High flexural strength is what an unreinforced concrete pavement needs if it is to transfer the wheel loads from large jet aircraft to the ground below. Heathrow in general and T5 in particular present a special challenge to the concrete technologist.
More than 50% of the airliners using the runways and taxiways are wide-bodied jets, with wheel loads of up to 23t in the case of a fully loaded and fuelled Boeing 747. Compare that to the world's busiest airport, Atlanta, which may boast of more plane movements than Heathrow, but can claim no more than 10% of them are big, wide-bodied planes.
Heathrow must also prepare for the advent of the daddy of them all - the monstrous double deck Airbus A380, with its projected 28t wheel loads. Many of the A380s that land at Heathrow will be handled by T5B. BAA knew well in advance of the T5 go-ahead that a major leap in concrete technology was needed to cope with the A380.
'If we had stuck with our long established F5 mix, the slab would have had to be 800mm thick, well beyond the capacity of modern slipform pavers, ' explains TPS T5 aircraft pavement design engineer Richard Moore.
'We needed a mix that could be slipformed in a single pass, preferably one that didn't need a lot of expensive admixtures.'
In 1998 the Pavement Team started slipforming with a new F6 mix, which for the first time had part of its Portland cement content replaced with PFA. The designation comes from the characteristic flexural strength - the long-term objective was to move onto even stronger mixes, with flexural strengths of 7N/mm 2or even more.