Volumetric mixers and self-loading mixers are two families of machines that make concrete at the place where it is needed, instead of receiving it ready-mixed from a batching plant. A volumetric mixer is a truck or trailer with separate compartments for sand, stone, cement, water and admixtures. At the site, it measures the materials by volume and mixes them continuously, producing exactly the quantity of fresh concrete required. A self-loading mixer is an off-road machine with a rotating drum and a loading shovel: it loads aggregates and cement with its own bucket, weighs or measures them, adds water, mixes the batch in the drum while driving and discharges it where needed.
Both machines answer the same problem: delivering fresh concrete in small or irregular quantities, at remote locations or over long working periods, where a conventional ready-mix supply would be impractical, expensive or wasteful.
Brief History of Volumetric and Self-Loading Mixers
Volumetric mixers were developed in the United States from the mid-1960s. The first mobile units combined bins for materials with a continuous mixing auger at the rear of a truck, so that concrete could be produced on demand. Companies such as Cemen Tech, Zimmerman and Reimer became established names in the segment, and the concept spread to the United Kingdom, Europe, Australia and other markets. Standards for concrete made by volumetric batching and continuous mixing, such as ASTM C685 in the United States, helped the method gain acceptance for structural work. The Volumetric Mixer Manufacturers Bureau was created in 1999 to represent the manufacturers.
Self-loading mixers grew out of site dumpers and small mixers in Europe. Italian manufacturers developed machines that combined a loading shovel, a rotating drum and a four-wheel-drive chassis, and the machine became established during the 1970s. Carmix, founded in Italy in 1976, became one of the best-known brands, and other Italian companies such as Fiori and Dieci built similar machines. From the 2000s self-loading mixers found a large market in developing countries, especially in India, where local production grew strongly, as well as in Africa, the Middle East and Latin America.
How Volumetric and Self-Loading Mixers Work
Volumetric Mixers
- Material bins. Separate compartments hold sand and coarse aggregate, with a cement bin, a water tank and admixture tanks.
- Metering. A conveyor belt under the aggregate bins and a cement feeder deliver materials in fixed proportions. Gate openings and belt speed set the proportions, which are calibrated by weight for each mix.
- Mixing. The materials drop into an inclined mixing auger, where water and admixtures are added. The concrete is mixed continuously as it moves up the auger and is discharged from the end, often via a chute.
- Control. Electronic controls record the materials used and allow the operator to change the mix quickly, for example from structural concrete to a lean mix or a screed.
Self-Loading Mixers
- Loading shovel. A bucket at the front of the machine, mounted in front of the drum opening, picks up aggregates and cement from stockpiles and bags and loads them into the drum.
- Weighing. Many machines weigh the bucket loads with load cells, and some record batch data electronically.
- Drum. The drum rotates to mix the materials with water from an on-board tank. On many models the drum can also slew so that it discharges to either side or to the front.
- Chassis. A four-wheel-drive articulated or rigid chassis with four-wheel steering allows the machine to work on rough ground and move around a site.
- Cab. The operator’s seat and controls can often be reversed so that the machine drives equally well in both directions.
Types of Volumetric and Self-Loading Mixers
Truck-Mounted Volumetric Mixers
The most common volumetric type, built on a road-going truck chassis. It can drive to several sites in a day and produce different mixes at each one.
Trailer and Stationary Volumetric Units
Towed or stationary units for sites where the machine stays in place, such as precast yards, rural projects and remote works.
Compact Self-Loading Mixers
Small machines with drums of about 1 to 2.5 m³, used on residential sites, farms, small infrastructure and places with narrow access.
Large Self-Loading Mixers
Machines with drums of about 3 to 5 m³ and more, used on roads, bridges, housing projects and remote sites where a batching plant is not available.
Size Classes
| Type | Typical capacity | Typical use |
|---|---|---|
| Compact self-loading mixers | about 1 to 2.5 m³ per batch | Small sites, farms, housing |
| Large self-loading mixers | about 3 to 5 m³ per batch and more | Infrastructure, rural works |
| Volumetric mixers | about 6 to 12 m³ of material on board | Small and multiple deliveries, repairs, night work |
Figures are indicative. Volumetric mixers produce concrete continuously, and their output depends on the mix and the machine.
Options and Equipment
Both machine families can be configured for particular work. Volumetric mixers are often fitted with admixture tanks for accelerators, retarders and colour, fibre feeders for fibre-reinforced concrete, extended or hydraulic chutes, and small conveyors or pumps for placing concrete away from the truck. Heated water systems and insulated bins allow work in cold weather. Self-loading mixers can be specified with electronic weighing and batch printers, water meters, larger or faster drums, cab heating and air conditioning, and special tyres for sand or rocky ground. Some models accept alternative tools on the loading frame, such as a pallet fork, so that the machine can also handle materials around the site.
Main Manufacturers
- Cemen Tech, Zimmerman, ProAll and Reimer, known for volumetric mixers.
- Carmix, the Italian self-loading mixer manufacturer.
- Fiori and Dieci, Italian manufacturers of self-loading mixers.
- Ajax Engineering, a major producer of self-loading mixers in India.
Main Uses
- Small and medium concrete pours where ready-mix would waste material.
- Repairs, kerbs, footpaths, fence posts and utility works.
- Remote and rural construction, including roads, bridges, irrigation and farm buildings.
- Night work and long, irregular pours where the supply must continue on demand.
- Mixes that ready-mix plants do not usually supply, such as rapid-setting mixes, screeds and lean mixes.
- Sites in countries where ready-mix networks are limited.
Choosing Between Volumetric, Self-Loading and Ready-Mix
- Quantity. Ready-mix is efficient for large, continuous pours. Volumetric mixers suit many small deliveries, and self-loading mixers suit steady production on a single site.
- Location. Self-loading mixers work where there are local aggregates and no plant within reach. Volumetric mixers need a base to reload but can travel on roads.
- Quality control. Specifications may require batching by weight or approval of volumetric batching. Calibration and records are essential for structural concrete.
- Labour. A self-loading mixer needs one operator to load, mix, transport and discharge. A volumetric mixer usually needs a driver who is trained to calibrate and run the mixing system.
- Waste. Both machines make only what is needed, reducing returned concrete.
Safety
Self-loading mixers combine a loader, a mixer and a dumper, and their stability changes as the drum fills and rotates. They should be driven with the bucket low and at moderate speed, especially on slopes and rough ground, and a roll-over protective structure and seat belt are essential. Visibility over the drum and bucket is limited, so people on site must keep clear during loading and manoeuvring. On volumetric mixers, the main hazards are the mixing auger, conveyor belts and moving parts, which must be isolated before cleaning or maintenance. Cement dust and wet concrete can cause irritation and burns, so gloves, eye protection and respiratory protection are needed when handling cement.
Trends and Technology
Electronic weighing and batch recording are improving the quality control of both machine types, making them more acceptable for structural work. Telematics tracks production, fuel use and location, and some machines print or send batch reports. Volumetric mixers are adding more admixture and fibre dosing systems for special mixes. Self-loading mixers are adopting cleaner engines, better cabs and automatic control of water addition based on moisture sensors. Electric and hybrid drive concepts are also being tested for smaller self-loading machines.
Key Terms
- Volumetric batching: measuring concrete ingredients by volume, calibrated against weight.
- Continuous mixing: mixing concrete in a steady flow rather than in batches.
- Mixing auger: the screw conveyor in which a volumetric mixer mixes the concrete.
- Self-loading bucket: the shovel a self-loading mixer uses to load its own drum.
- Calibration: checking that the metering system delivers the correct proportions.
- Slewing drum: a drum that can rotate to discharge in different directions.
Frequently Asked Questions
Is concrete from a volumetric mixer as good as ready-mix? When the machine is correctly calibrated and operated, it can produce concrete that meets standard specifications. Many projects accept it, but some specifications require approval or batching by weight.
Can a self-loading mixer be used on public roads? Some models are approved for road travel in certain countries, but they are mainly designed for use on site and at low speed.
How much concrete can a self-loading mixer produce in a day? It depends on the drum size, the distance to the pour and the loading cycle. A large machine can produce several batches per hour on a well-organised site.
Why choose a volumetric mixer for small jobs? It produces only the concrete needed, allows several deliveries of different mixes in one trip and avoids paying for part-loads of ready-mix.
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