In July 2003 a Gomaco slipform paving machine started laying pavement quality concrete that was thicker and stronger than anything ever attempted before. More than two years later there are three paving teams in full swing at London Heathrow - but such is the giant scale of the construction project that 'only' 35% of Phase 1 has been completed so far. Phase 1 totals 675,000m 2 of concrete taxiways and hardstandings up to 575mm thick, and terminal operator's BAA's T5 pavement team (BAA, TPS Consult and Amec) is quietly confident it will hit the November 2007 completion date with ease.
'Bigger areas are opening up to us now the bulk earthworks were completed at the end of July, ' reports T5 airfield project leader Matt Palmer. 'When we started, our single team could lay up to 900m 3 a week - now we're up to 3,300m 3.' Another reason for the accelerated rate of progress is the increasing sophistication of the Gomaco pavers themselves.
'The side plate depth can be adjusted hydraulically over a range of 350mm-600mm, ' TPS T5 aircraft pavement designer Richard Moore explains.
'This means we can vary the depth laid as the machine moves along, tailoring the thickness [of the PQ concrete] exactly to the design loading without stopping to reset the paver or keeping a constant depth to simplify the paving operation.
'So the average overall thickness is less than originally envisaged and the rate of progress higher.' When work began the paver ran on cement bound gravel excavated within the T5 site.
This source is now exhausted, and the team has reverted to a more traditional granular sub-base made from recycled site won materials topped with 115m of wet lean mix concrete using locally recycled concrete as aggregate. The sub base profile is varied to suit the service loading conditions, the lean mix is laid by the pavers to a constant thickness, and the PQ thickness is varied to achieve a level surface.
T5 sees the first large scale use of BAA's own specially developed F7 (7N/mm 2 flexural strength) PQ concrete. If the long established lower flexural strength F5 (5N/mm 2) mix had been chosen, some areas would have had to be at least 800mm thick to cope with the anticipated 28t wheel loads of the A380 super jumbo. This is well beyond the capacity of any current slipform paving technology.
'We've learned a lot about F7, ' Palmer declares. 'One of the most important aspects is the variability of silt content of the limestone aggregate, which comes from clay strata in the quarry.
'The aggregate complies with the British Standard, but we need tighter control. So we're currently experimenting with an aggregate washing plant at [aggregate supplier] Foster Yeoman's rail head at Colnbrook.' Moore adds: 'The silt affects the water demand and the bond between the aggregate and the cement matrix. We've always achieved our target strengths but the variability was too high.
Getting the silt content under control is the key to more consistent flexural strength - and workability.' Another difference between the F7 mix and the intermediate F6 mix used elsewhere in Heathrow is 'shelf life', Palmer says. 'F6 can survive a 40 minute journey from batcher to paver, which is often the norm in the central Heathrow area. F7 has a shelf life of more like 20 minutes, so it's best suited for a greenfield site like T5.' Also under scrutiny is contraction joint sealing. Normal practice is to make a 5mm wide transverse saw cut into the green concrete to induce controlled full depth cracking then come back 21 days later and widen out the original cut to 25mm before sealing with a polyurethane compound.
Joints between adjacent paver runs receive much the same treatment. And, apart from the initial cost and complexity of the sealing operations, the sealant rarely lasts longer than 10 years before needing replacement.
'It would be a massive saving on maintenance if we could make sealant-free joints work, ' Palmer points out. 'So we're currently experimenting with an unsealed joint based on a very high quality 3mm saw cut.
'Expansion joints and joints around service pits still have to be sealed - but two thirds could be left unsealed, including the longitudinal joints, ' he says.
Bringing up the rear of the paving train at the moment is a machine which sprays on a curing compound after applying a wire brush texture to the concrete surface. 'But a wire brush texture tends to wear off after 18 months or less, ' Moore points out.
'What we would like to do is leave the concrete with the finish achieved by the wet hessian drag on the rear of the paver itself.
This would provide essentially the same skid resistance as the surfacing on the main runway.
Britpave, the transport infrastructure group, has published a new design handbook explaining how to get the best out of hardstanding design and construction. It provides the latest advice on selecting appropriate concrete classes for hardstanding in accordance with the new European Standard BS EN 206 and incorporates the latest foundation classes developed by the Highway Agency, adapted for use with concrete pavements by the Transport Research Laboratory. Concrete Hardstanding Design Handbook is available from the Concrete Bookshop.