Getting the new mix design right was just part of the answer to the T5 PQ concrete challenge. Producing 350,000m 3of what is inherently a very stiff mix to the very tight workability limits needed for the paving operation at rates of up to 290m 3per hour is pushing current concrete production technology to its limits.
The paver itself also has to operate at the edge of the possible, way beyond the established 'string line technology' that has served so well so far.
Construction of the hardstandings and taxiways falls to Amec. Production leader Tony Heron says the big challenge was the fine control of water in the mix.
'We're using two Belgianmade Compactor plants combined into one facility, each with a 4.5m 3capacity horizontal twin screw mixer which mixes stiff concrete a lot faster than a pan mixer.
'Aggregates are fully surface saturated before they're batched, and we're currently developing microwave monitoring of workability in the mixer.'
The latest technology was also needed on the chosen paver, a Gomaco GHP 2800 four-track machine. This is a heavier, more robust version of the paver that conducted the Stansted trials, and came out of the US factory already fitted with remote guidance. String lines are out - instead, the paver communicates constantly with a network of Leica Geosystems automatic total stations for control in all three dimensions.
Also new, says Gomaco International managing director Rory Keogh, are sensored hydraulic vibrators. 'These are linked to the forward speed of the machine, so that the concrete always gets the correct amount of vibration, ' he says.
'Although we know of no other projects in the world where slabs this thick and this wide have been slipformed on this scale, we were always confident we could do it. After all, we regularly slipform motorway barriers up to 2.4m high, ' he adds.
To date, most of the slabs poured have been between 490mm and 510mm thick. The next phase of the T5 construction will include slab depths of up to 600mm. After a few of the inevitable teething troubles, says Heron, production has gone well.
'In fact, our surface level tolerances are the best the pavement team has ever achieved, ' he adds.