Monday, January 27, 2020

READY MIX CONCRETE

    Ready-mix concrete is concrete that is manufactured in a batch plant, according to a set engineered mix design. Ready-mix concrete is normally delivered in two ways. First is the barrel truck or in–transit mixers. This type of truck delivers concrete in a plastic state to the site. Second is the volumetric concrete mixer.
     This delivers the ready mix in a dry state and then mixes the concrete on site. Batch plants combine a precise amount of rock, sand, water and cement together by weight, allowing specialty concrete mixtures to be developed and implemented on construction sites.



     The first ready-mix factory was built in the 1930s, but the industry did not begin to expand significantly until the 1960s, and it has continued to grow since then. Ready-mix concrete is often preferred over other materials due to the cost and wide range of uses, from bird baths to high rise buildings and bridges. It has a long life span when compared to other products of a similar use, like road ways. It has an average life span of 30 years under high traffic areas compared to the 10 to 12 year life of asphalt concrete with the same traffic.
     Ready-mix concrete, or RMC as it's also known, refers to concrete that is specifically batched or manufactured for customers' construction projects. It is a mixture of Portland cement, water and aggregates: sand, gravel, or crushed stone. All aggregates should be of a washed type material with limited amounts of fines or dirt and clay. Ready-mix concrete is bought and sold by volume - usually expressed in cubic meters.

Metered concrete :
     As an alternative to centralized batch plant system is the volumetric mobile mixer. This is often referred to as on-site concrete, site mixed concrete or mobile mix concrete. This is a mobile miniaturized version of the large stationary batch plant. They are used to provide ready mix concrete utilizing a continuous batching process or metered concrete system.
     The volumetric mobile mixer is a truck that holds sand, rock, cement, water, fiber, and some add mixtures and color depending on how the batch plant is outfitted. These trucks mix or batch the ready mix on the job site itself. This type of truck can mix as much or as little amount of concrete as needed.



     The on-site mixing eliminates the travel time hydration that can cause the transit mixed concrete to become unusable. These trucks are just as precise as the centralized batch plant system, since the trucks are scaled and tested using the same ASTM (American standard test method) like all other ready mix manufactures. This is a hybrid approach between centralized batch plants and traditional on-site mixing. Each type of system has advantages and disadvantages, depending on the location, size of the job, and mix design set forth by the engineer.

Transit mixed ready mix vs. volumetric mixed ready mix :
 a) A centralized concrete batching plant can serve a wide area. Site-mix trucks can serve an even larger area including remote locations that standard trucks cannot.

 b) The batch plants are located in areas zoned for industrial use, while the delivery trucks can service residential districts or inner cities. Site-mix trucks have the same capabilities.

 c)Volumetric trucks often have a lower water demand during the batching process. This will produce a concrete that can be significantly stronger in compressive strength compared to the centralized batch plant for the same mix design using the ASTM C109 test method.

 d)Centralized batch systems are limited by the size of the fleet. It may take upwards of 10 minutes to batch and load out one 9 - 12 yard truck depending on the plant size and type. They are unable to change mix designs during the batch process.

 e)Volumetric mixers can seamlessly change all aspects of the mix design while still producing concrete. They can continuously mix quality concrete for an indefinite time while being continuously loaded with fresh materials. They can produce 1 yard of concrete in as little as 40 seconds depending on the mix design and batch plant size outfitted.

f) For short loads, (orders under 10 yards) Transit Mixers typically return to their batch plant after each delivery. Volumetric trucks can go directly from job to job until truck is emptied, reducing traffic and fuel consumption.

Advantages of Ready Mixed Concrete:
1. Quality assured concrete:- Concrete is produced under controlled conditions using consistent quality of raw material.
2. High speed of construction- Speed of construction can be vary fast in case RMC is used.
3. Reduction in cement consumption by 10 – 12 % due to better handling and proper mixing. Further reduction is possible if mineral admixtures or cementitious materials are used.
4. Versatility in uses and methods of placing: The mix design of the concrete can be tailor made to suit the placing methods of the contractor.
5. Since ready mixed concrete (RMC) uses bulk cement instead of bagged cement, dust pollution will be reduced and cement will be saved.
6. Conservation of energy and resources because of saving of cement.
7. Environment pollution is reduced due to less production of cement.

Limitations of Ready Mix Concrete:
1. As the Ready Mixed Concrete is not available for placement immediately after preparation of concrete mix, loss of workability occurs. In addition, there are chances of setting of concrete if transit time involved is more. Therefore, generally admixture like plasticisers/ super plasticisers and retarders are used. Addition of retarders may delay the setting time substantially which may cause placement problems. In addition, it may also affect the strength of concrete. Therefore, it is necessary that the admixtures i.e. plasticisers and super plasticisers/ retarders used in Ready Mixed Concrete are properly tested for their suitability with the concrete. In case loss of strength is observed, the characteristic strength may have to be enhanced so that after loss of strength, required characteristic strength is available.
2. Because of large quantity of concrete available in short span, special placing and form work arrangement are required to be made in advance. The placement methods of readymix concrete plays an important role as it affects the strength and durability of concrete structures. The time of delivery, quality checks and time of placements affects the ready mix concrete. Ready mix concrete is a concrete which is manufactured as per required mix ratio in batching plant,and then is transported to construction site on continuously mixing trucks.


Ready Mix Concrete Placement Methods :
Ready Mix Concrete placement methods include following basic principles:
1. When arriving to the site the concrete transport certificate must be checked for desired              characteristics of ordered concrete (quantity, class, maximum aggregate size, slump, temperature, type cement etc.) and time duration of transport.
 2. Concrete shall be delivered to the site and discharged from the truck completely and in the forms ready for vibration within 1-1/2 hours after batching.
3. Concrete shall be placed in maximum 15 minutes after its arrival to the site, and the finishing of placement will take place before the cement starts setting.
 4. Concrete shall be stored / deposited as near as (physically and economically) possible to its final position, in crane hoisted buckets, concrete pumps, chutes etc.
 5. The receptacles used for the transport and deposition of concrete shall be cleaned and washed out at the end of each day’s work and whenever concreting is interrupted for more than 30 minutes.
 6. If the concrete, due to transport, is segregated. It should be mixed again on clean platforms, without adding water, if not possible the batch should be refused.



EFFECT OF TRANSIT TIME ON READY MIX CONCRETE :
Ready mix concrete is generally produced in large quantity and is transported to distant places for placement in structural elements. Sometimes the distance can be in many kilometers or miles.

1. Loss of workability:
The concrete should always be laid in position without loss of time to avoid setting and stiffening of concrete to reduce its workability. When the transit time is high, it will have effect on workability of concrete. This happens due to hydration reaction taking place when cement mixes with water, evaporation of mixed water in concrete and due to absorption of water by aggregates. While the workability of concrete depends on many factors such as the constituent material, mixed proportion, ambient temperature, humidity and method of transport etc., the reduction in workability may lead to difficulty in placement of concrete.

2. Setting of concrete:
When the transit time of ready mix concrete is high, the initial setting of concrete may take place. To avoid setting of concrete, retarding admixtures can be used to prolong the setting of concrete. While permitting use of retarder, it should be ensured that the suitability and dose of retarder is decided after conducting necessary trials. It may be noted that generally retarding effect of retarder is smaller at higher temperature and sometimes few retarders seem to be ineffective at extremely high temperature. Thus, it is desirable to keep the temperature of concrete as low as possible.

3. Time period for delivery of concrete:
In order to control loss of workability and setting of concrete, the concrete should be delivered completely to the site of work within one and half hours (when the atmospheric temperature is above 200C) and within two hours ( when the atmospheric temperature is at or below 200C)of adding the mixing water to the dry mix of cement and aggregate or adding the cement to the aggregate whichever is earlier. Adequacy of the time period, required for delivery of concrete, should be checked. 

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