|Ready mix concrete|
Ready mix concrete is often preferred over on-site concrete mixing because of the precision of the mixture and reduced work site confusion.
Ready mix concrete, or RMC as it is popularly called, 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. 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. In 2011, there were 2,223 companies employing 72,924 workers that produced RMC in the United States.
A volumetric concrete mixer (also known as volumetric mobile mixer and metered concrete truck) is a truck that contains concrete ingredient materials and water to be mixed on the truck at the job site to make and deliver concrete according to the amount needed.
The mixer is a mixing device that measures the raw materials using volume rather than weight. The volumetric mixer blends the concrete mixture using an auger or a paddle or a drum device to mix the ingredients with water.
The volumetric mixing process starts with a batch metering system that allows the volume of raw materials to be measured prior to entering the mixing chamber. This process can be as simple as using a measured bucket, to highly sophisticated and computerized batch plants that feed the correct volume.
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The mixing chamber can vary depending upon the application and the mix design of the concrete.
In an auger type mixer, the material and water travels up the Archimedes' screw. Auger lengths can vary from about 5-feet to 15-feet, and vary in diameter and RPM. This type may be portable, but the length of the auger is a limiting factor.
A paddle mixer blends the concrete mix using a rotational motion and is used to blend finer gravels and sand, as in mortar. The majority of drum mixers are mounted on trailers and are pulled to a job site using a pick-up truck. These drums usually have a mixing capacity of between 1-yard and 1.75 yards (0.76 and 1.34m3) per batch.
A paddle mixer tumbles the mixture in a folding motion using curved fins and paddles. This is the most common type of volumetric mixing. Paddle mixers usually mix in smaller batches directly on the job site.
The history of volumetric mixing comes from several directions;
The mobile auger mixer was patented in 1964 by Harold Zimmerman, Ephrata, PA USA. Because of the patents, there was only one equipment manufacturer until the 1980s.
Equipment manufacturers created an association in 1999 with five charter members.
The drum for trailer mounted volumetric mixer was designed in 1979, by Fred Caron and Neal Surry, Sacramento, California USA. They mimicked the larger transit-mix truck's mixing drum, on a smaller scale and loaded the drum from a batching unit that metered the volume.
The paddle mixer's exact history is unrecorded, but these volumetric mixers have been in use by masons for at least 50 years. Usually these produce smaller batches.
source : wikipedia.org