Truck Mixers

A truck mixer, also called a transit mixer, concrete mixer truck or agitator truck, is a lorry chassis fitted with a large rotating drum that carries fresh concrete from a batching plant to the job site. The drum turns continuously on the road to keep the concrete homogeneous and workable, and at the site it reverses direction to discharge the load down a chute, into a pump hopper or into a skip. In most countries the truck mixer is the link that makes the ready-mixed concrete industry possible, and it is one of the most common heavy vehicles in any city with active construction.

Depending on how the plant is set up, the truck can receive concrete that has already been mixed in a central mixer and simply keep it agitated, or it can receive dry ingredients and water and do the actual mixing itself during the trip. Either way, the machine has to deliver concrete of a specified quality within a limited time, because cement starts to hydrate as soon as it meets water.

Brief History of Truck Mixers

Before truck mixers, concrete was mixed on site in small drum mixers or delivered in open vehicles, where it could segregate and begin to set. The idea of a motorised vehicle that mixed concrete while travelling appeared in the United States in the early twentieth century. Stephen Stepanian, an engineer based in Columbus, Ohio, applied in 1916 for a patent on a self-discharging motorised transit mixer. The application was rejected at first, and a patent was granted only in 1933 after a new filing. He is often described as a founder of the ready-mixed concrete industry.

During the 1920s and 1930s several designs mounted mixing drums on trucks, and the ready-mix business spread across North America and then Europe. After the Second World War, hydraulic drives replaced mechanical power take-off systems, drums became larger and lighter, and specialised mixer bodies were built by dedicated manufacturers. In Europe, for example, Stetter began producing truck mixers in the late 1950s. Later developments included front discharge mixers, popular in North America, lightweight drums made of high-strength steel or composite materials, and electronic control of drum speed and water addition. Today the truck mixer is a mature product, but it is changing again with electric drum drives and digital delivery tracking.

How a Truck Mixer Works

A truck mixer is built from a standard truck chassis and a mixer body, which is often supplied by a different manufacturer.

Drum. A pear-shaped steel drum is mounted at an angle on the chassis. Inside it, two spiral blades welded to the shell move the concrete towards the bottom of the drum when it turns one way, and towards the opening when it turns the other way.
Drive. The drum is usually driven by a hydraulic pump powered from the truck engine through a power take-off, with a hydraulic motor and a gearbox at the drum head. Some mixers use an auxiliary engine or an electric motor with a battery instead.
Charging and discharge. Concrete is loaded through a hopper at the top of the rear opening. On discharge, the drum reverses and the concrete flows down a collecting funnel onto a folding chute, which can be swung and extended by hand or by hydraulics.
Water system. A water tank on the body is used to wash the drum and chute after unloading and, where the specification allows, to adjust the mix.
Controls. Drum speed and direction are controlled from the cab and from the rear of the truck. Modern units can include drum rotation counters, slump monitoring sensors and telematics.

Types of Truck Mixers

Rear Discharge Truck Mixers

The standard layout worldwide. The drum opening faces the rear of the vehicle and the driver manoeuvres in reverse to the discharge point. Rear discharge mixers are mounted on two, three, four or more axle chassis depending on capacity and local weight rules.

Front Discharge Truck Mixers

Common in North America, these trucks have the drum opening above the cab, and the driver positions the truck and controls the chute from inside the cab. The layout allows the truck to drive forward to the pour, which can reduce the need for extra labour on site.

Semi-Trailer Mixers

A mixer drum on a semi-trailer, pulled by a tractor unit. This arrangement allows larger drums within some national weight limits and is used where long hauls and high volumes are common.

Truck Mixers with Conveyors or Pumps

Some mixers carry an extra device for placing concrete directly. A hydraulic conveyor belt mounted on the truck can deliver concrete over a distance, while a truck mixer pump combines a drum with a small concrete pump and a short placing boom, useful for small jobs where a separate pump would be too expensive.

Lightweight and Electric Mixers

Drums built from high-strength steel or composite materials reduce weight and allow a higher payload. Electric drum drives, powered by a battery charged at the plant or by the truck, allow the drum to turn with the engine switched off, which reduces noise, fuel and emissions during waiting and discharge.

Size Classes

Class	Typical drum volume	Typical chassis
Small	about 3 to 6 m³	Two-axle trucks, compact sites
Standard	about 7 to 9 m³	Three and four-axle trucks, ready-mix delivery
Large	about 10 to 12 m³ and more	Multi-axle trucks and semi-trailers

Figures are indicative. The legal payload depends on the chassis, the drum weight and national axle load limits, and the concrete volume carried is always less than the geometric volume of the drum.

Main Manufacturers

Mixer bodies are made by specialist companies and by the large concrete equipment groups, and mounted on chassis from truck makers such as Mercedes-Benz, Volvo, Scania, MAN, Iveco, Mack and Kenworth.

Liebherr, which builds truck mixers, including electric drum drive versions, as well as batching plants.
Schwing Stetter, the concrete equipment group formed when Schwing acquired Stetter.
CIFA, the Italian concrete equipment maker owned by Zoomlion.
McNeilus, part of Oshkosh, and Con-Tech in the North American market.
SANY and Zoomlion, the large Chinese construction equipment groups, which build mixers for domestic and export markets.

Main Uses of Truck Mixers

Delivering ready-mixed concrete from batching plants to building and infrastructure sites.
Supplying concrete pumps and placing booms during large pours.
Delivering concrete for foundations, slabs, walls, columns and floors.
Transporting mortar, screed and special mixes such as fibre-reinforced concrete.
Supplying on-site plants for precast yards, tunnels and dams where a dedicated fleet is used.

Buying and Operating Tips

Match drum size to deliveries. A large drum reduces the number of trips, but small loads carried in a big drum can be inefficient and the heavier truck may be restricted in some streets.
Watch the payload. The weight of the drum, the chassis and optional equipment reduces the concrete that can be carried legally. Lightweight drums and chassis are often chosen for this reason.
Respect delivery times. Specifications and standards usually set limits on the time between batching and discharge and on the number of drum revolutions. Long hauls, heat and traffic reduce the margin.
Control water addition. Adding water on site to make concrete easier to place lowers its strength. Many specifications forbid it or allow it only under strict control and with a record.
Wash out correctly. The drum and chute must be cleaned after each load, and wash water and returned concrete need to be managed according to environmental rules.
Inspect the blades. Worn mixing blades reduce mixing quality and slow down discharge, so the inside of the drum should be checked regularly.

Safety

Truck mixers are heavy, tall vehicles with a high centre of gravity that changes as the load moves in the drum. Rollover on bends, soft ground and slopes is a known risk, so site access roads must be firm and level near the discharge point. Reversing is the most frequent cause of accidents on site, and banksmen, cameras and reversing alarms are widely used. Operators must never enter the drum unless the drive is isolated and locked out, and climbing on the ladder to inspect the drum opening should be done only when the drum is stationary. Chutes are heavy and can trap fingers and hands when folded or extended. Contact with wet concrete can cause skin burns, so gloves, boots and eye protection are needed.

Trends and Technology

Several technologies are changing truck mixers. Electric drum drives and battery electric chassis are being introduced for urban deliveries where noise and emission limits apply. Sensor systems measure the slump and temperature of the concrete in the drum and record water or admixture additions, giving the producer and the contractor a digital record of each load. Telematics and dispatch software track trucks in real time, plan deliveries and send electronic delivery tickets. Lighter drums, better aerodynamics and automatic drum speed control reduce fuel consumption. Driver assistance systems, including camera monitoring of blind spots, are increasingly common because of the risks mixers pose to pedestrians and cyclists in cities.

Key Terms

Transit mixing: mixing concrete in the truck drum during the journey, starting from dry materials and water.
Central mixing: mixing concrete at the plant, with the truck only agitating it on the way.
Agitation speed: the slow drum rotation used to keep mixed concrete homogeneous.
Slump: a standard measure of concrete consistency and workability.
Chute: the folding steel channel that guides concrete from the drum to the pour.
Returned concrete: concrete brought back to the plant unused, which must be recycled or disposed of.

Frequently Asked Questions

How much concrete does a truck mixer carry? Most truck mixers in ready-mix fleets carry between about 7 and 9 m³, while small trucks carry less and large multi-axle or semi-trailer mixers carry more. The legal limit is set by the vehicle weight rules of each country.

Why does the drum keep turning during transport? Continuous rotation keeps the concrete mixed, prevents segregation of the aggregate and delays stiffening. If the drum stops for too long, the concrete can start to set inside it.

How long can concrete stay in a truck mixer? Many standards and specifications set a limit of about 90 minutes to two hours from batching to discharge, but the real limit depends on the mix, the admixtures used and the temperature.

What is the difference between a truck mixer and a volumetric mixer? A truck mixer carries concrete that has been batched at a plant. A volumetric mixer carries separate compartments of cement, aggregates and water and makes the concrete on site in the quantity required.