable.

        Admixtures are generally products used in relatively small quantities to improve the properties of fresh and hardened concrete. They are used to modify the rate of setting and strength development of concrete, especially during hot and cold weather. The most common is an air-entraining agent that develops millions of tiny air bubbles in concrete, which imparts durability to concrete in freezing and thawing exposure. Water reducing admixtures enable concrete to be placed at the required consistency while minimizing the water used in the mixture, thereby increasing strength and improving durability. A variety of fibers are incorporated in concrete to control cracking or improve abrasion and impact resistance.
        Architectural masterpieces like bridges, high-rise buildings, and dams require the highest standards of engineering. With the help of admixtures, concrete is able to fulfill this!
        Applicable to all types of ready mix concrete, from the very basic to ultra-high strength, our broad range of admixtures are able to improve slump retention, placing, pumping, finishing, appearance and in general, performance characteristic as desired.

Brands under this segment:-
        A new generation of versatile and high-performance admixtures based on polycarboxylate ether for hyper plasticized concrete. Used effectively for high performance, self compacting and high strength concrete.
        GLENIUM admixtures not only provide answers to old problems but unearth exciting opportunities for the construction industry for groundbreaking and cost effective solutions for today’s concrete. GLENIUM has an excellent track record of being used in the world’s most exciting architectural wonders including the present tallest Burj-Al-Arab Dubai!   
        The admixture used in local ready mix concrete plants is RHEOBUILD 918 it is a company product of BASF.



TYPES OF MIXING THE READY MIX CONCRETE



A. Transit Mixed (or "truck-mixed") Concrete:-
        In transit-mixed concrete, also called truck mixed or dry-batched, all of the raw ingredients are charged directly in the truck mixer. Most or all 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:
        Concrete mixed at the job site. While travelling to the job site the drum is turned at agitating speed (slow speed). After arriving at the job site, the concrete is completely mixed. The drum is then turned for 70 to 100 revolutions, or about five minutes, at mixing speed.
         Concrete mixed in the yard. The drum is turned at high speed or 12-15 rpm for 50 revolutions. This allows a quick check of the batch. The concrete is then agitated slowly while driving to the job site.
        Concrete mixed in transit. The drum is turned at medium speed or about 8 rpm for 70 revolutions while driving to the job site. The drum is then slowed to agitating speed. (More information on ready mixed concrete trucks can be found in the Delivery section).



B. Shrink Mixed Concrete:-
        Concrete that is partially mixed in a plant mixer and then discharged into the drum of the truck mixer for completion of the mixing is called shrink mixed concrete. Central mixing plants that include a stationary, plant-mounted mixer are often actually used to shrink mix, or partially mix the concrete. The amount of mixing that is needed in the truck mixer varies in these applications and should be determined via mixer uniformity tests. Generally, about thirty turns in the truck drum, or about two minutes at mixing speed, is sufficient to completely mix shrink-mixed concrete.

C. Central Mixed Concrete:-
        Central-mixing concrete batch plants include a stationary, plant-mounted mixer that mixes the concrete before it is discharged into a truck mixer. Central-mix plants are sometimes referred to as wet batch or pre-mix plants. The truck mixer is used primarily as an agitating haul unit at a central mix operation. Dump trucks or other non-agitating units are sometimes be used for low slump and mass concrete pours supplied by central mix plants. About 20% of the concrete plants in the US use a central mixer. Principal advantages include:

 Ø Faster production capability than a transit-mix plant 
 Ø Improved concrete quality control and consistency and
 Ø Reduced wear on the truck mixer drums.
 Ø There are several types of plant mixers, including:
 Ø Tilt drum mixer
 Ø Horizontal shaft paddle mixer
 Ø Dual shaft paddle mixer
 Ø Pan mixer
 Ø Slurry mixer


TYPES OF READY MIX CONCRETE

Precast:-
        Precast concrete is concrete cast elsewhere than its final position. Precast construction can be used to produce a wide variety of structural elements such as wall panels, bridge girders, pipes, poles, crash barriers, lift stations, etc. The concrete can be reinforced, lightly pre-stressed or pre-stressed depending upon its requirements. Precast concrete elements are typically transported from the plant to the job site through highways, rail, or barges. Tilt-up concrete is a form of precast concrete and is generally referred to as site pre-casting.

Tilt-Up:-
        Tilt-Up construction is "a construction technique of casting concrete elements in a horizontal position at the jobsite and then tilting and lifting the panels to their final position in a structure". The greatest advantage of tilt-up construction over cast-in-place concrete is the ease and speed of construction. Generally the concrete should attain a high early age flexural strength to facilitate lifting and handling. Tilt-up concrete is a form of precast concrete and is generally referred to as sit pre-casting.


Flow able Fill:-


        Flow able also known as cons trolled low-strength material (CLSM) is a self-compacted, cementations material used primarily as a backfill in lieu of compacted fill. The compressive strengths must be lower than 1200 psi and often is lower than 200 psi. Since the material flows into place no compaction is required which is a significant advantage over compacted back fill. Flow able fill can be made with very high amounts of non-standard materials.

Pervious Concrete:-


        Pervious concrete is concrete that does not contain fine aggregate. It typically has a voids content of 15% to 35%. This ensures that the concrete has the unique ability to allow storm water to pass through its mass into the ground underneath. Pervious concrete offers significant environmental benefits as it reduces the requirement for drainage facilities. Further it facilitates the recharge of ground water and the filtration process purifies the water as it percolates below.



TESTING & INSPECTION OF READY MIX CONCRETE

        Concrete is a manufactured product. Specific control tests and evaluations are required during the manufacturing process to produce predictable high-quality concrete. The customer may want to verify that concrete meets specifications. Some of the important properties of concrete that are measured by basic quality control tests are strength, temperature, slump, air content, and unit weight. In general concrete is tested at a frequency of 1 in 150 cubic yards


Ø Each test helps to determine the quality of concrete and it should be performed in accordance with standards. A certified Concrete Field Testing Technician – Grade I, must make the tests.

Ø Define the basis of purchase, cubic yards, and how it is measured.
Ø Define acceptable material specifications and acceptable industry practice and tolerances.
Ø Define strength testing procedures and acceptance criteria.
Set laboratory personnel qualifications. The testing laboratory must comply with ASTM C 1077, which is required in ASTM C 94.



DELIVERY OF RMC

        While ready mix concrete can be delivered to the point of placement in a variety of ways, a overwhelming majority of it is brought to the construction site in truck mounted, rotating drum mixers. Truck mixers have a revolving drum with the axis inclined to horizontal. Inside the shell of the mixer drum are a pair of blades or fins that wrap in a helical configuration from the head to the opening of the drum. To load or charge the raw materials from a transit mixed plant or central mixed plant into the truck, the drum must be turned very fast in the changing direction. After the concrete is loaded and mixed, it is normally hauled to the job site with the drum turning at the speed of less than 2 rpm. The truck mixer having front discharge units are more popular than the traditional ones having rear discharge units. Commonly used specifications for RMC stipulate that the concrete shall be discharged on the job site with in 90 minutes and before 300 revolutions after water is added to the cement. In certain situations, air entraining, water reducing, high range water reducing (HRWR) admixtures may need to be added to concrete prior to discharge, to compensate for the loss of air, high temp., or long delivery times



ADVANTAGES OF READY MIX CONCRETE OVER SITE MIX CONCRETE

 Ø A centralized concrete batching plant can serve a wide area.
 Ø The plants are located in areas zoned for industrial use, and yet the delivery trucks can service residential districts or inner cities.
 Ø Better quality concrete is produced.
 Ø Elimination of storage space for basic materials at site.
 Ø Elimination of procurement / hiring of plant and machinery
 Ø Wastage of basic materials is avoided.
 Ø Labor associated with production of concrete is eliminated.
 Ø Time required is greatly reduced.
 Ø Noise and dust pollution at site is reduced.
 Ø Reduce cost

DISADVANTIGES OF READY MIX CONCRETE OVER SITE MIX CONCRETE

 Ø The materials are batched at central plant, and the mixing begins at the plant, so the travelling time from the plant to the site is critical site over longer distances. Some sites are just too far away, though this is usually a commercial rather than technical issue.

 Ø Generation of additional road traffic; furthermore, access roads, and site access 2.5 tone per m3. This problem can be overcome by utilizing so-called ‘minimix’ companies, using smaller 4m3 capacity mixers able to access more restricted sites.

 Ø Concrete’s limited time span between mixing and going-off means that ready-mix should be placed within 120 minutes of batching at the plant.
 Ø It is cost effective   

Conclusion

 Ø It is Costlier then site mix concrete.
 Ø Due Taxies it is rear for Small construction
 Ø It is useful in the pre-stressed construction
 Ø Good in quality control concrete.
 Ø It have high strength with low w/c ratio
 Ø It requires skilled labors
 Ø This method help for rapid construction compared to site mix concrete





          It is useful in large construction, It helps to save the time for the construction. And labor should less as compare to site mixed concrete. Then better to use in large construction.