Concrete mixer
A concrete mixer is a device that homogeneously combines cement, aggregate, 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. However Stothert & Pitt's 1885 Price Book records Messers H. Lee & Co used a Lee's Mixer during the constuction of the Admiralty Pier at Dover, Kent. Their US Price Book distributor was E. & F. N. Spon, Booksellers, 35 Murray St, New York.
History
One of the first concrete mixers ever was developed in 1900 by T.L. Smith in Milwaukee. The mixer already exhibited the still common basic construction with a tiltable conical drum with blades. On February 9, 1904, the first portable concrete mixer was patented by Richard Bodlaender, an inventor from Breslau, Germany. This concrete mixer was horse-drawn and called 'Mortar Mixer'. It worked by replacing the front wheels with a large drum that held large paddles for mixing the cement.1925, at least two mixers, built 25 years ago, were still in use. The Smith Mascot in essence has the same construction of the small mixers used today. In the 1920s, the T.L. Smith Company in Milwaukee built the world's largest concrete mixers. Mixers of this company were used e. g. for the construction of the Wilson Dam, the first stadium of the Ohio State University and the Exchequer Dam. Roscoe Lee was granted a patent in 1934 for his transit concrete mixer design. This design made it possible for new trucks to be turned into concrete mixers by using a crane to put a drum on the back of the truck. It was less expensive than a cement truck dedicated to only carrying cement and allowed the trucks used to continue to be adapted for other things. Concrete mixers have continued to advance. Recently, research detailing a forward simulation model of an energy-saving concrete mixer using hydraulic technology was released based on Matlab/Simulink technology.Industrial mixers
Today's market increasingly requires consistent homogeneity and short mixing times for the industrial production of ready-mix concrete, and more so for precast/prestressed concrete. This has resulted in refinement of mixing technologies for concrete production. Different styles of stationary mixers have been developed, each with its own inherent strengths targeting different parts of the concrete production market. The most common mixers used today fall into three categories:- Twin-shaft mixers, known for their high intensity mixing, and short mixing times. These mixers are typically used for high strength concrete, RCC and SCC, typically in batches of.
- Vertical axis mixers, most commonly used for precast and prestressed concrete. This style of mixer cleans well between batches, and is favoured for coloured concrete, smaller batches, and multiple discharge points. Within this category, the pan mixers are losing popularity to the more efficient planetary mixers, as the additional mixing action helps in production of more critical concrete mixes.
- Drum mixers, used where large volumes are being produced. This type of mixer is capable of high production outputs.
Trucks and trailers
Concrete mixing transport trucks
Special concrete transport trucks 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. These trucks have an interior turbine that pushes the mixed concrete up against gravity inside the drum. 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. 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. Most cement trucks weigh a substantial amount empty, meaning they have very heavy tare weights. United States weight laws under the Federal Bridge Formula require a three-axle ready mixed concrete truck to weigh or under. This leaves only for concrete to be carried, as is the tare weight of the truck.
"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 use a rear engine and have controls inside the cab of the truck to allow the driver to move the chute in all directions. The first front discharge mixer, patented in 1974, 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 or drivetrain 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 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.
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. Some trucks in dry climates are equipped with the capability of adding water in spray form during the trip, especially on long trips. 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 or the company has to scrap the entire drum.
Stephen Stepanian filed a patent application for the first truck mixer in 1916.
Trucks weigh, and can carry roughly of concrete although many varying sizes of mixer truck are currently in use. The most common truck capacity is.
Most concrete mixers in the UK are limited to a speed of.
Concrete mixer trailers
A variant of standard concrete transportation is the concrete mixing trailer. These small versions of transit-mix trucks are used to supply short loads of concrete. They have a concrete mixing drum with a capacity of between. Cart-aways are usually pulled behind a pick-up truck and batched from smaller batching systems. The mixing trailer system is popular with rental yards and building material locations, which use them to supply ready-mix to their regular customer base.Metered concrete trucks
Metered concrete trucks or volumetric mobile mixers contain 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.On-site and portable concrete mixers
For smaller jobs, such as residential repairs, renovations, or hobbyist-scale projects, many cubic yards of concrete are usually not required. Bagged cement is readily available in small-batch sizes, and aggregate and water are easily obtained in small quantities for the small work site. To service this small-batch concrete market, many types of small portable concrete mixers are available.A typical portable concrete mixer uses a small revolving drum to mix the components. For smaller jobs the concrete made at the construction site has no time lost in transport, giving the workers ample time to use the concrete before it hardens.
Portable concrete mixers may be powered by gasoline engines, although it is more common that they are powered by electric motors using standard mains current.
These concrete mixers are further divided based on their loading mechanism. Cement, sand and other aggregates are loaded in a hydraulically operated hopper and then poured into the mixing drum for final mixing. They can be unloaded by tilting the drum. In hand-feed concrete mixers, cement, sand and other aggregates are directly added to the mixing drum manually. Both of these types of concrete mixers are popular in construction activities in Africa, some Middle Eastern countries and in the Indian subcontinent.