A concrete mixer (also commonly called a cement mixer) is a device that homogeneously combines cement, aggregate such as sand or gravel, and water to form concrete. A typical concrete mixer uses a revolving drum to mix the components. For smaller volume works portable concrete mixers are often used so that the concrete can be made at the construction site, giving the workers ample time to use the concrete before it hardens. An alternative to a machine is mixing concrete or cement by hand. This is usually done in a wheelbarrow; however, several companies have recently begun to sell modified tarps for this purpose.
Aside from the standard truck there are few special parts. there is a drive unit to turn the drum. The drum itself. The frame for the drum which has one front bearing and two rollers acting at bearings at the rear section of the drum. At the top there is a loading chute through which the ingredients of the mix is funnelled into the drum. and below that, a long chute, which can be swiveled to change angle relative to the position of the drum. (Chassis of the truck).
Inside the drum there are blades running longitudenly but set at an angle to the center axis of the drum.
The coment mis is introduced through the upper chute and the drum is set rotating but the hydraulic turning geat which is runn by the main engine. Water is added and the truck moves from the mixing plant to the buikding site. the turning action of the crum causes the mixture to roll along the inside surface of teh drum. But it is further aggitated (Turned) by the interferance of the paddles. the paddles are so set as to scoop this mixture towards the front of the drum.
When the truck arrives at the site the mixture is ready fro pouring. The rotation of the drum is reversed. Now, the angel of the internal blades scoops the mixture towards the mouth of the drum and further spilling out of the drum onto the chute.
The chute can be directed into forms in the ground or into a concrete pumper to be brought into a building site.
When the drum is empty water must be flushed thoroughly to prevent residual cememnt from hardning inside the drum. So also must all the material on the outside equipment. Concrete will harden even under water so the cleaning must be thorough.
On completion of the clean up, usually with water teken from a tank mounted on the area in front of the drum, the turning mechanism is stopped. the truck returns to the mixing yard for another load.
The important point in this system is that the concrete is mixed as the truck is in transit to the site. this makes it possible to reduce the time for delivery of concrete to the site.
There is also a limit to the amount of time the concrete can remain in the mixing drum. Regardless of movement, cement hardens as a result of an exothermic reaction. It will become thick and slowly harden even if the drum is turning. There are chemical retardents that can be added when necessary - the discharge of the coment is delayed by traffice or trouble at the site. This does affect the strength of the cement to some minor degree.
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