Equipment used in concrete road construction, and where the concrete comes from

A list of road construction equipment is a list of machines that shape material, which is a different question from where the material comes from. One widely read industry library divides its road machines into four groups, of which the paving group alone runs asphalt pavers, remixing transfer vehicles, cold planers, slipform pavers, chip spreaders, tack distributors, shoulder spreaders, line striping machines, asphalt plants, concrete mixers and pile drivers. A Kenyan construction-trade list of the same kind is shorter and harder: motor grader, asphalt plant, asphalt paver, road roller, excavator, wheel loader, truck crane, forklift. Eight machines, and no concrete mixing machine among them. Read the longer of the two and you have an account of how a pavement is laid, compacted and finished. Read the shorter and there is no concrete machine on it at all. Neither tells you how the concrete reaches the machine that lays it.

Where a mixer does appear, it appears once and generically. The industry library's entry reads in full: "Concrete mixers are used in roadway construction to mix cement, water and aggregate to form roadways, sidewalks and curbs." That sentence is true and it is not a supply plan. It does not say whether the mixer is a drum on a stand, a truck arriving from a plant, or a machine that charges itself at the stockpile, nor how many are needed, nor what happens once the work front has moved on from where they stood yesterday. Note what both lists carry on the production side: an asphalt plant, which the Kenyan list describes as combining aggregates, sand and filler and coating them with a binder. Where the pavement is asphalt, both lists name the plant that makes the material. Where it is concrete, the longer list names a mixer in one generic sentence and the shorter names nothing at all.

A paver forms and finishes concrete that is delivered to it. The encyclopaedia entry for the machine states the input directly: a paver lays asphalt concrete or Portland cement concrete and provides minor compaction, the asphalt being "added from a dump truck or a material transfer unit into the paver's hopper", and for the Portland cement concrete case, on the large freeways where slipform pavers are used, "trucks dump loads of readymix concrete" into the machine. The paver is the end of a supply chain, not the start of one, and the rollers and cutters that follow it are further from the material still. Whatever a paver can lay in a shift is conditional on that chain keeping up, and none of the equipment lists contains the chain. A list of machines answers what you hire; it does not answer what keeps them fed.

On a motorway contract the omission is reasonable, because the chain is assumed and usually exists: a batching plant sited for the job, a haul road, and enough trucks to hold a cycle. It stops being reasonable on the other kind of road work — village and rural roads, access roads, farm-to-market roads, widenings and short urban sections — where there is no plant inside cycle range and no haul road except the one being built. There the equipment question and the supply question are the same question, and a list that answers only the first has answered the easy half. What fails on that kind of job is rarely the paving machine. It is the second and third delivery of the morning, arriving from further away than the first.

A building site is one address for the duration. A road is a new address every few days, and the delivery allowance does not move with it. That allowance is a real number, written down, and it is worth reading what has happened to it. The international ready-mixed concrete specification, ASTM C94/C94M, carried a one-and-a-half-hour limit to the end of discharge from its original publication, and the current edition no longer states one: clauses 6.1.9 and 6.1.10 now require the purchaser to state a drum-revolution limit and a time limit, and where the purchaser states neither, the producer establishes them and communicates them before delivery, with the time limit printed on the delivery ticket. A buyer who reads the old figure into a new contract is reading a number that is no longer there. The number left the standard; the constraint did not.

The standard's own note on choosing that number is the part to read twice. The selection, it advises, should consider ambient conditions, the cementitious materials and admixtures used, placement procedures, and "projected transportation time between the batch plant and the point of delivery". On a road job that last input is not a constant. It is the one quantity the work increases every week, while the allowance agreed on the first morning stays where it was written. Some authorities have put a fixed figure back. California's health-facility regulator requires discharge completed within one and a half hours of the mixing water meeting the cement and aggregates, and within 300 drum revolutions, of which no more than 100 may be mixing revolutions. Neither limit may be waived by producer or purchaser except under an approved alternative compliance method, and an unplanned delay has its own written procedure, recording slump and temperature from ninety minutes and ending placement at 120.

It belongs on the supply side of the list, and nowhere else on it. A self-loading mixer charges its own bucket at the stockpile, mixes in its own drum, drives to the pour point and discharges there. It does not form, screed, texture, joint or cure anything. It is not a paver, not a kerb machine, not a canal finisher and not a pump; we supply none of those. What it removes is the plant-to-site leg: the batch is made where it is placed, so there is no haul to lengthen as the work advances. What it leaves untouched is everything after discharge — forming, compacting, jointing and curing all still need their own machines and their own crew, and the lists name machines for only some of that. What it does instead is the part the lists leave to somebody else: it turns a stockpile into a batch at the place the batch is needed.

The specifications that make it a supply machine rather than a yard mixer are the ones worth stating, and only where they are verified. A self-loading bucket is verified on all six mixer models published here. On-board water is carried on all six as standard equipment rather than as an option, in two forms: five of them, the SLM-18 among them, have an automatic water-fill controller with an external suction pump, and the sixth carries a water tank with a self-priming system. That is what matters on a linear front, where there is no standpipe. A drum that lifts and rotates through 270° is verified on five of the six. Maximum travel speed is verified on four: 0–23 km/h on the SLM-18 in full-time four-wheel drive, and 0–36 km/h on three larger models.

One machine in our wider records goes further, and it is worth recording precisely because it is a product decision rather than a claim. Of the sixteen self-loading mixers we hold specifications for, exactly one carries a synchronised-rotation drum — a variant whose configuration list opens by stating that the drum holds its mixing direction while the machine is travelling. That is the moving-front case designed into the hardware by somebody who had seen it. It is also not one of the six models published on this site: its discharge figure is in conflict between the English and Chinese specification sheets, its engine model is in the same conflict, and its dimensions are unverified, so it has no model page and cannot be ordered here. It is recorded here as evidence that the case is real, not as something on offer.

The machine has to make that trip on the formation being built, and what we can state about its ability to is thinner than what a brochure would state. Four-wheel drive is verified on four of the six published models and unverified — not absent — on the other two. Articulated steering is verified on three of the six. Articulated steering is not a turning circle and is not offered as one: no turning radius is recorded for any mixer in our files, so none appears here. Of the sixteen self-loading mixers we hold specifications for, fifteen carry a real tyre size; two of those are mountain-pattern and one is semi-solid, and the rest are ordinary construction or steel-belted tubeless sizes. An eight-wheel drive chassis appears on three of the sixteen, and none of those three is a published model.

Size decides the rest, and it has to be quoted as a band. Ten of the sixteen mixers carry a measured overall dimension: 1.60 m to 2.30 m in width and 4.35 m to 7.06 m in length. Width is the figure to plan a lane against, because it barely moves with the position of the drum. Height is a different matter. One of the ten, the SLM-18, is measured off a drawing that separates the height of the machine on its tyres from its height with the bucket raised; the other nine come from a supplier table that does not say which it measured, so no travelling height is quoted for them. Two of the ten are published models, one being the SLM-18 at 2.21 m wide and 7.06 m long, the longest of the ten. Quoting the narrowest of them, 1.60 m, as the width of self-loading mixers quotes one machine as though it were the range.

No cost per kilometre and no concrete volume per kilometre appears here, because we hold neither. Those are the two figures this query is most often asked alongside, and the honest position is that they depend on your pavement thickness, your formation width, your specified mix and your local material prices, none of which is ours to assume. The same refusal covers output: no hourly output figure is confirmed for any of the six published models, so any cubic metres per day or metres of road per shift would be manufactured here rather than reported. Nothing in our files records loads per shift, fuel consumption or crew size either, which is why no comparison against the size of a crew appears anywhere on this page. A page that cannot show its working on a number has no business printing the number.

One further gap belongs in the open rather than in a footnote, because it is a question a road contractor will ask, and the answer is that we do not have it. Water-tank capacity is not on record for any of the six published models — precisely the figure that tells you how many batches you get between refills on a front with no standpipe. We do not estimate it from drum size and we do not carry a figure across from one machine to another, so on this page it is simply missing. Ask for it in writing with the model named, before the machine is specified into a programme rather than after. A capacity quoted instantly and without a document is a capacity nobody has looked up, and the machine that has to live with it is the one working furthest from its water source.