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Keep it close to home

PFA

Quality pulverised fuel ash and sitewon aggregates are making the concrete on the UK's first tolled motorway much more sustainable.

Alan Sparks reports from Birmingham.

At two junctions, either side of the notorious bottleneck near Birmingham, weary travellers on the M6 will have seen the tips of the red soiled iceberg of a site that is being driven around the north east of England's second city.

Few are aware of the monster 43km dual three lane motorway and its more than 50 bridges that are under construction just out of sight. But by 2004 this will be a tolled alternative to the current M6 which promises to be less congested.

Right now, however, handling the sheer volumes of muckshift, concreting, asphalting and aggregates on what is the UK's largest road job is a real test for the CAMBBA consortium of Carillion, Alfred McAlpine, Balfour Beatty and Amec. This titanic team is tasked with delivering the £485.5M design and build job on the button.

'The scope of these works is quite phenomenal, ' says CAMBBA head of community relations Chris Jackson. 'Without doubt, none of us have been involved in a job of this size before. It really is difficult to express the full scale of it.'

As well as more than 50 bridges, there will be six toll stations, a new M6/M42 interchange, seven other junctions, culverts and retaining walls, environmental mitigation and landscaping. An Arup-Atkins joint venture is acting as structural engineer on these various elements.

Satisfying the unquenchable hunger for materials was an initial concern for the design team. From start to finish the M6 monster will consume over 250,000m 3of concrete, including 1.1Mt of aggregate.

The whole point of building the new road is to relieve congestion on the M6, so CAMBBA could not aggravate this with a mighty number of lorry journeys. Fortunately the solution lay beneath its feet. 'All aggregate used will have been won from the works, ' explains Jackson.

Another piece of good luck was the existence of a potential concrete works at Weeford, halfway along the site. This was under construction by concrete producer Hanson as a ready mixed mortar plant, but was swiftly converted to concrete production - including the installation of the biggest aggregate washing plant in the UK.

On the advice of Rugby Cement, CAMBBA decided to use pulverised fly ash (PFA) as a partial replacement for the Rugby CEM I cement in all concrete mixes, even though Hanson had never used PFA before.

'The benefits of PFA are well publicised and it was the reduced stresses and crack reduction that made it ideal for this job, ' explains CAMMBA laboratory quality manager Graham Booker. 'And for the structures, when done properly, the finish is much better.'

Around half of the road's length will be 220mm deep continuously reinforced concrete pavement, with either end built using a bituminous road base. Although this is a landmark project, it is not a playground for aspiring technologies. 'Cutting out risk is a key consideration here. Massive penalties face us should we fail to deliver on time, and so a team of contractors with expertise in just this type of scheme was fashioned, ' explains Jackson.

'This meant that we opted for the devil we know, in that we used techniques that were familiar to everyone involved rather than stacking our chips on developing technology - so cutting out risk.'

Booker agrees, 'We could have made changes to the design as we progressed, but any potential benefits were simply outweighed by the added risk.'

All concrete testing was undertaken at the main site offices. 'Over the course of the job we will have handled around 20,000 cubes and carried out 17,000 aggregate grading tests, ' says Booker.

This makes it the busiest testing laboratory in the country with 44 staff frantically ensuring that specified site quality was met during the peak of the works.

'Having the lab on site means that as all monitoring, testing and reporting is handled here, we can develop the mix designs quickly, with real savings in transport time and costs, ' acknowledges Jackson of his colleagues' weighty influence on the job's progress.

The job is rattling along at an average of 400m per day. Steel reinforcement sits at mid-depth of the four lane continuous concrete pavement laying, with no expansion joints required.

'The high level of PFA in the mix sufficiently reduces thermal expansion of the concrete and so far, with the road immediately sprayed with a bitumen coating, we have had no major cracks to report, ' says Booker.

For the 126,000m 3of C40 pavement quality concrete that was batched on site 25% of the 370kg/m 3cement content is being replaced with PFA, while for the 130,000m 3of the 390kg/m 3cement content structural concrete supplied by Hanson 30% PFA was used. In total over 22,000t of PFA will be delivered to the project As for the bridges, motorists will find that most of the structures along the route will be basically identical. Here, concrete abutments were replaced with reinforced earth and insitu concrete kept to a minimum. The major concrete formed on site is the central support columns.

Precast elements are arriving from Northern Ireland with the associated savings of site time, labour and formwork.

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