The 9 Stages of Making Asphalt in a UK Plant

From Quarry to Road

Ask ten people what’s under their tyres on the A1 and nine will say “tarmac”. The tenth will shrug. Fair enough, really. To most people the black stuff simply turns up in a lorry. A few lads in orange rake it about, a roller trundles over it, and by teatime the road’s open again.

I’ve spent a long time around asphalt plants, and I’ll be honest with you: that view drives me up the wall. Not because the public should know better. It’s because the people who should know better often don’t either. I’ve met procurement managers, council officers and even a few site agents who treat asphalt as a commodity, like buying sand. Order so many tonnes, job done.

It isn’t sand. A tonne of good surface course is a manufactured product. It’s built to a recipe, cooked at a precise temperature, timed to the second and delivered against the clock. Get any step wrong and the road tells you about it within a couple of winters. Usually in the form of a pothole with your council’s name on it.

So here’s my attempt to put that right. These are the nine stages a load of asphalt goes through in a typical UK plant, from a lump of rock in a quarry face to a finished, compacted road surface. I’ll tell you what happens at each one, and I’ll tell you where I think the industry gets it right and where it cuts corners.

One quick bit of housekeeping first. Almost nothing laid in Britain today is true “tarmac”. Proper tar macadam used coal tar, and that went out decades ago over health concerns. What you drive on now is bound with bitumen, a by-product of crude oil refining. I’ll say “asphalt” throughout, but I won’t sulk if you keep saying tarmac. Everyone does.

1. Quarrying and crushing the stone

Here’s the fact that surprises people most: asphalt is roughly 95 per cent stone by weight. The bitumen everyone obsesses over is a few per cent of the mix. So if you want to understand asphalt, start with rock.

In the UK the surface course, the bit your tyres actually touch, needs stone that resists polishing. Every car that passes buffs the aggregate a tiny bit smoother, and smooth stone means poor skid resistance. That’s why the industry rates stone by its Polished Stone Value, or PSV. High-PSV gritstones and certain igneous rocks from places like Wales, the Pennines and Leicestershire are prized for surfacing. Cheaper limestone tends to go into the lower layers, where grip doesn’t matter.

The rock is blasted from the quarry face, then fed through a primary crusher (usually a jaw crusher) and secondary and tertiary crushers (cone or impact types). It’s then screened into size fractions. You’ll see these written like 0/4, 6/10 or 10/14, meaning the range in millimetres.

My opinion? Shape matters as much as strength. Flaky, elongated stones snap under rollers and traffic. A good quarry manager obsesses over particle shape, and you can tell the difference in the finished road.

2. Stockpiling and the cold feed

The crushed fractions arrive at the plant and sit in separate stockpiles or bays. From there, a loading shovel tips them into a row of cold feed hoppers. Each hopper has a variable-speed belt feeder that meters out its stone in roughly the right proportions for the recipe being run.

Now, this is Britain, so let’s talk about rain. Stockpiles get wet. Every bit of moisture in that stone has to be boiled off in the next stage, and that costs fuel. The plants I rate most highly cover their fine aggregate stockpiles, slope their bays to drain, and don’t let shovel drivers dig into the soggy bottom of the heap. It sounds trivial. It isn’t. Wet sand can swallow a startling share of a plant’s energy bill.

The cold feed also does a first, rough job of proportioning. It doesn’t need to be perfect, because the real accuracy comes later. But a badly set cold feed starves or floods the hot bins, and then the plant stops and starts all day. Stop-start plants make inconsistent asphalt. Simple as that.

3. Drying and heating in the drum

The blended cold stone travels up a conveyor into the rotary dryer. This is a big inclined steel drum, often several metres long, with a burner firing into one end. Inside, flights (steel shelves welded to the drum wall) lift the stone and drop it through the flame like a curtain.

The aim is to drive off every trace of moisture and heat the stone to the target mixing temperature. For conventional hot mix that’s usually somewhere in the region of 150 to 190°C, depending on the binder grade and the product.

The dust has to go somewhere, and in a modern UK plant that somewhere is a bag filter house. Thousands of fabric filter bags catch the fines before the exhaust leaves the chimney. Those fines aren’t waste, by the way. Many plants feed them back into the mix as filler.

This is the stage where most of the carbon is spent. Burners have traditionally run on gas oil or natural gas, and the pressure to decarbonise is very real. I’ll come back to that at the end.

4. Screening into hot bins

Once hot, the stone climbs a bucket elevator to the top of the tower and drops through a vibrating screen deck. The screens split it back into size fractions, which fall into a set of insulated hot bins sitting above the mixer.

Why split it again when it was sized at the quarry? Because the cold feed was only roughly proportioned, and the drum mixes everything together. Re-screening hot gives the plant tightly controlled fractions to build the recipe from. That’s where precision really starts.

This is also why most UK plants are batch plants rather than the continuous drum-mix plants common in the US. Our market is fragmented. A single plant might run a dozen different recipes in a day, from a lorry of dense binder for a trunk road to six tonnes of fine surface course for a driveway contractor. Batch plants handle that chop-and-change far better. I think that’s the right call for Britain, even if the Americans can churn out more tonnes per hour.

5. Weighing out the recipe

Below the hot bins sits the weigh hopper. Gates open on each bin in turn, and the control system weighs out each fraction to the recipe. Filler is weighed separately. The bitumen is weighed in its own heated bucket.

And those recipes are not guesswork. In the UK, asphalt mixtures are specified under the BS EN 13108 series. That’s where you get names like AC 20 dense bin (a dense asphalt concrete binder course with a 20 mm maximum stone), SMA 10 surf (a stone mastic asphalt surface course) and HRA 30/14 (hot rolled asphalt, the old British classic, usually finished with coated chippings).

The binder grade matters too. A harder grade such as 40/60 penetration bitumen suits heavily trafficked roads. A softer 100/150 or 160/220 is more workable and forgiving for footways and smaller jobs. Polymer-modified binders cost more but resist rutting and cracking much better. Binder content usually lands somewhere around 4 to 6 per cent of the mix, depending on the product.

The plant runs Factory Production Control, a quality system required under the standard. Samples get tested for grading, binder content and volumetric properties. Ask any plant for its test records. A good one will be proud to show you.

6. Mixing in the pugmill

The weighed stone and filler drop into the pugmill, a twin-shaft mixer with heavy paddles turning in opposite directions. There’s a brief dry mix to blend the stone, then the bitumen goes in and the wet mix begins. The whole cycle typically takes under a minute, and a batch is commonly a few tonnes.

Every stone must be fully coated. Undermix and you get bald, uncoated particles that ravel out of the road. Overmix and you age the binder before it’s even left the yard. The timing is set per recipe, and good operators watch it like hawks.

This is also where recycled asphalt pavement, or RAP, comes in. Old road planings are crushed, screened and added to the mix, either cold onto superheated virgin stone or warmed in a separate parallel drum. I’m firmly in favour. Every tonne of RAP reused is a tonne of stone and bitumen not dug up or refined. But it has to be well managed. Badly characterised RAP is a lottery, and lotteries don’t belong in road surfaces.

Warm mix additives also go in around here. They let the plant mix and lay at lower temperatures, which saves fuel and fumes. More on that later.

7. Storing in hot silos

Most plants don’t load lorries straight from the mixer. Batches drop into insulated storage silos, which smooths out the gap between steady production and lorries arriving in fits and starts.

Storage is handy, but it isn’t magic. Leave some mixes too long and the binder starts to drain down or the material segregates, with big stones heading one way and fines another. Open-textured and binder-rich products are the worst offenders. A disciplined plant knows the safe storage time for every product and sticks to it. A sloppy one fills the silo at 5am for a job that doesn’t start laying until noon.

8. Haulage to site

The asphalt is loaded into tipper lorries, ideally with insulated bodies, and sheeted before leaving the yard. Sheeting isn’t optional decoration. It keeps the heat in and the rain off, and an unsheeted load on a cold, windy day can lose its usable temperature frighteningly quickly.

This is where I see the most avoidable failures. The plant makes a perfect batch, then the lorry sits in a queue on the M62 for 50 minutes. Or it arrives on site and waits an hour because the paver broke down. The top of the load crusts, the material gets too cold to compact properly, and someone lays it anyway because nobody wants the argument about sending it back.

Site teams should check delivery temperatures with a probe, and BS 594987 sets out minimum rolling temperatures for different materials. Reject cold loads. Every time. It’s cheaper than digging up the road in three years.

9. Laying and compaction

Finally, the road itself. The surface below is cleaned and given a bond coat, a sprayed bitumen emulsion that glues the new layer to the old. Skip it, or spray it patchily, and the layers slide on each other and delaminate. I’ve seen whole surface courses peel off like a plaster because someone was stingy with the bond coat.

The lorry tips into the hopper of a paver, which spreads the material to width and thickness behind a heated screed. Then come the rollers: usually tandem steel-drum rollers, sometimes with a pneumatic-tyred roller for certain materials. They work in a set pattern while the mat is still hot enough to compact.

Compaction is the whole game. Air voids that are too high let in water and oxygen, and water is asphalt’s sworn enemy. Joints matter just as much. A cold, poorly cut longitudinal joint down the middle of a carriageway is where many British potholes are born.

Everything that came before, the careful quarrying, the drying, the weighing and the timing, is either rewarded or wasted in the next half hour on site.

The bit nobody wants to hear

Here’s my honest view after years of watching this process. The UK asphalt industry is very good at making asphalt. Our plants are well run, our standards are thorough, and the people behind the control desks know their trade.

Where we fall down is the handover. The plant optimises the batch, the haulier optimises the round trip, and the surfacing gang optimises the shift. Nobody owns the whole journey from silo to roller. That gap is where temperature gets lost, joints get rushed and roads fail early.

The next ten years will change the plant end of things a lot. Burners are moving to lower-carbon fuels, recycled content is climbing, and warm mix asphalt is finally getting the take-up it deserves. Mixing and laying at lower temperatures saves energy, cuts fumes for the crews, and gives you a longer window to compact. Clients have been slow to ask for it. They should start.

But no amount of clever chemistry fixes a cold load laid by a crew racing the clock. If you buy, specify or lay asphalt, my advice is simple. Treat it like the manufactured product it is. Ask the plant for its test data. Check the temperature when the lorry arrives. And never, ever skimp on the bond coat.

The road will thank you. So will everyone who drives on it.

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Article by Jobling Purser asphalt suppliers in Newcastle UK