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T5 Concrete

More than 700,000m 2of high technology concrete paving will surround the T5 complex. Dave Parker reports on the unique challenges involved.

Over the decades, many hectares of concrete paving have been laid at Heathrow. Most of these are founded more or less directly onto high quality substrates, mainly gravels. T5 is different - there was a very good reason why this was the last area within the Heathrow perimeter to be developed. The pavements will lie on an area of clay fill several metres thick.

Incentives to re-use the soft clays excavated during the massive earthworks programme on site are irresistible. Even without the Landfill Regulations, the daunting logistical puzzle that the £4bn project represents can only be solved by minimising the movement of bulk materials through the huge site's single access point.

So the pressure was on to reuse and recycle as much of the material on site as possible, even if this meant adopting radical solutions and ditching some of BAA's long-proven pavement practices.

Responsibility for coming up with the answer fell to the T5 Pavements Task Team - an integrated team consisting of BAA, TPS Consult and Amec. The T5 Team worked in close partnership with the BAA/Amec Pavement Team which has been responsible for a number of major projects at all London's main airports.

T5 would be a different degree of challenge, however. Not only would a much stronger mix be needed (see box), but a whole new pavement design as well.

'Typically the clay fill has a CBR of 2% as opposed to the 20% with the materials we normally work with at Heathrow, ' explains BAA T5 airfield pavement design engineer Richard Moore.

'This required a complete rethink of the PQC slab and base layer design.'

Instead of the 300mm of classic Type 1 sub-base topped with 150mm of wet lean concrete that would have underlain the earlier design of 450mm thick slab, TPS and Amec came up with a radical alternative. On top of the fill would sit a simple 250mm thickness of in situ cement bound material (CBM) - using gravel largely sourced from within the T5 site.

T5 airfield project leader Peter Neale says that the decision to eliminate the wet lean concrete was only taken after long analysis.

The advantages were obvious.

'CBM production rates were higher - up to 4,000m 2a day - no paver was needed, and on the areas we're working on now all the gravels come from site, ' he says.

An added bonus is the contribution to sustainability, with truck movements reduced by 13,000 overall. In practice, CBM production became part of the earthworks programme. Laing O'Rourke's main earthworks subcontractor CA Blackwell brought in two stabilising rotavators to mix the cement and gravels together, handing over a strong, durable working platform to Amec on which to place the PQ concrete.

Before any concrete is laid, however, an equally crucial operation has to be progressed.

T5 is a complex organism, perhaps the most highly serviced airport development yet designed. Under the concrete 'skin' of its surrounding pavements lies a vital network of nerves and arteries - ducts and pipelines conveying everything from computer cables to jet fuel. So many of these services have to be accessible through the slab that these service openings could have seriously interfered with the paving process 'The head of stand service areas would have been particularly difficult, ' says Amec production leader Tony Heron. 'Using conventional technology would have meant up to 40% of the pavement couldn't be slipformed - our target was no more than 10%.'

The team increased the percentage of machine laid concrete by co-ordinating the pavement bay layout design with the service pit locations and by paving straight over the pits. 'We involved the paving foreman at an early stage of the design process to ensure that the pits were coordinated correctly with the bay layouts, ' says Moore.

Working with its precast concrete subsidiary C V Buchan, Amec developed a range of slim precast pits and access chambers with integral jacking systems so that they could be levelled precisely. The chamber covers can be set initially 80mm low so that the paver can pass directly over them without trouble.

'Then we survey the slab very accurately, saw through the 80mm of PQ concrete and jack the chamber up into its final level and position, ' Heron explains.

Taxiways and stands will all be made up of 7.5m square bays.

This will keep the wheel loads from wide-bodied jets away from the longitudinal joints and hence minimise edge stresses. An improved construction joint detail maximises load transfer; a new, neater precast Elkington Gatic slot drain takes away rainwater.

Like all the T5 teams, the PQC team has to work to a complex construction programme with many interfaces, both with the rest of T5 and the operating airport itself. It will be more than three years before all the PQ concrete is laid and all services installed - by which time no doubt both the mix and the production process will have been developed even further.

Working in partnership with Amec and admixture specialist Grace, the team set up a series of full-scale slipforming trials at Stansted. These confirmed that it would be possible to produce single pass slipformed F7 concrete up to 640mm deep and 7.5m wide - provided the supply to the paver could be maintained.

The mix was also included in a number of pavement projects at Heathrow, Gatwick and Stansted.

This allowed the team to identify the sensitivities of the F7 mix and produce the detailed specifications required for the T5 batching plant and slipform paver.

The current mix contains a cementitous blend of 70% Hope CEM II cement from Lafarge and 30% PFA. Aggregates are Mendip Limestone. Admixtures include both high range water reducers and an air entraining agent (aea).

'We did a value engineering exercise to see if we needed the aea, ' says Moore. 'In fact we found that a 5% air content resulted in an overall cost saving.'

Trials continue - PFA replacements of 40% are under evaluation; an F8 mix remains a tantalising possibility.

Moving to a higher strength mix does a lot more than solve the challenge of the A380. Pavements across the entire project can be significantly reduced in depth, by up to 200mm. The savings on a project of this size are obviously phenomenal.

Sustainability also benefits.

There will be 14,000 fewer truck movements as a result of the thinner slabs. Savings in cement will equate to a reduction of 60,000t of CO 2coming out of the cement kilns. And the use of borehole water on site cuts demand for potable water by 54M. litres.

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