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A greener route to road construction

RECYCLING

Recycled aggregates are playing an increasingly important role in delivering a sustainable transport infrastructure.

Paul Thompson takes a trip up the M6 to look at the first application of recycled road base on the motorway network

THIS summer, when thousands of holiday-makers return from their two-week break in the Lake District or Scotland, they will have little idea that the queue of traffic they are tailing is meting out punishment to some of the most advanced road surfacing material on the motorway network.

Fleets of caravans will trundle over the first piece of surfacing to have been recycled and replaced in a motorway road base and given the all-clear by the Highways Agency.

But far from being a test site, work on a 2 km-long stretch of carriageway near M6 Junction 37 in Cumbria is a natural progression for recycled wearing course, says Gary Cook, director of planing subcontractor Bruce Cook Road Planing and managing director of Roadstone Recycling, the Tarmac/Bruce Cook joint venture company carrying out the recycling.

'The Highways Agency has developed the application - called Foamix - well. It has been tested through the range of surfacing from estate roads through to trunk roads and now it is using it on our motorways. We are grateful. It is a bold, brave step, ' he says.

Mr Cook is equally appreciative of Tarmac, which has taken on the promising Foamix method but has not found it easy to persuade authorities or companies to use the system.

'Nobody has ever refused to use Foamix but we have had to underwrite the design life of a road to persuade some clients, ' he says.

He insists this is the best way of persuading clients to accept the method. It is certainly one way of bypassing some of the more intransigent planning staff in some county halls. While Mr Cook has some sympathy for local authority specifiers, he accepts that many are not used to dealing with risk.

'You cannot blame the authorities, ' he says. 'I often wonder if I would put my neck on the block if I was in their position. There is no personal gain for them and I would want every possible eventuality covered as well.'

But the Highways Agency has kept a close eye on the recycling method since its first application on an industrial estate in Walsall four years ago and now it is getting its big chance.

The Foamix recycling method used on the M6 by main contractor Tarmac is an 'ex-situ' system. The road surface is planed out, transported to a temporary off-site reprocessing centre then relaid using conventional paving plant and rollers.

In fact, the only real difference between the Foamix application and standard practice is that the material is laid cold rather than hot. Of course, this means workers don't get to warm their feet during the laying process - a distinct disadvantage on a cold day in the Lake District but, as far as reducing harmful emissions into the atmosphere goes, it's a big plus.

'The amount of emission from the Foamix plant is minimal compared with standard asphalt plants. Couple that with the bonus of reducing the volume of virgin aggregates quarried and the environment benefits begin to stack up, ' says Mr Cook.

The philosophy behind Foamix is relatively simple.

Road surface planings are mixed with a binding agent (in this case pulverised fuel ash), passed through a machine that injects hot bitumen into the mix, and is then taken back to the site and re-laid.

The hot bitumen is injected into the mix alongside compressed air and water under pressure. This causes the bitumen to foam and the PFA filler 'snatches' it so that no bitumen actually coats the aggregate, unlike with more traditional surfacing methods.

'It is a similar reaction to that which occurs when you pour a fizzy drink over ice cubes. The liquid froths up and that is when the filler material grabs hold of it, ' explains Mr Cook.

With the filler holding on to the bitumen in microscopic bubbles, the resulting material is virtually dry as it leaves the recycling plant.

The road base is laid with conventional plant.

'When it is put down, it looks like it should not work but, after it has been rolled, the surface is excellent, ' says Mr Cook.

It is the rolling of the material that plays a the most significant part in giving it its final strength.

'In simple terms, a conventional material relies on the aggregate being coated by bitumen to gain its strength.

In this case, we want the bitumen to stay off the aggregate until it is rolled, ' says Mr Cook.

Bitumen is held on the fine filler in microscopic bubbles, which burst when the mixture is rolled. This causes the bitumen to spread throughout the mix and gain its structural strength.

Mr Cook is convinced the Foamix system holds the key for the future of road construction but says it needs further backing at Government level.

'The next stage is to get it used as the norm rather than the exception - and that can only come from the highest level. The roads of Britain could become its quarries.'

'The more we recycle, the less we quarry'

MARTIN RILEY, project manager for contractor Tarmac National Contracting, has become accustomed to showing off his site.

On a windswept section of the M6, near Killington Lake, Highways Agency engineers and others have come to watch the Foamix material placed on the motorway network. He has no misgivings over the use of the recycled road base.

'It has been working very well. The Highways Agency knows the material from previous applications and it has complete confidence in it, ' he says.

A 150 mm-thick layer is being laid on the inside lane of the southbound motorway. The hard shoulder, middle and outside lanes are being reconstructed using a conventional surfacing material.

Some 340 mm of material is being planed out of the inside lane, 150 mm from the hard shoulder and 100 mm from the middle and outside lane. These road planings will make up the stone for the recycled road base.

It may not seem a massive amount of aggregate but, to Mr Riley, it is better than nothing.

'We are just on the cusp of increasing the use of recycled materials. The more recycled aggregate we use, the less virgin stone is quarried and that helps our existing land banks to last longer, ' he says.