What capacity means on a self-loading mixer, and what it does not

Ask for the capacity of a self-loading mixer and you can be handed any of three numbers. The first is the litre figure built into a model designation, which is a naming convention rather than a measurement: seven of the sixteen self-loading mixer records we hold carry one, and the other nine are identified by the volume they discharge instead. The second is the drum's geometric volume, the physical internal volume of the vessel, and we hold a measured figure for that on four of those sixteen. The third is the discharge capacity, the volume of mixed concrete that actually comes out, and we hold that on fourteen of the sixteen. Three quantities, three different populations of machine, and a buyer weighing one supplier's designation against another's is usually comparing two naming conventions.

On three of the four records where we hold both, the designation and the measured drum volume are the same number. On the fourth the designation is a round litre figure several hundred litres above the drum that was actually measured. That is the whole trouble with reading a designation as a specification: sometimes it is the drum volume and sometimes it is not, and nothing on the badge tells you which case you are looking at. Where we hold a measured drum volume we publish it, where we hold a discharge figure we publish that, and we never derive one from the other, because the ratio between them is not a constant. On the SLM-18, the one model here with a complete published sheet, a 2,900 litre drum discharges 1.6 to 1.8 cubic metres, so the drum figure is 1.61 to 1.81 times the concrete.

A mixing drum cannot be filled. The material needs headroom to tumble, fold and turn over; concrete that fills the vessel does not mix, it slumps. The ready-mixed concrete industry puts a figure on that headroom, and it is the basis on which drums are rated in the first place: a rated maximum capacity of 63 per cent of gross drum volume where the drum is doing the mixing, and 80 per cent where it is only agitating concrete mixed elsewhere. At the mixing rating, a drum volume offered as a capacity overstates what the machine will place by 37 per cent of the drum's own volume, and it does so legitimately. It is answering a different question. That percentage is the rating ready-mixed truck drums are designed around — the nearest published benchmark, not a rule these machines are certified against — so the distance between the two numbers is not a rounding error and no supplier is at fault for it.

We can test that ratio on exactly two of the sixteen mixer records, because only two carry both figures verified. The other fourteen give one or the other, and a ratio cannot be taken from a single number. On the SLM-18, 1.6 to 1.8 cubic metres out of a 2,900 litre drum is 55.2 to 62.1 per cent of drum volume, under the 63 per cent mixing rating across the whole band. The other record sits higher, at 61.5 to 69.2 per cent, and the top of that band is above the rating. That would be consistent with the upper figure describing what the drum will carry rather than what it will mix, but the source does not say so, and we are not going to settle it on the plant's behalf. Both percentages are worked out here by division. Neither is a declaration by anybody.

The model codes here are the discharge figure multiplied by ten, with SLM for self-loading mixer, and the published range runs from 1.0 to 4.5 cubic metres across six models. The rule is fixed in the naming convention this range is built on, so a model here is not named after its drum. The reason for choosing that number out of the three is that it is the only one which turns up in the buyer's own arithmetic. Nobody orders litres of drum. A contractor works out how many machine loads a column, a raft or a bay of floor is going to take, and the figure he divides by is cubic metres of concrete placed. Naming a machine after its drum puts the wrong number in front of that calculation.

Five of the six published models carry a verified discharge figure, and three of those five are bands rather than single values, which is how they were given to us. We do not quietly collapse a band to its top figure to make a table look even, and a band is not vagueness either: it is the honest width of a figure that moves with the mix design, the slump and how full the drum is actually run. The other two of the five are single values and are published as single values. The sixth model shows no discharge capacity at all. Its model code appears to contradict that, and the explanation is worth setting out, because the code was issued from a commercial document that gives a single figure for the machine.

The two technical specification sheets we hold for it then turned out to disagree with each other: one states the same single value the name came from, the other a slightly wider band that brackets it. Our standing rule where two sources conflict is that the row does not render and nobody here picks the winner. So the machine keeps the name it was issued and shows an empty cell beneath it. That is an uncomfortable thing to publish and it is the accurate state of what we hold. One written confirmation from the plant closes it, and until that arrives the cell stays empty rather than being filled with the more flattering of the two readings. It is also the one row in this range that a single email would close, which is why it sits on a list of questions rather than in a footnote.

The arithmetic belongs to you and it is short: divide the pour by the discharge figure and you have the number of batches. A four cubic metre raft is four loads at the bottom of the published range and one at the top. What this page will not do is turn that into a figure for the day. Our own specification sheet for every model carries a cubic-metres-an-hour line, and it is empty on all six, because nobody has put a figure there in writing; there is no cycle time and no batches-per-shift line anywhere in our records at all. How long one cycle takes depends on the haul from the aggregate pile, on how the mix is specified and on who is operating, none of which is knowable by a supplier in another country.

For a sense of scale against the alternative, the ready-mixed concrete industry describes the typical truck mixer as a 9 to 11 cubic yard unit, which converts to 6.88 to 8.41 cubic metres. Taken against both ends of both bands, the largest machine here places 47.6 to 65.4 per cent of one truck mixer load, and the smallest 11.9 to 14.5 per cent. The comparison matters less than it looks, because the pours these machines are bought for are not the pours a ready-mix truck gets dispatched to. A machine that batches on site carries no minimum load, no delivery window and nothing idling on the road outside, which is the entire argument on a site with no batching plant within reach, on a rural self-build, or on a paving job where the pour front moves all day.

A discharge figure is a rating, and reaching it on site is a question of how the batch is made up. All six models draw their own water. Five list a self-filling controller with an external suction pump, and the sixth a large-capacity tank with its own filling system, so the water side of the mix does not wait on a water truck. Batch weighing is where the range separates. One of the six carries an on-board electronic hydraulic weigh scale in the standard build we hold. On four of the other five it is listed as an option rather than fitted as standard, and for the fifth we hold no list of options at all. Ordered without it, the proportions are set by bucket loads rather than read off a display. That changes nothing about the cubic metres the drum will discharge, and a good deal about how repeatable the mix is from batch to batch.

Capacity and access are independent, and on the sites these machines are bought for it is usually access that binds first. Width is the test that decides, because a width failure is terminal: every standard container is 2.352 metres wide inside, and a roof can come off but the walls cannot. Ten of the sixteen mixer records carry measured overall dimensions, and their widths run from 1.60 to 2.30 metres. Against the 2.29 metre usable door opening of a 40 ft box, that spread straddles the line, which is why width is checked on the specific machine before anything else is discussed. Where the binding constraint is an old-district lane, a site gate or a ramp rather than the size of the pour, the capacity question gets answered with a tape measure.

Height is harder, and this page is careful with it. Exactly one of those ten machines has a dimensioned drawing that separates the height it travels at from the height with the bucket raised: the SLM-18, at 2.58 metres travelling against 3.45 metres at full lift. That travelling height stands 45 millimetres above the 2.535 metre usable door height of a 40 ft high cube, so the machine is quoted out of gauge. On the other nine the height came from a table that does not state which posture it was measured in, and posture moves a height by far more than a container tolerance, so those nine get no door verdict here. Width is barely moved by posture, which is why the widths above remain usable while the heights behind them do not. The container guide on this site works the same test through model by model.

The gaps are worth stating plainly, because they are the reason some of the six models have no page of their own. Across the sixteen mixer records, fourteen carry a discharged volume, ten carry overall dimensions and four carry a measured drum volume. Narrowed to the six published models: five have a discharge figure, three a drum volume, two a set of overall dimensions and two a confirmed operating weight. The pattern is not random. What is present is what could be measured off a drawing or read from a sheet already in our hands; what is missing is the material that has to arrive from the plant in writing. There is no price on this page and no output figure of any kind. Every gap is a question on a list, and this page changes when an answer arrives, not before.