Readymix Concrete
Ready-mixed concrete is created by mixing Portland cement, water, and aggregates like sand, gravel, or crushed stone in a batching plant according to standard specifications. The alternate of Readymix concrete is site mix concrete, which is made at the site using hand tools or small mixing equipment like mini cement mixers. There are three types of Readymix concrete.
1. Transit Composite Concrete mix:
The transit composite concrete is often referred to as dry-batch concrete because all of its essential components are immediately loaded into the truck mixer. On the truck, the mixer drum is rotated at the charging speed during the loading process and then at conventional mixing speed afterward.
2. Shrink Mixed Concrete mix:
A truck-mounted drum mixer mixes a portion of the concrete in transit and the rest in the concrete plant mixer. The time of mixing carried out at the central mixing plant determines the amount of mixing required within the transit mixer.
To determine whether drum mixers mounted on trucks should be used or not for mixing, tests on the workability of concrete are conducted.
3. Central Mixed Concrete:
This process, also known as central batching, involves adding concrete to a truck right after mixing has been completed. These plants are also called wet mix plants or pre-mixed plants. Occasionally, non-agitating units or dump trucks will also be used, if the travel distance is short.
Materials used in ready mixed concrete:
Aggregates:
As important components of concrete, aggregates account for 70–80% of its volume. According to their size, aggregates are divided into two categories:
· Fine aggregate
· Coarse aggregate
With a dimension of over 4.75 mm, it is considered a coarse aggregate. With a dimension of 4.75 mm or less, it is considered a fine aggregate.
Cement:
The four main components of cement are Dicalcium Silicate (2Ca0 Si), Tricalcium Silicate (3Ca0 Si02), Tricalcium Aluminate (3Ca0 Al203), and Tetra-Calcium Al203 Fer203. These compounds are identified as C2S, C3S, C3A, and C4AF, where C represents calcium oxide (lime), S represents silica, and A represents alumina and iron oxide. Besides alcohol and magnesia, alkalis and other elements are present in small quantities.
Fly ash:
The coal used in plants is mostly composed of combustible elements including carbon, hydrogen, and oxygen, as well as non-combustible impurities (from 10 to 40%) such as clay, shale, quartz, feldspar, and limestone. The average size of fly ash is 20 microns and ranges from 1.0 to 100 microns.
Water:
Water should have a pH between 6.0 and 8.0.
How does it differ with site mix concrete?
· Due to being produced in batching plants in large quantities, Readymix concrete has much better consistency than the site mix concrete which is rather mixed by the hands.
· Readymix concrete allows quicker construction times than the site mix concrete.
· The quality of raw materials can be monitored much efficiently in the case of Readymix concrete.
· By opting for Readymix concrete large quantities of concrete can be supplied to the site.
· Working conditions with Readymix concrete are much safer than the site mix concrete as the later involves direct exposure of labor to concrete dust and other hazardous raw materials
· In the case of Readymix concrete applications, the need for storing aggregates and cement on site is eliminated. This allows better area management of the construction site.
· For smaller projects, where the stationary batching plant is located at a far distance and mobile batching plants are becoming an expensive alternative, site mix concrete can be a better solution.