A concrete mixer truck (often called a cement truck, agitator truck, concrete truck, cement mixer or concrete mixer) is a rigid truck that has a rotating drum on the back that keeps pre-mixed concrete moving so that it doesn’t set. They are built on a regular lorry chassis, then fabricated by adding the concrete mixing equipment. This way, they can be specified according to the weight of concrete carried (e.g. 10×4, 8×4 or 6×4), and the type of truck that the driver or company prefers.
The rotating drum contains blades that stir the material. When in transit, the drum rotates one way which pushes the concrete down the bottom of the drum and forces it to be mixed; this is called charging. When it’s being unloaded, the drum rotates the other way which brings the concrete up to the top of the drum into the chute.
The drum doesn’t rotate very fast in transit – usually just 1-3 revs per minute. The drum is usually powered by PTO which takes its power from the engine, but can be powered by an auxiliary engine.
The trucks range in capacity from around 2 cubic meters to 9 cubic meters of concrete. Smaller trucks are used both for smaller jobs and also where access is difficult.
The chute diverts the concrete down to the ground. It can be rotated 180 degrees around the back of the vehicle. Up to around 0.7m3 of concrete can be ejected per minute, depending on the mixer’s specifications.
Because concrete sets hard, the drum must be cleaned as soon as possible, otherwise it is difficult to get it clean.
Concrete mixer truck drivers must be careful when cornering as the centre of gravity can shift dramatically if the vanes in the drum pick up the concrete (this can happen if the concrete starts to thicken).
Volumetric mixers don’t carry pre-mixed concrete, they carry the separate components that are then mixed on-site, as and when they are needed, which can reduce waste. It also makes the truck less likely to be affected by a change in the centre of gravity of a rotating drum.