Anyone who has ordered concrete the traditional way knows the two anxieties. The first is ordering too little, watching the drum empty with the last corner of the slab still bare, and waiting half a day for a top-up load that costs almost as much as the first. The second is ordering too much, paying for a cubic metre that goes back on the lorry, and being charged a disposal fee for the privilege.
Both anxieties come from the same place. A drum mixer leaves the plant with a fixed quantity of a fixed mix, already turning, and it has to be used within a couple of hours. The volumetric mixer was invented to remove that constraint, and on domestic and small commercial jobs it changes how the day goes.
What a volumetric mixer actually is
A volumetric lorry carries the ingredients separately: sand and aggregate in bins, cement in a sealed hopper, water in a tank, admixtures in their own containers. Nothing is mixed until the lorry is parked at the job. The operator sets the mix on the control panel, starts the auger, and fresh concrete comes out of the chute at whatever rate the pour needs. When the pour is finished, the auger stops. The ingredients left in the bins go on to the next customer.
The quantity is measured as it is produced, which is where the “pay for what you use” promise comes from. On a 6 cubic metre estimate that turns out to need 5.4, you pay for 5.4. On one that needs 6.3, there is no second lorry, no wait and no cold joint across the slab.
Changing the mix without changing the lorry
The second advantage gets less attention and matters more on mixed jobs. Because the mix is made on site, it can be changed on site. A footing that wants a wet C25 can be poured first, then the operator adjusts the panel and the same lorry delivers a stiffer C35 for the slab, then a screed mix for the floor. One delivery, three products.
With a drum mixer that is three deliveries, three minimum charges and three waiting periods, or one compromise mix that is not quite right for any of the three.
Small pours stop being expensive
Ready-mix plants price small loads harshly, because the lorry and driver cost the same whether the drum holds 1 cubic metre or 8. A garden wall footing, a shed base, a couple of fence post holes and a patio sub-slab are all jobs that have historically been done with bags and a hired mixer because the delivered price was absurd.
A volumetric lorry can deliver small quantities economically because it carries enough for several jobs and only mixes what each one needs. For builders in the Romford and Brentwood belt who run two or three small jobs a day, that is the difference between delivered concrete and a labourer on a mixer all afternoon. Anyone pricing volumetric concrete in Essex should ask about the minimum charge, because on small pours it is the number that decides the job.
Access and timing
Volumetric lorries are big. They need the same access as a standard mixer and sometimes a little more, because the operator works at the back of the vehicle. On a tight residential street, measure the approach, warn the neighbours and clear the parked cars. A pump or a conveyor can be used where the lorry cannot reach the pour.
Timing is more forgiving than with a drum. Because nothing is mixed in advance, a delay on site, a late shuttering fix or a slow start does not cost you the load. The concrete is as fresh at the end of the pour as at the beginning, which also means the last barrow is as workable as the first.
What to tell the supplier
A good volumetric supplier will ask what the concrete is for, not just how much you want. Tell them the application, the approximate volume, whether it is going into shuttering or being laid and floated, whether a pump is needed, and the access. They will specify the strength class and the slump and, on the day, the operator will adjust in front of you if the first barrow comes out too stiff or too wet.
Ask for a delivery ticket that records the mix and the volume produced. On any job that building control will inspect, that ticket is your evidence.
When a drum mixer still makes sense
Large continuous pours, where a single mix is needed in quantities of 20 cubic metres and more, are still the territory of the ready-mix plant and a fleet of drums. The volumetric lorry earns its place on everything smaller, on anything where the quantity is uncertain, and on any job that needs more than one mix in a day. For most of the building work that happens in gardens, driveways and extensions, that is almost all of it.
