Concrete Mixer Trailers: Drum Volume, Legal Payload and Cost per m³
Autor STU Trailers Engineering Team30 lipca 20269 min czytania
A 14.7 m³ drum does not mean 14.7 m³ of concrete on every route. Concrete weighs roughly 2.4 tonnes per cubic metre, so the weight limit in your country decides how much of the drum you can legally fill. Here is how to work it out before you buy.
Concrete weighs roughly 2.4 tonnes per cubic metre. That single figure governs everything about mixer trailer specification, because it means a large drum fills up on weight long before it fills up on volume. A 14.7 m³ drum holds around 35 tonnes of concrete when full, and in markets operating to a 40-tonne gross combination limit that is far more than the combination can legally carry. Understanding the gap between drum volume and legal volume is the difference between buying the right unit and buying a drum you can never fill.
The short answer
Take your national gross combination weight limit, subtract the tractor unit and the trailer's tare weight, and divide what remains by 2.4. That number is the cubic metres of concrete you can legally deliver per trip. In a 40-tonne market that typically lands between 9 and 10 m³. In markets with higher limits, or on site work where road limits do not apply, larger drums come into their own. Specify the drum to the limit you actually operate under, not to the largest figure in the catalogue.
Why drum volume and legal volume are different numbers
A mixer drum has two published figures. Geometric or nominal capacity is how much the drum physically holds. Rated or mixing capacity is how much concrete it can carry while still rotating and mixing properly, usually around 55 to 65 percent of geometric volume depending on drum design.
Neither figure accounts for road weight limits. That is a separate calculation, and in most European operations it is the binding one. Work it in this order: legal gross combination weight, minus tractor weight, minus trailer tare weight, gives payload in tonnes. Divide by 2.4 and you have your deliverable volume in cubic metres.
If that result is well below your drum's rated capacity, the extra drum volume is carrying air on every journey while costing you tare weight. If the result is close to rated capacity, the specification is matched.
Where mixer trailers beat truck mixers
A truck mixer is a rigid vehicle with the drum permanently mounted. A mixer trailer is a semi-trailer, which means the tractor unit is separable. That distinction drives most of the operating economics.
Volume per trip is higher. Because a semi-trailer distributes weight across more axles than a rigid four-axle truck mixer, it can usually carry more concrete within the same national weight limit. For a batching plant measuring cost per cubic metre delivered, that difference compounds across every load of every day.
Tractor utilisation improves. A truck mixer sits idle whenever the drum is not in use. A tractor unit can drop one mixer trailer and pick up another, or move a different trailer type entirely. Fleets running mixed work often justify mixer trailers on tractor utilisation alone.
Replacement cost separates. When a drum reaches end of life, you replace a trailer rather than a complete vehicle. When a tractor reaches end of life, you replace it without touching the mixing equipment.
Where truck mixers still win is manoeuvrability on constrained sites and short urban delivery cycles. A rigid four-axle mixer turns inside spaces a 16.5-metre combination cannot enter. Most batching plants that run mixer trailers keep some truck mixers for exactly that reason.
Drum drive systems
The drum has to keep rotating in transit or the concrete begins to set. Two arrangements are available, and STU supplies both.
A power take-off drive runs the drum's hydraulic circuit from the tractor. It removes a second engine from the trailer, which means less weight, less fuel and less maintenance. The condition is that every tractor pulling the unit must carry a compatible PTO and hydraulic circuit, and the drum stops turning whenever the trailer is uncoupled. For fleets running fixed tractor and trailer pairings, this is usually the more economical arrangement.
An auxiliary engine mounted on the trailer drives the drum independently, typically a Deutz or equivalent industrial diesel unit. This frees the trailer from any requirement on the tractor, keeps the drum turning while the unit is dropped at a plant or on site, and allows any tractor in the yard to pull it. The trade-off is a second engine to fuel, service and maintain, plus its weight on the chassis.
The decision is driven by fleet structure rather than by engineering. Dedicated pairings favour PTO. Mixed fleets and drop-and-swap operations favour the auxiliary engine.
Water tank and washout
Every mixer needs onboard water for adjusting slump at the point of delivery and for washing the chute and drum entry after discharge. STU supplies a 600-litre water tank and a 40-litre admixture tank as standard, with larger capacities specified where delivery distances or the number of loads between plant returns require it.
Under-specifying the tank is a common and expensive mistake. A driver who runs out of washout water leaves hardened concrete in the chute and around the drum opening, which accumulates load after load, adds dead weight, and eventually requires a chipping operation that takes the unit out of service.
Drum wear and blade life
The inside of a mixer drum is an abrasive environment. Aggregate scours the drum wall and the mixing blades continuously, and worn blades stop moving concrete efficiently, which lengthens mixing time and reduces discharge speed.
Drum shell thickness and blade material are therefore direct operating cost decisions, not construction details. A thicker shell and harder blade material cost more at purchase and last longer under aggregate abrasion. For a plant running multiple loads a day, blade replacement intervals affect availability more than most buyers anticipate at the specification stage.
Discharge, chute reach and site access
Concrete has to arrive where the pour is, not merely at the site gate. Chute length and articulation determine how far from the trailer you can place concrete without a pump. Discharge height determines whether you can load a pump hopper or a skip directly.
Where the pour point is beyond chute reach, a concrete pump is required, which changes the cost of the job. Specifying adequate chute reach for the work you typically do reduces how often that happens.
Approach also matters. Mixer trailers are long, and batching plant yards and construction site approaches are frequently not. Where site access is a recurring constraint, the honest answer is sometimes a truck mixer rather than a larger trailer.
Specifying by the market you operate in
Weight limits vary substantially, and mixer specification should follow them rather than a global standard.
In markets operating to a 40-tonne gross combination limit, the practical delivery volume is around 9 to 10 m³ and drum specification should reflect that. In markets permitting higher combination weights, larger drums translate directly into more concrete per trip and a genuinely lower cost per cubic metre. On closed sites such as dams, mines and large infrastructure projects, road limits may not apply at all, and drum capacity becomes the only constraint.
This is why the same manufacturer offers a range of drum volumes rather than one size. The right drum is the one matched to the legal payload where the unit will work.
What catches operators out
Three things, repeatedly. Buying drum volume that the local weight limit will never allow you to use, and paying tare weight for it on every journey. Under-specifying the water tank and losing service time to hardened build-up. And comparing a mixer trailer to a truck mixer on purchase price alone, when the real comparison is cost per cubic metre delivered across the asset's working life.
Frequently asked questions
How much does a cubic metre of concrete weigh?
Around 2.4 tonnes for standard structural concrete, with a normal range of roughly 2.3 to 2.5 tonnes depending on the mix and aggregate. Lightweight and heavyweight mixes fall outside this range and should be calculated separately.
How much concrete can a mixer trailer carry legally?
Subtract the tractor unit weight and the trailer tare weight from the national gross combination limit, then divide the remaining payload by 2.4. Under a 40-tonne limit this typically produces 9 to 10 m³. Under higher national limits the figure rises proportionally.
Is a mixer trailer better than a truck mixer?
For volume per trip and tractor utilisation, generally yes. For manoeuvrability on tight urban sites, no. Most batching plants running significant volume operate both, using trailers for bulk delivery and truck mixers for constrained access.
Does the drum have to keep turning during transport?
Yes. Concrete begins to set from the moment it is batched, and continuous rotation keeps the mix homogeneous and workable. Drive comes either from the tractor's power take-off or from an auxiliary engine mounted on the trailer, and that choice determines whether the drum can keep turning while the trailer is uncoupled.
What drum volume should I order?
The one that matches your legal payload, not the largest available. Calculate deliverable volume from your national weight limit first, then choose the drum whose rated mixing capacity sits closest to that figure. STU produces 12.7 m³ and 14.7 m³ drums as standard; 9, 10 and 12 m³ units are built to order and subject to a minimum quantity.
How STU builds mixer trailers
STU Trailers produces concrete mixer trailers for batching plants and ready-mix producers across Europe, the Middle East and Africa. Standard production volumes are 12.7 m³ and 14.7 m³. Drums of 9, 10 and 12 m³ are built to order and subject to a minimum quantity, so operators working under tighter weight limits should raise volume requirements at the enquiry stage rather than assuming a stock size.
Drum drive is supplied either as a power take-off from the tractor or as an independent auxiliary engine, and water and admixture tanks are fitted at 600 and 40 litres as standard. Chassis are laser-cut and welded from S690QL or Strenx 700MC high-strength steel, which keeps tare weight down and converts directly into deliverable concrete within a given weight limit. Axles are available from SAF, BPW, GIGANT, VALX, TRAX and MPD in 9, 11, 12 and 14-tonne capacities, with WABCO or Knorr-Bremse EBS braking. Drum volume, drive arrangement, tank capacity, chute configuration and axle count are specified per order against the customer's batching cycle, delivery profile and national weight limits.