A concrete mixer (often colloquially 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 by hand. This is usually done in a wheelbarrow; however, several companies have recently begun to sell modified tarps for this purpose.
The concrete mixer was invented by Columbus, Ohio industrialist Gebhardt Jaeger.
Special concrete transport trucks (in-transit mixers) are made to mix concrete and transport it to the construction site. They can be charged with dry materials and water, with the mixing occurring during transport. They can also be loaded from a "central mix" plant; with this process the material has already been mixed prior to loading. The concrete mixing transport truck maintains the material's liquid state through agitation, or turning of the drum, until delivery. The interior of the drum on a concrete mixing truck is fitted with a spiral blade. In one rotational direction, the concrete is pushed deeper into the drum. This is the direction the drum is rotated while the concrete is being transported to the building site. This is known as "charging" the mixer. When the drum rotates in the other direction, the Archimedes' screw-type arrangement "discharges", or forces the concrete out of the drum. From there it may go onto chutes to guide the viscous concrete directly to the job site. If the truck cannot get close enough to the site to use the chutes, the concrete may be discharged into a concrete pump, connected to a flexible hose, or onto a conveyor belt which can be extended some distance (typically ten or more metres). A pump provides the means to move the material to precise locations, multi-floor buildings, and other distance-prohibitive locations. Buckets suspended from cranes are also used to place the concrete. The drum is traditionally made of steel but on some newer trucks, fibreglass has been used as a weight reduction measure.
"Rear discharge" trucks require both a driver and a "chuteman" to guide the truck and chute back and forth to place concrete in the manner suitable to the contractor. Newer "front discharge" trucks have controls inside the cab of the truck to allow the driver to move the chute in all directions. The first front discharge mixer was designed and built by Royal W. Sims of Holladay, Utah, United States.
Concrete mixers are equipped with two or more axles. Four-, five- and six-axle trucks are the most common, with the number being determined by the load and local legislation governing allowable loads on the road.
The axles are necessary to distribute the load evenly, allow operation on weight restricted roads, and reduce wear and tear on normal roads. A two- or three-axle truck during the winter when road weight limits are reduced has no usable payload in many jurisdictions. Other areas may require expensive permits to operate.
Additional axles other than those used for steering ("steers") or drivetrain ("drives") may be installed between the steers and drives, or behind the drives. Mixers commonly have multiple steering axles as well, which generally result in very large turning radii. To facilitate maneuvering, the additional axles may be "lift axles", which allows them to be raised off the ground so that they do not scrub (get dragged sideways across the ground) on tight turns, or increase the vehicle's turning radius. Axles installed behind the drives are known as "tag axles" or "booster axles", and are often equipped to turn opposite to the steering axle to reduce scrubbing and automatically lift when the truck is put into a reverse gear.
Tractor trailer combination mixers where the mixer is installed on a trailer instead of a truck chassis are used in some jurisdictions, such as the province of Quebec where even six-axle trucks would have trouble carrying a useful load.[clarification needed]
Concrete mixers generally do not travel far from their plant, as the concrete begins to set as soon as it is in the truck. Many contractors require that the concrete be in place within 90 minutes after loading. If the truck breaks down or for some other reason the concrete hardens in the truck, workers may need to enter the barrel with jackhammers.
Stephen Stepanian filed a patent application for the first truck mixer in 1916.
Trucks weigh 20,000 to 30,000 pounds (9,070 to 13,600 kg), and can carry roughly 40,000 pounds (18,100 kg) of concrete although many varying sizes of mixer truck are currently in use. The most common truck capacity is 8 cubic yards (6.1 m3).
Most concrete mixers in the UK are limited to a speed of 56 miles per hour (90 km/h).