Ready Mix Concrete

 

What is Ready Mixed Concrete (RMC)?

Ready-mix concrete (RMC) is a type of concrete which is manufactured in a cement factory, or specifically known as the batching plant, according to a given set of proportions, and then delivered to a work site, by truck mounted with mixers. This results in a precise mixture, allowing specialty concrete mixtures to be developed and implemented on construction sites. The fist factory which produced ready mix concrete was built in 1930s; however, the industry didn’t begin to expand until late 80s. Since then it has continued to grow significantly.

Ready mix concrete is sometimes preferred over on-site concrete mixing because of the volume it can produce with precision of proportion of mixtures and also due to reduced work site confusion. Using a pre-determined concrete mixture reduces flexibility, both in the supply chain and in the actual components of the concrete.

Ready-mix concrete is also termed as the customized concrete products for commercial purpose. Ready-mix concrete (RMC) refers to concrete that is specifically manufactured for delivery to the customer’s construction site in a freshly mixed and plastic or unhardened state. Concrete itself is a mixture of Portland cement, water and aggregates comprising sand and gravel or crushed stone. In traditional work sites, each of these materials is procured separately and mixed in specified proportions at site to make concrete. Ready-mix concrete is bought and sold by volume – usually expressed in cubic meters (cubic yards in the US).

Ready-mix concrete is manufactured under controlled operations and transported and placed at site using sophisticated equipment and methods.

Types of Ready Mixed Concrete

There are three types of ready mix concrete (RMC) depending upon the mixing of the various ingredients as given below:

1. Transit mixed concrete
2. Shrink mixed concrete
3. Central mixed concrete

1. Transit mixed concrete

It is also called dry batched concrete because all the basic ingredients including water are charged directly into the truck mixer. The mixer drum is revolved fast at charging speed during the loading of the material and after that it continues rotating at a normal agitating speed. In this type of ready mix concrete, also three types of variations are possible as given below: Concrete mixed at job site While being transported towards the destination, the drum is revolved at a slow or agitating speed of 2 rpm, but after reaching the site just before discharging the material, it is revolved at maximum speed of 12 to 15 rpm for nearly 70 to 100 revolution for ensuring homogeneous mixing. Concrete mixed in transit The drum speed is kept medium during the transit time, i.e. approximately 8 rpm for about 70 revolutions. After 70 revolutions, it is slowed down to agitating speed of 2 rpm till discharging the concrete. Concrete mixed in the yard The drum is turned at high-speed of 12 to 15 rpm for about 50 revolutions in the yard itself. The concrete is then agitated slowly during transit time.

2. Shrink mixed concrete

The concrete is partially mixed in the plant mixer and then balance mixing is done in the truck mounted drum mixer during transit time. The amount of mixing in transit mixer depends upon the extent of mixing done in the central mixing plant. Tests should be conducted to establish the requirement of mixing the drum mixer.

3. Central-mixed concrete

It is also called central batching plant where the concrete is thoroughly mixed before loading into the truck mixer. Sometimes the plant is also referred as wet-batch or pre-mix plants. While transporting the concrete, the truck mixer acts as agitator only. Sometimes, when workability requirement is low or the lead is less, non-agitating units or dump trucks can also be used.

Process of Ready-mix concrete

Ready mix concrete has cement, aggregates, sand, water and other chemicals, which are weigh- batched at a centrally located plant for a premium quality. The concrete is then delivered to the construction site in transit mixers and can be used straight away without any further treatment. The automatic plant monitors weigh-batching, water-cement ratio, dosage of admixture, moisture content, with precision to produce quality concrete.

All ingredients used for the preparation of ready mix concrete are thoroughly tested for their quality and physical properties in a well equipped laboratory attached to the plant for conformity to relevant international standard codes. The moisture probe determines the water content in the sand and aggregates. This accordingly helps in fixing the proportion of water to be added for the preparation of the mix. Trial mixes are carried out and tested to ensure that each and every batch of concrete coming out of the plant meets various mix designs as per the client’s requirement with different grades of concrete.

Advantages of Ready Mixed Concrete

1. Quality concrete is obtained as a ready-mix concrete mix plant make use of sophisticated equipment and consistent methods. There is strict control over the testing of materials, process parameters and continuous monitoring of key practices during the manufacture.Poor control on the input materials, batching and mixing methods in the case of site mix concrete is solved in a ready-mix concrete method.
2. Speed in the construction practices followed in ready mix concrete plant is followed continuously by having mechanised operations. The output obtained from a site mix concrete plant using a 8/12 mixer is 4 to 5 metric cubes per hour which is 30-60 metric cubes per hour in a ready mix concrete plant.
3. Better handling and proper mixing practice will help to reduce the consumption of cement by 10 – 12%. Use of admixtures and other cementitious materials will help to reduce the amount of cement.
4. The concrete mixed is used with high versatility. It is placed by following best concrete placing methods.
5. Cement is saved and the dust caused is reduced as ready mix concrete make use of bulk concrete instead of bags of cement.
6. Cement saving will conserve the energy and the resources.
7. Less consumption result in less production of cement hence less environmental pollution.
8. More durable structure is obtained thus increasing the service life and saving the life cycle costs.
9. Ready mix concrete manufacture have less dependency on human labours hence the chances of human errors is reduced. This will also reduce the dependency on intensive labours.
10. Small or large quantities of concrete as per the specification is delivered timely at the site.
11. This demands no space for storing the raw materials at site. There is no delay due to site based batching plant erection/ dismantling; no equipment to hire; no depreciation of costs.
12. Petrol and diesel consumed is less thus noise and air pollution is reduced.

Disadvantages of Ready Mixed Concrete

1. The transit time from the time of preparation of concrete to the delivery site, will result in loss of workability. This will demand for additional water or admixtures to maintain the workability as per the specification. At site, the QA/QC engineer are supposed to check the workability through slump test before using it for construction.
2. Traffic during the transit of concrete can result in setting of concrete. This will hence require addition of admixtures to delay the setting period. But unexpected traffic is a great problem.
3. The formwork and placing arrangement must be prepared in advance in large area as the concrete can be bought in larger amounts.