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Tarmac paves the way for aggregate savings

Demolition/Recycling; 'Phenomenal' is not too strong a word to describe Tarmac's innovative mix of recycled and low-energy materials going into an experimental road near Edinburgh. Ty Byrd reports

VIRTUALLY the only virgin materials being used on the Echline project - an experimental road pavement at South Queensferry - are bitumen and cement, with a little new stone thrown in. Nearly everything else is recycled or secondary aggregates.

The wearing course is a cold-lay bitumen emulsion asphalt including steel slag coarse aggregate, blastfurnace slag fine aggregate and the aforementioned bitumen - a high-performing polymer modified emulsion binder from bitumen specialist Nynas.

A hot-mix asphalt incorporating recycled asphalt planings and foundry sand is being used for the base course. The road base is formed of cold lay slag-bound material made up of recycled aggregates bound with granulated blast furnace slag and PFA, activated with gypsum and lime.

Under this, the sub base layer consists of spent oil shale aggregates bound with PFA and cement. Below this, in situ stabilisation of the subgrade has been carried out with PFA and cement.

All this has come about as a result of an unusual and progressive contract let by the Scottish Executive Development Department, which invited a number of contractors to bid for the chance to partner it in the construction of a sustainable pavement. Tarmac was the preferred bidder.

Tarmac technical director Colin Loveday says: 'We had total freedom to come up with the concepts which we then had to justify to the client and its consulting engineer, Scott Wilson. It enabled us to put into practice a number of ideas we had been researching over the past few years.'

Typical trunk road design constraints of 100 msa (million standard axles) and 40-year lifespan were demanding and meant that 'cheap and cheerful' recycling was not an option.

'We opted for a lot of stabilisation, both in the ground and in the secondary granular materials using secondary hydraulic binders like PFA and slag wherever possible,' Mr Loveday says. As a result, the road looks like being even stronger than a conventional one, with savings that are being described as 'phenomenal'.

Mr Loveday says: 'Taken along a kilometre of dual carriageway, our Echline design would use about 5,000 tonnes of virgin aggregate compared with 45,000 tonnes on a standard design.

'In practice it is unlikely that all these techniques would be used together in this way on a full-scale job, but Echline points out what can be done in the future.'

The Echline experimental road is a 100 m long by 6 m wide single carrigeway that will not be trafficked in the medium term. Soil was broken on site last November but work began in earnest this spring and was scheduled to be completed this month. The care that has been taken in designing every layer is best exemplified in the wearing course, which has been the subject of much research.

Cold lay emulsion asphalt is still relatively new in Britain, although its advantages are well charted. Cold lay materials use less energy in their production; unlike hot materials they do not have to be placed soon after they are mixed; and they are particularly user friendly.

Tarmac, together with Nynas, pioneered a cold lay stone mastic asphalt in Britain during the '90s which was trialed on the A52 trunk road in Staffordshire. The 120 m-long stretch was opened to traffic immediately after laying in mid 1997 and continues to perform well.

The key to a cold mix is the bitumen binder. 'The binder being used for the Echline project is similar to the one we supplied for the A52,' says Nynas asphalt engineer Paul Acock. 'It's a polymer modified emulsion especially developed for cold lay applications, produced at our refinery at Ellesmere Port.'

The Echline mix differs from that used on the A52 in that basic oxygen steel slag is being used for the coarse aggregate, with blast furnace slag (BFS) as the fine aggregate, replacing conventional virgin stone and sand. Both are industrial by-products from the production of iron and steel.

'Steel slag can display many of the beneficial characteristics of premium wearing course aggregate and is potentially a valuable sustainable resource,' says Tarmac research and development manager Howard Robinson.

The material has a comparatively high density, hence the use of low-density BFS dust to counter this. Trials of the asphalt mix were carried out in March at Tarmac's Darlton Quarry in Derbyshire, where the validated material for Echline's wearing course was eventually dispatched to South Queensferry. Darlton is geared to the manufacture of cold mix asphalt, and was a more viable option than commissioning a Scottish plant to make the required tonnage.

Moving down through the Echline road's layers, use of recycled asphalt planings in hot mixes is not common in Britain although it does occur in other parts of the world. Echline's base course includes 10 per cent by volume of such planings, as well as 10 per cent foundry sand.

Tarmac plays a leading role in promoting the use of recycled aggregates and it was the company's recycling depot at Addiewell, West Lothian, which was the source of the recovered stone used at South Queensferry.

Echline's slag bound material (SBM) roadbase is based on proven continental European technology and is another sustainable technology little employed in Britain, although it has been the subject of trials in recent times. SBM is a slow cementing continuously graded roadbase material containing granulated blast furnace slag (GBS) as the binder.

GBS is similar to cement in that, in a high alkaline environment, it hydrates. It has been used at Echline with added lime to help the hydration process to bind recycled aggregates.

'The French did much to develop SBM which is known as Graves-laitier,' says Mr Robinson. 'The Highways Agency recognises it as a proven technology in Europe, and will include SBM this year in the Specification for Highway Works.'

Another industrial by-product which has gone into the Echline road is Red Blaes, or spent oil shale. Throughout the 19th century, Edinburgh supported a small but vibrant oil industry, oil bearing shale being mined in the area with the oil being extracted and the spent shale piled in unsightly 'bings'.

Blaes is a comparatively weak, porous material, much like a weak limestone.

Tarmac reckoned it would make a good sub-base material and turned to consultant TRL's Edinburgh office for guidance.

TRL was retained by Tarmac for general advice on mix design and to be the design validator called for in its contract with the SEDD.

'We also acted as the SEDD's observer during construction of the Echline road, charged with keeping an accurate written and photographic account of the project,' says TRL's principal engineer in Edinburgh Dr Mike Winter.

TRL confirmed to Tarmac that use of blaes would be suitable, especially if the material was stabilised with a hydraulic binder such as PFA/cement.

It all adds up to one of the most adventurous experiments in sustainable construction so far.

The Scottish Executive has a declared policy of encouraging research into greater use of recycled materials in road building and repair; and Echline is part of the Executive's involvement with the Construction Industry Research & Information Association's programme of demonstration projects on the use of recycled materials.

'This experiment will help determine what secondary materials can viably be used in trunk road construction,' says Forbes Macgregor, a senior engineer in the SEDD's trunk roads divisions. 'The bottom line is about reducing the amount of virgin primary aggregates being used and also ameliorating environmental impact, of past activity as well as current.'