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Cracking good idea

Airports Heathrow runway

Heathrow Airport is preparing for the superjumbo A380. One technological breakthrough should help control costs, reports Dave Parker.

The biggest, heaviest aircraft using London's Heathrow Airport are fully loaded 420t Boeing 747-400s with 69m wingspans. But that is soon to change.

Preparing the world's busiest wide body jet airport for the arrival of the Airbus A380 - whose wings stretch out a further 11m and which can weigh up to 550t at take-off - poses many special challenges for airport operator BAA's pavement and infrastructure team.

Terminal 5 was designed to take the A380 from day one, but when the first A380 touches down in the spring of 2006 it will offload its five or six hundred passengers at Terminal 3.

Already the reconstruction of Pier 6 at T3 has involved building a new set of stands to take three A380s. These used 450mm of BAA's proven F6 pavement quality (PQ) concrete mix on 150mm of cement bound material (CBM), rather than the normal 350mm of lower flexural strength F5 PQ.

That was the easy part, says BAA infrastructure delivery team leader Duncan Pickard.

'Getting the A380 from the runway to the stand is the real challenge. Its wider wingspan means we have to move the centrelines of the taxiways between 10m and 33m away from the runway and increase their width by 2m on average.

'Luckily, a lot of these taxiways were coming up for reconstruction anyway, ' he adds.

In the past BAA would have broken up the concrete taxiways, crushed and recycled them as aggregate, and relaid another PQ pavement. But with the pressure on across Heathrow to minimise the disruption caused by construction traffic and save time, alternative methods of rehabilitation had to be considered.

The pavement and infrastructure team consists of BAA, consultant TPS and contractor Amec.

A value engineering exercise soon threw up at least one very promising alternative rehabilitation technique, although Amec continued to plan for the traditional option.

And it was on the UK road network that the team found what they were looking for.

Concrete highways in the UK are now routinely upgraded by a technique dubbed 'crack and seat'. This uses some form of falling weight to create hairline cracks through the depth of the slab at close centres.

The cracked pavement is then overlaid with a relatively thin asphalt layer. Close spacing of the cracks reduces the risk of any reflection cracking failure at the asphalt surface.

Assuming the technique could be adopted for PQ pavements, the potential benefits for BAA were obvious. All the original concrete would remain insitu, with only the asphalt and the extra PQ concrete needed for widening brought in from outside.

'But we didn't know if crack and seat would work in our circumstances, which are very different from roads, ' Pickard reports. 'On highways you have lots of relatively light vehicles moving fast. On our taxiways the speed limit is 30mph, but we have very heavy vehicles at relatively low frequency.'

So in January last year BAA commissioned consultant WSP to carry out a desk study with TPS into any previous use of crack and seat on airfield taxiways anywhere in the world.

The study revealed that there had indeed been instances of the use of the technique on taxiways on several airfields both civil and military - but none carried traffic of the weight and frequency that Heathrow's new taxiways would experience.

'It also showed there was absolutely no data anywhere on how pavements behave under aircraft loading', Pickard adds. 'So WSP had to carry out full scale trials on one taxiway, using deflection gauges specially made by TRL.'

Four areas were monitored;

a control area of original taxiway and three 'cracked and seated' areas with different crack spacing ranging from 750mm to 1.5m. No overlay was used. BAA senior project manager Matt Palmer says the results surprised everyone involved.

'We were expecting deflections of anything up to 5mm. In the event we could hardly measure them, ' he says, adding that Heathrow's largely excellent ground conditions made a significant contribution. 'They were way below 1mm, and there was very little difference between the different crack patterns.'

The trials allowed WSP to verify its computer model of the pavement and a 1m crack spacing was selected.

On highways the cracks induced are purely transverse.

But at Heathrow, with pavement widths of at least 25m, cracking was needed in both directions, producing 1m square 'platelets'. An American pneumatic-tyred guillotine breaker was imported by USowned cracking sub-contractor Antigo Breakers to cope with the 350mm depth of 30 year old concrete on the taxiways.

'This breaker has a variable drop height, ' Palmer explains.

'It's vital not to 'overcrack' - the interlocking friction of the cracked faces is needed to transfer load properly.'

Work began on the first crack and seat section of the £50M southern taxiway rehabilitation project at the end of May. Cracking is complete and re-installing the web of services under way. Getting power to the Aircraft Guidance Lighting (AGL) was the second headache, according to Palmer.

'Traditionally the cables are threaded through ducts below the PQ slab, ' he explains. 'This wouldn't be cost effective in this situation.'

Further analysis revealed that the cabling between individual lights hardly ever failed at all. Any faults occurred with the lights themselves.

'So we decided to lay the cabling in chases in the lower layer of stone mastic ashphalt (SMA) and cover it in the final pass, ' Palmer reports.

This was an important decision. 'Lose too many lights and the taxiway has to close, ' Pickard points out. But the combination of crack and seat and cables in the asphalt has so many potential benefits that BAA took the plunge. And the results to date have fully met expectations, he reports.

'There's been a 90% reduction in construction traffic compared to conventional rehabilitation, helped by using up to 10% asphalt planings in the resurfacing.

Time on site has been cut by more than 15%.

'In terms of cost per square metre crack and seat, with 110mm of SMA, is 25% cheaper than a PQ pavement. And overall it saves at least 10% on the project costs.'

Reducing lorry movements brings more benefits than simply improving relationships with the local community, important though this is. Much of the traffic has to cross live runways; a complicated logistical challenge and one that has been minimised by the choice of crack and seat.

Once the SMA is down on the first phase the big red guillotine will get to work on the next section. This is planned for later this year.

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